BackgroundSupplemental vitamin D modulates inflammatory cytokines and skeletal muscle function, but results are inconsistent. It is unknown if these inconsistencies are dependent on the supplemental dose of vitamin D. Therefore, the purpose of this study was to identify the influence of different doses of supplemental vitamin D on inflammatory cytokines and muscular strength in young adults.MethodsMen (n = 15) and women (n = 15) received a daily placebo or vitamin D supplement (200 or 4000 IU) for 28-d during the winter. Serum 25-hydroxyvitamin D (25(OH)D), cytokine concentrations and muscular (leg) strength measurements were performed prior to and during supplementation. Statistical significance of data were assessed with a two-way (time, treatment) analysis of variance (ANOVA) with repeated measures, followed by a Tukey's Honestly Significant Difference to test multiple pairwise comparisons.ResultsUpon enrollment, 63% of the subjects were vitamin D sufficient (serum 25(OH)D ≥ 30 ng/ml). Serum 25(OH)D and interleukin (IL)-5 decreased (P < 0.05) across time in the placebo group. Supplemental vitamin D at 200 IU maintained serum 25(OH)D concentrations and increased IL-5 (P < 0.05). Supplemental vitamin D at 4000 IU increased (P < 0.05) serum 25(OH)D without altering IL-5 concentrations. Although serum 25(OH)D concentrations correlated (P < 0.05) with muscle strength, muscle strength was not changed by supplemental vitamin D.ConclusionIn young adults who were vitamin D sufficient prior to supplementation, we conclude that a low-daily dose of supplemental vitamin D prevents serum 25(OH)D and IL-5 concentration decreases, and that muscular strength does not parallel the 25(OH)D increase induced by a high-daily dose of supplemental vitamin D. Considering that IL-5 protects against viruses and bacterial infections, these findings could have a broad physiological importance regarding the ability of vitamin D sufficiency to mediate the immune systems protection against infection.

Vitamin D supplementation was the most significant positive predictor of vitamin D status. Collectively, these data point to the practicality of utilizing vitamin D supplements to reduce hypovitaminosis D in adults throughout the United States.

Recent evidence suggests that circulating vitamin D levels above the current RDA (20ng/ml) may be associated with better health outcomes. However, many factors impact vitamin D status, but their influence on serum levels of 25(OH)D remains unclear. In this retrospective, cross‐sectional study, we sought to identify predictor variables of vitamin D status in free living subjects. Serum levels of 25(OH)D were measured, and the variables gender, ethnicity, height, weight, age, geographic location, duration of daily sun exposure, use of sunscreen and sun‐obscuring clothing, dietary intake, supplement usage, latitude of residence and BMI were used to predict vitamin D status in a summer and winter cohort of subjects. Supplemental vitamin D3 intake was the most significant positive predictor ‐ whereas BMI was the most significant negative predictor ‐ of vitamin D status in both the summer and winter cohort. Additional positive predictors were fortified beverage consumption in the summer, and dairy consumption in the winter. Negative predictors were African American and Asian ethnicity in the summer, while latitude of residence >;36°N and Asian ethnicity were negative predictors in the winter. These results indicate that vitamin D supplementation is the most significant positive predictor of vitamin D status, and point to the practicality of utilizing vitamin D supplements to reduce hypovitaminosis D.