People, including a lot of nutritionists and diet doctors, tend to treat people as if they were more or less homogenous all the way through, like a hard boiled egg: some fat on the outside and a metabolism on the inside. So, when they talk about diets and losing weight, they assume that it’s just all stuff going in versus stuff going out of a sort of blob in the middle. This results in the naive picture of weight regulation where the number of kilocalories you eat (measured by burning the food to ash in a calorimeter) goes in, and it’s either burned up or deposited in the egg white as new fat.
Real organisms aren’t that way, of course. When you eat something, there are long chains of complicated processes going on to transform the chicken meat and carrots and noodles in your chicken soup into amino acids, and fatty acids, and various ions in solutions in the bloodstream; a whole bunch (a whole bunch) of free riders are eating the food too, converting it to other forms that they use to breed their own descendants; some of the result of that turns into nutrients in our blood stream, some of it turns into bacteria, and a whole lot of that eventually turns into something I’m far too delicate to mention.
Once it’s in the bloodstream, there are lots of other complicated processes going on. I talked about them a little bit two weeks ago, but it’s worth remembering that sugars cause the body to release insulin, insulin causes adipocytes (fat cells) to store triglycerides, plump adipocytes release leptin, leptin reduces appetite, which means less food and less sugar, which makes the adipocytes release triglycerides, and so on. There is a complicated feedback going on there, and in a lot of people this feedback results in essentially perfect control of body fat and weight.
We tend to forget this, as talk about the “epidemic of obesity” gets around, but the fact that roughly one-third of adults are obese means that roughly two-thirds of adults are not obese. Most of those not-obese people eat the same general diet, live similar lifestyles, go to the same movies, watch TV and drink sugary sodas, and yet they stay more or less skinny.