Vitamin D status in winter and summer among nurses and implications for cancer etiology
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Background: Recent research indicates that inadequate vitamin D status may incur increased cancer risk, particularly for breast and colon cancer. Cutaneous generation of vitamin D through direct absorption of summer sunlight is considered the principal source of vitamin D; however, at latitudes above 40ºN, UVB rays are insufficient to permit cutaneous synthesis during winter. Vitamin D can also be obtained from natural and fortified dietary sources and through supplement use, although dietary reference intake values do not seem to compensate for restricted cutaneous synthesis in winter. Objectives: To determine the prevalence of vitamin D insufficiency post-winter and post-summer, to determine the seasonal variation in 25-hydroxyvitamin D (25(OH)D) levels, and to investigate determinants of 25(OH)D concentrations. Methods: A cross-sectional study of vitamin D status post-winter and post-summer was conducted among 49 premenopausal rotating shift nurses at Kingston General Hospital. In each season, vitamin D levels were measured using a radioimmunoassay to quantify serum 25(OH)D from fasting blood samples, and personal, lifestyle, and dietary factors potentially influencing vitamin D levels were assessed from a self-report questionnaire. Nurses were classified as vitamin D sufficient (>30ng/mL) or insufficient (≤30ng/mL). Results: A statistically significant difference was found between mean serum 25(OH)D concentrations post-winter (32.6ng/mL) and post-summer (42.5ng/mL) (p=0.0003). The prevalence of vitamin D insufficiency post-winter was 49% compared to 24% post-summer p=0.02). Tanning bed use was associated with significantly higher winter serum 25(OH)D levels was tanning bed, while controlling for age, BMI, and vitamin D-fortified yogurt consumption (R2=0.41, p=0.007). For summer, predictors of higher serum 25(OH)D levels were supplement use and tanning bed use in the past six months, while controlling for age, BMI, and beef/veal consumption (R2=0.53, p=0.001). Conclusions: Vitamin D insufficiency was prevalent both post-winter and post-summer, but with much higher prevalence post-winter. Controlling for age and BMI, tanning bed use was a determinant of serum 25(OH)D concentrations in both seasons, while fortified yogurt consumption also contributed in winter, and beef/veal consumption and supplement use contributed in summer. These findings support the need to improve vitamin D status among this population, and to increase the dietary reference intake values and supplement recommendations.