“Calories in – calories spent = weight gain.” Sounds simple and rather obvious – conservation of energy, right? But as applied, it makes some rather bad assumptions. And as many will testify, it doesn’t seem to work.
To start with, caloric input is NOT the same as the calories you eat. To some extent this is recognized. Cardboard has calories, but there is no way a human body can use them. While fiber (cellulose or soluble fiber) is often excluded from calorie counts, even digestible calories may not always be digested. The true caloric input is the calories your body is able to turn into glucose and lipids in your blood stream. I suspect that people vary quite a lot in how efficient their digestive systems are, and that may even vary with time for the same person. Certainly variation with time could help explain the “set point” for body weight.
Inefficiency in our digestive system? There are digestible calories in what comes out the other end, and not just in diabetics who lose sugar in the urine. Pigs and dogs scavenge human feces, among other things, if given a chance. It is the difference in calories between what we eat and what comes out that is the important energy input, and there has been very little study of how much that form of energy out might vary.
Then there is energy usage. Certainly exercise, even walking, burns more calories than simply sitting. But it takes energy to keep our body temperature up, our heart beating, our lungs expanding and contracting, and especially to keep those big brains operating. Sitting as quietly as you can in a cool room may burn a good many calories, though I wouldn’t recommend it as a way to lose weight. (It is, however, recognized as one of the reasons people working in the cold may need more calories. If your body is very efficient at all these “housekeeping” tasks (low basal metabolism) you may need fewer calories to maintain constant weight than someone whose basal metabolism is higher.
For that matter, some people may use their bodies in exercising more efficiently than others.
I strongly suspect this is an oversimplification of what seems to be a near-epidemic of excess weight. I certainly wouldn’t argue with the idea that something in our environment (including our food environment) is tinkering with the efficiency of our digestive processes, though I suspect serving size has a lot to do with it. But why don’t we ever consider calories out? It would be simple enough in test animals, if not in humans.