In the following article published in Scientific American and written by nutrition diva Monica Reinagel, she answers a listeners question who asks:
“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?”
"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!"