When we need energy, can’t the body just burn the stored fat? Isn’t that the whole point of stored fat? Why will we feel hungry, weak, lightheaded, etc. at all? I understand if the body doesn’t have enough fat (if you’re super skinny), it would make sense to feel hungry, but I don’t understand why would that be the case if there’s enough fat to go around.
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There are 3 primary energy pathways that the body uses. These are used in order and you can physically feel when they switch.
Phosphagen phase is the first. Creatine Phosphate (stored in the muscles) is rapidly turned to ATP. This is your initial burst of energy but it runs out very quick. If you start a run, this is the first 30s where you feel really good. It mostly serves as fight or flight to give you instant quick burst of energy.
Anaerobic Glycolysis is the second pathway. This kicks in pretty quickly as the phosphagen phase is so short lived. This begins rapidly turning available glucose (carbs) into ATP. This has a great advantage in that it doesn’t require oxygen. This means you can get a lot of energy while the body is starting to bring oxygen in more rapidly via increased breathing rate. When you are running this is where your breathing starts building. Your body knows it can only sustain a few minutes in this phase and so it starts prepping the 3rd stage.
Aerobic Glycolysis is stage 3. This is what everyone calls the fat burning stage but that’s not necessarily true. Your heart is now beating fast and lungs breathing heavy so the body has enough oxygen to begin converting stored carbohydrates and fats into ATP. Your body will prioritise carbs as it requires less oxygen for the same amount of ATP even though fats are generally provide more energy overall. This is where you see a difference in trained individuals Vs untrained. The stronger the heart and lungs, the more oxygen can be brought in at any time and the more the body can use the stored fat. The bigger the body/fat stores, the more oxygen is required. It’s possible in those really untrained or with reduced lung function (via illness or damage eg smoking) that very little fat is actually being burned in this stage.
Once you have finished your exercise your body will remain in a state where it is still bringing in more oxygen. This is known as Excess Post-Exercise Oxygen Consumption. The body is still bringing in high amounts of oxygen to quickly recover the ATP etc in the muscles so it can go again when it needs to. This area still needs a lot of research but is believed to be why things like weightlifting and High Intensity Interval Training (aka HIIT) are really good for fat loss even though they burn less calories during the actual exercise than something like running, cycling or swimming.
So in short, to use fats in this way, your body would need to be able to rapidly bring in oxygen, but it can’t just jump up the heart rate and breathing so it uses what it can first. Without that extra oxygen the body will prefer to use available carbs.
I’ll also add in an extension, this is why good sleep is so valuable during fat loss diets. during REM sleep, your body increases heart rate and breathing rate allowing for making use of burning fats over carbs especially while the body. The more quality sleep you get, the more REM sleep you get, the more the body uses fats.
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