Got to thinking today (dangerous for me...lol)! To me, the very fact that the human body converts excess sugar (read excess carbs) or calories into adipose tissue (read fat) is biological proof that the human genome has adapted to burn fat as the most efficient fuel. If evolution or natural selection (survival of the fittest) has any merit whatsoever, would it not be logical to conclude the biological response is to store the most efficient source of energy as fuel to burn during times of want? What say the experts???
Really great thought. Most people conclude that carbs (glucose) must be the preferred fuel source, since the body with burn it first. However, when you flip that around, and say that the body is trying to get rid of that fuel (and/or store excess as fat) things become a little more clear.
But, I agree with Alligator, that we are well adapted for the use of both fuels . . . but the proportions should tell us something. Short bursts of high intensity (catching prey, or eluding predators) uses glucose - normal activity (walking, gathering, playing, etc.) uses ketones. Over consumption of carbs shuts down our fat burning machinery (or at least turns it off) - and should be an indication that we are running sub-optimally . . . or as Tommy (Snatch) would say "It's not in sync with evolution."
You are only seeing part of the picture. We are all fat burners at low intensities, because i takes more time to break fat down for use. At higher intensities, we are all glucose burners. If you don't consume glucose, then the excess protein in your diet or the protein in your muscles broken down for energy. You see? We are fat burners at low intensities, and sugar burners at high intensities.
Using your same logic, one could just as well say that we store (1200-2000 calories) as glycogen within our muscles and liver..it's proof we're sugar burners!
We burn sugar when it's conducive to X activity, and we burn fat when it's conducive to Y activity. Try burning fat when sprinting. You simply CANNOT. You burn fat after sprinting though.
That is one hypothesis. There are other theories on why we store energy as fat, a common one being that fat is more energy dense than carbs and this means more efficient energy storage.
If you want to evaluate this theory on the basis of species adaptation, look at other animals; it's pretty common for fat to be the basis of energy storage in the body regardless of the macronutrient content of the animal's natural diet. I'm currently not aware of a species that stores excess carbs (or other macros) as huge amounts of glycogen or another carbohydrate storage polymer, though there could be.
Fat could be the preferred energy substrate, but such an observation is not proof. It leads to a hypothesis, which can then be tested. But it's not proof.
Yup. Also reducing carbohydrates (glucose, fructose, etc) are reactive molecules. They contain aldehydes and ketones that can undesirably react with other biological moieties, such as amines (think: protein/DNA). Fats are not nearly as reactive towards other molecules. So just based on the stability for storage, fats make sense.
1 gram of fat = 9 calories 1 gram of carbs = 4 calories 1 gram of protein = 4 calories
Fat has more energy per gram than the other macros. It makes more sense to store fat as a reserve energy source for this reason. Healthy BF% levels are from 10% to 25% for Men (up to 30% for women). This means a person weighing 150 pounds with 20% BF is carrying approximately 30 pounds of 'dead' weight as energy. To carry the same energy in another macro form would require 67.5 pounds or an additional 37.5 pounds of 'dead' weight. I imagine this would put you at a disadvantage when you are running from a tiger on the serengeti. :)
Stored body fat is not proof humans are fat burners. We can just store more energy in fat cells. In these times of plenty there is more likelihood of over-stuffing our fat cells. We were never meant to have such a surplus of energy hanging about.
Perhaps stored body fat is merely a reflection of our environment, which does not prove that fat is better than carbs or protein really. We live in an obesogenic environment is all. Sure some say that humans evolved the ability to store more body fat than lean tissue as compared to most animals as a keen survival mechanism. But perhaps our exquisite ability to store body fat easily is more a sign that we must try to remove the abundance of extremely calorie dense foods from our environment, or introduce a lot more daily activity (or both), rather than a sign that we should be dipping our steaks in coconut oil.
Anyway, I thought this was a good read: http://blogs.scientificamerican.com/bering-in-mind/2010/11/02/the-fattest-ape-an-evolutionary-tale-of-human-obesity/
Could one not also ask "stored muscle... proof humans are protein burners?" Or, stored glycogen...proof humans are sugar burners?
In men, the only amount of bf you need is enough to protect your internal organs. BF beyond 10% likely contributes to hormonal imbalances like exess estrogen and reduced insulin sensitivity. Having excess body fat would also be a huge disadvantage in the wild.
Also, the reason there seems to be a threshold on how much muscle one can naturally build, in a non-metabolically derranged individual, there is likely also a threshold on how much adiposity one can accumulate (especially given only actually paleo foods). Without activity, the reason one tends to store fat preferential to muscle is because msucle is metabolically expensive to carry.
My point is that all stored body fat really says is that we're able to draw on that as reserve energy, but so are we able to draw on muscle, and glycogen.
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