humans. Nevertheless, results from animal feeding experiments regarding vitamin C, vitamin E, selenium, and β-carotene and other carotenoids were included in the evidence reviewed in developing the decisions concerning the ability to specify the Dietary Reference Intakes (DRIs) for these nutrients.

Human Feeding Studies

Controlled feeding studies, usually in a confined setting such as a metabolic ward, can yield valuable information on the relationship between nutrient consumption and health-related biomarkers. Much of the understanding of human nutrient requirements to prevent deficiencies is based on studies of this type. Studies in which the subjects are confined allow for close control of both intake and activities. Complete collections of nutrient losses through urine and feces are possible, as is recurring sampling of biological materials such as blood. Nutrient balance studies measure nutrient status in relation to intake, whereas depletion-repletion studies measure nutrient status while subjects are maintained on diets containing marginally low or deficient levels of a nutrient, followed by correction of the deficit with measured amounts of the nutrient. However, these studies have several limitations: typically they are limited in time to a few days or weeks, so longer-term outcomes cannot be measured with the same level of accuracy. In addition, subjects may be confined, and therefore findings cannot be generalized to free-living individuals. Finally, the time and expense involved in such studies usually limit the number of subjects and the number of doses or intake levels that can be tested.

In spite of these limitations, feeding studies play an important role in understanding nutrient needs and metabolism. Such data were considered in the DRI process and were given particular attention in the absence of reliable data with which to directly relate nutrient intake to disease risk.

Observational Studies

In comparison, observational epidemiological studies are frequently of direct relevance to free-living humans but lack the controlled setting of human feeding studies. Hence they may be able to establish convincing evidence of an association between the consumption of a nutrient and disease risk, but they are limited in their ability to ascribe a causal relationship. A judgment of causality may be supported by a consistency of association among studies in di-

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