Hughes et al., 1991, 1995; Lamberg-Allardt et al., 1989, 1993; Sorva et al., 1991; Webb et al., 1990).
The recommendation for how much vitamin D is required to maintain adequate calcium metabolism and good bone health for all ages may be considered the easiest, as well as at times the most difficult, to determine. Humans of all ages, races, and both sexes can obtain all of their body's requirement for vitamin D through exposure to an adequate amount of sunlight. However, the sunlight-mediated synthesis of vitamin D in the skin is profoundly affected by a wide variety of factors, including degree of skin pigmentation, latitude, time of day, season of the year, weather conditions, and the amount of body surface covered with clothing or sunscreen (Holick, 1994). Therefore, it is very difficult to determine an accurate value for an Estimated Average Requirement (EAR) as most of the studies are subject to one or more of these variables, especially exposure to sunlight, which is difficult to quantitate.
Vitamin D is a hormone, and therefore, when considering the requirements for vitamin D, EARs would represent gross estimates of the need for the active hormone. The only studies that provide an approximation of how much vitamin D is required to maintain an individual's serum 25(OH)D concentration above that associated with abnormalities in BMD are ones that have been conducted in the winter at far northern and southern latitudes where exposure to sunlight does not produce any significant quantities of vitamin D (Ladizesky et al., 1995; Markestad and Elzouki, 1991). However, these studies still do not account for subjects' exposure to sunlight in the spring, summer, and fall when the cutaneous synthesis of vitamin D occurs and it is stored in the body fat for use in the winter.
Another limitation of the reported studies is the assumption made regarding the vitamin D content of various foods. Despite government mandates for vitamin D fortification of milk in both the United States and Canada, actual analysis has shown this fortification to be highly variable (Chen et al., 1993; Holick et al., 1992; Tanner et al., 1988). Furthermore, the amount of vitamin D found naturally in foods, such as fish liver oils, fatty fish, and egg yolks, is very dependent on the time of the year these foods are harvested. Studies that report the dietary intake of vitamin D based on the expected amount of vitamin D fortification of milk, margarine, cereals, and breads are highly suspect because the analysis of the vitamin D con-