The first consideration is often whether it is necessary to have an accurate quantitative estimate of nutrient bioavailability, or whether an estimate of relative bioavailability compared to a known standard nutrient source will suffice. An accurate quantitative measure of bioavailability might be necessary when the intention is to provide data to derive a recommendation for dietary intake to meet a nutrient requirement. In this case, it is important to have a reasonably good estimate of the true fraction of a given dose of the ingested nutrient that could be absorbed and utilized, for example, to replace endogenous losses of the nutrient.
A common application of bioavailability measurements is to compare relative bioavailability between two or more sources of a nutrient. For example, one might be concerned with determining the calcium bioavailability from milk compared to calcium derived from a vegetable source such as broccoli or calcium-fortified orange juice. There are many techniques available to measure relative nutrient bio-availability under in vivo conditions based on a comparison of the rise in plasma level (or urinary excretion) of the nutrient or rate of appearance in plasma of a radioactively labeled nutrient after an oral test dose. An important technical advance in measuring food mineral bioavailability in humans was the validation of an extrinsic tag method. Extrinsic tag studies were validated by measuring the extent of absorption of a mineral isotope mixed exogenously (the 'extrinsic tag') with a food compared to that of an intrinsic tag where the absorption of the isotope is determined from an intrinsically labeled food source. The intrinsic tag is often achieved by growing plants hydroponically in a solution enriched in a radioactive or stable mineral isotope to label the plant food of interest during growth, or by supplying the mineral isotope tag to a growing animal used for meat, or one that was used for milk production, for example. These studies have shown that in most cases the ratio of absorption of the extrinsic to the intrinsic isotope was approximately one, indicating that the extrinsi-cally added isotope tag became homogenously incorporated into the pool of absorbed mineral found endogenously in the food of interest. The use of the extrinsic tag method has greatly facilitated the study of relative bioavailability of minerals from food in human subjects.
A large and growing number of people are consuming dietary supplements. However, due to the relative difficulty of labeling these supplements, in most cases, little information on the bioavailability of the nutrients in the supplements is available. A study of vitamin and mineral bioavailability from a popular multinutrient supplement found good absorption of the water-soluble vitamins (B vitamins and vitamin C) from the tablet but relatively poor absorption of copper and zinc.
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