FitProven
Evidence Breakdown
Based on 7 studies
THE BIGGER QUESTION
Whether this one statement holds is settled above. What to actually do about it is a wider question, weighed across every claim that bears on it.
Evidence map
For & against, at a glance
Meta-Analysis
Pro
Beaudart C et al. · 2014The Journal of Clinical Endocrinology and MetabolismMeta-analysis of 30 randomized controlled trials (5,615 participants, mean age 61.1 years) of vitamin D supplementation. It found a small but statistically significant positive effect on global muscle strength (SMD 0.17, P = .02), with no effect on muscle mass (SMD 0.058, P = .52) or muscle power (SMD 0.057, P = .657). The effect was concentrated where you would expect it: it was larger in adults aged 65+ (SMD 0.25) than in younger subjects (SMD 0.03), and stronger in participants whose baseline 25(OH)D was below 30 nmol/L. This is the strongest quantitative support for the claim, and even here the strength effect is small.
0.85
Meta-analysis of 30 randomized controlled trials (5,615 participants, mean age 61.1 years) of vitamin D supplementation. It found a small but statistically significant positive effect on global muscle strength (SMD 0.17, P = .02), with no effect on muscle mass (SMD 0.058, P = .52) or muscle power (SMD 0.057, P = .657). The effect was concentrated where you would expect it: it was larger in adults aged 65+ (SMD 0.25) than in younger subjects (SMD 0.03), and stronger in participants whose baseline 25(OH)D was below 30 nmol/L. This is the strongest quantitative support for the claim, and even here the strength effect is small.
Design Meta-Analysis (1.0) × quality 0.85 = impact 0.85
View sourceMeta-Analysis
Neutral
Stockton KA et al. · 2011Osteoporosis InternationalSystematic review and meta-analysis of 17 RCTs (5,072 participants) that split the analysis by baseline vitamin D status, which is why it lands as genuinely conditional rather than for or against. In adults with baseline 25(OH)D above 25 nmol/L, supplementation had no significant effect on grip strength (SMD -0.02, 95% CI -0.15 to 0.11) or proximal lower-limb strength (SMD 0.10, 95% CI -0.01 to 0.22). In the two trials of frankly deficient participants (25(OH)D below 25 nmol/L), pooled hip muscle strength improved enormously (SMD 3.52, 95% CI 2.18 to 4.85). The claim is therefore false as a general statement and plausibly true only in severe deficiency — but that deficient-subgroup estimate rests on just two small trials.
0.85
Systematic review and meta-analysis of 17 RCTs (5,072 participants) that split the analysis by baseline vitamin D status, which is why it lands as genuinely conditional rather than for or against. In adults with baseline 25(OH)D above 25 nmol/L, supplementation had no significant effect on grip strength (SMD -0.02, 95% CI -0.15 to 0.11) or proximal lower-limb strength (SMD 0.10, 95% CI -0.01 to 0.22). In the two trials of frankly deficient participants (25(OH)D below 25 nmol/L), pooled hip muscle strength improved enormously (SMD 3.52, 95% CI 2.18 to 4.85). The claim is therefore false as a general statement and plausibly true only in severe deficiency — but that deficient-subgroup estimate rests on just two small trials.
Design Meta-Analysis (1.0) × quality 0.85 = impact 0.85
View sourceMeta-Analysis
Con
Bislev LS et al. · 2021Journal of Bone and Mineral ResearchThe largest synthesis on this question: a systematic review and meta-analysis of 54 randomized placebo-controlled trials (8,747 participants) covering muscle strength, physical performance and lean mass. Supplementation did not improve any muscle-health outcome; if anything it was slightly worse than placebo, with a slower timed-up-and-go test (mean difference +0.15 s, 95% CI 0.03 to 0.26) and lower knee flexion strength (-3.3 N, 95% CI -6.63 to -0.03). The authors concluded that available evidence does not support a beneficial effect of vitamin D on muscle health and that possible adverse effects should be considered.
0.95
The largest synthesis on this question: a systematic review and meta-analysis of 54 randomized placebo-controlled trials (8,747 participants) covering muscle strength, physical performance and lean mass. Supplementation did not improve any muscle-health outcome; if anything it was slightly worse than placebo, with a slower timed-up-and-go test (mean difference +0.15 s, 95% CI 0.03 to 0.26) and lower knee flexion strength (-3.3 N, 95% CI -6.63 to -0.03). The authors concluded that available evidence does not support a beneficial effect of vitamin D on muscle health and that possible adverse effects should be considered.
Design Meta-Analysis (1.0) × quality 0.95 = impact 0.95
View sourceMeta-Analysis
Con
Han Q et al. · 2024Frontiers in NutritionUpdated systematic review and meta-analysis of 10 RCTs in healthy athletes (354 randomized, 318 completing; 185 vitamin D3 vs 169 placebo). Supplementation significantly raised serum 25(OH)D (MD +14.76 nmol/L, 95% CI 8.74 to 20.77), but the pooled effect across four strength measures was not significant (SMD 0.18, 95% CI -0.02 to 0.37, P = 0.08). Only one isolated sub-outcome, quadriceps contraction, reached significance (SMD 0.57, P = 0.04). The authors' own conclusion is that the evidence cannot warrant significant overall enhancement of muscle strength in athletes made replete by supplementation.
0.75
Updated systematic review and meta-analysis of 10 RCTs in healthy athletes (354 randomized, 318 completing; 185 vitamin D3 vs 169 placebo). Supplementation significantly raised serum 25(OH)D (MD +14.76 nmol/L, 95% CI 8.74 to 20.77), but the pooled effect across four strength measures was not significant (SMD 0.18, 95% CI -0.02 to 0.37, P = 0.08). Only one isolated sub-outcome, quadriceps contraction, reached significance (SMD 0.57, P = 0.04). The authors' own conclusion is that the evidence cannot warrant significant overall enhancement of muscle strength in athletes made replete by supplementation.
Design Meta-Analysis (1.0) × quality 0.75 = impact 0.75
View sourceMeta-Analysis
Pro
Beaudart C et al. · 2014The Journal of Clinical Endocrinology and MetabolismMeta-analysis of 30 randomized controlled trials (5,615 participants, mean age 61.1 years) of vitamin D supplementation. It found a small but statistically significant positive effect on global muscle strength (SMD 0.17, P = .02), with no effect on muscle mass (SMD 0.058, P = .52) or muscle power (SMD 0.057, P = .657). The effect was concentrated where you would expect it: it was larger in adults aged 65+ (SMD 0.25) than in younger subjects (SMD 0.03), and stronger in participants whose baseline 25(OH)D was below 30 nmol/L. This is the strongest quantitative support for the claim, and even here the strength effect is small.
0.85
Meta-analysis of 30 randomized controlled trials (5,615 participants, mean age 61.1 years) of vitamin D supplementation. It found a small but statistically significant positive effect on global muscle strength (SMD 0.17, P = .02), with no effect on muscle mass (SMD 0.058, P = .52) or muscle power (SMD 0.057, P = .657). The effect was concentrated where you would expect it: it was larger in adults aged 65+ (SMD 0.25) than in younger subjects (SMD 0.03), and stronger in participants whose baseline 25(OH)D was below 30 nmol/L. This is the strongest quantitative support for the claim, and even here the strength effect is small.
Design Meta-Analysis (1.0) × quality 0.85 = impact 0.85
View sourceMeta-Analysis
Neutral
Stockton KA et al. · 2011Osteoporosis InternationalSystematic review and meta-analysis of 17 RCTs (5,072 participants) that split the analysis by baseline vitamin D status, which is why it lands as genuinely conditional rather than for or against. In adults with baseline 25(OH)D above 25 nmol/L, supplementation had no significant effect on grip strength (SMD -0.02, 95% CI -0.15 to 0.11) or proximal lower-limb strength (SMD 0.10, 95% CI -0.01 to 0.22). In the two trials of frankly deficient participants (25(OH)D below 25 nmol/L), pooled hip muscle strength improved enormously (SMD 3.52, 95% CI 2.18 to 4.85). The claim is therefore false as a general statement and plausibly true only in severe deficiency — but that deficient-subgroup estimate rests on just two small trials.
0.85
Systematic review and meta-analysis of 17 RCTs (5,072 participants) that split the analysis by baseline vitamin D status, which is why it lands as genuinely conditional rather than for or against. In adults with baseline 25(OH)D above 25 nmol/L, supplementation had no significant effect on grip strength (SMD -0.02, 95% CI -0.15 to 0.11) or proximal lower-limb strength (SMD 0.10, 95% CI -0.01 to 0.22). In the two trials of frankly deficient participants (25(OH)D below 25 nmol/L), pooled hip muscle strength improved enormously (SMD 3.52, 95% CI 2.18 to 4.85). The claim is therefore false as a general statement and plausibly true only in severe deficiency — but that deficient-subgroup estimate rests on just two small trials.
Design Meta-Analysis (1.0) × quality 0.85 = impact 0.85
View sourceMeta-Analysis
Con
Bislev LS et al. · 2021Journal of Bone and Mineral ResearchThe largest synthesis on this question: a systematic review and meta-analysis of 54 randomized placebo-controlled trials (8,747 participants) covering muscle strength, physical performance and lean mass. Supplementation did not improve any muscle-health outcome; if anything it was slightly worse than placebo, with a slower timed-up-and-go test (mean difference +0.15 s, 95% CI 0.03 to 0.26) and lower knee flexion strength (-3.3 N, 95% CI -6.63 to -0.03). The authors concluded that available evidence does not support a beneficial effect of vitamin D on muscle health and that possible adverse effects should be considered.
0.95
The largest synthesis on this question: a systematic review and meta-analysis of 54 randomized placebo-controlled trials (8,747 participants) covering muscle strength, physical performance and lean mass. Supplementation did not improve any muscle-health outcome; if anything it was slightly worse than placebo, with a slower timed-up-and-go test (mean difference +0.15 s, 95% CI 0.03 to 0.26) and lower knee flexion strength (-3.3 N, 95% CI -6.63 to -0.03). The authors concluded that available evidence does not support a beneficial effect of vitamin D on muscle health and that possible adverse effects should be considered.
Design Meta-Analysis (1.0) × quality 0.95 = impact 0.95
View sourceMeta-Analysis
Con
Han Q et al. · 2024Frontiers in NutritionUpdated systematic review and meta-analysis of 10 RCTs in healthy athletes (354 randomized, 318 completing; 185 vitamin D3 vs 169 placebo). Supplementation significantly raised serum 25(OH)D (MD +14.76 nmol/L, 95% CI 8.74 to 20.77), but the pooled effect across four strength measures was not significant (SMD 0.18, 95% CI -0.02 to 0.37, P = 0.08). Only one isolated sub-outcome, quadriceps contraction, reached significance (SMD 0.57, P = 0.04). The authors' own conclusion is that the evidence cannot warrant significant overall enhancement of muscle strength in athletes made replete by supplementation.
0.75
Updated systematic review and meta-analysis of 10 RCTs in healthy athletes (354 randomized, 318 completing; 185 vitamin D3 vs 169 placebo). Supplementation significantly raised serum 25(OH)D (MD +14.76 nmol/L, 95% CI 8.74 to 20.77), but the pooled effect across four strength measures was not significant (SMD 0.18, 95% CI -0.02 to 0.37, P = 0.08). Only one isolated sub-outcome, quadriceps contraction, reached significance (SMD 0.57, P = 0.04). The authors' own conclusion is that the evidence cannot warrant significant overall enhancement of muscle strength in athletes made replete by supplementation.
Design Meta-Analysis (1.0) × quality 0.75 = impact 0.75
View sourceShowing the 4 strongest of 7 studies. Tap any node to expand its detail.
Evidence
PRO (1)
PRO Meta-Analysisn=56150.85 Beaudart C, Buckinx F et al. (2014)
Small but significant improvement in global muscle strength (SMD 0.17, P = .02); no effect on muscle mass (SMD 0.058, P = .52) or muscle power (SMD 0.057, P = .657). Larger in adults 65+ (SMD 0.25) than in younger subjects (SMD 0.03), and stronger when baseline 25(OH)D was below 30 nmol/L
Meta-analysis of 30 randomized controlled trials (5,615 participants, mean age 61.1 years) of vitamin D supplementation. It found a small but statistically significant positive effect on global muscle strength (SMD 0.17, P = .02), with no effect on muscle mass (SMD 0.058, P = .52) or muscle power (SMD 0.057, P = .657). The effect was concentrated where you would expect it: it was larger in adults aged 65+ (SMD 0.25) than in younger subjects (SMD 0.03), and stronger in participants whose baseline 25(OH)D was below 30 nmol/L. This is the strongest quantitative support for the claim, and even here the strength effect is small.
Weighted 0.85 — 30 RCTs, 5,615 participants (mean age 61.1 y) — large, well powered, with the effect concentrated exactly where biology predicts (older and deficient subjects), which is a strong internal-coherence signal. Only discounted from the top band because the strength effect is small and the meta-analysis pools heterogeneous doses and regimens. Funding could not be verified, so the field is omitted.
The Journal of Clinical Endocrinology and Metabolism
AGAINST (4)
AGAINST Meta-Analysisn=87470.95 Bislev LS, Grove-Laugesen D et al. (2021)
No improvement in any muscle-health outcome; if anything slightly worse than placebo — timed-up-and-go slower by 0.15 s (95% CI 0.03 to 0.26) and knee flexion strength lower by 3.3 N (95% CI -6.63 to -0.03)
The largest synthesis on this question: a systematic review and meta-analysis of 54 randomized placebo-controlled trials (8,747 participants) covering muscle strength, physical performance and lean mass. Supplementation did not improve any muscle-health outcome; if anything it was slightly worse than placebo, with a slower timed-up-and-go test (mean difference +0.15 s, 95% CI 0.03 to 0.26) and lower knee flexion strength (-3.3 N, 95% CI -6.63 to -0.03). The authors concluded that available evidence does not support a beneficial effect of vitamin D on muscle health and that possible adverse effects should be considered.
Weighted 0.95 — 54 randomised placebo-controlled trials, 8,747 participants — the largest and best-powered synthesis on the question, academically funded, covering strength, physical performance and lean mass together. Precise enough to exclude a meaningful benefit rather than merely fail to find one. Near the ceiling; held off 1.00 only for the usual heterogeneity of dose, baseline status and population across five decades of trials.
Funding: Aarhus University
Journal of Bone and Mineral Research
AGAINST RCTn=4110.90 Grimnes G, Kubiak J et al. (2019)
Serum 25(OH)D rose substantially, yet hip flexion, biceps flexion, pectoralis and handgrip strength were unchanged versus placebo
Placebo-controlled randomized trial in 411 vitamin D-insufficient adults (208 vitamin D, 203 placebo) with mean baseline 25(OH)D of roughly 33-35 nmol/L — i.e. exactly the population in which supplementation is supposed to work. Participants received a 100,000 IU loading dose then 20,000 IU weekly for four months. Serum 25(OH)D rose substantially in the treatment arm, yet hip flexion, biceps flexion, pectoralis and handgrip strength did not change in either group, with no between-group differences. This is direct evidence that correcting insufficiency (as opposed to severe deficiency) does not by itself produce strength gains.
Weighted 0.90 — 411 of 422 randomised participants were analysed - by far the largest and best-powered trial on this question, placebo-controlled, publicly funded, and run in exactly the vitamin D-insufficient population where an effect is claimed. Supplementation demonstrably raised 25(OH)D, which makes this a direct, well-powered null rather than a failure of dose.
Funding: North Norway Regional Health Authorities (SFP1277-16) and UiT The Arctic University of Norway
PLoS One
AGAINST Meta-Analysisn=3180.75 Han Q, Xiang M et al. (2024)
25(OH)D rose +14.76 nmol/L (95% CI 8.74-20.77), but the pooled strength effect was null (SMD 0.18, 95% CI -0.02 to 0.37, P=0.08)
Updated systematic review and meta-analysis of 10 RCTs in healthy athletes (354 randomized, 318 completing; 185 vitamin D3 vs 169 placebo). Supplementation significantly raised serum 25(OH)D (MD +14.76 nmol/L, 95% CI 8.74 to 20.77), but the pooled effect across four strength measures was not significant (SMD 0.18, 95% CI -0.02 to 0.37, P = 0.08). Only one isolated sub-outcome, quadriceps contraction, reached significance (SMD 0.57, P = 0.04). The authors' own conclusion is that the evidence cannot warrant significant overall enhancement of muscle strength in athletes made replete by supplementation.
Weighted 0.75 — 318 completers of 354 randomised across 10 RCTs. Supplementation clearly worked biochemically, which makes the strength null informative rather than a dosing failure. Still a modest pooled sample, and the one significant sub-outcome (quadriceps contraction) looks like multiplicity across four strength measures. Publicly funded, no commercial relationships.
Funding: National Key R&D Program of China (2019YFF0301700)
Frontiers in Nutrition
AGAINST Meta-Analysisn=4360.70 Sist M, Zou L et al. (2023)
Supplementation reliably raised serum 25(OH)D but produced no significant improvement in maximal strength or power (baseline <75 nmol/L: upper-body SMD 0.25, 95% CI -0.44 to 0.95; lower-body SMD 0.26, 95% CI -0.13 to 0.65)
Systematic review and meta-analysis of 11 RCTs (436 athletes) on maximal strength and power. Supplementation reliably raised serum 25(OH)D but produced no statistically significant improvement in maximal strength or power. Stratified by baseline status, athletes starting below 75 nmol/L showed only small, non-significant effects on upper-body (SMD 0.25, 95% CI -0.44 to 0.95) and lower-body strength (SMD 0.26, 95% CI -0.13 to 0.65), and those already at or above 75 nmol/L showed a trivial effect on power (SMD 0.15, 95% CI -0.42 to 0.72).
Weighted 0.70 — k=11 RCTs (12 comparisons), 436 athletes pooled -- a modest evidence base. The authors' own RoB2 assessment rates only 2 studies low risk, 8 'some concerns' and 2 high risk, and their GRADE ratings are one moderate, two low and one very-low quality outcome, mostly downgraded for unclear randomisation, allocation concealment and blinding. No funding statement is given; one author was employed by AstraZeneca (unrelated to vitamin D), so funding is omitted rather than asserted. A well-executed synthesis of weak primary trials.
Frontiers in Nutrition
NEUTRAL (2)
NEUTRAL Meta-Analysisn=50720.85 Stockton KA, Mengersen K et al. (2011)
No effect on grip strength (SMD -0.02, 95% CI -0.15 to 0.11) or proximal lower-limb strength (SMD 0.10, 95% CI -0.01 to 0.22) when baseline 25(OH)D was above 25 nmol/L; hip strength improved markedly (SMD 3.52, 95% CI 2.18 to 4.85) only in the severely deficient
Systematic review and meta-analysis of 17 RCTs (5,072 participants) that split the analysis by baseline vitamin D status, which is why it lands as genuinely conditional rather than for or against. In adults with baseline 25(OH)D above 25 nmol/L, supplementation had no significant effect on grip strength (SMD -0.02, 95% CI -0.15 to 0.11) or proximal lower-limb strength (SMD 0.10, 95% CI -0.01 to 0.22). In the two trials of frankly deficient participants (25(OH)D below 25 nmol/L), pooled hip muscle strength improved enormously (SMD 3.52, 95% CI 2.18 to 4.85). The claim is therefore false as a general statement and plausibly true only in severe deficiency — but that deficient-subgroup estimate rests on just two small trials.
Weighted 0.85 — k=17 RCTs, 5072 participants pooled -- large and well powered, and the pre-specified split by baseline vitamin D status is exactly what makes the finding interpretable rather than an averaged-out null. Strong within its class. Held below the top band for one reason: the dramatic deficient-subgroup estimate (SMD 3.52) rests on just two small trials, so the conditional half of the conclusion is far less secure than the null half. Funding statement not retrievable; PubMed tags it non-industry, non-US-government support, but the funder is not named, so funding is omitted.
Osteoporosis International
NEUTRAL RCT0.25 Close GL, Russell J et al. (2013)
Serum 25(OH)D rose from 29 +/- 25 to 103 +/- 25 nmol/L (P = 0.0028) on 5,000 IU/day for 8 weeks; the supplemented group improved 10 m sprint time and vertical jump (both P = 0.008) while placebo did not change
The trial most often cited as showing a performance benefit in athletes. After screening 61 UK athletes and 30 controls in winter (62% and 73% respectively had inadequate 25(OH)D), a small placebo-controlled arm gave 5,000 IU/day of vitamin D3 for 8 weeks. Serum 25(OH)D rose from 29 ± 25 to 103 ± 25 nmol/L (P = 0.0028) and the supplemented group significantly improved 10 m sprint time and vertical jump (both P = 0.008), while placebo did not change. The caveat is severe: the supplementation arm was only about 5 per group, and the reported gains are within-group changes rather than a between-group comparison, so this single result carries far less weight than the pooled athlete meta-analyses that fail to reproduce it.
Weighted 0.25 — The supplementation arm is roughly 5 per group (about 10 analysed), which is at the very bottom of the credible range — the cross-sectional screening arm (61 athletes, 30 controls) is much larger but answers a different question. Worse, the reported performance gains are WITHIN-group changes, not a between-group comparison against placebo, so the headline result does not test the hypothesis it is cited for. Pooled athlete meta-analyses do not reproduce it.
Funding: Medical Research Council (UK) grant G0801462 (from indexed grant metadata; the paper's own funding statement is behind a paywall and could not be read)
Journal of Sports Sciences