What does it actually mean to “burn fat” during exercise?

Submitted by orange-robin t3_10nemof in askscience

I understand that the body can use carbs and/or fat as an energy source during exercise, in differing proportions depending on intensity, but what does “burning fat” during exercise actually mean, and how is it different to actually losing fat?

If you do a long, low intensity workout where your body predominantly uses fat as its fuel source, will your body fat percentage be lower when you’re done? Is your body burning fat for fuel the same thing as when it loses fat due to a calorie deficit?

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Beginning_Cat_4972 t1_j6985ur wrote

The short answer to all your questions is, yes. Adipocytes (fat cells) store energy in large lipid droplets that are encased in their own single-layer phospholipid membrane. People tend to describe lypolysis (the breakdown of of fat) as though it doesn't happen until your body has completely exhausted all blood sugar and stored carbohydrates. That's probably (I'm saying probably because I didn't look this part up, but I'm pretty confident) not true, it's more like you are using all forms of energy all the time, but you may increase or decrease the retrieval of different energy stores depending on your metabolic needs.

Lipids tend to be stored as triglycerides (larger, more complex lipids) and then are broken down into free fatty acids (smaller, simpler lipids) and transported out of adipocytes in a few different ways. Some are carried by proteins, some are carried by small vesicles. It probably depends on the pathway that is being used/what process has led to lypolysis.

Different signals that induce lipolysis are going to come from fasting, growth, stress, and disfunction of things like the thyroid. All the pathways lead to some amount of stored lipids leaving adipocytes in one way or another. For different people these processes may be more or less efficient. There are a lot of steps in signaling lypolysis and if any of them are impaired, it will be harder to release stored lipids.

I imagine that you have a balance of storing and releasing lipids that is constantly happening. If you expend more energy than you consume, the balance will shift towards fat loss. When this happens, your fat cells essentially shrink as their droplets become depleted. Since you are (again, educated assumption) constantly breaking down and replacing lipids, the effects of the shift won't be noticeable immediately after initiation. But if you keep the balance of fat loss and production on the loss end, you will eventually lose enough lipid mass that you see a difference.

So, even though the signaling that leads to "burning fat" may be somewhat different during exercise or fasting, the end result for your fat cells is more or less the same. Following exercise, depending on how efficiently you can signal for lypolysis, you may have a minute net loss of lipids in your fat cells. But they have to be further broken down and the byproducts have to leave your body at a higher rate than you generate new lipids for you to notice a loss of body fat.

I hope this helps!

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Ausoge t1_j6ajakm wrote

I think the reason people talk about the body using up all its carbs before starting to munch on fat reserves is that sugar is the "fast" energy source. It is the easiest energy source for the body to use, because:

  • it requires little to no processing/metabolism/breaking down before its chemical energy can be accessed, as it is a fairly simple molecule.
  • it is stored primarily in the liver and in muscle cells, so it is always immediately accessible during the initial period of exercise.
  • It is water-soluble, so readily dissolves into blood and is easy to transport around the body.

Fats are far more complex molecules than a sugar like glucose. They are made of the same atoms as sugars (C,H,O), so fat can be processed into sugar to replenish depleted reserves, but this requires a level of energy investment before the energy payoff is reached.

For the above reasons, I think it's pretty fair to assume that carbs and sugars will be used preferentially over fats and proteins, simply because of their ease of use and accessibility.

One thing I find really cool about weight loss is how the mass actually leaves the body. I don't remember the exact process, but ultimately it is mostly excreted via your lungs - all that carbon and oxygen is breathed out as carbon dioxide.

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Glasnerven t1_j6aykjk wrote

> One thing I find really cool about weight loss is how the mass actually leaves the body. I don't remember the exact process, but ultimately it is mostly excreted via your lungs - all that carbon and oxygen is breathed out as carbon dioxide.

If my memory serves me correctly, when you want to know how many calories a person is burning--and to measure it accurately, instead of just estimating by their level of activity--you do it by measuring how much carbon dioxide they produce. Metabolism is combustion.

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Objective_Regret4763 t1_j6bw595 wrote

Actually fats are not much more complex than sugar molecules, if at all. Fats are essentially just 3 straight chain hydrocarbons that may or may not be saturated. Once lipolysis begins it is actually a faster process than breaking down glycogen and sugar. It’s just the body will delay initiating the process because our body prioritizes the use of glucose over fats for energy for various reasons.

Google a picture of triglycerides and a picture of glycogen and see which is more complex.

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exphysed t1_j6ap4f1 wrote

Your body saves carbs as much as possible. If it can use fat, it will. Think of carbs as only being used if your body has to - to go fast (or in the brain)

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Triabolical_ t1_j6gnn5c wrote

This is a bit confusing and there's a lot of incorrect information out there.

The aerobic system - which gives you power for low-intensity efforts - is dual-fuel. There is a pathway where it can be fueled by glucose (glycolysis/pyruvate oxidation) and another one where it can be fueled by fatty acids (beta oxidation). Both of those paths feed into what is known variously as the Krebs cycle or the TCA cycle.

The obvious question is how the body determines whether to burn fat or glucose. It's a bit complicated.

If your blood glucose is elevated from carb intake, your body will preferentially burn glucose to try to get the blood glucose to go back down.

If blood glucose is normal, then the body will burn glucose and fat based on the kind of training you have done. Mostly high carb training, you will burn glucose. Mostly low-carb training, you will burn more fat.

At higher intensities, the additional power only comes from glucose. So a highly-trained aerobic system is better for burning fat.

So, if you want to burn a lot of fat, you need to train your body with extended periods of zone 2 training without much glucose around. Fasted is the best, but you need to transition gradually or you can run out of glucose ("bonk"), which is no fun.

This also means that the idea that you should "eat carbs to fuel your workout" is bad advice for those who want to lose weight.

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