| We don't suck up all calories from what we eat. We know this because our fecal matter burns. What you have on dietary labels are dietary calories, which is what a typical human body can absorb. Not the "fuel" calories. I think the understanding about dietary calories comes from the agricultural world where calories are much more involved in the economics of animal growth and weight gain. So we might not even be that sure that what we consider dietary calories are accurate for humans. Assuming they are accurate, each individual digests a different % of all calories they ingest and it depends on many factors. It's a very imprecise science. If you want, you can eat a given number of calories each day of just one meal (like Huel or Soylent) for a month. Then, for the last week of the month, you can see how much weight you gained or lost over 7 days. You will also need a medic to estimate your basal metabolic rate and you would need to stick to your daily physical activity habits consistently. And then you can arrive at some number of what % of dietary calories you absorbed for that particular food with those particular eating times, exercise habits, and so on. I think it's a very impractical test. Maybe it might be easier to do in a very controlled environment in terms of energy expenditure, like in weightlessness. But then the weightlessness is a factor in digestion, too. If someone found a way to accurately determine what % of dietary or fuel calories humans really absorb, the data would probably be as impactful in our understanding of diets as the Minnesota starvation experiment. |