Losing weight is seldom the goal—losing fat is, while preserving muscle. Unfortunately, your body is reluctant to give up its reserves of energy (its fat) and all too quick to metabolize your hard-earned muscle when the caloric deficit gets just a little too high.

Fortunately, what you eat while on a calorie-restricted diet can affect where the weight loss comes from. Diets high in protein, in particular, have benefits with regard to energy metabolism, appetite, overall caloric intake, and muscle retention.

Appetite and energy metabolism

Protein reduces appetite (and thus food intake) by increasing the levels of certain peptides, such as glucagon-like peptide 1 (GLP-1).[reference|url=https://pubmed.ncbi.nlm.nih.gov/11549680|title=A meta-analysis of the effect of glucagon-like peptide-1 (7-36) amide on ad libitum energy intake in humans|published=2001 Sep|authors=C Verdich, A Flint, J P Gutzwiller, E Näslund, C Beglinger, P M Hellström, S J Long, L M Morgan, J J Holst, A Astrup|journal=J Clin Endocrinol Metab|]

In addition, your body needs to expend 20–30% of the caloric value of protein to metabolize and store it, compared to 5–10% for carbs and just 0–3% for fat.[reference|url=https://pubmed.ncbi.nlm.nih.gov/19400750|title=Dietary protein, weight loss, and weight maintenance|published=2009|authors=M S Westerterp-Plantenga, A Nieuwenhuizen, D Tomé, S Soenen, K R Westerterp|journal=Annu Rev Nutr|][reference|url=https://pubmed.ncbi.nlm.nih.gov/15466943|title=The effects of high protein diets on thermogenesis, satiety and weight loss: a critical review|published=2004 Oct|authors=Halton TL, Hu FB|journal=J Am Coll Nutr|][reference|url=https://pubmed.ncbi.nlm.nih.gov/8878356|title=Thermic effect of food and sympathetic nervous system activity in humans|published=1996|authors=Tappy L|journal=Reprod Nutr Dev|] In other words, the thermic effect of food (TEF) is highest for protein. Whether this makes a practical difference in human beings outside of a clinical setting is a much-debated topic,[reference|url=https://pubmed.ncbi.nlm.nih.gov/25926512|title=The role of protein in weight loss and maintenance|published=2015 Jun|authors=Leidy HJ, Clifton PM, Astrup A, Wycherley TP, Westerterp-Plantenga MS, Luscombe-Marsh ND, Woods SC, Mattes RD|journal=Am J Clin Nutr|] but diets high in protein have also been shown to mitigate the decline in resting energy expenditure (the calories the body burns at rest) caused by calorie-restricted diets,[reference|url=https://pubmed.ncbi.nlm.nih.gov/23097268|title=Effects of energy-restricted high-protein, low-fat compared with standard-protein, low-fat diets: a meta-analysis of randomized controlled trials|published=2012 Dec|authors=Wycherley TP, Moran LJ, Clifton PM, Noakes M, Brinkworth GD|journal=Am J Clin Nutr|] perhaps because protein promotes muscle retention.[reference|url=https://pubmed.ncbi.nlm.nih.gov/19179060|title=High protein diets decrease total and abdominal fat and improve CVD risk profile in overweight and obese men and women with elevated triacylglycerol|published=2009 Oct|authors=P M Clifton, K Bastiaans, J B Keogh|journal=Nutr Metab Cardiovasc Dis|][reference|url=https://pubmed.ncbi.nlm.nih.gov/18752682|title=A controlled trial of protein enrichment of meal replacements for weight reduction with retention of lean body mass|published=2008 Aug 27|authors=Leo Treyzon, Steve Chen, Kurt Hong, Eric Yan, Catherine L Carpenter, Gail Thames, Susan Bowerman, He-Jing Wang, Robert Elashoff, Zhaoping Li|journal=Nutr J|]

A trial randomized 130 overweight people between a high-protein group and a low-protein group. Each day, the high-protein group consumed 1.6 grams of protein per kilogram of body weight (1.6 g/kg, so 0.73 g/lb) and the low-protein group 0.8 g/kg (0.36 g/lb). The trial’s 500 kcal daily deficit led to a weight loss of 9.9–11.2% over a year with no difference between groups, but the high-protein group lost more fat (14.3%  ± 11.8%) than the low-protein group (9.3% ± 11.1%), and men more fat than women.[reference|url=https://pubmed.ncbi.nlm.nih.gov/22691622|title=Effects of protein intake and gender on body composition changes: a randomized clinical weight loss trial|published=2012 Jun 12|authors=Ellen M Evans, Mina C Mojtahedi, Matthew P Thorpe, Rudy J Valentine, Penny M Kris-Etherton, Donald K Layman|journal=Nutr Metab (Lond)|]

The higher protein intake still benefited women, though, as it did in other trials. When a 10-week trial enrolled 11 obese women to compare a high-protein diet (30% of daily calories as protein) with a high-carb diet (55% of daily calories as carbs), the high-protein diet led to greater weight loss (4.4 kg, so 9.7 lb), most of which was fat (3.7 kg, so 8.2 lb).[reference|url=https://pubmed.ncbi.nlm.nih.gov/15806847|title=Effects of protein vs. carbohydrate-rich diets on fuel utilisation in obese women during weight loss|published=2003|authors=Idoia Labayen, Nieves Díez, Alvaro González, Dolores Parra, J Alfredo Martínez|journal=Forum Nutr|] Relatedly, in a 12-week trial with 100 overweight or obese women, the high-protein and high-carb groups lost the same weight, but the high-protein group lost more fat,[reference|url=https://pubmed.ncbi.nlm.nih.gov/15941879|title=Effect of an energy-restricted, high-protein, low-fat diet relative to a conventional high-carbohydrate, low-fat diet on weight loss, body composition, nutritional status, and markers of cardiovascular health in obese women|published=2005 Jun|authors=Noakes M, Keogh JB, Foster PR, Clifton PM|journal=Am J Clin Nutr|] which is the same result as in the trial presented in the paragraph above.[reference|url=https://pubmed.ncbi.nlm.nih.gov/22691622|title=Effects of protein intake and gender on body composition changes: a randomized clinical weight loss trial|published=2012 Jun 12|authors=Ellen M Evans, Mina C Mojtahedi, Matthew P Thorpe, Rudy J Valentine, Penny M Kris-Etherton, Donald K Layman|journal=Nutr Metab (Lond)|]

Of course, adding exercise can help. In a 16-week trial, 90 overweight or obese women followed the same exercise regimen (aerobic exercise and resistance training) and were randomized to three groups: high protein, high dairy; adequate protein, medium dairy; and adequate protein, low dairy. The first group experienced greater fat loss and muscle gains.[reference|url=https://pubmed.ncbi.nlm.nih.gov/21775530|title=Increased consumption of dairy foods and protein during diet- and exercise-induced weight loss promotes fat mass loss and lean mass gain in overweight and obese premenopausal women|published=2011 Sep|authors=Andrea R Josse, Stephanie A Atkinson, Mark A Tarnopolsky, Stuart M Phillips|journal=J Nutr|]

And while the four trials above had enrolled young and/or middle-age women,[reference|url=https://pubmed.ncbi.nlm.nih.gov/22691622|title=Effects of protein intake and gender on body composition changes: a randomized clinical weight loss trial|published=2012 Jun 12|authors=Ellen M Evans, Mina C Mojtahedi, Matthew P Thorpe, Rudy J Valentine, Penny M Kris-Etherton, Donald K Layman|journal=Nutr Metab (Lond)|][reference|url=https://pubmed.ncbi.nlm.nih.gov/15806847|title=Effects of protein vs. carbohydrate-rich diets on fuel utilisation in obese women during weight loss|published=2003|authors=Idoia Labayen, Nieves Díez, Alvaro González, Dolores Parra, J Alfredo Martínez|journal=Forum Nutr|][reference|url=https://pubmed.ncbi.nlm.nih.gov/15941879|title=Effect of an energy-restricted, high-protein, low-fat diet relative to a conventional high-carbohydrate, low-fat diet on weight loss, body composition, nutritional status, and markers of cardiovascular health in obese women|published=2005 Jun|authors=Noakes M, Keogh JB, Foster PR, Clifton PM|journal=Am J Clin Nutr|][reference|url=https://pubmed.ncbi.nlm.nih.gov/21775530|title=Increased consumption of dairy foods and protein during diet- and exercise-induced weight loss promotes fat mass loss and lean mass gain in overweight and obese premenopausal women|published=2011 Sep|authors=Andrea R Josse, Stephanie A Atkinson, Mark A Tarnopolsky, Stuart M Phillips|journal=J Nutr|] there’s evidence that postmenopausal women on a calorie-restricted diet also lose less muscle when they eat more protein.[reference|url=https://pubmed.ncbi.nlm.nih.gov/18589032|title=Lean mass loss is associated with low protein intake during dietary-induced weight loss in postmenopausal women|published=2008 Jul|authors=Melanie J Bopp, Denise K Houston, Leon Lenchik, Linda Easter, Stephen B Kritchevsky, Barbara J Nicklas|journal=J Am Diet Assoc|]

In a 20-week trial, 24 obese postmenopausal women took either 15% or 30% of their daily calories as protein. The 30% group lost less weight (8.4 vs. 11.4 kg, so 18.5 vs. 25.1 lb) but nearly as much fat (7.0 vs. 7.1 kg, so 15.4 vs. 15.7 lb), suggesting greater muscle retention.[reference|url=https://pubmed.ncbi.nlm.nih.gov/18810296|title=Effects of dietary protein on the composition of weight loss in post-menopausal women|published=2008 Oct|authors=M M Gordon, M J Bopp, L Easter, G D Miller, M F Lyles, D K Houston, B J Nicklas, S B Kritchevsky|journal=J Nutr Health Aging|]

Similarly, in a 6-month trial, 31 overweight or obese postmenopausal women (aged 65.2 ± 4.6) were prescribed a diet limited to 1,400 kcal (15% as protein, 65% as carbs, 30% as fat), with one group also taking 50 grams of whey protein twice a day and the other 50 grams of carbs twice a day. The protein group lost more weight (−8.0% ± 6.2%) than the carb group (−4.1% ± 3.6%). The protein group lost more muscle relative to their weight loss, but less muscle relative to their fat loss.[reference|url=https://pubmed.ncbi.nlm.nih.gov/21798863|title=The effects of a higher protein intake during energy restriction on changes in body composition and physical function in older women|published=2011 Nov|authors=Mojtahedi MC, Thorpe MP, Karampinos DC, Johnson CL, Layman DK, Georgiadis JG, Evans EM|journal=J Gerontol A Biol Sci Med Sci|]

By eating more protein when on a weight-loss diet, you can lose more fat and less muscle.

What is the effect on blood lipids?

In a 12-week weight-loss trial, 100 obese people were randomized to high daily protein (2.2 g/kg, so 1.0 g/lb) or low daily protein (1.1 g/kg, so 0.5 g/lb). Both groups lost the same weight, but here again, the high-protein group lost more fat, and it also experienced reductions in LDL-C and total cholesterol not observed in the low-protein group.[reference|url=https://pubmed.ncbi.nlm.nih.gov/18752682|title=A controlled trial of protein enrichment of meal replacements for weight reduction with retention of lean body mass|published=2008 Aug 27|authors=Leo Treyzon, Steve Chen, Kurt Hong, Eric Yan, Catherine L Carpenter, Gail Thames, Susan Bowerman, He-Jing Wang, Robert Elashoff, Zhaoping Li|journal=Nutr J|] In another 12-week weight-loss trial with 215 overweight or obese people, a high-protein diet led to greater reductions in fat, total cholesterol, and triglycerides.[reference|url=https://pubmed.ncbi.nlm.nih.gov/19179060|title=High protein diets decrease total and abdominal fat and improve CVD risk profile in overweight and obese men and women with elevated triacylglycerol|published=2009 Oct|authors=P M Clifton, K Bastiaans, J B Keogh|journal=Nutr Metab Cardiovasc Dis|]

Does the kind of protein matter?

Trials usually control for protein quantity rather than quality, but studies show that proteins with a higher percentage of essential amino acids (EAAs) tend to better correlate with fat loss[reference|url=https://pubmed.ncbi.nlm.nih.gov/22284338|title=Quality protein intake is inversely related with abdominal fat|published=2012 Jan 27|authors=Jeremy P Loenneke, Jacob M Wilson, Anssi H Manninen, Mandy E Wray, Jeremy T Barnes, Thomas J Pujol|journal=Nutr Metab (Lond)|] and bone health.[reference|url=https://pubmed.ncbi.nlm.nih.gov/21124023|title=Short report: Relationship between quality protein, lean mass and bone health|published=2010|authors=Jeremy P Loenneke, Abhishek Balapur, Austin D Thrower, Georganne Syler, Maureen Timlin, Thomas J Pujol|journal=Ann Nutr Metab|] Most EAA-rich proteins are animal proteins: meat, fish, eggs, and dairy. The protein in milk is 80% casein and 20% whey protein.

In general, animal proteins are richer in essential amino acids than plant proteins, and so are better for fat loss, but the advantage is small. For fat loss, protein quantity trumps protein quality.

For more information, see our Optimal Protein Intake Guide and Protein Intake Calculator.

Protein and the Kidneys

Don't worry about it if you have healthy kidneys and control your protein intake if you have damaged kidneys. It may be prudent to gradually increase protein intake to higher levels rather than jumping in both feet at a time, but there isn't much on this topic.

It is generally recommended to consume more water during periods when protein intake is being increased. Whether or not this has biological basis is not known, but it may be prudent to do

Healthy kidneys

When looking at active male athletes and measuring urinary creatinine, albumin, and urea no significant changes were seen in dosage ranges of 1.28-2.8g/kg bodyweight.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/10722779|title=Do regular high protein diets have potential health risks on kidney function in athletes|published=2000 Mar|authors=Poortmans JR, Dellalieux O|journal=Int J Sport Nutr Exerc Metab] The above study lasted 7 days, but survey research supports this lack of association (in post-menopausal women).[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/21653574|title=Higher biomarker-calibrated protein intake is not associated with impaired renal function in postmenopausal women|published=2011 Aug|authors=Beasley JM, Aragaki AK, LaCroix AZ, Neuhouser ML, Tinker LF, Cauley JA, Ensrud KE, Jackson RD, Prentice RL|journal=J Nutr] Although 'high protein' was defined as 1.1+/-0.2g/kg bodyweight, it was associated with better glomerular filtration rate. [reference|url=http://www.ncbi.nlm.nih.gov/pubmed/21653574|title=Higher biomarker-calibrated protein intake is not associated with impaired renal function in postmenopausal women|published=2011 Aug|authors=Beasley JM, Aragaki AK, LaCroix AZ, Neuhouser ML, Tinker LF, Cauley JA, Ensrud KE, Jackson RD, Prentice RL|journal=J Nutr] The Nurse's study (survey) corroborates these results, but also suggests that this apparent lack of harm does not hold true for renal insufficiency (damage) and that non-dairy animal proteins are associated to damage to a greater degree than other proteins.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/12639078|title=The impact of protein intake on renal function decline in women with normal renal function or mild renal insufficiency|published=2003 Mar 18|authors=Knight EL, Stampfer MJ, Hankinson SE, Spiegelman D, Curhan GC|journal=Ann Intern Med]

There do appear to be functional changes in the kidneys related to protein intake[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/8933120|title=Effect of chronic dietary protein intake on the renal function in healthy subjects|published=1996 Nov|authors=Brändle E, Sieberth HG, Hautmann RE|journal=Eur J Clin Nutr]. As protein does modulate renal function,[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/8329667|title=Dietary protein and renal function|published=1993 May|authors=King AJ, Levey AS|journal=J Am Soc Nephrol][reference|url=http://www.nutritionandmetabolism.com/content/2/1/25#B1|title=Dietary protein intake and renal function] these interactions may lead to damage if imposed acutely onto mice (from 10-15% of the diet, up to 35-45% of the diet immediately)[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/21535903|title=A diet with 35% of energy from protein leads to kidney damage in female Sprague-Dawley rats|published=2011 Sep|authors=Wakefield AP, House JD, Ogborn MR, Weiler HA, Aukema HM|journal=Br J Nutr][reference|url=http://www.ncbi.nlm.nih.gov/pubmed/21059282|title=Effects of high-whey-protein intake and resistance training on renal, bone and metabolic parameters in rats|published=2011 Mar|authors=Aparicio VA, Nebot E, Porres JM, Ortega FB, Heredia JM, López-Jurado M, Ramírez PA|journal=Br J Nutr] and one study in healthy humans going from 1.2g/kg to 2.4g/kg (doubling) was associated with higher than normal blood values of protein metabolites; a trend was noted for adaptation (increasing GFR) but it was not enough to clear uric acid and BUN over 7 days.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/19812175|title=Effect of short-term high-protein compared with normal-protein diets on renal hemodynamics and associated variables in healthy young men|published=2009 Dec|authors=Frank H, Graf J, Amann-Gassner U, Bratke R, Daniel H, Heemann U, Hauner H|journal=Am J Clin Nutr]

These studies are likely indicative of a 'too much, too fast' sitation, as controlled changes do not lead to adverse changes in kidney function.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/2301386|title=Controlled changes in chronic dietary protein intake do not change glomerular filtration rate|published=1990 Feb|authors=Wiegmann TB, Zlomke AM, MacDougall ML, Kipp DE|journal=Am J Kidney Dis] Thus, it would be prudent to slowly change protein intake over a moderate length of time.

Damaged or unhealthy kidneys

Restricted protein diets are recommended for those with kidney damage, as it slows the seemingly inevitable progression of kidney damage.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/8629624|title=Effects of dietary protein restriction on the progression of advanced renal disease in the Modification of Diet in Renal Disease Study|published=1996 May|authors=Levey AS, Adler S, Caggiula AW, England BK, Greene T, Hunsicker LG, Kusek JW, Rogers NL, Teschan PE|journal=Am J Kidney Dis][reference|url=http://www.ncbi.nlm.nih.gov/pubmed/8989740|title=Effects of dietary protein restriction on the progression of moderate renal disease in the Modification of Diet in Renal Disease Study|published=1996 Dec|authors=No authors listed If protein was not controlled for in those with renal damage, it would accelerate (or at least not reduce) the decline in function.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/12639078|title=The impact of protein intake on renal function decline in women with normal renal function or mild renal insufficiency|published=2003 Mar 18|authors=Knight EL, Stampfer MJ, Hankinson SE, Spiegelman D, Curhan GC|journal=Ann Intern Med]

Protein and the Liver

In healthy persons and rats, there is no evidence to suggest a relatively normal style of protein intake is harmful to the liver when habitually consumed as part of the diet. There is some preliminary evidence, however, that very high protein refeeding after prolonged fasting (>48 hours) may cause acute injuries to the liver.

When is damage seen?

The current standards for treatment of hepatic diseases (cirrhosis) recommend a reduction in protein intake due to the possibility of ammonia build-up in the blood[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/19052853|title=Dietary and nutritional indications in hepatic encephalopathy|published=2009 Mar|authors=Merli M, Riggio O|journal=Metab Brain Dis][reference|url=http://www.ncbi.nlm.nih.gov/pubmed/22230269|title=Cirrhosis: diagnosis, management, and prevention|published=2011 Dec 15|authors=Starr SP, Raines D|journal=Am Fam Physician] which may contribute to encephalopathy.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/12637132|title=Correlation between ammonia levels and the severity of hepatic encephalopathy|published=2003 Feb 15|authors=Ong JP, Aggarwal A, Krieger D, Easley KA, Karafa MT, Van Lente F, Arroliga AC, Mullen KD|journal=Am J Med]

At least one animal model suggests that damage may be seen when cycling periods (5 days) of sufficient protein intake, and periods of protein malnutrition.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/20878513|title=Alternation between dietary protein depletion and normal feeding cause liver damage in mouse|published=2011 Mar|authors=Caballero VJ, Mendieta JR, Giudici AM, Crupkin AC, Barbeito CG, Ronchi VP, Chisari AN, Conde RD|journal=J Physiol Biochem] Similar effects were seen after 48 hours of fasting when fed a diet containing 40-50% casein.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/21902856|title=Refeeding with a high-protein diet after a 48 h fast causes acute hepatocellular injury in mice|published=2011 Sep 9:1-10|authors=Oarada M, Tsuzuki T, Nikawa T, Kohno S, Hirasaka K, Gonoi T|journal=Br J Nutr] The latter study noted that the 35% and 50% casein groups had higher AST and ALT levels than the lower protein controls, effectively controlling for refeeding syndromes in general and its adverse effects on liver enzymes.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/14671676|title=The changes of hepatic metallothionein synthesis and the hepatic damage induced by starvation in mice|published=2003 Oct|authors=Sogawa N, Sogawa CA, Fukuoka H, Mukubo Y, Yoneyama T, Okano Y, Furuta H, Onodera K|journal=Methods Find Exp Clin Pharmacol][reference|url=http://www.jstage.jst.go.jp/article/internalmedicine/47/16/47_1447/_article|title=Hepatocellular Injuries Observed in Patients with an Eating Disorder Prior to Nutritional Treatment] The increases in liver enzymes seen in this study were concurrent with a decrease in the expression of the cytoprotective gene HSP72 and increases of c-Fos and nur77, which are upregulated in response to injury.

Thus, said animal study is some preliminary evidence that high protein refeeding (35-50%) after 48 hour fasting may harm the liver. Shorter fasts were not examined.

Finally, aflatoxin (a toxic mold that is produced from some species of nuts and seeds) is known to be more carcinogenic (cancer producing) when the diet is very high in protein[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/4294825|title=The effect of dietary protein on carcinogenesis of aflatoxin|published=1968 Feb|authors=Madhavan TV, Gopalan C|journal=Arch Pathol] and subsequently less potent in diets lower in protein.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/6131741|title=Effect of high and low dietary protein on the dosing and postdosing periods of aflatoxin B1-induced hepatic preneoplastic lesion development in the rat|published=1983 May|authors=Appleton BS, Campbell TC|journal=Cancer Res][reference|url=http://www.ncbi.nlm.nih.gov/pubmed/1423844|title=Effect of dietary protein level on aflatoxin B1 actions in the liver of weanling rats|published=1992 Oct|authors=Mandel HG, Judah DJ, Neal GE|journal=Carcinogenesis][reference|url=http://www.ncbi.nlm.nih.gov/pubmed/1346757|title=Influence of different levels of dietary casein on initiation of male rat liver carcinogenesis with a single dose of aflatoxin B1|published=1992 Feb|authors=Blanck A, Lindhe B, Porsch Hällström I, Lindeskog P, Gustafsson JA|journal=Carcinogenesis] This is due to the toxin being bioactivated by the P450 enzyme system, which has its overall activity increased when dietary protein increases. This phenomena also has effects on drugs metabolized by P450, in which the dosage may need to be increased due to faster metabolism.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/18363537|title=Potential interaction between warfarin and high dietary protein intake|published=2008 Apr|authors=Hornsby LB, Hester EK, Donaldson AR|journal=Pharmacotherapy]

The above is not an adverse effect of high protein diets per se (as it requires aflatoxin ingestion, which can be avoided) but should otherwise still be noted.

The only other relevant information on the topic is a 1974 study showing that a diet of 35% casein led to increased ALT and AST levels in rats;[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/4435148|title=Plasma enzyme activities in rats with diet-induced alterations in liver enzyme activities|published=1974 Nov 15|authors=Bolter CP, Critz JB|journal=Experientia] this study does not seem to have been replicated.

Beyond the above situations, there are no more adverse interactions between dietary protein per se and the liver. It is typically seen as safe to consume protein given you have a healthy liver.

Amino acids are acids, right? What about acidity?

Evidence is theoretically sound, but the acidity of excessive amino acids does not appear to be a clinical concern. It's not potent enough to cause harm to most individuals.

Bone Mineral Density

In looking at large survey research, there appears to be no relation between protein intake and bone fracture risk (indicative of bone health) except for when total calcium intake was below 400mg per 1000kcal daily, although the relation was fairly weak (RR=1.51 when compared against the highest quartile).[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/18665794|title=Proteins, dietary acid load, and calcium and risk of postmenopausal fractures in the E3N French women prospective study|published=2008 Dec|authors=Dargent-Molina P, Sabia S, Touvier M, Kesse E, Bréart G, Clavel-Chapelon F, Boutron-Ruault MC|journal=J Bone Miner Res] Other reviews not similar 'lack of correlation despite logic' relations.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/22127335|title=Protein intake, calcium balance and health consequences|published=2011 Nov 30|authors=Calvez J, Poupin N, Chesneau C, Lassale C, Tomé D|journal=Eur J Clin Nutr][reference|url=http://www.jissn.com/content/1/1/45|title=High-Protein Weight Loss Diets and Purported Adverse Effects: Where is the Evidence?]

One intervention study noted that protein intake was actually positively associated with bone mineral density, but this correlation only was shown when the acidic effects of sulfate (from sulfur amino acids) was controlled for.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/18156408|title=A positive association of lumbar spine bone mineral density with dietary protein is suppressed by a negative association with protein sulfur|published=2008 Jan|authors=Thorpe M, Mojtahedi MC, Chapman-Novakofski K, McAuley E, Evans EM|journal=J Nutr]

Soy protein itself seems to have additional protective effects on bone mass in post-menopausal women, which may be due to the isoflavone content.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/16157834|title=Prospective cohort study of soy food consumption and risk of bone fracture among postmenopausal women|published=2005 Sep 12|authors=Zhang X, Shu XO, Li H, Yang G, Li Q, Gao YT, Zheng W|journal=Arch Intern Med] For more information, please read our FAQ page on Soy Isoflavones.

The role of the Kidneys

Kidneys can acutely increase the Glomerular filtration rate (GFR), or the rate of filtration of the blood. They do this in response to dietary protein intake[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/3207474|title=Glomerular filtration rate in response to an acute protein load|published=1988|authors=von Herrath D, Saupe J, Hirschberg R, Rottka H, Schaefer K|journal=Blood Purif], and the lack of this compensation in some forms of kidney damage are a reason protein intake is controlled for in kidney disease management.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/6650549|title=Renal functional reserve in humans. Effect of protein intake on glomerular filtration rate|published=1983 Dec|authors=Bosch JP, Saccaggi A, Lauer A, Ronco C, Belledonne M, Glabman S|journal=Am J Med]

Additionally, the kidneys serve to regulate acid-base balance in the body via the sodium-bicarbonate buffering system.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/21170887|title=Acid-base transport by the renal proximal tubule|published=2010 Nov-Dec|authors=Skelton LA, Boron WF, Zhou Y|journal=J Nephrol] Disorders in acid:base balance can further pathophysiology (symptoms and signs of the disease) of renal complications.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/20539229|title=Acidosis and progression of chronic kidney disease|published=2010 Sep|authors=Yaqoob MM|journal=Curr Opin Nephrol Hypertens][reference|url=http://www.ncbi.nlm.nih.gov/pubmed/20526632|title=Consequences and therapy of the metabolic acidosis of chronic kidney disease|published=2011 Jan|authors=Kraut JA, Madias NE|journal=Pediatr Nephrol]

These protective measures appear to be preserved in healthy kidneys, but begin to fail when they kidneys are otherwise damaged.

The role of Resistance Training

When rats are subjected to an dramatic and acute increase in dietary protein and experience a decline in renal function, resistance training is able to alleviate some of the adverse changes and exert a protective effect.[reference|url=http://www.ncbi.nlm.nih.gov/pubmed/21059282|title=Effects of high-whey-protein intake and resistance training on renal, bone and metabolic parameters in rats|published=2011 Mar|authors=Aparicio VA, Nebot E, Porres JM, Ortega FB, Heredia JM, López-Jurado M, Ramírez PA|journal=Br J Nutr]

Next: Learn about about how much protein you need every day.

Glutamine is the most abundant amino acid in your body. Your body can synthesize it, so it is not an essential amino acid (EAA), but your requirements may at time outpace your rate of synthesis, making glutamine conditionally essential. Historically, however, glutamine has been considered conditionally essential only in the critically hurt or sick — burn victims and other people in whom physical stress is exceptionally high and catabolism (body-tissue breakdown) rampant.[reference|url=https://pubmed.ncbi.nlm.nih.gov/2080048|title=Is glutamine a conditionally essential amino acid?|published=1990 Aug|authors=Lacey JM, Wilmore DW|journal=Nutr Rev|]

It is only recently that glutamine studies have started exploring the lesser stress and catabolism caused by strenuous exercise. Some of these studies examined the effect of glutamine on body composition, others on muscle recovery, and yet others on the immune system.

Body composition

The benefits of glutamine in the critically hurt or sick have led some researchers to propose that it might be a useful supplement for athletes engaged in heavy exercise, which is also catabolic in nature.[reference|url=https://pubmed.ncbi.nlm.nih.gov/9916176|title=Glutamine: a potentially useful supplement for athletes|published=1999 Feb|authors=Antonio J, Street C|journal=Can J Appl Physiol|] These researchers tested their hypothesis through a double-blind RCT involving 6 resistance-trained men who consumed glutamine or glycine (0.3 grams per kilogram of body weight) one hour before a weightlifting session.[reference|url=https://pubmed.ncbi.nlm.nih.gov/11834123|title=The effects of high-dose glutamine ingestion on weightlifting performance|published=2002 Feb|authors=Antonio J, Sanders MS, Kalman D, Woodgate D, Street C|journal=J Strength Cond Res|] Glutamine did not benefit performance.

Another group of researchers tested glutamine (0.9 g/kg) against placebo in 31 resistance-trained men and women during a 6-week resistance-training program.[reference|url=https://pubmed.ncbi.nlm.nih.gov/11822473|title=Effect of glutamine supplementation combined with resistance training in young adults|published=2001 Dec|authors=Candow DG, Chilibeck PD, Burke DG, Davison KS, Smith-Palmer T|journal=Eur J Appl Physiol|] Even such a high daily dose of glutamine did not affect strength or lean-body mass (LBM) more than did placebo (strength and LBM increased in both groups).

Of course, neither study exposed its participants to the high levels of stress experienced by, for instance, burn victims. An RCT involving 18 collegiate male wrestlers aimed to address this issue by comparing placebo with glutamine (0.35 g/kg) during an intensive 12-day cut.[reference|url=https://pubmed.ncbi.nlm.nih.gov/24688278|title=Glutamine Supplementation did not Benefit Athletes During Short-Term Weight Reduction|published=2003 Dec 1|authors=Finn KJ, Lund R, Rosene-Treadwell M|journal=J Sports Sci Med|] Both groups lost 2 kg, with no significant differences between groups with regard to changes in LBM or fat mass.

A 2018 meta-analysis of 5 studies also found no benefit from glutamine on body composition.[reference|url=https://pubmed.ncbi.nlm.nih.gov/29784526|title=The effect of glutamine supplementation on athletic performance, body composition, and immune function: A systematic review and a meta-analysis of clinical trials|published=2018 May 9|authors=Ramezani Ahmadi A, Rayyani E, Bahreini M, Mansoori A|journal=Clin Nutr|] Although glutamine does play a part in muscle synthesis (it is an independent activator of mTOR[reference|url=http://www.jbc.org/content/early/2018/09/07/jbc.RA118.004972.abstract|title=Phospholipase D-Dependent mTORC1 Activation by Glutamine|published=2018|authors=Bernfeld E, Menon D, Vaghela V, Zerin I, Faruque P, Frias M, Foster D|journal=Journal of Biological Chemistry]), what we obtain through foods seems to suffice; supplementation doesn’t appear to confer additional benefits.

Glutamine supplementation has no effect on lean mass or fat mass, even during aggressive dieting.

Muscle recovery

So glutamine supplementation has no effect on body composition; but might it facilitate recovery from resistance-training sessions?

To help answer that question, researchers gave placebo or glutamine (0.3 g/kg) to 15 recreationally active men right after a muscle-damaging exercise (100 drop-jumps) and for the next four days. They reported that, compared to placebo, glutamine significantly reduced muscle soreness and improved strength recovery.[reference|url=https://www.sciencedirect.com/science/article/pii/S1728869X12600070|title=Glutamine Supplementation in Recovery From Eccentric Exercise Attenuates Strength Loss and Muscle Soreness|published=2011|authors=Street B, Byrne C, Eston R|journal=Journal of Exercise Science and Fitness]

However, a study of 17 untrained young men reported that taking glutamine (0.1 g/kg) thrice a week for 4 weeks had no effect on muscle soreness, range of motion, or EMG activity up to 48 hours after a muscle-damaging exercise (eccentric leg extensions at 75% of 1-RM).[reference|url=https://pubmed.ncbi.nlm.nih.gov/23997909|title=Effect of L-glutamine supplementation on electromyographic activity of the quadriceps muscle injured by eccentric exercise|published=2013 Jun|authors=Rahmani Nia F, Farzaneh E, Damirchi A, Shamsi Majlan A|journal=Iran J Basic Med Sci|] Importantly, this latter study involved untrained men and used a smaller dose, a different dosing schedule, and a different exercise protocol — all factors that could explain the discrepancy between the two studies.

A contraction is isometric when the muscle’s length does not change, and isotonic otherwise. An isotonic contraction is called concentric when the muscle shortens under load (as when you lift a dumbbell) and eccentric when it lengthens under load (as when you control the dumbbell on its way down). Your one-repetition maximum (1-RM) is the heaviest weight you can lift (concentric contraction) for a given exercise.

Most recently, a study of 23 resistance-trained men investigated the effects of taking glutamine with leucine.[reference|url=https://pubmed.ncbi.nlm.nih.gov/29770871|title=The effects of acute leucine or leucine-glutamine co-ingestion on recovery from eccentrically biased exercise|published=2018 May 16|authors=Waldron M, Ralph C, Jeffries O, Tallent J, Theis N, Patterson SD|journal=Amino Acids|] The men were randomized into three groups and took either leucine (0.087 g/kg), leucine with glutamine (0.087 g/kg + 0.3 g/kg), or a placebo 30 minutes before and after a muscle-damaging exercise (100 drop-jumps), and again before and after recovery tests conducted 24, 48, and 72 hours afterward. Leucine led to better strength recovery at 72 hours only. Leucine with glutamine led to better strength recovery at 24, 48, and 72 hours. Muscle soreness, however, didn’t differ between groups.

Note that these three studies were conducted in men only. Another study recruited 8 men and 8 women, all recreationally active, and gave them placebo or glutamine (0.3 g/kg) one hour before and after a muscle-damaging exercise (80 eccentric contractions at 125% of 1-RM), and again before recovery tests conducted 24, 48, and 72 hours afterward.[reference|url=https://pubmed.ncbi.nlm.nih.gov/25811544|title=The Influence of Oral L-Glutamine Supplementation on Muscle Strength Recovery and Soreness Following Unilateral Knee Extension Eccentric Exercise|published=2015 Oct|authors=Legault Z, Bagnall N, Kimmerly DS|journal=Int J Sport Nutr Exerc Metab|] Strength recovery was modestly improved in men but not in women, although both sexes experienced significant reductions in muscle soreness.

In recreationally active men, glutamine supplementation around exercise appears to improve strength recovery and might reduce muscle soreness. Only one study lasted more than 72 hours, though, which precludes drawing conclusions about the effects of chronic supplementation. Likewise, only one study included women, which precludes drawing conclusions about the effects of supplementation in women.

Illness

Glutamine is a major fuel for cells of the immune system.[reference|url=https://pubmed.ncbi.nlm.nih.gov/17403271|title=Amino acids and immune function|published=2007 Aug|authors=Li P, Yin YL, Li D, Kim SW, Wu G|journal=Br J Nutr|] Plasma glutamine levels are reduced after prolonged endurance exercise, and this reduction correlates with an increased risk of infection.[reference|url=https://pubmed.ncbi.nlm.nih.gov/11985938|title=Can glutamine modify the apparent immunodepression observed after prolonged, exhaustive exercise?|published=2002 May|authors=Castell LM|journal=Nutrition|]

An early study of endurance athletes (marathoners and ultra-marathoners) reported that taking 5 grams of glutamine right after an athletic event and 2 hours later significantly reduced the occurrence of infections over the following week.[reference|url=https://pubmed.ncbi.nlm.nih.gov/8803512|title=Does glutamine have a role in reducing infections in athletes?|published=1996|authors=Castell LM, Poortmans JR, Newsholme EA|journal=Eur J Appl Physiol Occup Physiol|] Specifically, 19% of the glutamine group reported illness, compared to half of the placebo group.

No other trial has studied the effects of glutamine on infection as an outcome in athletes, but other trials have looked at various aspects of the immune system,[reference|url=https://pubmed.ncbi.nlm.nih.gov/29784526|title=The effect of glutamine supplementation on athletic performance, body composition, and immune function: A systematic review and a meta-analysis of clinical trials|published=2018 May 9|authors=Ramezani Ahmadi A, Rayyani E, Bahreini M, Mansoori A|journal=Clin Nutr|] such as white-blood-cell function[reference|url=https://pubmed.ncbi.nlm.nih.gov/11546663|title=Effect of glutamine supplementation on exercise-induced changes in lymphocyte function|published=2001 Oct|authors=Krzywkowski K, Petersen EW, Ostrowski K, Kristensen JH, Boza J, Pedersen BK|journal=Am J Physiol Cell Physiol|][reference|url=https://pubmed.ncbi.nlm.nih.gov/10722780|title=Effect of oral glutamine supplementation on human neutrophil lipopolysaccharide-stimulated degranulation following prolonged exercise|published=2000 Mar|authors=Walsh NP, Blannin AK, Bishop NC, Robson PJ, Gleeson M|journal=Int J Sport Nutr Exerc Metab|] and salivary IgA concentrations,[reference|url=https://pubmed.ncbi.nlm.nih.gov/11457800|title=Effect of glutamine and protein supplementation on exercise-induced decreases in salivary IgA|published=2001 Aug|authors=Krzywkowski K, Petersen EW, Ostrowski K, Link-Amster H, Boza J, Halkjaer-Kristensen J, Pedersen BK|journal=J Appl Physiol (1985)|] and none has found a relationship between an exercise-induced decrease in plasma glutamine levels and changes to the immune system.[reference|url=https://pubmed.ncbi.nlm.nih.gov/12183472|title=Exercise-induced immunodepression- plasma glutamine is not the link|published=2002 Sep|authors=Hiscock N, Pedersen BK|journal=J Appl Physiol (1985)|]

Glutamine’s benefits may be mediated by its effects on the intestinal barrier, as more recent evidence has begun to suggest. Prolonged endurance exercise is known to cause leaky gut — a condition in which heat stress and reduced blood flow to the gastrointestinal tract cause intestinal cell damage that loosens tight junctions between cells, allowing for the absorption of things that are not supposed to pass through the intestinal barrier.[reference|url=https://pubmed.ncbi.nlm.nih.gov/26359485|title=Intestinal epithelial barrier function and tight junction proteins with heat and exercise|published=2016 Mar 15|authors=Dokladny K, Zuhl MN, Moseley PL|journal=J Appl Physiol (1985)|]

In a recent study, glutamine (0.25, 0.5, and 0.9 g/kg) showed a dose-dependent reduction in exercise-induced intestinal permeability.[reference|url=https://pubmed.ncbi.nlm.nih.gov/29058112|title=Glutamine supplementation reduces markers of intestinal permeability during running in the heat in a dose-dependent manner|published=2017 Dec|authors=Pugh JN, Sage S, Hutson M, Doran DA, Fleming SC, Highton J, Morton JP, Close GL|journal=Eur J Appl Physiol|] In an earlier study, the reduction in intestinal permeability from glutamine supplementation had correlated with reductions in serum endotoxin and in inflammatory markers.[reference|url=https://pubmed.ncbi.nlm.nih.gov/25062931|title=The effects of acute oral glutamine supplementation on exercise-induced gastrointestinal permeability and heat shock protein expression in peripheral blood mononuclear cells|published=2015 Jan|authors=Zuhl M, Dokladny K, Mermier C, Schneider S, Salgado R, Moseley P|journal=Cell Stress Chaperones|]

Glutamine is an important source of energy for intestinal cells and the immune system. Supplementation may reduce exercise-induced dysfunctions of the intestinal tract and might decrease the risk of falling sick from prolonged endurance exercise.

In Summary

There is no evidence that supplemental glutamine helps build muscle or improve body composition.

There is evidence that supplemental glutamine improves strength-training recovery. It may also help maintain the integrity of the gastrointestinal tract during prolonged endurance exercise and thus decrease the risk of getting sick afterward.

So if your goal is to build muscle, don’t waste your money on glutamine. Other supplements are more likely to help, notably creatine, and of course you should ensure you get enough protein.

Our constantly updated Fitness Guide is a convenient all-in-one resource that tells you which supplements to take, when, and in what dosages, based on human studies.

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Nutrition sure can seem complicated. You already had to worry about getting enough protein in your diet, should you now make sure you only get quality proteins?

And what makes a “quality protein” anyway?

Different scales have been developed to answer this last question. They rate and rank proteins based on two criteria: bioavailability and amino acid profile.

Bioavailability

Biological Value (BV), Net Protein Utilisation (NPU), and Nitrogen Balance (NB) rate proteins based on nitrogen measurements. They measure how much nitrogen people excreted, calculate how much protein that represents, and compare this number to how much protein was ingested. In such a way, they determine the protein’s bioavailability.

All three scales are based on two assumptions, both of which have been challenged: first, that dietary protein is the body’s sole source of nitrogen; second, that all nonexcreted protein has been used to make bodily proteins. In truth, some of the protein we ingest can be converted to glucose, especially if the protein’s digestion is fast and the body’s glycogen stores are low, and some can be fermented by our microbiota,[reference|url=https://pubmed.ncbi.nlm.nih.gov/22468341|title=Bacteria, colonic fermentation, and gastrointestinal health|published=2012 Jan-Feb|authors=Macfarlane GT, Macfarlane S|journal=J AOAC Int|] especially if the protein’s digestion is slow.

The BV scale is still in use today, though mostly in promotional material and in the media, and so it needed mentioning despite being outdated. The current official scale, used notably by the FDA, is the Protein Digestibility Corrected Amino Acid Score (PDCAAS), which takes into account not just the bioavailability of a protein but also its amino acid profile.

A protein is considered highly bioavailable if it’s easy to digest, absorb, and (after conversion into its constituent amino acids) make into other proteins. Some protein rating scales, such as the BV scale, rank proteins based solely on bioavailability.

Amino acid profile

Proteins are composed of amino acids, some of which your body can synthesize and others not. The nine you need yet cannot synthesize, and thus need to ingest, are called essential amino acids (EAAs). Among those, branched-chain amino acids (BCAAs) are crucial to your muscles,[reference|url=https://pubmed.ncbi.nlm.nih.gov/7810616|title=Branched-chain amino acids augment ammonia metabolism while attenuating protein breakdown during exercise|published=1994 Dec|authors=MacLean DA, Graham TE, Saltin B|journal=Am J Physiol|] with leucine being especially anabolic.

mg/kg/day = daily requirement in milligrams (of a given amino acid) per kilogram (of body weight) per day
Complete/Milk/Pea/Rice/Soy/Whey = milligrams of amino acid per gram of complete/milk/pea/rice/soy/whey protein (mg/g)
References: World Health Organization. Protein and Amino Acid Requirements in Human Nutrition, page 245, table 49. 2007.[reference|url=https://pubmed.ncbi.nlm.nih.gov/18330140|title=Protein and Amino Acid Requirements in Human Nutrition|published=2007|authors=Joint WHO/FAO/UNU Expert Consultation|journal=World Health Organ Tech Rep Ser|] Kalman. Foods. 2014.[reference|url=https://pubmed.ncbi.nlm.nih.gov/28234326|title=Amino Acid Composition of an Organic Brown Rice Protein Concentrate and Isolate Compared to Soy and Whey Concentrates and Isolates|published=2014 Jun 30|authors=Douglas S Kalman|journal=Foods|] Gorissen et al. Amino Acids. 2018.[reference|url=https://pubmed.ncbi.nlm.nih.gov/30167963|title=Protein Content and Amino Acid Composition of Commercially Available Plant-Based Protein Isolates|published=2018 Dec|authors=Stefan H M Gorissen, Julie J R Crombag, Joan M G Senden, W A Huub Waterval, Jörgen Bierau, Lex B Verdijk, Luc J C van Loon|journal=Amino Acids|] USDA Food Composition Databases (accessed: 2018 Sep)

A protein is called complete when, proportionally to its overall amino-acid content, it has enough of each EAA. The main advantage of animal proteins is that most are complete.

Most, but not all. Take beef protein powders: you might assume they’re made from meat, which is to say from the animal’s muscles, when most are actually made from collagen boiled from the animal’s skin, bones, and other connective tissues. Now, dietary collagen is far from useless; it’s been shown to promote skin and joint health, and it probably promotes bone health too; but it isn’t a complete protein.[reference|url=https://pubmed.ncbi.nlm.nih.gov/13276342|title=The amino acid composition of mammalian collagen and gelatin|published=1955 Dec|authors=EASTOE JE|journal=Biochem J|] Rich in glycine and proline but poor in BCAAs, it isn’t a good primary source of protein, and is probably not the best muscle builder (though it has shown benefit in elderly women on a low-protein diet[reference|url=https://pubmed.ncbi.nlm.nih.gov/19465192|title=Effects of whey and fortified collagen hydrolysate protein supplements on nitrogen balance and body composition in older women|published=2009 Jun|authors=Hays NP, Kim H, Wells AM, Kajkenova O, Evans WJ|journal=J Am Diet Assoc|] and in elderly men[reference|url=https://pubmed.ncbi.nlm.nih.gov/26353786|title=Collagen peptide supplementation in combination with resistance training improves body composition and increases muscle strength in elderly sarcopenic men: a randomised controlled trial|published=2015 Oct 28|authors=Zdzieblik D, Oesser S, Baumstark MW, Gollhofer A, König D|journal=Br J Nutr|]).

Conversely, most plant proteins are incomplete, but according to the table above, the proteins in soy, pea, and rice are nearly complete: rice is relatively poor in lysine; soy and pea, in methionine. Of course, incomplete proteins can complement one another — this is true of proteins from foods as well as from supplements. The amino acid profile of a 70:30 pea:rice protein blend is similar to that of whey.

Alas, soy and pea protein powders are usually very high in salt. Salt is used in the process that makes soy and pea protein powders, and it cannot all be washed away. Check the label of your soy or pea protein powder to ensure you don’t end up exceeding your tolerable upper intake of salt (sodium) for the day: 2.3 g for most adults.

Although pea and rice are gaining in popularity, alone and in combination, soy is still the most popular vegan source of protein powder. On the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) scale, soy protein isolates scores 0.97 (97%), so at first glance it appears to be the virtual equals of any animal protein. However, this is because the PDCAAS scale truncates any number superior to 1 (100%), with the rationale that any amount of amino acids beyond the requirement pattern confers no additional benefit to the individual consuming the protein. Otherwise, whey protein concentrate could score a more-than-perfect 1.07 (107%).[reference|url=https://pubmed.ncbi.nlm.nih.gov/28382889|title=Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins may better describe protein quality than values calculated using the concept for protein digestibility-corrected amino acid scores (PDCAAS)|published=2017 Feb|authors=Mathai JK, Liu Y, Stein HH|journal=Br J Nutr|]

A dietary protein is considered complete if it contains enough of each of the amino acids your body needs yet cannot synthesize. Some protein rating scales, such as the PDCAAS scale, rank proteins based not just on bioavailability but also on amino acid profile.

The real world

The PDCAAS scale has displaced the BV scale, not because it is more accurate, but because it is more pertinent: it rates protein not only based on bioavailability but also on amino acid profile, so as to better reflect human needs. Better, but not perfectly, which is why the Food and Agriculture Organization has already proposed to replace it with yet another scale: the Digestible Indispensable Amino Acid Score (DIAAS).

The two scales differ notably by where the sample is taken. To determine the PDCAAS of a protein, you analyze the feces. To determine the DIAAS of a protein, you analyze the content of the ileum. In other words, PDCAAS looks at how much protein was absorbed after it has gone through your small and large intestines, whereas DIAAS looks at how much protein was absorbed after it has left the small intestine.

The food you ingest passes through your small intestine before passing through your large intestine, where most of your gut’s microbiome (the bacteria inhabiting your gut) can be found. Since this microbiome can use amino acids and peptides that you yourselves never absorbed, PDCAAS may overestimate a protein’s bioavailability.[reference|url=https://pubmed.ncbi.nlm.nih.gov/28382889|title=Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins may better describe protein quality than values calculated using the concept for protein digestibility-corrected amino acid scores (PDCAAS)|published=2017 Feb|authors=Mathai JK, Liu Y, Stein HH|journal=Br J Nutr|] DIAAS overcomes this issue.

DIAAS = Digestible Indispensable Amino Acid Score | PDCAAS = Protein Digestibility-Corrected Amino Acid Score
Reference: Mathai et al. Br J Nutr. 2017.[reference|url=https://pubmed.ncbi.nlm.nih.gov/28382889|title=Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins may better describe protein quality than values calculated using the concept for protein digestibility-corrected amino acid scores (PDCAAS)|published=2017 Feb|authors=Mathai JK, Liu Y, Stein HH|journal=Br J Nutr|]

The DIAAS scale, however, doesn’t address the main problem of all protein rating scales, which is that, even when they take into account bioavailability, amino acid profile, and ileal digestibility, and even if they took into account gluconeogenesis, digestion speed, and the effects of exercise on endogenous protein (i.e., muscles), they would still fail to reflect real life.

Trials need to eliminate confusing factors, so a subject is fed only one kind of protein, and that on an empty stomach. But when do you eat only one kind of protein over a whole day? And, except at breakfast maybe, when is your stomach really empty? Other food components, such as fiber and anti-nutrients (trypsin inhibitors, tannins, etc.), can all affect how much of your protein you absorb.

Rating each protein in isolation fails to reflect real life. You don’t need each protein you ingest to be complete; in a balanced diet, incomplete proteins, rich and poor in different amino acids, can complement one another (especially when eaten at about the same time). But a protein’s rating can be one of the criteria you consider when selecting a protein powder (alongside price, mixability, digestion speed, etc.).