Objectives: To investigate the effects of vitamin D supplementation on muscle strength in healthy individuals.
Design: A systematic review with meta-analysis.
Methods: In October 2013 a computerised literature search of three databases (Pubmed, Web of Knowledge and Scopus) was performed. Included in the review were controlled and randomised controlled trials, published in English, which measured muscle strength and serum Vitamin D concentration in participants 18-40 years old. References of identified articles were then cross-checked and citations scanned for additional articles. Quality was assessed using the PEDro scale. Muscle strength and Vitamin D levels were extracted for a meta-analysis on upper and lower limb strength with standardised mean differences calculated to analyse effect.
Results: Six randomised controlled trials and one controlled trial were identified and quality assessment showed all seven trials were of ‘good quality’. Data was extracted from 310 adults, 67% female, with mean ages ranging from 21.5 to 31.5 years. Trials lasted from 4 weeks to 6 months and dosages differed from 4000 IU per day to 60,000 IU per week. Upper and lower limb muscle strength had a standardised mean difference of 0.32 (95% CI = 0.10, 0.54) and 0.32 (95% CI = 0.01, 0.63) respectively, suggesting Vitamin D supplementation significantly increased muscle strength in the experimental group for upper (P = 0.005) and lower limbs (P = 0.04).
Conclusion: Vitamin D supplementation increases upper and lower limb strength. Further research should focus on its effect on muscle power, endurance and maximal strength.
Alex’s Notes: I’m sure the title caught your eye as well as it did mine. Vitamin D needs no introduction, and a meta-analysis on strength with supplementation? No more delays.
Studies were included in the meta-analysis and review if they were published in English, had a control group, and measured muscle strength as a primary outcome. No studies involving persons under the age of 18-years, animals, or “non-healthy” subjects were included. Unfortunately, the authors don’t define what they consider healthy. Seven papers were included in the meta-analysis involving a total of 310 subjects, of which 67.4% were female with an average age of 24.1-years. The average quality of these studies as assessed by the PEDro score was 8.7 (out of a possible 10), and all but one used a vitamin D3 oral supplement with various dosing regimens and durations.
There was quite literally no homogeneity among the studies, and strength was assessed with a variety of methods ranging from grip-strength via a hand held dynamometer to a 1RM bench press, leg press, and free-weight squat to isometric quadriceps contraction. Extremely importantly, only five of the studies reported baseline vitamin D status, which was found to average 12.3ng/mL, and four of these studies reported post-intervention levels that exceeded 20ng/mL. In other words, this meta-analysis must be viewed in the context of going from a vitamin D deficient state to a barely adequate state.
Honestly, that is probably the largest let-down of this meta-analysis, as its results only apply to adults that are deplete of vitamin D and achieving “normal” levels with supplementation. It also makes the results unsurprising given the importance of vitamin D and its status as a pseudo-hormone. For upperbody and lowerbody strength, the meta-analysis found a significant but small effect size of 0.32 favoring supplementation.
There are countless reasons to get your vitamin D levels to a minimum of 30ng/mL, and improved muscle strength can now be added to the list. Personally, I would like to see this review/meta-analysis repeated in people going from this low end of adequate to the often stated optimal range of 50-70ng/mL.