Skeletal muscle is packed with vitamin D receptors, and every hard training session triggers a flood of signaling molecules meant to rebuild that tissue stronger. A new narrative review in the Journal of the International Society of Sports Nutrition argues that when vitamin D status is low, those signals do not land the way they should.
The paper focuses on a class of molecules called exerkines, and it makes a case that has largely been overlooked in sports nutrition: vitamin D is not just a bone nutrient athletes should top up in winter. It may be a gatekeeper for how well the body actually adapts to training.
What exerkines actually do
Exerkines are signaling molecules released by muscle, fat, liver, and other tissues during and after exercise. The term grew out of research on myokines, a category first popularized by Bente Klarlund Pedersen, a physician-researcher at Rigshospitalet in Copenhagen who helped establish that contracting muscle behaves like an endocrine organ.
These molecules travel through the bloodstream and tell other tissues what to do next. Some trigger mitochondrial biogenesis, the process of building new energy-producing structures inside cells.
Others reduce inflammation, promote blood vessel growth, or even cross into the brain to support cognitive function. Collectively, they are a major reason a single workout can improve metabolic health, not just muscle size.
Where vitamin D fits into the signal
Vitamin D does not just regulate calcium absorption. Its active form binds to the vitamin D receptor, a protein found throughout skeletal muscle tissue, where it influences gene expression tied to protein synthesis, calcium handling within muscle fibers, and mitochondrial function.
The new review proposes that this receptor activity overlaps directly with exerkine pathways. When vitamin D is sufficient, the authors argue, it appears to support the signaling cascades that turn a workout into an adaptation.
When it is deficient, those same pathways may be blunted, even if training volume and intensity stay identical. That distinction matters because two athletes can follow the same program and see different results depending on a nutrient status that rarely gets tested.
A deficiency hiding in plain sight
Vitamin D deficiency is common among athletes, particularly those who train indoors, compete during winter months, or live at higher latitudes. Graeme Close, a professor of applied sport and exercise physiology at Liverpool John Moores University, has spent over a decade studying vitamin D status in professional athletes and has repeatedly found insufficiency rates far higher than most training staff expect, including among elite footballers and rugby players tested during the competitive season.
Indoor sports athletes and those training at night are especially exposed, since the body’s main source of vitamin D is skin synthesis from ultraviolet light. An athlete can eat well, sleep enough, and periodize training carefully, and still be working against a hormonal deficit that standard nutrition screens do not catch.
What the review actually found
The authors compiled evidence linking low vitamin D status to impaired muscle repair, slower recovery from eccentric exercise damage, and reduced markers of mitochondrial adaptation. They frame this within the exerkine model rather than treating vitamin D as an isolated variable, which is what separates this review from older vitamin D and athletic performance literature.
Instead of asking whether vitamin D improves strength directly, the review asks whether vitamin D status changes how effectively the body responds to the stimulus of exercise itself. That reframing has practical weight, because it suggests supplementation timed around vitamin D deficiency correction could matter more during heavy training blocks, when exerkine signaling is working hardest.
The review is explicit that this is a narrative synthesis of existing mechanistic and observational research, not a new clinical trial. But it consolidates a plausible biological pathway that had been scattered across separate fields, endocrinology, exercise physiology, and molecular biology, without much crosstalk.
What this means for training and recovery
Blood testing remains the only reliable way to know vitamin D status, since symptoms of deficiency, including fatigue and slower recovery, overlap heavily with normal training fatigue. Sports medicine guidelines generally consider serum 25-hydroxyvitamin D levels below 30 ng/mL insufficient and below 20 ng/mL deficient, though optimal ranges for athletic performance remain debated.
Athletes training indoors through fall and winter, or those with limited sun exposure due to schedule or skin pigmentation, are reasonable candidates for testing rather than guessing. Correcting a genuine deficiency is a controlled medical decision, not a reason to self-prescribe high-dose supplements, since excess vitamin D carries its own risks.
For most athletes with normal levels, there is no evidence that megadosing vitamin D beyond sufficiency will supercharge exerkine signaling further. The research supports fixing a deficit, not chasing a ceiling effect that the data does not show exists.
The bigger picture
Sports nutrition has spent years focused on what athletes put into their bodies before and after training, protein timing, carbohydrate loading, creatine saturation. This review is a reminder that some of the most consequential variables are hormonal and often invisible without a blood panel.
Vitamin D will not replace a training program, and it will not turn a mediocre block of work into a great one. But if the body’s own repair signals are being dampened by something as fixable as a nutrient deficiency, that is a gap worth closing before blaming programming, sleep, or genetics for stalled progress.
