Science Questions

Do we absorb all the calories in food?

Sun, 21st Mar 2010

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Harriet, Cambridge asked:

Are all the calories in food actually absorbed by the body?


We put this to Susan Jebb from the Medical Research Council's Unit of Human Nutrition Research in Cambridge. 

Susan -  Not all of the calories that are actually in a food will be absorbed, digested and available for the body to use.  What happens is that the calories which are in food, which will be released if we were just to burn it, as we might do in the bomb calorimeter in the laboratory, those calories cannot all be absorbed by the body.  A pair of cheeseburgersSome will be lost in the faeces and the remainder will be digested.  About, perhaps, 10% of the total calories we consume might actually appear at the other end of the gut.  Once calories have been absorbed, again, they're not all fully available.  Some will be lost in urine for example.  The final loss of calories happens because some of the energy is fermented by the bacteria in the gut and so, itís not available to humans.  Itís actually burned off by the bacteria that are living inside us.  And so, the consequence of all of that is that not all the calories that are actually in the food will be available for the body to use.  But in fact, the losses are proportionately quite small.  The calories that you see written on the back of a food pack have already had all of these adjustments made for the amount that will be digested and absorbed.  And so, the calories you see on the packet is actually not the total calories in that food.  Itís the so-called metabolisable energy, the amount of energy which is going to be available to the body.


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Fiber contains calories, but I'm not sure if these calories are included in labels on packages.

I've heard most people have some protein in their urine, so that energy in the protein is also not being used.

Jessica H, Thu, 18th Mar 2010

The gut bacteria must get some of it too. Bored chemist, Thu, 18th Mar 2010

Indeed, but there's another interesting aspect to this - evidence suggests that a significant aspect of our ability to derive calories from food is down to the microbes we carry inside us.

One researcher at Washington University, St Louis - Jeff Gordon - has been comparing the microbial communities inhabiting the guts of fat and thin mice.

He's found that there are significant differences in the relative proportions of different microbial families between the two and "transplanting" the microbes of fat mice into their thinner counterparts can make the lean animals gain weight. Also, fat mice that slim down see a switch in their gut bugs so that the populations instead reflect those of thin mice.

All this adds up to the suggestion that the bacteria are altering the caloric load made available to the individual.

Some people may therefore get less energy from their Mars bar than others owing to the spectrum of intestinal bacteria with which they are coonised...

Chris chris, Fri, 19th Mar 2010

So "I've got big bones" will be replaced by "I've got efficient bacteria".

If particular intestinal microbiota are the cause of obesity, (rather than occurring as consequence of a diet rich in fats and carbs), then I have a great business idea: "treat" obesity with fecal bacteriotherapy using sh1t from thin donors. RD, Fri, 19th Mar 2010

Brilliant comment RD!

Alternatively, come and work at my hospital, where the staff catering and the food sold on the concourse are so unappetising that the anorexia ward is opening extra beds...

That said, I've now heard the answer to this question (Diana played it to Ben and me this afternoon) and I have to say it is really interesting. It made us go "Oh, wow, I didn't know that..."

So tune in on Sunday when the answer's revealed, or suffer the suspense until Tuesday night when the podcast comes out... chris, Fri, 19th Mar 2010

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