Abstract
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.
Highlights
Vitamin D status is influenced by a variety of factors, including but not limited to geographical latitude, season and supplement use
Serum 25(OH)D is the best indicator of vitamin D status [6], but 1a,25-dihydroxyvitamin D3 (1,25(OH)D) is the active form of vitamin D that regulates biological functions
We previously demonstrated that serum 25(OH)D concentration is influential on muscle strength recovery following ligamentous injury and surgery in active adults [19], it is unknown if supplemental vitamin D increases muscular strength simultaneous with serum 25(OH)D concentrations in young, reportedly active and healthy adults
Summary
Vitamin D status is influenced by a variety of factors, including but not limited to geographical latitude, season and supplement use. Serum 25(OH)D is the best indicator of vitamin D status [6], but 1a,25-dihydroxyvitamin D3 (1,25(OH)D) is the active form of vitamin D that regulates biological functions. In kidney and immune cells, 1a-hydroxylase (1a-OHase) converts 25(OH)D to 1,25 (OH)D. This enzymatic conversion is limited by substrate availability since 1a-OHase operates below its Km. an adequate circulating 25(OH)D concentration is necessary to maintain substrate availability for the enzymatic conversion of vitamin D to its active form. Supplemental 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. 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
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