Scientific American presents Nutrition Diva by Quick & Dirty Tips. Scientific American and Quick & Dirty Tips are both Macmillan companies.
Nutrition Diva listener Andrew writes:
“We can measure the amount of vitamins and minerals in a food, but how do we know how much of that our bodies actually absorb? If a banana contains 422 mg of potassium, for example, do our bodies take in 100% of that? What factors determine how much (or how little) nutrition we get from our food?”
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There are, in fact, lots of things that influence what percentage of vitamins and minerals are absorbed, such as the other foods you eat at the same meal, how they are prepared, drugs or supplements you may be taking, even your age and the time of day. Taking all of these into account, you might absorb anywhere from 10 to 90% of a given nutrient from a given food!
Before I get into some specific examples, however, I want to clear up a common misunderstanding about the nutrient content of foods.
The Nutrient Content of Foods Varies…a Lot
The USDA maintains a huge database of the nutritional composition of thousands of foods. If you look up "banana" in the National Nutrient database, you will see that a medium banana contains 422 mg of potassium. But that is just an average—in this case, based on 14 different samples. In fact, the amount of potassium in these 14 samples ranged from 364 mg to 502 mg per medium banana.
The amount of vitamins and minerals in a raw food depends on the variety, growing conditions, weather, timing of harvest, and storage conditions, not to mention natural variation. Although the variables are fewer, even dietary supplements may contain slightly more or less than the amount shown on the label.
So, even if there were a way to predict exactly what percentage of a nutrient your body will absorb (which there isn’t), we’d still be starting with an approximate figure.
With that in mind, let's take a look at some of the many things that influence nutrient absorption.