Most people think that the body deals with energy from foods in a specific order.
The common analogy is that carbs are fast burning twigs or paper, protein is a log and fat is long burning coal and that you have to burn through them in order. They seem to suggest that you only get to fat burning when you first burn through carbs and protein. This is fundamentally and actually comically wrong.
Take this video from my least favorite company. The MLM (pyramid scheme) company Pruvit. They sell those useless ketone supplements that they claim will have you burning fat just from drinking ketones. Just in case you want to ask, they don’t help you do anything other than have ketones in your blood. Waste of money.
This is how it actually works. I will try to get as detailed as I can without being too detailed and I will mostly focus on how it works in a person who is metabolically healthy and eats mostly a ketogenic diet as this creates the most healthy metabolic fueling system.
When we eat a mixed meal of carbs/protein/fat all food gets broken down into smaller parts in the stomach. It then moves to the small intestine where it gets further broken down into its base components.
- Protein gets broken down into amino acids.
- Fats get broken down into fatty acids
- carbohydrates get broken down into simple sugars
According to the rocket surgeons above, you would have to burn up all the carbs and protein and all the fat would be stored. In a very basic sense, yeah that is sort of what happens in the short term but that is so basic that it doesn’t look past the surface at all. It is like looking at the earth from space and saying “Oh it’s just a ball.” There is so much more to it than that and there is so much more to energy partitioning in the body.
This is what actually happens. Again, this is in the context of a mostly ketogenic dieter.
- The carbs initially spike insulin
- This shuts down the use of fat and puts the body into storage mode. At this time it is true that the body will use only carbohydrate for fuel or at least more so. It really depends on the inulinagenic load of the meal. Lets assume it was very high. For right now, as insulin is high, carbs are the majority fuel source. At the same time as insulin is shoving fats into fat cells it is also shoving glucose into muscles (reserved for high intensity exercise) and liver (reserved for blood glucose management). We have storage in these locations for carbohydrate just as we have fat cells for storage of fatty acids. The form changes which actually locks the fuel away in these locations. The form is glycogen and the fuel needs to be converted back from that form in order to be used for fuel again. This is why we don’t use all the glucose first. More on that later. The difference is that these stores are limited where fat cells are nearly unlimited. Once all glucose is put away into this storage, insulin levels return to normal and we start using fat again. Regardless if we have used up all the carbs or not. This will happen faster the more insulin sensitive a person is.
- Since insulin is high, fat cells are not releasing any fats. They can only store.
- All fat in the meal will go to storage so the body must use glucose as fuel until insulin levels drop. Again, the more insulin sensitive you are the faster insulin will drop and the faster you can use fat again. As they drop there will be an incremental increase in the amount of fat being used. Already we can see that the above video is factually incorrect. We rarely ever use only one fuel source. We are usually using a different precent of fats vs glucose depending on insulin levels. More insulin means less fat and more glucose and less insulin means more fat and less glucose.
- Protein has been broken down into amino acids. The building blocks of the body.
- Unlike the uneducated scam artists at pruvit, protein is not a fuel. It can be used as a fuel but only in emergency situations. The only time it will be used for fuel is in the rare instance where insulin is high but glucose levels are low. This is a rare occurrence. The only time this could ever really happen is in someone with severe insulin resistance. Insulin levels could be very high for prolonged periods of time which could potentially cause exhaustion of glycogen stores. Once the glycogen is all used, if insulin is still high, the body will turn to protein because the insulin is holding fat hostage. Other than this instance, protein is never used as a fuel source. It is either used to rebuild tissue, used to create enzymes for other bodily processes or if eaten in excess of body needs, will be converted to glucose in which case it could be used as fuel.
Now that we have that broken down properly lets look closer at what happens as we go on about our day after the meal.
If we are just sitting around doing nothing or just working at a desk, the body does not require any fast acting fuel for this. It is most content to run on fat and it will do so primarily. Now what about those stores of glucose? The glucose that was stored in the liver and muscles as glycogen has a specific use. Since glucose is a fairly valuable commodity and is in limited reserve, it is locked up pretty well. It needs a chemical change to make it available as glucose again. Until it is changed it is locked up and will not leave the glycogen storage.
The liver is a fairly good about letting go of glycogen. The primary job of the liver glycogen is to maintain blood glucose levels. Thinking in context of a ketogenic dieter, we don’t need much glucose. The brain will use some blood glucose while the liver is full of glycogen and the brain pulling glucose from the blood will cause a low blood glucose state. This low glucose state will spike glucagon a bit but only slightly. That is all that is needed to release glycogen from the liver. This will keep the blood levels at a safe range. We are still using fat at this time as insulin is not raised above baseline. This use by the brain will deplete the liver pretty quickly. There is only about 100g of glycogen in the liver. The brain can and will use about 130g in a day and there are other things that require glucose lke red blood cells. This 100g will not last long. Once the liver is depleted, the body will return to making ketones from fat regardless of muscle glycogen levels. The brain will start using ketones and the need for glucose is now dramatically diminished.
There is a large process that has to happen for muscle glycogen to get released to the blood so this doesn’t happen unless it is critically needed as in the insulin resistant scenerio. It stays locked up there simply due to the complexity of the process. Once liver is depleted you can still have as much as 400g of glycogen in muscles but it will not be released to general circulation and will not be used unless there is a large energy demand that requires glucose. Like if a bear is chasing you. Only then will that glycogen get used. If there is no bear chasing you, you absolutely won’t be using this source of glucose. Light exercise will use some of this but will still use more fat than glycogen. The percentage of fat vs glycogen will shift with intensity of exercise. Yet again we see how the majority of the fitness and diet world are dead wrong. We don’t have to burn up all our carbs to use fat. We can have 400g of carbs sitting in our muscles and it will happily sit there until it is needed for escaping a bear. Or just high intensity exercise. I like the bear analogy better. 🙂
So no, we don’t have 3 sources of fuel in our body. We have 2 sources of fuel and one building block that can be used for fuel in an emergency. This is a huge reason why so many are confused. There is far too much misinformation out there coming from people who have little to no actual understanding of how the body actually partitions fuel.
I hope this sheds a little more light onto how we actually use fuel in the body. Below you will find some resources if you want to learn more about glycogen metabolism as well as the bodies different energy systems.