Vitamin D and VO2max: Why Supplementation Falls Flat

Vitamin D status and VO2max move together in population studies, but taking vitamin D does not appear to raise aerobic fitness. A 2026 meta-analysis put a number on the association: serum 25-hydroxyvitamin D and cardiorespiratory fitness correlated at r=0.17 overall. That is statistically significant, but small. When the same review pooled randomized trials of supplementation, the effect on fitness was essentially zero.

This distinction matters if you are deciding whether vitamin D belongs in a performance stack. Correcting a documented deficiency can matter for health. It may also improve inflammatory markers during training. But the evidence does not support vitamin D as a way to increase VO2max.

A real association, with a modest effect size

Chen Chen et al.'s 2026 systematic review and meta-analysis combined 20 observational studies with 14,554 participants. Higher serum 25(OH)D was associated with higher cardiorespiratory fitness, with a pooled correlation of r=0.17 (95% CI, 0.11 to 0.24; P<0.0001).

A correlation of 0.17 is not meaningless, but it is not a large explanation for why one person has a higher VO2max than another. The observational studies also differed sharply: heterogeneity was high (I²=88%). They enrolled different populations, measured vitamin D and fitness in different ways, and occurred across different seasons and settings. The pooled number summarizes a pattern; it does not establish a single biological effect.

The age subgroup result makes the causal story even less tidy. In adults, the correlation was r=0.26 (95% CI, 0.17 to 0.34; P<0.0001). In children, it was r=0.05 (95% CI, 0.01 to 0.09; P=0.027), and the difference between subgroups was statistically significant (P<0.0001).

Outdoor exercise offers a straightforward explanation. Adults who run, cycle, or play field sports outside can raise VO2max through training while also making more vitamin D through sun exposure. The vitamin D result may therefore track the behavior that improves fitness rather than cause the improvement itself.

Other observational work fits that interpretation. In a 2022 cross-sectional study of free-living Iranian adults, vitamin D status was one of several correlates of VO2max, alongside adiposity and fasting glucose. Adiposity complicates this association: it relates to lower VO2max and can lower circulating 25(OH)D through sequestration in adipose tissue. A correlation that survives a spreadsheet is not automatically a causal pathway.

Likewise, the FIT-AGEING study found that sedentary time, physical-activity intensity, cardiorespiratory fitness, and 1,25-dihydroxyvitamin D concentrations were interrelated in middle-aged sedentary adults. That study cannot prove reverse causation, but it shows why activity behavior is a serious alternative explanation.

Randomized trials do not show a VO2max benefit

Randomization changes the question. Instead of asking whether people with higher vitamin D levels tend to be fitter, trials ask whether assigning vitamin D changes fitness relative to a comparator.

In the Chen meta-analysis, nine randomized controlled trials including 733 participants produced a standardized mean difference of 0.03 for cardiorespiratory fitness (95% CI, −0.12 to 0.17; P=0.896). The confidence interval crosses zero and the result was not statistically significant. More strikingly, heterogeneity was 0%: the trials did not produce a patchwork of large benefits and large harms that averaged into a null result. They pointed in the same direction—toward no detectable effect.

Observational studies say vitamin D status and fitness are related. Randomized trials say supplementation has not raised cardiorespiratory fitness in the populations studied.

The most useful test of the "perhaps it only works in deficiency" idea came from Savolainen et al. in 2022. The randomized trial enrolled 69 vitamin D-deficient young men with baseline 25(OH)D below 20 ng/mL. Participants took 2,000 IU of vitamin D3 daily for eight weeks while completing resistance training. Supplementation corrected biochemical deficiency, yet it did not improve VO2max.

The trial did find improved inflammatory markers. That result does not establish faster recovery, greater training adaptation, or better long-term performance; those are separate outcomes that need direct testing. It does, however, show that vitamin D can affect biology in deficient trainees without producing a measurable aerobic-fitness gain.

Longer and higher-dose trials have not rescued the hypothesis

Short trials can miss slow adaptations, so duration deserves scrutiny. The VITAL trial secondary analysis followed 25,871 older adults assigned to 2,000 IU/day vitamin D3 for five years. It found no improvement in physical-performance measures. These measures are not interchangeable with directly measured VO2max, but the study strongly weakens the argument that years of supplementation produce broad performance gains in older adults.

Pediatric trials point the same way. In a peer-reviewed 2024 Mongolian schoolchildren trial, 8,851 children received weekly 14,000 IU vitamin D3 or placebo for three years. Nearly all had baseline 25(OH)D concentrations below 75 nmol/L. Vitamin D did not improve cardiorespiratory fitness, grip strength, or explosive leg power; VO2peak was measured in a subset of 632 children using a 20-meter multistage shuttle run.

An independent ViDiKids trial in South African schoolchildren also found no benefit for cardiorespiratory fitness, grip strength, or explosive leg power. Different geography and baseline sunlight exposure did not change the result.

Why the mechanism has not become a performance effect

The original hypothesis was biologically reasonable. Vitamin D receptors occur in skeletal muscle, and vitamin D signaling has been linked in laboratory work to muscle-cell processes and mitochondrial respiration. Calcium handling in cardiac cells and endothelial nitric-oxide production are also plausible pathways. A 2023 systematic review of nutrition in sarcopenia describes vitamin D's theoretical relevance to mitochondrial activity and muscle performance in older adults.

Mechanistic plausibility is a reason to run trials, not a substitute for them. Cell and pathway findings cannot tell us whether a supplement produces a clinically detectable change in whole-body oxygen uptake. So far, the human trials have not shown that it does.

What the evidence can and cannot rule out

The intervention evidence is consistent, but it is not exhaustive. The nine cardiorespiratory-fitness trials in the meta-analysis included only 733 people. Small effects in severely deficient people, or effects in elite athletes, remain possible because those groups have not been adequately tested. Doses ranged roughly from 800 to 4,000 IU/day across the pooled trials, and some studies lasted only weeks. Several studies also used predicted or indirect fitness measures rather than direct gas-exchange VO2max testing.

Those limitations justify restraint, not wishful extrapolation. The large and long VITAL trial addresses duration for general physical performance, while the Savolainen trial directly tested deficiency correction in young men and still found no VO2max improvement. Neither result proves that every conceivable athlete subgroup will respond identically. Together, they make a useful ergogenic effect unlikely under the conditions tested.

The practical verdict

Use vitamin D to correct deficiency and support general health, ideally guided by a 25(OH)D blood test and a clinician when dosing needs to be individualized. Do not take it expecting a higher VO2max. At 2,000 IU/day, the dose used in both Savolainen's trial and VITAL, supplementation did not improve the relevant fitness or physical-performance outcomes.

For deficient athletes, the inflammatory-marker finding is worth following, but it is a biomarker result rather than proof of better recovery or performance. The reliable ways to raise VO2max remain training, adequate recovery, and enough total energy to support both. Vitamin D can correct a deficiency. It has not shown that it can replace intervals.