The ketogenic diet is likely not the ideal diet for most athletes, but in some cases it could be useful. A ketogenic diet could help with weight loss for athletes in bodybuilding and for sports that have weight classes. The ketogenic diet has not been conclusively shown to increase the risk of losing lean body mass while in a caloric deficit, so it can be an option for weight loss, muscle growth, or maintenance diets.^[23][8] Current evidence also suggests that ketogenic diets do not negatively affect aerobic fitness or endurance performance, although more research is needed in this area.^[24][25][23]

Overall, the evidence remains inconsistent on the effect a ketogenic diet might have on endurance, sprint, and strength athletes.^[26][27]

At present, there is insufficient evidence to support the use of a ketogenic diet as a treatment in cancer of any type. In in vitro studies on cancer cells, the ketogenic diet slowed tumor growth, delayed the initiation of new tumors, and in some cases sensitized the cancer cells to chemotherapy. However, this was not the case for all types of cancer. In a mouse study of melanoma, the ketogenic diet actually increased the growth of the cancer cells,^[28] and in some types of cancer, the diet had no impact on the cancer at all. Some case reports in humans have shown positive results when the ketogenic diet is combined with standard treatments. Before the ketogenic diet could be recommended as an adjunct to cancer treatments, studies will need to focus on each type of cancer individually, and randomized controlled trials will need to support the claims made by anecdotal evidence.^[29]

It could, however, be a safe dietary pattern for people with cancer who also have obesity or overweight and are trying to lose weight.^[30]

A ketogenic diet may be useful for people with type II diabetes. A 2023 review showed that following a ketogenic diet could improve fasting glucose, HbA1c, and quality of life while also assisting with weight loss.^[31] This supported findings from a 2022 meta-analysis which showed a ketogenic diet to be more effective than a low-carbohydrate diet in improving glycemic control in people with diabetes.^[32]

Safety in the long term has not yet been established, and one concern is that a ketogenic diet could increase cardiovascular risk factors in people with type 2 diabetes. In people who use insulin as part of their treatment for diabetes, blood sugar should be closely monitored during a ketogenic diet to avoid potentially dangerous hypoglycemic (low blood sugar) episodes.^[21] While a ketogenic diet might help people with diabetes lose body fat and improve glycemic control, the restrictive nature of the diet can be difficult to adhere to in the long term, and it is not superior to other more balanced, calorically controlled diets that can achieve similar results.^[31] People with diabetes who would like to try a ketogenic diet should discuss it with their doctor, who can help them assess the risks, benefits, and alternative options.

A ketogenic diet is not currently recommended for people with type 1 diabetes, and there is not a lot of research available in this population yet. Several published case studies and small observational studies have shown that the ketogenic diet could improve HbA1c levels and keep blood sugar levels more stable. However, this diet strategy is not without risks, because it can lead to hypoglycemia (low blood sugar) and increase LDL cholesterol levels and cardiovascular disease risk.^[33][34][35]

Particularly in children, the risks seem to outweigh the benefits. Children on a ketogenic diet are more at risk for hypoglycemic episodes and do not tolerate the severely restricted diet. This restriction can cause long-term psychological effects.^[36]

In adults with type 1 diabetes who choose to try the ketogenic diet, several strategies can help decrease risk. People who use insulin will likely need to decrease the dose and should discuss this with their doctor. Any other medication should also be reviewed, and all SGLT2 inhibitors (sodium-glucose cotransporter-2 inhibitors), such as empagliflozin, should be stopped, because they increase the risk for euglycemic diabetic ketoacidosis (a type of diabetic ketoacidosis in which the typical features of raised blood sugar and urinary ketones are hidden and make it more difficult to diagnose). Slowly lowering carbohydrate intake over a period of weeks (instead of dropping immediately down to 50 grams daily) may lower the risk of hypoglycemia. Finally, a continuous glucose monitor should be used, because it is necessary to monitor glucose levels very closely during the transition period.^[35][34][33]

The ketogenic diet is not currently recommended during pregnancy. Studies done in mice and rats have shown that ketosis could negatively affect the growth of the fetus. These rodent studies showed some neurodevelopmental delays, low birth weights, and abnormal organ development.^[37][38] A study done on mice also showed that a ketogenic diet decreased the viability of the embryo after implantation, which might mean an increased risk for miscarriage.^[39] In one study it was noted that there was improvement in these abnormalities if the mother was given a normal diet at a later stage in the pregnancy and during lactation.^[37] All of these studies have been done in rodent populations, so they cannot be translated directly to human pregnancy. However, given the concerning findings, it is not considered safe to follow a ketogenic diet during pregnancy.

The ketogenic diet is not recommended during breastfeeding, because safety has not been clearly established. Breastfeeding itself can induce ketosis because it increases energy demand, and these demands can be met by an increased carbohydrate intake, gluconeogenesis (where the liver uses stored glycogen to release glucose into the blood), or the use of fatty acids as an energy source.^[40]

Lactation ketoacidosis occurs in breastfeeding mothers who are severely undereating, although it is quite uncommon in humans. The resulting acidosis in the body can be severe and usually requires treatment in a hospital. The added ketosis produced by a ketogenic diet can cause lactation ketoacidosis, and there has been a case report of severe lactation ketoacidosis in a breastfeeding mother on a ketogenic diet.^[41]

A nutritional ketosis leads to a moderate increase in the concentration of ketones in the body. This is a slow and controlled process that does not significantly change the pH level of the blood. This is very different from ketoacidosis, which is often seen in people with type 1 diabetes (although it can also occur in people with type 2 diabetes). Ketoacidosis is a rapid increase in the concentration of ketones that leads to an acidic blood pH and needs urgent medical attention.^[21]

Low carb dieting seems pretty simple at first: just eat fewer carbs. But fewer than … what?

There are three carb cutoffs that researchers use to define “low-carb”, plus a fourth option that involves carb timing. Let’s quickly see what those are.

Option 1: Eat less than ~50 grams of carbs (aka ketogenic dieting)

Once you go below 50 grams of carbs a day, most studies classify that as a Very Low Carbohydrate Ketogenic Diet (VLCKD). You’ll see the overlapping term “VLC” thrown around more frequently on websites than “VLCKD”.

When you think of the Atkins Diet, or even medical diets based on Atkins, they fall under this category. This carb level is supposed to lead to extra fat loss magic, which has been mostly disproved by recent experiments. Regardless, the lower you go below 50 grams of carbs, the more ketones you produce. And for some conditions (especially epilepsy), ketones could be beneficial. Other people just feel better eating VLC diets.

Eating less than 50 grams of carbs guarantees you admission to the low-carb club. Even your body knows about this club membership, since your liver is forced to make ketones in order to preserve glucose.

Option 2: Eat less than ~150 grams of carbs (aka the amount stored in your liver)

Your liver stores carbohydrate in the form of glycogen, maxing out at around 150 grams of stored carbs. If you fast for a day or two, or just eat very low carb intakes, that’ll exhaust your liver’s glycogen stores.

This is also roughly the maximum carb intake that real life low-carbers would consider “low-carb”. And it would come with a qualification like “yeah, I eat fairly low carb”.

Anecdotally, when people eat below 100-120 grams of carbs a day, some of them feel really crappy. The reasons for this can vary by person. Some simply crave more carbs and are dissatisfied with what’s going into their bellies. Also, our intestines can be finicky, leading to altered bowel habits and altered microbiomes even after just a day of carb changes. And since neurotransmitters abound in that area (such as the 90% of your body’s serotonin that’s produced in the gut), your brain can be impacted by what impacts your gut.

If you eat somewhere between 100-150 grams of carbs, you’re way lower than average, and your body is quickly burning through its liver carb stores. If you eat extra protein, that can provide a buffer to replenish these stores.

Option 3: Eat less than ~250 grams of carbs (aka less than the average person eats)

The average American eats a little over 300 grams of carbs a day.

So if you instead eat a little over 200 grams of carbs a day, you might not be welcome at your local low-carb meetup, but you are definitely firmly in the lower-carb end of things -- slashing carb intake by almost half.

This carb level can be a nice middle-of-the-road intake, especially since it doesn’t seem to impair muscle performance. Then again, it doesn’t appear to provide the metabolic benefits of lower carb intakes in overweight people with diabetes.

Absolutely nobody would call this a low-carb diet. But it’s lower than what most people eat, and is easily sustainable for almost everyone.

Option 4: Eat carbs only at certain times or days (aka targeted or cyclical ketogenic dieting)

Everyone you know has actually tried a low-carb diet before. That’s because everyone’s low-carb between meals!

Most people are only low-carb for around ten hours: eight hours of sleep plus a couple more hours. Others are low-carb for many years. But there’s actually a standardized low-carb diet that falls between those extremes.

Its called the cyclical ketogenic diet (CKD), which basically involves low-carb weekdays and carb refeeds on the weekend. This diet is especially beneficial for athletes or those interested in fine tuning body composition, since you can drastically lower calories for most of the week then fuel exercise with carbs for a day or two.

CKD is just one form of intermittent carbohydrate restriction, which has shown benefits in a few trials. There’s one other way you can restrict carbs, by only eating carbs around workout time (TKD, or targeted ketogenic diet). All of these diets are ways to play around with energy restriction based on timing, with the most popular form being intermittent fasting.

You can modify carb intake in creative ways other than just eating less every day. For example, eating less on certain days, not eating much carb when you’re not working out much, or just fasting for a large portion of the day.

There is no standard definition of low-carb, and hence everybody and their mother and their health guru has their own definition. The only definitions that matter are those that your body understands, such as the level that depletes liver glycogen, or the level that reliably produces ketones, or most importantly the level where you still enjoy eating but also doesn’t make you gain too much extra weight.