For years, vitamin D has been associated almost exclusively with bones. And that makes sense: its role in calcium absorption and bone tissue maintenance is very well established. But today we know that its biological function goes far beyond that. Vitamin D acts as a molecule with hormone-like behavior, participates in the regulation of multiple physiological processes, and has receptors in many tissues of the body. However, recognizing its importance doesn't mean attributing miraculous effects to it or making it the explanation for everything.

A regulator with action in many tissues

The active form of vitamin D acts through the vitamin D receptor, known as VDR, which is involved in regulating gene expression. This receptor is present in numerous tissues, not only those related to bone metabolism. Therefore, in addition to contributing to calcium and phosphorus homeostasis, vitamin D also plays a role in processes linked to neuromuscular function, the immune response, and other relevant physiological mechanisms.

Simply put, vitamin D isn't just "the bone vitamin." It has a broader range of effects. But it's important to clarify: its involvement in many systems doesn't mean that indiscriminate supplementation will improve all of them. In fact, the Endocrine Society's 2024 clinical guidelines emphasize that the benefit of taking more vitamin D to prevent disease in healthy individuals is unclear, and that there isn't a well-defined "universal optimum" level for that purpose.

How it is activated in the body

The vitamin D we obtain from sun exposure, food, or supplements doesn't act directly in its initial form. First, it's transformed in the liver into 25-hydroxyvitamin D, or 25(OH)D, which is the form measured in the blood to assess vitamin D status. Then, primarily in the kidneys, it's converted into 1,25-dihydroxyvitamin D, the biologically active form. Furthermore, some tissues can activate vitamin D locally, which helps explain why its effects aren't limited to bone.

This point is important because “active vitamin D” is often confused with “blood vitamin D level.” In clinical practice, the reference marker is 25(OH)D, because it better reflects the total vitamin D intake from sunlight, diet, and supplements.

Its role in overall health

The strongest evidence regarding vitamin D remains its role in the musculoskeletal system. It contributes to the intestinal absorption of calcium, helps maintain adequate calcium and phosphorus levels, and promotes normal bone mineralization. When levels are clearly insufficient, the risk of disorders such as rickets in children, osteomalacia in adults, and other problems related to bone and muscle health increases.

Beyond that, vitamin D has also been linked to immune, neuromuscular, and metabolic processes. However, it's important to be rigorous here: the existence of biological plausibility or observational associations does not mean that increasing supplementation will automatically improve overall health in any individual. Current guidelines emphasize interpreting this data with caution and avoiding simplistic messages.

Vitamin D and athletic performance: what we really know

In the sports world, vitamin D is of great interest due to its relationship with muscle function, contraction, bone health, and injury risk. The strongest message today is not that "more vitamin D improves performance," but rather that maintaining adequate levels is important, especially in people at risk of deficiency or with confirmed low levels.

The available evidence suggests that correcting suboptimal levels may provide benefits for musculoskeletal health and, in some cases, improve parameters such as strength, power, balance, or reaction time. However, the results are not consistent and do not allow us to conclude that vitamin D supplementation consistently improves performance in any athlete, especially if they already have adequate levels.

Therefore, from a practical perspective, vitamin D should be understood more as a physiological support factor than as a universal ergogenic aid. Its importance is greater in contexts such as limited sun exposure, indoor training, winter months, or a history of bone and muscle problems.

What can happen when there is a deficit?

When vitamin D levels are low, both bone health and muscle function can be compromised. In athletes, a deficiency has been associated with a higher risk of bone injuries, musculoskeletal pain, and a poorer physiological capacity to tolerate training loads.

However, it wouldn't be accurate to say that a deficiency "always causes" fatigue, poor performance, or muscle atrophy. A more precise statement is that low levels are associated with a less favorable environment for musculoskeletal function, and that correcting this deficiency can be relevant when a genuine deficiency exists.

What blood level is considered adequate?

This is where the most confusion often arises. The marker used to assess vitamin D status is 25(OH)D in the blood. According to the NIH and the National Academies of Sciences, Engineering, and Medicine, these are the most commonly used reference points in the general healthy population: levels below 12 ng/mL are associated with deficiency, levels between 12 and 20 ng/mL may be inadequate for some people, and 20 ng/mL or higher is considered sufficient for most of the population. Furthermore, concentrations above 50 ng/mL may be associated with potential adverse effects.

This means that presenting a general range of 30–50 ng/mL as a “universal optimum” does not accurately reflect the current consensus. In sports, some practical frameworks use higher targets in specific situations, but this does not equate to a general clinical consensus nor does it justify supplementation without justification.

Neither deficiency nor excess: balance matters

With vitamin D, more isn't always better. Deficiency is a real problem, but too much can be. Excessive supplementation can lead to hypercalcemia and other associated complications, especially when continued over time. Toxicity is usually due to overuse of supplements, not sun exposure or a typical diet.

Therefore, the best strategy is not to assume that everyone needs more vitamin D, but to assess context, sun exposure, diet, lifestyle and, when indicated, resort to professional evaluation and analysis.

Conclusion

Vitamin D is essential for bone health and plays important roles in other systems, including muscle function. In sports, maintaining adequate levels can be crucial, especially when there is a deficiency or an increased risk of low levels. However, the evidence does not support making it a universal solution for improving performance or a "magic" molecule capable of optimizing health on its own.

The most useful and honest message remains this: vitamin D matters, but it matters especially when it is lacking, and it should always be interpreted within the clinical, nutritional, and lifestyle context of each individual.

And if you want to better understand how your actual diet fits with what your body needs, you can rely on tools such as Caloo, designed to help you interpret what you eat in a more visual, practical and personalized way.

 

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Disclaimer: This content is for informational purposes only and does not replace individual assessment by a healthcare professional. The interpretation of vitamin D levels and the decision to supplement should be made based on the clinical context, diet, sun exposure, time of year, physical activity, and other personal factors.