T Nation

Efficiency of Digestion


I was thinking about the efficiency of digestion today and wanted to bounce a few questions off those in the forums.

The amount of calories in a food is obtained by burning it in a bomb calorimeter, if I'm not mistaken. The amount of calories is gathered by the number of degree celsius 1 gram of water is raised while the substance is burned, or something of that nature.

Anyhow my question is this. If an apple has 200 calories, and you eat it, its assumed you have taken those 200 calories in and you use them as you will.

However, what comes out the other end is also highly burnable if Im not mistaken. So how many calories are you really taking in?


Sometimes there are situations in science where something that would seem pretty obvious and quite relevant gets generally ignored.

The point you are raising seems to me to be one of those things.

Occasionally one does see a study where, for example, nitrogen content of the stool is measured and compared to nitrogen intake, so that efficiency of absorption of a given protein can be evaluated.

But by far the most commonly, nutritional and fat-loss studies simply look at food intake and do not substract out the entirely-measurable caloric content of the stool.

Because of the relatively rarity or perhaps lack of studies looking at this next point, I can't prove my expectation on this one.

But, when I consider cases of fairly sedentary or only quite moderately active people who consume a very high number of calories per day and do not gain fat from it, and I consider the fact that there is no evidence whatsoever that these people are producing more body heat than other people, I have to figure that a large part of perhaps all of their "excess" (relative to normal) caloric intake is simply not absorbed.

A person who actually burned say, 6000 calories per day while weighing only say 160 lb and who did not do much physical activity would inevitably be throwing off a lot of body heat: twice as much or more as the average person in that situation.

The simplest explanation is that they are not burning the extra 3000-3500 calories: rather, they are failing to absorb it. This might be due to gut flora outcompeting the body for absorption of nutrients: in other words, bacteria manage to take in a high percentage or even a majority of the macronutrients.

As you point out it would be entirely doable to measure the caloric content of the stool to prove the point, but I can't point to where that has been done. Which doesn't prove it's never been done, but it certainly isn't routine in nutritional and fat-loss studies.

On your specific question, I'd have to say as a personal guess that the absorption varies according to the individual.

A more informal evaluation is stool volume. Fiber can of course provide volume without necessarily there being any nutrients that were not absorbed, but other than fiber, bile and compounds excreted via the bile (which do not add up to much), and shed intestinal cells, as I understand it the entire dry mass of the stool is bacteria or dead bacteria.

So if for example consuming only low-or-no-fiber MRP's and/or milk and/or eggs, etc. for the day and still having a quite substantial stool volume, I think it would be correct to say that this would be strong evidence or perhaps actual proof of a substantial amount of the nutrients not being absorbed.

Gas is also evidence of this. It is produced by bacterial metabolism rather than our own metabolism.



Seems to me as though this is one of those situations where when an answer is uncovered, it potentially leads to more questions - not that this is necessarily a bad thing.

I've often pondered the same questions. My body temperature is constantly elevated to the point where my perception of room temperature is drastically different than anyone else's in the room. However, it never occured to me to think of a possibility that I could be failing to absorb the food I consume - perhaps quite the opposite.


read up on the thermal efficiency of food. Different macros are digested at different levels of efficiency. Fat and carbs are quite efficient, but protein can have around 10% of the energy lost during digestion.

This is unrelated to Bill's treatise on poop. I strongly suspect that for that for the vast majority of people, that's not an issue, as the human body is usually quite good at making use of food. That's why so many people are fat.


Yes, I don't think that is a substantial issue for the large majority of people. I don't think I would use the phrase "vast majority," though, as it does not seem that rare to me to encounter individuals who really do consume a very-much-above-average amount of food and are not physically active, and yet are neither fat nor gain fat nor do they seem in the slightest bit to be human furnaces.

Skwasny, your point on perceived room temperature is an interesting one. I've thought about it a number of times and have never been able to figure it out (of course, others may well have.) I don't know if it correlates with metabolic rate. I tend to think that the perception of temperature is independent of that, though the sensation of being personally overheated or not is definitely related to internal heat production.


Bill, perceived room temperature is a constant issue for me (and my co-workers!). Although you hint at the perception of temperature remaining independent of metabolic rate, I'm of the opinion that there is a relationship between the two. However, I'm also willing to wager that it could be a small one and I expect there to a large degree of variance from one person to the next.

You suggest the sensation of being personally overheated is definitely related to internal heat production. Would this, in turn, suggest that internal heat production is influenced by the foods (and more specifically, the food sources) we consume? The thermic effect of food is a well established concept and has been brought up many times on this site alone. Based on my personal experiences alone, I think its thermic effect largely influences one's perception of heat.


Yes, raising of metabolic rate due to the eating of food can definitely cause one to feel warmer than would be the case without the meal or with only a smaller one.

What I was meaning was that the sensation of the room's temperature may not change on that account. The body can feel hot without there being a sense that the room is hot or that the cause is the room temperature.


Are you aware of any scientific studies conducted to measure, or at least observe, one's perception of room temperature, and what factors might influence that perception? I can't imagine this phenomenon has yet to be explored any further.


I'm not but in that case the area in question (perception) is quite far from what I study so it could be that there is an absolute ton of information and work done on it, but completely unknown to me.

It is an interesting question. My most interesting experience with it was some years back when having either pneumonia or bronchitis (I forget which) and holing myself up in a hotel room with enough heat to be comfortable and with a humidifier on high, for the lungs.

When feeling partially better the next day, I went out to get some groceries and thought the temperature was in the mid-60s.

According to the radio, the temperature was actually low 80s.

Which had me wondering how hot I had gotten the room! Because I had thought the room was high 80s at most.

Definitely my temperature perception was vastly different from its usual at that time.


I notice when I take in an abundance of calories (15% + more than usual) I always feel hot. My girlfriend thought I was getting a fever, but I noticed the temp raise was always after a series of big meals.


This also brings up the interesting point. Are calories burned the same as calories restricted in the diet

For example : If one eats 5,000 calories a day and burns 3,000 is that the same as only consuming 2000 with no exercise.


I've also heard (although not really confirmed) that increased water intake may result in a higher body temperature, either indirectly or directly.

I'm very interested to see if anyone can find any research relating to either the affect of weight training or weight training diet on body heat, as I'm consistently overheating (many jokes made at my expense) even in 40-50F weather. I often sit and point a fan directly at me.

This heat generation does seem to increase around exercise. I just assumed it was due to a larger muscle mass than average, and thus a higher heat output.


Don't know about water, but when I drink 8, 10 beers I get real hot. I assume it's the calories.


Would not the increase in temp. have something to do with with training as well? When I pick of my frequency 2x a day I feel warm/hot through out the day.


Thats weird or maybe I am weird, when I eat really big meals I get cold especially my feet and hands.


My guess is that all the blood is going to your stomach.


We'd have to figure calories burned in the same way for each comparison.

Sometimes people mean calories burned by exercise, above and beyond what would have been burned without it; while sometimes people mean the total that the body burns.

If one ate 5000 calories per day and burned 3000 calories attributable to exercise (plus X from resting metabolic rate) then the fat gain or loss that day would I suppose be the same as consuming 2000 calories per day and having none of that exercise.

But that was stated in a precise way: "attributable to exercise." If simply counting the extra calories burned during the exercise itself then it would not work out quite that way, because there is increased metabolism post-exercise. That can fall into the "attributable to exercise" category but perhaps would not be counted as calories burned in exercise.

And for practical purposes there is another difference in that if the body is trained to routinely burn large amounts of calories in exercise, then the resting metabolic rate tends to go up so that even on non-exercise days there will be more calories burned.

The short answer would be that the person who chooses to avoid exercise on the theory that simply eating correspondingly fewer calories will be equivalent, will be wrong.


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Thanks Bill Very informal.


X2 Bill and BBB