control group (4.5 µg or 180 IU/day). Serum PTH concentrations were significantly higher in the vegetarians (57 ng/liter) compared with the control group (28 ng/liter), and 3 out of 10 subjects had concentrations that were higher than the upper reference limit (65 ng/liter), indicating secondary hyperparathyroidism.

Based on the available literature, both sunlight and diet play an essential role in providing vitamin D to this age group. Because cutaneous vitamin D synthesis is markedly diminished or absent in the winter, young and middle-aged adults who live in northern (> 40°N) and southern latitudes (> 40°S) can become vitamin D deficient (Kinyamu et al., 1997). During the winter months in Omaha, Nebraska, most women with an average intake of 3.3 to 3.4 µg (131 to 135 IU)/day of vitamin D had serum 25(OH)D concentrations greater than 30 nmol/liter (12 ng/ml) (Kinyamu et al., 1997). Because there are no data for men only, other than those from submariners at a high supplement intake, it is assumed that the AI for men is similar to that for women. To cover the needs of adults ages 19 through 50 years, regardless of exposure to sunlight, the above value is rounded down to 2.5 µg (100 IU) and then doubled for an AI of 5.0 µg (200 IU)/day.

Serum 25(OH)D. Although this age group also depends on sunlight for most of its vitamin D requirement, this population may be more prone to developing vitamin D deficiency, owing to a variety of factors that reduce the cutaneous production of vitamin D₃. Increased use of clothing to cover the skin and prevent the damaging effects of sunlight (Matsuoka et al., 1992) and increased use of sunscreen can increase the risk of vitamin D deficiency (Holick, 1994). Aging also decreases the capacity of the skin to produce vitamin D (Holick et al., 1989; Need et al., 1993).