Journal Title (Medline/Pubmed accepted abbreviation):Med. Sci. Sports Exerc.
Page numbers: 157-162
Background: It has been observed that serum 25-hydroxyvitamin D (25(OH)D) is correlated with muscle strength and exercise performance in some, but not all, trials. However, it is unknown if the relationship is affected by other variables such as age, gender, physical activity level, or body mass index (BMI).
Hypothesis: Serum vitamin D levels will correlate with muscle strength and exercise performance across all ages and genders.
Experimental Design: cross-sectional (no intervention)
Subjects: A total of 419 people, 205 men and 214 women, age about 25- 65 y
Protocol: Participants were recruited to participate in a study that assessed the effects of atorvastatin, a drug used to lower cholesterol, on muscle strength and exercise performance. This research paper looks at just one aspect of the study- vitamin D levels in blood, and how they correlate with muscle strength and exercise performance before pharmaceutical intervention.
At the first visit, participants were assessed for height and weight, from which BMI was calculated; vital signs including blood pressure and pulse; serum 25(OH)D (vitamin D) concentrations; and maximum oxygen capacity (VO2max) using a graded treadmill protocol. Physical activity was monitored for 4 days using an accelerometer. The participants were first familiarized with the exercise protocol, and then completed on two subsequent days in order to provide muscle strength and exercise performance data. Dominant hand strength was determined by how tightly the participant could squeeze a hydraulic handgrip. Lower and upper body strength was measured using isometric and isokinetic elbow and knee flexion/extension exercises. Isokinetic exercises were performed at rates of 60°/sec and 180°/sec. Average peak torque during three or four maximal continuous contractions was assessed.
- In total, 65% of the participants displayed vitamin D levels above 30 ng/mL, which is often considered “good” vitamin D status.
- The participants that were evaluated for vitamin D status in the spring and summer tended to have higher serum vitamin D concentrations than those tested in the fall and winter.
- There was not a correlation between serum vitamin D status and hand grip strength.
- There was a positive correlation between serum vitamin D concentration and average peak torque on all six of the elbow flexion/extension exercises (p ≤ 0.04).
- There was a positive correlation between serum vitamin D concentration and average peak torque for isometric extension (p = 0.03) but not isokinetic extension exercises (p = 0.12-0.13). There was a positive correlation between vitamin D concentrations and both isokinetic flexion exercises (p ≤ 0.03).
Key practice applications: When accounting for age and gender, vitamin D levels were positively associated with muscle strength, especially upper body strength. These authors suggest that the reason for this correlation is that vitamin D deficiency can lead to atrophy of type II muscle fibers. Although these results cannot confirm this suggestion, it important to maintain good vitamin D status to maintain healthy bones, a healthy immune system, healthy absorption of calcium and phosphorous, and other biological processes that depend on vitamin D.
- The authors did not assess vitamin D intake.
- These results are correlational and do not determine causation (whether increased vitamin D status can increase muscle strength, increased muscle strength leads to increased concentrations of circulating vitamin D, or neither). For example, it could be true that the people in this study who exercise more, and are therefore stronger, consume more vitamin D-fortified milk which increases their vitamin D status.
- Other vitamins and hormones in the blood that are related to vitamin D metabolism (ex. calcium, parathyroid hormone) were not measured. Therefore, it is unknown how extensive effects of vitamin D deficiency were.