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Many of us spend a considerable amount of time and money buying and preparing foods that will help keep us healthy—and for good reason.  There are far more nutrients in whole foods than in anything you can buy in a box at the supermarket.  When food is processed, many vitamins, minerals and important micronutrients are removed.  Some are added back in by the manufacturer, but it is never the same as what you would get from the original food.

But even if you cook whole foods from scratch, is there really any way to be sure that your body is actually getting all those good nutrients you’re putting into it?  Even if you take appropriate vitamin and mineral supplements, is there really any guarantee they actually get to where they’re needed?  Understanding how absorption and the bioavailability of nutrients works can help ensure you are getting the nutrients you need.

The technical definition of bioavailability according to the Oxford Dictionary is “the proportion of a drug or other substance which enters the circulation when introduced into the body and so is able to have an active effect.”  In other words, it’s the amount of a nutrient that gets into your bloodstream that determines its bioavailability.  The many things that affect bioavailability include the food’s original nutrient content, food processing, efficiency of digestion, absorption, how the circulation distributes the nutrient and whether it is able to pass through cellular walls.

To become bioavailable, nutrients must first be absorbed from food.  The first step in absorption is breaking your food down so the nutrients contained inside it can be released.  To some extent, this happens when the food is cooked or otherwise processed.  Lycopene, the phenol contained in tomatoes, is a great example.  It is more bioavailable if the tomatoes have first been processed in a product such as tomato sauce.  The tough cell walls of raw tomatoes do not allow for as much lycopene to be released through the normal digestive process.  Prior processing aside, food begins to be broken down when you chew and swallow.  It continues to be broken down by digestive juices in your stomach.

Once food is broken down in the stomach by digestive enzymes, it passes into the small intestine, which is where the most nutrients are absorbed.  If there are any problems with the digestive system at any point, bioavailability may be reduced.  For instance, many elderly people have been found to have low levels of vitamin B12, despite eating foods high in the nutrient.  As we get older, our digestive system often does not release sufficient amounts of pepsin and digestive acids to be able to free B12 from the protein in the food to which it is bound.

Bioavailability can also be affected by the combination of nutrients being consumed at the same time.  Some types of nutrients help and some hinder the absorption of certain other nutrients.  For example, calcium and iron interfere with one another’s absorption, so if you are trying to increase either your intake of calcium or iron (or both), be sure to eat them at separate meals.  On the other hand, vitamin C actually enhances iron absorption, so sautéed spinach with red bell peppers is a good food combination from this this point of view.

If you are in good health, have a good digestive system and eat healthy foods, then you are likely getting sufficient nutrients from the foods you eat.  Bioavailability becomes more of an issue if these basic conditions are not met or if you’re asking your body to perform at a level that benefits from having specific types of nutrients available faster (during elite athletic competitions for example).  If either of these situations applies to you, your doctor can suggest supplements you can take (or injections, if necessary) to ensure that you are not deficient in any important nutrients.  If you’re involved in competitive athletics, you should also be careful to know and follow the rules of your sport as they apply to nutrition, supplements and any other substances that may be introduced to improve performance.

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