Just a simple blog post about the reactions in the body about ingested fat and carbohydrate in the diet.

Every now and then, a topic or infograph pops up that claims that 'fats make you fat' in a unanimous sense, or perhaps that 'fats don't make you fat' with some insulin fear-mongering. Pinpointing either macronutrient as increasing bodyfat alone, without context to the diet as a whole, is fallacious. However, balanced infographs aren't eaten up as easily as overly dramatic ones that paint something 'pure evil'; whatever said 'thing' is.

The body can easily handle both carbs and fats, either alone or together; the purpose of this blog post is to just briefly elucidate what happens when you eat them.

What happens when you ingest fats?

In regards to systemic hormones, not much actually. The body tends to have a very similar hormonal profile after solely fat ingestion when compared to not ingesting anything at all.

Fatty acids, after being digested, route around the body in chylomicrons (transport systems) and get deposited in cells. The enzyme lipoprotein lipase (LPL) is usually pinpointed for its role in taking the glycerol backbone off fats so they can diffuse into fat cells.

While fats are entering the cell, they can also be used up for energy in beta-oxidation processes. There is no real hindrance to fatty acids being used for fuel when fats are ingested.

So without carbs, fats both enter cells and are burnt for energy.

(It should be noted that this 'lack' of systemic hormonal response is similar to fasting. The only difference is that in the ingestion of fats there is a influx and efflux of fatty acids; during fasting there is only efflux)

What happens when you ingest carbs?

Most dietary carbohydrates cause insulin to rise, which acts to transfer the body form a 'fat burning mode' to a 'glucose burning mode'. This is due to the body having a seeming 'preference' to burn glucose over fatty acids when glucose is in higher than normal levels.

Temporarily, LPL is hindered and beta-oxidation is hindered as well; these two effects reduce the amount of fat used for energy.

At the same time, insulin increases the rate of glycolysis (breaking of glucose for use as energy) and facilitates glucose getting into cells from the blood stream.

So when carbohydrate is eaten, passive fat burning is hindered in order to burn the carbohydrate just ingested. Overall metabolic rate stays relatively the same.

What about when you ingest both?

Due to carbohydrate having precedence over fat, the same hormonal response occurs in carb+fat as would with solely carb; this is in part due to lack of a hormonal response from fatty acids alone.

Although LPL is hindered in the presence of insulin, fatty acids can still enter a cell via a hormone called ASP (Acylation Stimulating Protein). Beta-oxidation, however, is still hindered.

The end result is some gain in fat storages as their efflux is hindered, and a burning of glucose for immediate fuel needs.

So what makes you fat?

One can be pedantic and say 'fats make you fat' as, technically, the fat molecules do get directly dropped off in the fat cells and accumulate.

Additionally, one can legitimately say 'carbs make you fat' as by eliminating fatty acid efflux, any dietary fat influx with automatically result in a net gain.

The truth is that 'getting fat' is highly related to overall caloric intake, especially from non-protein nutrients (fats and carbs). As long as the metabolic rate demands X amount of calories per day to be used, they need to come from somewhere.

If they come form fat, great. You have a depletion of fat and, if the depletion (from the metabolic rate) exceeds intake you will lose body fat.

If they come from carbs, great. Assuming you don't overeat calories the intake of carbs inadvertently means less fat eaten. There isn't much influx of fats anyways, so the limited amount of efflux that occurs during sleep (when you aren't eating) is enough to negate the influx and result in a net loss.