Vitamins are typically associated with protective functions that support overall health and wellness. For example, vitamin C has antioxidant properties that mitigate the many negative effects associated with oxidative damage to biomolecules such as DNA within the human body.1 Vitamin D is somewhat different. Like other vitamins, it promotes overall health— but it also plays an active role in promoting muscle growth and strength.
Vitamin D is a fat-soluble, steroid-like vitamin that functions as a prohormone— aiding many different processes such as the absorption and metabolism of calcium and phosphorous, promoting bone health.2 Furthermore, low levels of vitamin D— which are prominent in the western world – correlate with several diseases such as cancer and cardiovascular disease.
Some of the muscle-promoting properties associated with vitamin D apparently stem somewhat from the similar chemical structure between vitamin D and steroid molecules like testosterone. Studies have shown vitamin D can bind the androgen receptor3, perhaps mimicking some of the muscle-building properties of testosterone. Vitamin D can also bind and activate the vitamin D receptor – which directly regulates the expression of hundreds of genes4, with several of the genes turned on by vitamin D directly involved in generating muscle growth and strength.
Increased Testosterone and Androgen Receptor Levels
Among the genes regulated by vitamin D, a few appear to be involved in the production of testosterone.5 A study by Wehr et al.6 investigated the association of vitamin D levels with testosterone in over 2,200 men. The study showed a strong correlation between vitamin D levels and testosterone. The group with the lowest vitamin D levels had the lowest testosterone measurements, and the group with the highest amount of vitamin D possessed the highest testosterone.
Interestingly, the study also found that higher levels of vitamin D produced lower amounts of the sex hormone binding globulin protein (SHBG). Since testosterone circulates in the bloodstream bound mostly to SHBG and only a small fraction is unbound – and thus biologically active where it can activate the androgen receptor – vitamin D’s ability to lower SHBG levels will give way to greater testosterone activity.
Thus, vitamin D not only increases the production of testosterone but it also increases testosterone activity by diminishing SHBG’s inhibitory influence on testosterone function.
Since the production of vitamin D can be produced in the human body by exposure to sunlight, the differences in sunlight-induced vitamin D production should vary throughout the year. Thus, in another part of this study, Wehr et al. further validated the relationship between vitamin D and testosterone production by uncovering the tight correlation between vitamin D levels and testosterone production throughout the year. The researchers demonstrated that high vitamin D production in the summer months corresponds with greater testosterone levels, and vice versa during the winter months.
Altogether, this study highlights vitamin D’s capability to raise or maintain testosterone levels. The research emphasizes the importance of consuming enough dietary vitamin D— especially during the long winter months, when exposure to sunlight is diminished and the need for dietary vitamin D is the greatest— in order to stimulate or maintain testosterone production.
In addition to increasing testosterone levels, vitamin D has been shown by researchers at the Dutch company Organext Research to stimulate the expression of the androgen receptor in isolated skeletal muscle cells. The increase in androgen receptor stimulated by vitamin D promoted the proliferation of muscle satellite cells into new muscle fibers, potentially leading to increased muscle growth. In addition to the influence of vitamin D on the androgen receptor, the researchers found that the anabolic steroid nandrolone decanoate stimulated the expression of the vitamin D receptor in isolated skeletal muscle cells— and that the combination of nandrolone decanoate and vitamin D had a overwhelmingly synergistic effect on satellite cell conversion into muscle tissue in isolated skeletal muscle cells.
All in all, vitamin D’s ability to increase the amount of testosterone and androgen receptor should powerfully stimulate muscle growth – not only by initiating satellite cell proliferation, but also by cranking up protein synthesis in muscle tissue – considering that testosterone and the androgen receptor function cooperatively to stimulate muscle cell protein synthesis.
Anti-aromatase Activity
According to an investigation by Krishnan et al.,7 vitamin D considerably decreases the expression of the aromatase enzyme – most potently in fat cells. Since aromatase activity in fat cells can decrease testosterone levels by catalyzing the conversion of testosterone into the estrogen-like compound estradiol, vitamin D’s ability to decrease aromatase represents another mechanism in its arsenal for increasing testosterone.
In the above study, mice were given large quantities of the steroid molecule androstenedione— which can be converted to the estrogen-like molecules estradiol or estrone by aromatase. Some of the mice were also given a daily injection of vitamin D, while the control group received no vitamin D injections. The mice that received vitamin D showed a significant decrease in the expression levels of aromatase and a lower amount of estradiol or estrone, specifically in fat cells, compared to the control group that received no vitamin D.
In addition, the researchers in this study showed that vitamin D diminishes aromatase levels and activity in isolated human breast cancer cells, which is a cell type that typically expresses large amounts of the aromatase enzyme. This finding implies that vitamin D functions similarly in humans by diminishing the quantity and activity of aromatase. Moreover, the researchers also investigated the influence of vitamin D in combination with aromatase inhibitors such as Arimidex on aromatase activity, also in isolated human breast cancer cells. Interestingly, their results showed that vitamin D enhances the effect of aromatase inhibitors. Since some athletes consume aromatase inhibitors during anabolic steroid cycles to prevent the conversion of steroids into estrogen, vitamin D in combination with aromatase inhibitors should more potently decrease aromatase’s ability to convert anabolic steroids into estrogen – thus preventing some of the unpleasant side effects associated with anabolic steroid use such as the development of breast tissue, also known as gynecomastia.
In conclusion, vitamin D has the powerful ability to regulate the expression of specific genes that are intimately involved in muscle growth. Moreover, most people appear to have suboptimal amounts of vitamin D due to dietetic inadequacy and a lack of sunlight exposure. Consequently, vitamin D is evidently a necessary and influential supplement for the athlete or recreational weightlifter for gaining muscle and increasing strength.
References:
- Pauling L. Vitamin C and common cold. Jama 1971;216(2):332.
- Holick MF. Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. Am J Clin Nutr 2004;80(6 Suppl):1678S-88S.
- Proal AD, Albert PJ, et al. Dysregulation of the vitamin D nuclear receptor may contribute to the higher prevalence of some autoimmune diseases in women. Ann N Y Acad Sci 2009;1173:252-9.
- Ramagopalan SV, et al. A ChIP-seq defined genome-wide map of vitamin D receptor binding: associations with disease and evolution. Genome Res 2010;20(10):1352-60.
- Reichel H, Koeffler HP, et al. The role of the vitamin D endocrine system in health and disease. N Engl J Med 1989;320(15):980-91.
- Wehr E, et al. Association of vitamin D status with serum androgen levels in men. Clin Endocrinol (Oxf) 2010;73(2):243-8.
- Krishnan AV, et al. Tissue-selective regulation of aromatase expression by calcitriol: implications for breast cancer therapy. Endocrinology 2010;151(1):32-42.