# What is known about the variance in the efficiency of the human digestive system to metabolize food?

**URL:** <https://boards.straightdope.com/t/what-is-known-about-the-variance-in-the-efficiency-of-the-human-digestive-system-to-metabolize-food/544823>\
**Category:** Factual Questions\
**Created:** [June 28, 2010, 7:11pm UTC](https://boards.straightdope.com/t/what-is-known-about-the-variance-in-the-efficiency-of-the-human-digestive-system-to-metabolize-food/544823 "2010-06-28T19:11:26Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![RTFirefly](https://avatars.discourse-cdn.com/v4/letter/r/c77e96/32.png) [@RTFirefly](https://boards.straightdope.com/u/RTFirefly)\
**Post date:** [June 28, 2010, 7:11pm UTC](https://boards.straightdope.com/t/what-is-known-about-the-variance-in-the-efficiency-of-the-human-digestive-system-to-metabolize-food/544823/1 "2010-06-28T19:11:26Z")

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Some of us are tall, some of us are short. Some of us can run fast, some can’t. Some of us can do complicated math in our heads, others can barely do addition on paper. Some can sing beautifully, others sing like me.

It goes without saying that attributes we _can’t_ directly observe must vary from one person to another as well; the only question is, by how much? And how can we tell? There is a tendency to believe that what we can’t or don’t measure doesn’t really have much effect. But that’s often far from the truth.

In particular, I am curious about the variance in our digestive system’s capacity to metabolize the calories we throw its way. If a thousand people eat essentially identical 300-calorie PB&Js, do we know anything about how different people in that group will turn the PB&J into different quantities of [ATP](http://en.wikipedia.org/wiki/Adenosine_triphosphate)?

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**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\
**Post date:** [June 28, 2010, 9:22pm UTC](https://boards.straightdope.com/t/what-is-known-about-the-variance-in-the-efficiency-of-the-human-digestive-system-to-metabolize-food/544823/2 "2010-06-28T21:22:15Z")

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The first place I’d look is [Biochemical Individuality](http://www.amazon.com/Biochemical-Individuality-Roger-Williams/dp/0879838930/ref=sr_1_1?ie=UTF8&s=books&qid=1277760014&sr=8-1) by Roger Williams. If I hadn’t lost my copy somewhere along the way, I’d check it myself, but since I did, I can’t. It was written in the 50s, so it won’t have current (or even recent) research, but you’d be amazed at what was known back then and hasn’t trickled at all into the popular press.

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**Author:** ![RTFirefly](https://avatars.discourse-cdn.com/v4/letter/r/c77e96/32.png) [@RTFirefly](https://boards.straightdope.com/u/RTFirefly)\
**Post date:** [July 7, 2010, 1:37pm UTC](https://boards.straightdope.com/t/what-is-known-about-the-variance-in-the-efficiency-of-the-human-digestive-system-to-metabolize-food/544823/3 "2010-07-07T13:37:07Z")

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Seems that [our gut bacteria](http://www.newsweek.com/2010/07/06/don-t-just-blame-calories.html?from=rss) have a lot to do with it:

> [@](#):
>
> The calories that matter are not simply the number printed on grocery items, fast-food menus, and those guilt-inducing signs next to Starbucks’ brownies. The calories that count are those extracted by your digestive enzymes and—as more and more research is showing—the trillions of bacteria in your intestine. People whose gut bacteria are better at digesting fats and carbs than their neighbor’s will absorb all 1,500 calories in a Friendly’s Ultimate Grilled Cheese BurgerMelt, while the neighbor will absorb fewer. So even in people with identical metabolisms, the effects of eating identical foods can be different.