Food type dictates how well nutrients will be absorbed

Our Jan. 12 article stressed the critical importance to health of providing the body with an adequate supply of all essential nutrients. To do this requires the intestine to absorb an adequate supply of these essential nutrients. For many food sources of nutrients, only a small percentage of some nutrients is absorbed into the body by the intestine. If a nutrient is not absorbed, it is just taking a trip through the tube that starts at our mouth and exits at the rear. The nutrient never gets to the cells in the body where it can function.

Since nutrients in the body are continuously breaking down or being lost from the body through various routes, the nutrients we absorb into the body need to be adequate to balance these losses.

There is no ideal single food that provides all of the 40-some essential nutrients. All individual foods and food groups have their own nutrient strengths and weaknesses — therefore, the common recommendation is to consume a wide variety of foods from all the key food groups. However, just because a food contains a nutrient does not mean it is bioavailable.

Question: What does bioavailability mean?

Answer: Bioavailability refers to how well a nutrient actually gets from a food into the body. Some nutrients in foods are absorbed relatively efficiently whereas others are not. This depends on a number of factors in the food that might make digestion difficult or compromise absorption.

Q: What causes a nutrient to be poorly absorbed?

A: It depends on the nutrient and a number of factors. For example, some food proteins are not efficiently digested which limits how much can be absorbed. In general, animal proteins are more efficiently digested than plant proteins. For example, the Institute of Medicine estimates that the digestibility of bean protein is almost 20 percent less than milk or meat protein. Consequently, greater amounts of protein need to be consumed to meet protein needs with a vegetarian diet.

The nutrients with the lowest bioavailability are minerals. Both iron and zinc are two essential minerals that are well known to have low bioavailability in many foods, especially plant foods.

Q: What factors enhance or inhibit mineral absorption?

A: Foods often have components that bind minerals in the intestine, preventing them from being absorbed. Many types of phytochemicals, or plant chemicals, can inhibit mineral absorption, including oxalic acid in many vegetables, phytates in grains and beans, and tannins in teas and red wines.

Spinach is a great example of how complicated bioavailability can be. Based on nutrient food tables, spinach looks like a good source of both calcium and iron. Spinach, however, is also high in oxalic acid and therefore only about 5 percent of calcium is absorbed compared to over 30 percent absorption of the calcium from milk. To complicate matters, the calcium in spinach inhibits iron absorption, but adding high vitamin C to the meal can decrease the inhibitory effects of calcium. Therefore, adding a tomato or mandarin orange to a spinach salad can help enhance the absorption of iron.

Although there are not many studies on mineral bioavailability from dietary supplements, studies indicate that compounds in dietary supplements have similar inhibitory and enhancing effects as foods.

As always, the best way to assure that a person is getting adequate amounts of all of their essential nutrients is to consume a wide variety of nutrient-rich foods.

Alan Titchenal, Ph.D., C.N.S., and Joannie Dobbs, Ph.D., C.N.S., are nutritionists in the Department of Human Nutrition, Food and Animal Sciences, College of Tropical Agriculture and Human Resources, University of Hawaii-Manoa. Dobbs also works with University Health Services.