Objective: The purpose of this study was to provide an estimate of vitamin D status in young women residing in south-east Texas and to determine factors that predict 25-hydroxyvitamin D (25-OHD) concentration.
Design: A cross-sectional study was conducted on 800 non-Hispanic white, non-Hispanic black, and Hispanic women 16-33 years of age, who were seen in an outpatient clinic.
Measurements: Information was obtained on race, smoking, exercise and dietary intake of vitamin D. Percentage total body fat (%TBF) was assessed using dual-energy X-ray absorptiometry (DXA). Exposure to sunlight was estimated by examining national records of temperature, hours of daylight and UV index for the latitude of the study site. To determine the relationship between 25-OHD and %TBF, season, race, body mass index (BMI), dietary vitamin D, age and smoking in a multivariate context, stepwise linear regression analysis was performed.
Results: Serum 25-OHD levels differed among the racial groups (all pairwise differences P < 0.001), with the lowest value among non-Hispanic blacks (37.7 nmol/l) and the highest value among non-Hispanic whites (71.8 nmol/l). Among Hispanics, mean serum 25-OHD was 47.9 nmol/l. Serum 25-OHD was negatively associated with %TBF (r = -0.28), BMF (r = -0.36) and TBF (r = -0.33), all P < 0.001, and positively associated with dietary vitamin D (r = 0.10) and pack years of smoking (r = 0.11), both P < 0.01. In the summer months, serum 25-OHD values were higher (55.4 nmol/l) than in the winter months (48.1 nmol/l), P < 0.001. The final regression model predicting serum 25-OHD levels included race, %TBF and season (all P < 0.05) and explained 36% of the variance in 25-OHD.
Conclusions: Favourable environmental conditions do not result in sufficient vitamin D status for young women, especially non-Hispanic blacks, Hispanics and the obese.