QotW: How long before food becomes 'me'?

From digestion to absorption to incorporation: how long does it take for food to become part of us?
03 September 2021


Food market



Listener Martin asked "How long does it take the food I eat to become part of me?"


Eva Higginbotham set off to find out the answer...

Eva - The first step in incorporating the food we eat into our bodies is breaking it down to its constituent parts through digestion. Carbohydrates like rice and bread break down into simple sugars like glucose, which we use as energy. We break down fats into molecules called fatty acids, which make up the membranes of our cells. And then there’s proteins: these get divided into the amino acid molecules that make them up, and our cells then use those amino acids to build and repair body tissues. There’s also minerals - things like calcium, which we absorb and use for lots of purposes, including, of course, maintaining our bones.

In terms of how long digestion takes, that depends on both the food being eaten and the person eating it. Simple carbs like white rice can get out of the stomach and into the small intestine in as little as 30 minutes, with the glucose ready for absorption into the bloodstream shortly after. But, if you eat that rice with something fatty and proteiny like peanut butter, that can slow down the process by several hours - although, now I’ve said it, rice and peanut butter sounds… interesting! 

Lots of people interested in sports have studied how quickly protein is incorporated into the body from the angle of muscle building - think about the, er, delicious looking protein shakes you see people drink after going to the gym. Most of these studies have been done by radioactively labelling the proteins and then scanning the body to see where they’ve gone. Protein shakes normally contain really simple proteins, like whey or soy protein, which don’t need as much breaking down as others, and the amino acids in them have been shown to be available to the body in less than an hour.

One study showed that about 20% of amino acids consumed will go towards skeletal muscle, like those all important biceps, whereas the rest will either be excreted or used in other kinds of tissues. So, to put it simply, if you workout at the gym and drink a protein shake, within a few hours you can be pretty sure that some of those amino acids have become a part of you already. If you’re not a fan of the protein shake, another study looked at cow’s milk, and showed that, after drinking, some amino acids became available in the blood within an hour and the numbers kept rising for the next 5 hours.

In terms of how long the nutrient you ate stays part of you, that depends on where it’s gone. Some cells in your body have very high turnover, like those that line your intestines, whereas others will last for years and years and just need some maintenance. Your bones will turnover about 500mg of calcium every day, and replace it with new calcium from your diet - so it’s a good idea to keep up with those leafy greens if you’re not a fan of dairy.

So, how long does it take the food you eat to become part of you? The answer, as Dr Tristan Dew from the University of Nottingham said, is ‘not as long as you might think!’ and I certainly agree with that.



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