I had an epiphany several years ago during a lunch time conversation with a co-worker. He was bemoaning the fact that I could wolf down huge quantities of food and still remain slim while he would eat half the quantity and still be pudgy. I jokingly told him that his body was just more efficient at digesting the food than mine was, and that I probably shit out a perfectly good meal.
That conversation has sat with me for years, lurking in the back of my mind. It brought up this question: If the human body has so many natural variations, why do we treat the unit of energy derived by digesting food (the calorie) as a universal constant? Individuals have huge variations in blood pressure, VO2 max, cholesterol, IQ, perspiration, etc. Why do we assume that the energy derived from a chocolate square is the same for me as it is for my wife? Considering how complex the human digestive system is, I’m surprised that calories consumed for a particular food are assumed to be constant across ALL people. In fact, I wouldn’t be surprised if the caloric benefit that I derive from a chocolate square varies day-by-day, week-by-week. It probably varies greatly based on what other foods I’ve eaten.
In fact, we know that the caloric absorption varies based on what we eat: Chitosan, Conjugated linoleic acid, Green tea extract and Guar gum are popular weight-loss supplements which work by messing with how food is absorbed by the intestines (hence the nasty side-effects like diarrhea, constipation, and “other gastrointestinal problems”). If OTC laxatives move food more easily through the intestines, doesn’t it make sense that some of that caloric benefit cannot be extracted?
Of course, measuring the truly “digested calorie” would require some nasty lab work. Typically, the food is burned in a calorimeter and the calories calculated from there. I (unfortunately) think the proper way is to have people eat the food (and only that food), then burn their poo in a calorimeter, and subtract the result. I think the most interesting results would be in the variations: for 1000 calories of food consumed, would some people have 100 calorie poo, and others have 400 calorie poo? Then there could be whole industries around measuring your personal poo calories so that you know the best and worst foods for you. Maybe I better patent this right now…