Fat or Glucose for Fuel?

A POV on the ongoing debate

In both the nutrition and medical sciences, there has been an ongoing debate over whether the human body prefers glucose or fat for fuel. After over a year of secondary research on nutrition, metabolism and weight management, I offer the following point-of-view.

Define “Preferred”

To me, the notion that the human body, on its own, has a preference for one thing over another is rather absurd. From my point of view, the body has requirements — things it needs to survive. These requirements can be obtained in two ways: We can eat them, or we can synthesize them from what we eat. Not all of our requirements can be synthesized. In nutrition and medical science, these requirements we need to eat to obtain are called “essential nutrients.”

The Essentials

Water. We are mostly water — so yes, we need to drink it.

Vitamins. There are 13 essential vitamins: A, C, D, E, K, B1 (thiamine), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyridoxine), B7 (biotin), B9 (folate), and B12 (cobalamin).

Minerals. There are 15 essential minerals: potassium, chloride, sodium, calcium, phosphorus, magnesium, iron, zinc, manganese, copper, iodine, chromium, molybdenum, selenium and cobalt. Carbon, hydrogen, oxygen, and nitrogen are also essential for human life, but there is so much of each in the food we typically eat that they aren’t even considered nutrients.

Amino Acids. We need 20, but only 9 are essential — that is, if we eat enough of the 9, we can synthesize the rest. The 9 we need to eat are: phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. We get amino acids from eating protein.

Fatty Acids. There are only 2 essential fatty acids: alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid). We get fatty acids from eating fat.

Those are the essentials.

But Something Is Missing…

I know — I’ve only listed two of the three macronutrients. I left carbohydrate off the list. But it’s not a mistake. Carbohydrate is a non-essential nutrient for humans. It is true that if we eat carbohydrate it gets broken down into glucose, and our body needs a certain amount of glucose. Given that is true, why is carbohydrate non-essential? Carbohydrate is non-essential because the body can synthesize glucose. In fact, the body can synthesize all the glucose it needs from the fat we eat and the fat we store on our body.

Hold That Thought

So put a pin in that factoid about being able to synthesize glucose from fat and let’s now think about the food environment as we evolved from our primate ancestors and the critical branch we took that likely allowed early humans to evolve to what we are today.

Much of the vegetation found in the cradle of the human species is made of cellulose — which is undigestible by current-day humans. To have the gut (gastric system) suitable to digest a wide variety of wild (not cultivated) vegetation, it needs to be big and it needs to have a microbe population that can break down cellulose. Big guts require a lot of energy to run.

Another organ that requires a lot of energy is the brain. The food environment 2 million years or so ago required species to choose, as it were, between a small brain with big gut that could digest the abundant vegetation or a big brain and a small gut that needed to eat animals to survive — as animals are easier to digest and are more nutrient-dense. Current-day humans evolved from the earlier primates that survived on animals (including fish), which allowed them to develop, and fuel, a bigger brain.

It makes sense that we evolved eating mostly protein and fat. While there were some digestible plants like fruit and leafy greens, it was easier to hunt game than to gather these plants — remember, this is pre-cultivation times and getting enough calories from raw vegetables and fruits (there were no oils back then, either) would have been difficult unless you spent the entire day eating. It also makes sense that this is why we don’t have any essential carbohydrates — we evolved in a low-carb environment (in terms of carbs we could digest with our small gastric system), so we evolved the ability to be fueled by nutrients other than carbohydrate and the glucose it contains.

Most of our cell types evolved to run on what are called ketone bodies. Ketone bodies are produced by the liver and are derived from fat (dietary and body). Yes — the human body will create fuel out of body fat. But it will do this only under the right conditions. I believe it is the interpretation of these conditions that leads some to the notion that the human body prefers to run on glucose versus fat.

Interpreting the Process

The conditions under which the body will run on fat (via the production of ketone bodies) relate to the level of insulin in the blood. Fat oxidation (using fat for fuel) is driven by a hormone called glucagon. Glucagon is produced in the pancreas. Beta cells in the pancreas create insulin; alpha cells in the pancreas create glucagon. High levels of insulin in the blood causes a person to store fat and blocks the use of fat for fuel. Low levels of insulin allow glucagon to direct the use of stored fat for fuel.

The level of insulin in the blood is directly related to the amount of glucose in the blood. Remember, there are two ways the human body can get glucose in the blood. If we eat very little or no carbohydrate (the dietary source of glucose), the liver will produce just the amount of glucose the body needs to function normally — and this is associated with a low level of insulin in the blood. If we eat a lot of carbohydrate we end up with more glucose in the blood than the body needs, and more insulin in the blood to help store it: some goes into cells for use as fuel and some gets converted into fat and stored in the liver and/or in our fat cells. While this use and storage of glucose is going on, the use of fat for fuel is halted in favor of ridding the blood of excess glucose.

It is this prioritization of dealing with glucose that, in my opinion, gets misinterpreted by some as the body’s preference for using glucose over fat.

An Alternative Perspective

A key point to remember here is that too much glucose in the blood can lead to tissue damage and result in blindness, kidney failure, cardiovascular disease and, of course, Type 2 diabetes. My view is the body simply reacts to what we eat — it prioritizes dealing with blood glucose first because high levels of blood glucose are dangerous. The body has a safety net to guard against glucose levels getting too low — the liver will make glucose out of body fat if that happens. But the only way to deal with too much glucose in the blood is to use it right away or, if there is too much to use right away, store it — even if it has to be converted into fat and stored in fat cells.

If there is such a thing as a preference in fuels for the body it should not be confused with prioritization. If I had a fireplace in my living room with piles of wood to fuel it, it would be safe to say I prefer to burn wood in that fireplace. But if someone tossed a pile of oil-soaked rags into the room, it would also be safe to say I’d prioritize burning those potentially dangerous oily rags before burning any more wood.

Rather than preference, I think it’s more useful to think in terms of what fuel the body uses by default. That is, when fulfilling its need for required nutrients, what fuel would the body predominantly use? The answer to that question — given carbohydrate is a non-essential nutrient — is clearly that the body would predominantly use fat (ketones) for fuel.

I’m sure this article won’t end the debate, but I hope it gets some thoughts percolating!

I wrote a book about nutrition, metabolism and weight management where I cover topics like this in more depth — but not so much that you need an advanced degree to understand it! See my profile page for more details, if you are interested. It’s available on Amazon.

References

Young VR (August 1994). “Adult amino acid requirements: the case for a major revision in current recommendations” (PDF). The Journal of Nutrition. 124 (8 Suppl): 1517S–1523S. doi:10.1093/jn/124.suppl_8.1517S. PMID 8064412.

“Dietary Reference Intakes: The Essential Guide to Nutrient Requirements”. Institute of Medicine’s Food and Nutrition Board. Archived from the original on 5 July 2014. Retrieved 14 July 2014.

Goodhart RS, Shils ME (1980). Modern Nutrition in Health and Disease (6th ed.). Philadelphia: Lea and Febinger. pp. 134–138. ISBN 978–0–8121–0645–9.

Ellie W, Rolfes SR (2008). Understanding Nutrition (11th ed.). California: Thomson Wadsworth.