(An update of something I posted on facebook many years ago on applying some simple thoughts related to machine efficiency to the topic of human weight loss and gain and "slow/fast" metabolism).
I sometimes run into people (usually young males) who want to gain weight but claim they can't because they feel they have a fast metabolism. The first question I ask is what did they have for breakfast. The answer was nothing or not much, which is typical. Literally >80% of people that I've run into that say they can't gain because of metabolism don't eat a substantial breakfast. It is extremely hard to make it over the bar with two meals a day rather than with three or four. I have grown very suspicious of this talk of "metabolism" differences. They may be there but my guess is that they are overstated.
What would it mean for someone to have a fast metabolism?
If your body composition and weight is at steady state, the following must be true, because a fuel that is not stored must be either burned, used for work (which also produces some heat), or thrown out (can you think of another option?):
Food in = Food out (excreted) + Work done + Heat produced.
In this scheme, the only way you can eat more than someone of identical mass but remain at steady weight is if:
- You excrete more.
- You do more work. Maybe you run more than someone, or you run the same but inefficiently. Maybe you are a fidgeter as opposed to a laid-back relaxed type. This could include having a more muscular composition, which is said to require greater work to maintain.
- You expel more heat. This could be some combination of the following factors:
- You have a harder time keeping your body temperature because of higher surface area, less clothing, or colder ambient temperature.
- You have a substantially higher body temperature.
Considering that heat loss is proportional to the temperature difference between you and your surroundings, a 10% faster metabolism that doesn't involve more excretion, more work or higher surface area means that your temperature difference with the environment is at least 10% larger. For a normal human being at room temperature (25C), this would mean your body temperature is at least a full degree celsius warmer: from 37C to 38C. And that is just for a 10% faster metabolism.
So if you want to claim that you have a fast metabolism, an implication is that you are likely running hot. The other option is a high heat transfer coefficient.
More likely, a close examination of total food intake and activity will show that systemic differences are smaller than they seem.
Metabolism slowing down when you get older
Looking at things this way, it reduces the plausibility that anybody's metabolism 'slows' down when they grow older. That used to make sense - oh it slows down, like cars or other devices with age.
However, this way of thinking implies that the design goal of human physiology is to be able to eat more without increasing in weight. Our long-ago ancestors would strongly disagree that this was the main problem they were facing.
A 'slowdown' in metabolism in the sense people usually seem to mean (I am doing everything the same as when I was younger but I am gaining weight) actually would imply an increase in work efficiency of the body. You work more efficiently or you expel less heat. In this sense, a person's metabolism slowing down with age is like a car's gas mileage increasing the longer you have it.
More likely, as a friend pointed out, an older person with a "slower" metabolism is simply doing less physical work (perhaps due to learning how to do things more efficiently), not exercising, not fidgeting, living in comfortable climate-controlled surroundings, etc.
So in both cases: decline with age and apparent variation between people, perhaps best to first carefully look at what one eats and what one does before concluding that the difference is in one's own system.