We noted very low concentrations of 25(OH)D and a very high prevalence of insufficiency (61%) in a sample of adult African American men in the Philadelphia region, indicating a more severe problem of hypovitaminosis D than has been reported previously among African American men [6
]. Comparison with previous studies is difficult because different cutpoints have been used to define insufficiency, and also because we did not use an external control to permit comparison with results from other laboratories. The lower mean 25(OH)D concentration in our sample than in African American men in Massachusetts (17–25 ng/mL) [6
] and Washington, DC (18 ng/mL) [18
] may be due to variability in methods [22
]. However, mean 25(OH)D concentration during summer months in our sample is only slightly lower than the mean of ~20 ng/mL estimated for black participants in the Third National Health and Nutrition Examination Survey, which was conducted during the summer in northern states and during the winter in southern states [21
]. It is comparable to mean concentrations estimated for black adults in regions with greater sun exposure – in particular, Arizona (18.2 ng/mL) [20
], California (18.1 ng/mL) [25
], and the South (19.0 ng/mL) [17
]. This context supports the plausibility of the low vitamin D status observed in our sample.
We found large differences in vitamin D status by season, confirming the substantial contribution of season, particularly winter, to severe vitamin D insufficiency. We also found an association of physical activity with vitamin D status but could not assess whether the association could be attributed to sun exposure with outdoor activity [26
]. The significant associations for intakes of supplemental vitamin D and milk indicate the importance of measures to increase vitamin D intake to improve vitamin D status. In multivariate analyses, vitamin D supplement use increased mean 25(OH)D concentrations by 4.5 ng/mL, and milk consumption of at least 3.5 times per week increased concentrations by 3.3 ng/mL relative to consumption of less than once per week. However, even among men with total (dietary and supplemental) vitamin D intake of >400 IU/day, the recommended intake for men over 50 years of age [5
], 55% had 25(OH)D concentrations <15 ng/mL, suggesting that current dietary recommendations are not adequate to achieve optimal vitamin D concentrations in this population.
The significant association between BMI and 25(OH)D concentrations in bivariate analyses was attenuated after adjustment for season of blood draw. In our sample, BMI was significantly higher in the winter (31.2 kg/m2
) than in the summer (28.2 kg/m2
), a phenomenon that has been noted in other studies [29
]. The lack of association between BMI and 25(OH)D concentrations in our sample differs from previous studies, conducted primarily in white populations, that have noted an inverse association between the two [31
]. Other evidence exists to suggest that adiposity may not be as strong a predictor of low vitamin D status among African Americans as it is in whites [14
A limitation of our study is that detailed information on sun exposure, such as time spent outdoors, sunscreen use, and other sun protection behaviors, was not available. Additionally, 21% (N = 40) of our sample completed their questionnaires over a year before collection of their blood sample used in these analyses; however, analyses excluding these men produced no meaningful differences from results based on the full sample. Self-selection into the high risk program may have biased our estimates or may limit generalizability of our results, but African American men were required to meet no eligibility criteria to enroll in the program other than having no personal history of prostate cancer. Further, we saw no association of family history of prostate cancer with 25(OH)D levels (data not shown), nor is there reason to expect that self-selection into the program would be related to major determinants of vitamin D status. Our analysis included only 194 of the 440 African American men enrolled in PRAP. Men in our sample were more likely to be never smokers than were men not included in the analysis (62% vs. 48%), but the two groups were otherwise similar with respect to age, education level, BMI, and participation in recreational physical activity (data not shown).