The calories in poo.

No, it’s not what you think.

Lets say a person is on a diet. They eat 1500 calories a day. How much gets ‘left behind’ and does it vary?

Yeah, I know about corn and peanuts and olestra and that it could change depending on the types of food. But does the body’s ability (desire?) to extract all the potential energy out of the food stay consistent?

And what IS that efficiency? Many moons ago, we had a friend who’s campsite had a propane powered outhouse. (Woe to the person who used the facilities before it fully cooled off) But that’s effectively what happens when you perform a calorie test: You set something on fire and see how much energy it gives off. So there’s probably SOME energy left, but how much?

Too many variables, but here is one:

Processed food and cooked foods are more likely to be absorbed completely.

Raw foods not so much.

If a lab test shows a cup of beans to have 160 calories, it might wind up being more like 100 when the body processes it.

If a lab test reveals a can of cola to have 180 calories, it will probably wind up being 180 calories (or damn close to it) when the body processes it.

Gastrointestinal issues can reduce absorption, too. Heavy caffeine intake, overindulgence and other variables (stress) can cause the body to propel food forward at a rate that does not allow maximum absorption.

I read nice article about guys who studies human waste to determine some of the things I mentioned above, but can’t find the article. They also happened to substantiate the point about calcium (not calcium in supplement form) reducing fat absorption. So, replace some calories with calcium-rich foods and more fat will end up in the stool – unabsorbed.


I ask as it seemed like, while dieting, I continued to lose weight, even on days I was “bad”. I didn’t fall off the wagon, it was just a day where I had something deep fried, or some ice cream, or I tied one on. The weight kept going down even though I had more that enough calories to send it in the other direction, if you assume the body always extracts a certain percentage.

IIRC, the byproduct of burning fat is mostly water, which goes out through your breath first, and in your piss. Your lungs remove more weight from your body than any other source. Your poop is mostly fiber and dead bacteria, again, IIRC.

Malabsorbtion is defined (in part) if there’s more than 5 gm/d of fat in the stool. That is equivalent to 45 kcal of energy. In other words, your gut is very efficient (when healthy) and little nutrition is wasted.

I should also note that studies done years ago showed that even when people deliberately abuse laxatives in an effort to lose weight (by decreasing time available for nutrients to be absorbed by the gut), there is little effect on caloric absorption.


This was my first thought.

Then I thought, wouldn’t methane be more efficient/green, allowing you to utilize the outgassings of the contents?

Then I realized this thread wasn’t about that at all.


(Really, although the topic is interesting there are some misleading breadcumbs around here!)

The energy content of animal and human waste has been covered before - the moisture content is the key variable. Totally dry it ranges from 4,000 to 7,000 Btu/lbm.

How is the moisture content going to change the energy content? Wet fuel won’t release as much heat when burned because of the high heat capacity and heat of vaporization of water, but isn’t the net energy released the same both wet and dry?

That makes me wonder about how the calorie content is measured for food. If they’re just burning the food in a calorimeter, will high-moisture foods give artificially low results?

It changes the energy on a mass basis. Just as we don’t eat totally dry food most of the time, we need too keep things on a consistent or moisture-corrected basis. It’s all very well to say “this food has X kCal/kg and poo has Y kCal/kg”, but the point is unless you correct both to the same moisture content, or look at them on a dry basis, you’re not doing a fair comparison. In short, you and I are talking about closely related but different ways of looking at the energy content.

Oh right. I was thinking more in terms of net energy in and out. I wonder what the daily dry output of a human is. The number in the link is for dry sewage sludge. I’m assuming that’s something like 98+% human waste, but does anyone know for sure?


When I first read your post I thought about
how in survival manuals they suggest
cooking moose and rabbit droppings in a stew.

Rabbits and Moose do something called external digestion.
the digestive track in these animals is very short (for a herbivore)
and the dung is left to ferment (increasing its food value)
and the animal will come back later for the second half of the meal:smack: