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Ausoge t1_j6ajakm wrote

Reply to comment by Beginning_Cat_4972 in What does it actually mean to “burn fat” during exercise? by orange-robin

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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