I personally am not interested in fasting for any more than 20 hours at a time. However…
For someone with a really fast metabolism (in excellent cardiovascular shape, with a low body fat percentage), would their metabolic rate and body composition affect their ability to fast for extended periods of time (5+ days) without feeling hungry, weak, lethargic etc.?
This question is coming from a place of knowing that higher metabolisms require a greater amount of energy to maintain bodily functions. So in the case of someone who is overweight and sedentary and does a 10 day fast, for example, they may be taking in extremely minimal calories but their bodies are still using less energy to function than an elite athlete who did the exact same 10 day fast.
So I guess I’m just wondering if my theory of the elite athlete having a harder time doing an extended fast than an overweight/sedentary person holds any validity
I’d say you’re right on the money. Fasting converts stores of triacylglycerol into free fatty acids that the mitochondria can use for muscle energy (ketone bodies produced in the liver) while the glycerol moiety is converted into glycogen (by the liver aswell) and with the help of Glut transporter down regulation in the muscles and periphery tissue is made exclusively available to the CNS tissue (the blood sugar that is)
Now your brain needs roughly 200g of glucose per day to maintain homeostasis and keep an adequate membrane potential so your neurons can send signals through the entire length of the nervous system… but if your slim and fit there is simply not enough available TAG to be converted into energy and glucose so thats when the body starts to break down protein for gluconneogenesis but auto-phagy is not always the best state for our bodies long term but once in a while is okay.. but If there are no excess adipose cells to break down for the bodies protein needs You’ll soon notice hair stops growing and being unusually lethargic so thats when it’s time to re-feed and replenish your glycogen and protein.
I think it depends on how lean you are talking – 15% body fat is pretty lean and atheletic , but the body only needs about 5-7% fat essential. So there is still much fat (although not obvious like a big belly) for the person to be in fast; thinking of our hunter-gatherer ancestors , I would imagine they were pretty lean and there would be times whereby they went without feeding for a good while, and natural selection would favour those who are able to survive longer without feeding . So those lean people will also benefit from the autophagy and the benefits a fast give.