Nutrient bioavailability is defined as the fraction of a nutrient in a food that is absorbed and utilized. In practice, however, measurements of bioavailability have focused on either direct measurement of absorption or determination of the change in some functional or biochemical endpoint reflecting absorption and utilization of the nutrient. In general, the bioa-vailability of all nutrients can be estimated by measuring absorption alone because, once absorbed, nutrients are freely available for biological utilization, irrespective of their original dietary source. For example, consider the case of iron bioavailability. Iron absorption can be measured directly from a food by a variety of methods (described in more detail later). In addition, absorbed iron enters the plasma iron pool carried on the protein transferrin. In turn, this absorbed iron will be used in large part (about 80%) immediately in the synthesis of hemoglobin by erythrocyte precursor cells in the bone marrow. The fraction of iron that is utilized for hemoglobin synthesis is not dependent in any way on the food source of that iron. Thus, food iron bioavailability can be conveniently measured and compared in relative terms among various sources by determining the change in blood hemoglobin after consumption of various forms of iron in iron-deficient subjects. Thus, nutrient bioavailability can be estimated by measuring these appropriate endpoints, such as hemoglobin incorporation for iron, hepatic tissue or mineral content of bone for various bone-seeking minerals, or more generally as growth stimulation under nutrient limiting conditions, etc.

An exception to the 'absorption equals bioavailabil-ity' rule is selenium. One form of dietary selenium is selenomethionine. This selenium-containing compound is handled by the body exactly like the amino acid methionine and gets readily incorporated into methionine-containing proteins. However, the selenium found in selenium-dependent enzymes is in the form of a special amino acid called selenocysteine, which must be synthesized in the body during the process of incorporation of selenocysteine into these selenoproteins. Selenomethionine catabolism will result in the release of this selenium into an active endogenous selenium pool, which serves as the source of selenium for selenocysteine synthesis. Thus, selenium in selenomethionine is not immediately available to support selenium-dependent functions in the body.

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