Ten trials. Three hundred fifty-four athletes. One supplement that raised blood vitamin D by a mile — and moved the strength needle barely an inch.
Across ten trials, vitamin D supplements raised blood levels by nearly 15 nanograms per milliliter in 354 athletes. Bench press, vertical jump, and handgrip strength showed no significant gain. But one muscle group broke the pattern.
Your blood test came back optimal. The vitamin D number climbed exactly the way the bottle promised. And then ten trials checked whether your muscles got the same memo.
They didn't.
A 2024 meta-analysis published in Frontiers in Nutrition pooled data from ten randomized controlled trials — 354 athletes across judo, taekwondo, soccer, rugby, and swimming — and found that vitamin D3 supplements raised blood vitamin D levels by nearly 15 nanograms per milliliter.
The pooled increase was highly significant (p < 0.0001).
Then they measured strength. Bench press, vertical jump, handgrip, and quadriceps contraction — four ways to ask whether the supplement that fixed the blood test also improved performance. The overall answer: no significant improvement. The combined strength effect was so small it could have been random noise.
The supplement did exactly what it promised for one measurement. And no significant gain for the one that matters to you between sets.
Your blood test confirms the supplement raises your levels. Ten trials of 354 athletes found no significant overall strength gain — with one exception, quadriceps contraction, in two small trials.
- The researchers pooled ten trials of vitamin D3 in 354 athletes and found no significant overall improvement in muscle strength — despite a significant rise in blood levels.
- Bench press, vertical jump, and handgrip strength showed no significant improvement. Quadriceps contraction was the only test that reached statistical significance — from just two of the ten studies.
- The authors suggest vitamin D may act through receptors in muscle cells and on fast-twitch fibers, but say the quadriceps result falls short of conclusive evidence.
- The researchers themselves concluded that vitamin D supplementation "cannot warrant significant overall enhancements" in muscle strength even when blood levels reach sufficient ranges.
- Roughly one in three adult athletes has insufficient vitamin D levels without knowing it — the first question isn't whether to supplement, but where you currently stand.
The Answer That Kept Coming Back
Han first ran this analysis in 2019 with fewer studies. Same answer. So they expanded the dataset to ten trials and 354 athletes. The conclusion did not change — the same one nobody wanted to hear the first time.
Bench. Jump. Grip. Quads.
The overall number hides something specific.
Vitamin D3 did not significantly change bench press strength — athletes taking the supplement performed no better than placebo on upper-body pressing. For most lifters, that is the number they check first.
Vertical jump showed no significant gain. Neither did handgrip. Two more measurements where the supplement made no clear difference.
Three tests. Three non-significant results. And then the fourth.
Quadriceps contraction was the only strength measure that cleared the statistical bar. Tested on equipment that isolates leg extension force at controlled speeds, the quad result was moderate in size and statistically significant (p = 0.04). Among four ways to measure whether vitamin D made athletes stronger, quads stood alone.
Bench, jump and grip showed no significant gain. The quadriceps test did.
Most athletes in these trials started with low vitamin D — the group you would expect to benefit most. Even so, overall strength did not improve significantly.
A Suggested Mechanism, Not a Tested One
Why would the quadriceps respond when bench, jump and grip did not? The meta-analysis does not answer that. Its authors call the precise mechanism behind vitamin D's effects on performance a subject of ongoing research, and describe a general suggestion from earlier work, not one specific to the quadriceps: muscle cells carry vitamin D receptors, which could conceivably affect calcium binding during contraction, and over the long run vitamin D may contribute to the enlargement and multiplication of fast-twitch fibers.
None of the papers cited here says the quadriceps carry more of those receptors than other muscles, or that they responded for that reason.
The animal work points in a general direction. Mice lacking the receptor throughout the body had weaker grip and smaller muscle fibers. In rats, muscle fibers shrank, particularly type IIB fibers, when the receptor was knocked down in one muscle, and type II fiber area fell on a vitamin D-deficient diet [3]. Mice lacking the receptor in muscle cells alone were weaker too, but their fibers were larger, with no clear difference in fiber types [3].
An earlier review describes the receptor in bird, mouse and human muscle cells, while noting that one paper found none in skeletal muscle, and two possible routes of action: a rapid release of calcium inside the cell, and slower effects through gene expression, including the proteins muscle uses to contract [4].
That makes a role for vitamin D in muscle plausible. It does not explain the quadriceps result.
Zinc and testosterone follow the same conditional logic. Restricting zinc intake in healthy men crashed testosterone 73%, but supplementing men with adequate zinc did nothing measurable for testosterone or body composition — a result confirmed by two independent labs.
What Two Studies Can and Cannot Prove
Now here is where the excitement needs guardrails.
The quadriceps result came from two of the ten trials in the meta-analysis. Two studies. Limited participants.
When researchers calculated the range of plausible effects, the bottom end sat at 0.04 — a hair above zero, which is the line between “the supplement did something real” and “this could just be noise.” One more negative study could push the entire result across that line.
The researchers said it themselves: the limited availability of data for quadriceps contraction means the finding needs confirmation before anyone builds a plan around it.
A mechanism has been suggested, not tested. The data barely makes statistical sense. Both things are true at the same time, and holding them together without collapsing into either “it works” or “it doesn’t” is the honest position.
The ten studies ranged from ten to fifty-seven athletes each, with doses spanning two thousand to nearly nineteen thousand units per day. That spread carries real findings and real noise.
The quad finding stood on only two of ten studies — a signal with fragile statistics that the authors call short of conclusive.
Are You in the Thirty Percent?
Most of the athletes in these trials were vitamin D insufficient at baseline. The supplement raised their blood levels, and it raised them in the two trials that started with adequate levels too.
This matters because the question is not “does vitamin D help strength” but “does it help your strength?” This meta-analysis did not split its strength results by starting level. A second one that did found no significant effect on strength or power either way [2].
About one in three adult elite athletes has vitamin D levels below the insufficiency line [1]. Among adolescent athletes, the proportion rises to nearly two in five [1].
If you train indoors, live in a northern climate, or go months without meaningful sun exposure, the odds shift further. The first question is not whether vitamin D works in general. It is whether you are low.
The Largest Trials Agree
The skepticism is not fringe. It comes from the biggest and most expensive trials in the field.
The VITAL trial — twenty-five thousand people, five years — found no benefit for muscle or bone in people who already had adequate vitamin D. A Lancet editorial went further: maybe low vitamin D is a sign of poor health, not a cause of it.
Those conclusions align with Han’s overall finding. Across ten trials, combining bench, jump, grip, and quads together, the supplement did not produce a significant strength improvement. The scientific establishment and the data are on the same page.
What the large trials did not do was isolate the quadriceps. They measured grip. They measured general function. They never tested the quadriceps of deficient athletes training through winter.
An independent meta-analysis of eleven trials in 436 athletes reached the same overall conclusion — no significant effect on upper body strength, lower body strength, or muscle power [2]. The null finding is consistent across research teams, though six of Han's ten trials are in both analyses. Testing everything together can hide a signal in one test, but this one rests on two small trials, and the second meta-analysis found no significant effect on lower-body strength measured by squat and leg press [2].
Roughly one in three adult elite athletes is vitamin D insufficient. The first question is not ‘should I take it’ — it is ‘where do I stand?’
What the Blood Test Actually Tells You
Your blood test is real. The nearly fifteen-nanogram increase in vitamin D across these trials was highly significant (p < 0.0001). The supplement does what it promises for blood chemistry.
What it does not do — based on ten trials — is translate that blood response into significant strength gains across bench press, vertical jump, or handgrip. The only test that broke the pattern was quadriceps contraction, and that finding stands on two studies with a narrow margin of confidence.
The honest position: if you are deficient, correcting the deficiency is supported by the data. The blood response is real; a general mechanism has been suggested, not shown to explain the quadriceps result. If you are already sufficient, more vitamin D is unlikely to make you stronger — that conclusion has held across two versions of this meta-analysis and an independent one [2].
The decision is not “take vitamin D” or “skip vitamin D.” It is “find out where you stand, and then choose.”
If one significant test out of four, in two small trials, is all a single nutrient showed for strength, the next question is already forming. Your multivitamin targets thirty. Does any of it land?
Your blood test tells you whether the supplement reached your bloodstream. It doesn't tell you whether it reached your muscles.
The distinction matters because this meta-analysis found a massive gap between those two outcomes. Blood levels rose significantly in the pooled analysis of all ten trials. Strength scores — bench press, vertical jump, handgrip — showed no significant gain.
The first practical step is knowing where you stand. A standard blood draw measures your vitamin D level directly. Below 20 nanograms per milliliter is considered deficient. Between 20 and 30 is insufficient. Above 30 is sufficient. Most athletes in these trials started below 25 ng/mL, and supplementation raised their levels without a significant gain in overall strength.
If you're below that line, correcting the deficit has broad health support beyond muscle function. If you're above it, the authors conclude that supplementation cannot warrant significant overall strength gains once athletes reach adequate levels, and a second meta-analysis that split athletes by starting level found no significant strength gain in those starting at 75 nmol/L (30 ng/mL) or above.
What other research found
What this means for you
The strongest case for supplementation lives here. Supplementation raised blood levels in deficient athletes, though not every trial got them above 30 ng/mL: after the 8-day trial's single dose, the average stayed below it.
The catch: that blood correction didn't translate into overall strength gains across the ten trials. The one exception — quadriceps contraction — came from only two studies, and the authors call it short of conclusive.
Correcting a deficiency has health benefits beyond muscle function. But if your reason for supplementing is specifically to get stronger, this meta-analysis says expect a blood test improvement, not a strength test improvement.
The evidence here has a clear message for athletes above the sufficiency line: more vitamin D didn't produce more strength.
In the two trials that started with adequate levels, blood levels still rose. This meta-analysis did not split strength by starting level; a second one that did found no significant strength gain in athletes starting at 75 nmol/L (30 ng/mL) or above. The data supports maintaining sufficient levels, not chasing higher ones.
The supplement budget might do more elsewhere. Training load, sleep quality, and protein intake each have larger effect sizes for strength than vitamin D supplementation in athletes who aren't deficient.
Sprinters, cyclists, soccer players, and anyone whose performance depends on knee extension force — the quadriceps finding is the most relevant signal in this meta-analysis.
Quadriceps contraction was the only strength measure that reached statistical significance. Why it responded is not known: the authors describe only a general suggested mechanism, through receptors in muscle cells and fast-twitch fibers, and no paper here shows the quads carry more receptors than other muscles.
The honest calibration: that finding came from two studies with a confidence range that barely cleared the significance line. A suggested mechanism, fragile statistics. If you're deficient and your sport loads the quads, this is the result to watch — but it is preliminary evidence, not a guarantee.
Before you change anything
This meta-analysis pooled 354 athletes across ten trials — judo, taekwondo, soccer, rugby, and swimming. Most were young adults training competitively, and most started with vitamin D levels below the insufficiency line.
Seventy percent of the trials were conducted in northern Europe during winter, a season the authors describe as having minimal sunlight exposure. That geographic and seasonal clustering means the results map most clearly to athletes training through dark months at higher latitudes.
The data does not cover recreational exercisers or elderly populations, and only two trials started with adequate vitamin D levels. If you train outdoors in a sunny climate year-round, these athletes are not your comparison group.
No dose-response analysis despite doses ranging from 2,000 to 18,750 IU per day across trials. Whether higher doses produce different strength effects remains unanswered.
The trials measured strength using different protocols — different machines and testing protocols — leg extension tests at various speeds, different one-rep-max methods — which adds noise to the pooled results. Supplementation periods ranged from just one week to twelve weeks, and muscle adaptation may need longer to manifest.
No trial measured muscle size. The meta-analysis can speak to strength but not to whether vitamin D affects muscle growth. Diet was not controlled or uniformly reported, leaving a potential confounding variable unaddressed across all ten trials.
The blood response is rock-solid. A nearly 15-nanogram increase in vitamin D levels, pooled across ten trials (p < 0.0001). If you supplement, your blood levels will rise. The overall strength null is well-supported. Two meta-analyses from different teams (Han 2024 with 354 athletes, Sist 2023 with 436 athletes) both found no significant strength effect. The VITAL trial with over 25,000 adults agrees. When three independent evidence sources converge on the same null, the finding carries weight.
The quadriceps exception is preliminary. Two studies. A confidence range that starts at 0.04 — barely above zero. A mechanism the authors suggest, which no trial here was designed to test. Promising lead, not proven finding.
Vitamin D showed one significant strength result, in two small trials of the quadriceps. Everything else on the strength scorecard showed no significant gain.
That raises a question about the supplement sitting next to it on the shelf. If a single nutrient shows this little for strength, what happens when you swallow thirty nutrients at once? A multivitamin scorecard across six body systems tested that question in over 5.5 million people — and the results look nothing like the label promises.
What This Study Found
All findings from this paper, in plain language.
- Pooling all four strength tests across ten trials, vitamin D supplements produced no significant overall improvement in muscle strength for athletes.
- Quadriceps contraction was the only strength measure that improved significantly with vitamin D supplementation — but it came from just two studies.
- Bench press, vertical jump, and handgrip strength showed no meaningful change with vitamin D supplements across the included trials.
- Vitamin D supplements raised blood levels by nearly 15 nanograms per milliliter — a highly significant increase in the pooled analysis of all ten trials (p < 0.0001).
- Athletes who started with low vitamin D levels raised their levels with supplementation doses ranging from 2,000 to 18,750 IU per day, though after the 8-day trial's single dose the average stayed below 30 ng/mL.
- This is an update of the same team's 2019 analysis — with more data, the overall null finding for strength held steady.
- Both the quadriceps and handgrip findings rested on only two studies each, limiting how much weight either result can carry.
- Removing studies where many athletes dropped out didn't change the blood-level finding — the vitamin D increase was consistent regardless.
- Vitamin D3 taken for four to twelve weeks at doses above 2,000 IU per day reliably corrected winter-season deficiency in athletes at northern latitudes.
- Even athletes who started with adequate vitamin D levels saw their blood levels climb higher with supplementation — the meta-analysis did not analyze their strength separately.
- The researchers concluded that vitamin D supplementation cannot warrant significant overall strength improvements in athletes who reach adequate blood levels through supplementation.