36.5 ± 6.4 years) and 70 African American (age 37 ± 6.7 years) premenopausal women living in the New York City area, who were active and exposed to sunlight. With an average daily intake of 3.5 ± 2.1 µg (139 ± 84 IU) and 3.6 ± 1.8 µg (145 ± 73 IU)/day, respectively, as measured by food frequency questionnaires and interviews, the Caucasian and African American women had similar normal serum 25(OH)D and PTH concentrations. However, BMD in both the lumbar spine and radius was significantly higher in the African American women (Meier et al., 1991). During the winter months (November through May) in Omaha, Nebraska, all except 6 percent of a group of young women aged 25 to 35 years (n = 52) maintained serum 25(OH)D levels greater than 30 nmol/liter (12 ng/ml) when daily vitamin D intake was estimated to be 3.3 to 3.4 µg (131 to 135 IU)/day (Kinyamu et al., 1997). Taken together, these studies suggest that adults younger than 50 years of age in the United States depend on sunlight for most of their vitamin D requirement. Physiological reliance on dietary vitamin D probably only occurs in the winter months in a small proportion who are not exposed to sunlight during the summer.
Indirect evidence of the importance of sunlight to vitamin D status was obtained from a recent study in 22 young male submariners (aged 18 to 32 years) who were not exposed to sunlight but who maintained their serum 25(OH)D at concentrations similar to those measured just before entering the submarine for 3 months with 15 µg (600 IU)/day of vitamin D (Holick, 1994). Those submariners who did not receive a vitamin D supplement had a 38 percent decline in serum 25(OH)D concentration after 1.5 and 3 months in the submarine. Lower supplement doses were not studied. The serum 25(OH)D levels of the nonsupplemented group increased by more than 80 percent when the submariners were exposed to sunlight for one month.
The importance of dietary sources of vitamin D was demonstrated by observations of significantly lower serum 25(OH)D (67.5 ± 47.5 nmol/liter [27 ± 19 ng/ml]) concentrations in males and females who were strictly vegetarian (mean age 42 ± 10 years) in the winter in Helsinki, Finland, compared with a control group of healthy omnivorous women with mean serum 25(OH)D concentration of 117.5 ± 37.5 nmol/liter (47 ± 15 ng/ml). Six of the 10 strict vegetarians had 25(OH)D levels below the lower reference limit of the group (62.5 nmol/liter [25 ng/ml]) indicating vitamin D deficiency (Lamberg-Allardt et al., 1993). Based on a 2-week food record, vitamin D intake in the strict vegetarians (0.3 µg or 10 IU/day) was markedly lower than in the