Comment by waterhouse
3 years ago
Let's consider a specific example: Lactose intolerance. The Wiki page for it is pretty good. So, despite the name, lactose intolerance refers to lacking the enzyme lactase, which is used to break down lactose. If you have lactase, then your human cells can use the calories from lactose; if you don't, then the bacteria get the lactose. Wiki says "Symptoms may include abdominal pain, bloating, diarrhea, gas, and nausea", which sounds like the bacteria do indeed eat it; if the bacteria couldn't extract calories from it either, then I assume it would just pass through and there would be no symptoms.
Thus, if Bob and Joe eat the exact same diet that has some milk products, and Bob is lactose intolerant while Joe isn't, then Bob's cells get fewer calories from that diet than Joe's. If Bob and Joe's bodies are otherwise identical and follow identical exercise routines, then I would assume that, like, if the diet is exactly enough to maintain Joe's weight, then Bob would lose weight; and if it's exactly enough to maintain Bob's weight, then Joe would gain weight. (Right? For sake of illustration, we could imagine that most of the diet's calories are from milk, and assume Bob can tolerate the nausea.) Yet "calories in" (measured as food entering stomach) are identical, and "calories out" (measured as CO2) might also be identical.
Lactose is a specific, well-understood example of some people absorbing nutrients much better than others. I think there are other examples, and I expect there's a lot of variation in absorption efficiency that's less known. When you hear about people who eat lots of food and remain thin, I suspect this is part of the explanation. And whether their bacteria get the calories instead, or whether it passes through untouched, might show up in CO2 measurements but I don't think it would be related to body fat accumulation.
In a conservation-of-energy sense, "calories in" certainly gives you an upper bound on how many observed "calories out" you can produce without losing weight. But I don't think there's a lower bound on how inefficient someone's digestive system can be (except "zero"), or a practical upper bound on how much their cells might burn energy without us noticing (without close observation). I think, if you wanted a complete accounting of calories-out that would actually match the input, you'd need to add up (a) the heat a person puts out (via contact with air and surfaces, also infrared emissions), (b) the work they do in a physics sense (e.g. lifting heavy objects), and (c) the amount of un-burned calories in their stool (or any other excreted substances).
> Lactose is a specific, well-understood example of some people absorbing nutrients much better than others. I think there are other examples, and I expect there's a lot of variation in absorption efficiency that's less known. When you hear about people who eat lots of food and remain thin, I suspect this is part of the explanation. And whether their bacteria get the calories instead, or whether it passes through untouched, might show up in CO2 measurements but I don't think it would be related to body fat accumulation.
Yeah, the blind assertions I have seen that people basically always absorb pretty much 100% of the calories of the food is weird. We already know that some forms of calories like fiber have very low absorption rate. The Atwater indirect system of calculating calories takes that into account, but is still just an estimation, since it uses average calorie values for protein, carbs, and fat.
And so I've just noted that calories on packaging is just an approximation. An individual food may actually have more above average proteins than below average, etc, which can skew the real calorie count. For example, nuts are known to have less calories than indicated on the label, with whole almonds having 20% fewer. (https://doi.org/10.1093/jn/138.9.1741S) This is also ignoring the fact that many foods, especially those prepared by people are not always super consistent in sizing. A subway sandwich prepared correctly should on average have the calories subway claims, but we all know that some locations will follow the correct procdure more closely than others. Etc.
The other issue with just blindly going with calorie counting (which is really on tracking calories in) is that the differences in the calories one takes in will cause changes in the calories one expends. Eating fairly substancially less than your body is used to will almost certainly cause one to feel bad, and avoid doing as much, meaning fewer calories out, at least partially offsetting the the calorie reduction. But people are generally not measuring the calories expended very closely, if they bother doing it at all.
> or a practical upper bound on how much their cells might burn energy without us noticing
Well, heat dissipation places a bound on that.