Saturday, May 7, 2016

Maintaining Weight

Many of us gain weight as we age, but some people seem to stay at the same weight for most of their adult lives. How do they do it?

A pound of fat contains 3500 calories, and dietitians and are fond of telling us that if we eat an extra 3500 calories a year, we’ll gain 1 pound a year, so we’ll gain 10 pounds a decade and 50 pounds between high school and retirement age and that this partly explains the “obesity epidemic.” The idea is that just overeating by a tiny bit will result in significant weight gain in the long run.

But that 1 pound a year works out to about an extra 10 calories a day.

Now, we all know that the body/brain does miraculous things, but it’s never made any sense to me that our bodies, and especially our brains, would be able to control our food intake so closely that we would eat within 10 calories of what we needed. A stick of chewing gum contains about 10 calories.

So I was interested to read a recent article about male Barbary macaques. It seems that these monkeys regulate the amount of thyroid hormone they produce during mating season, when they need extra energy to fight with other males as well as mating with as many females as they can.

Some monkeys actually double their levels of thyroid hormone at the peak of the mating season. Extra thyroid hormone would speed up their metabolism, burning more food instead of storing it, which would provide more energy during this stressful season. Conversely, when food is scarce, they produce less thyroid, slowing their metabolism down. This is analogous to what some humans find when they eat a lot less in order to lose weight.

Of course, this might make one think that if you're overweight, all you have to do is take a little thyroid hormone and your weight will melt away. Unfortunately, that doesn't work unless you start out deficient in thyroid hormone, and too much can be dangerous.

However, it suggests to me that the body could compensate for a little overeating or a little undereating by simply increasing or decreasing the amount of active thyroid hormone in the bloodstream to keep our weight constant. Massive overeating or undereating would still have massive consequences for weight. But that extra stick of gum every day would not make us obese.

1 comment:

  1. The reason why an increase in Ein doesn't result in increasing weight ad infinitum is because ↑ weight → ↑ Eout (independently of thyroid hormone levels).

    Ref: The last section of Nonequilibrium thermodynamics and energy efficiency in weight loss diets, by Richard D Feinman and Eugene J Fine.

    An increase in Ein of 100kcals results in an increase in weight of ~10lbs.