Vitamin D

Vitamin D, known for its role in bone health and calcium metabolism, has also been implicated in skin health, including the regulation of hair growth. Historically, vitamin D was discovered in the early 20th century during investigations into rickets, a bone disorder in children caused by vitamin D deficiency. Since then, research has expanded into the vitamin's broader effects on human health, revealing its significance in immune function, cardiovascular health, and skin condition, among others.

In the realm of alopecia, vitamin D's mechanisms of action are complex and multifaceted. It interacts with the skin and hair follicles through the vitamin D receptor (VDR), which is present in various skin cells, including keratinocytes and hair follicle cells. Animal and laboratory studies suggest that vitamin D signalling through this receptor plays a role in the normal cycling of hair follicles, including the transition from the resting phase (telogen) to the growth phase (anagen). This has not been shown directly in people. Deficiency in vitamin D has been linked to various forms of hair loss, including alopecia areata (AA), a condition characterized by patchy hair loss, and possibly even androgenetic alopecia (AGA), the common form of hair thinning and balding.

Research on vitamin D as it relates to hair, hair growth, or loss has produced intriguing findings. For example, a review highlighted the importance of vitamin D in skin health, suggesting that deficiency might contribute to conditions like psoriasis, acne, and alopecia. Studies have shown that patients with AA often have lower levels of serum 25-hydroxyvitamin D, implying a potential link between vitamin D status and autoimmune hair loss (Lee et al., 2018, a meta-analysis of 14 observational studies, PMID 29633370). These studies show an association, not cause and effect. However, the relationship between vitamin D and hair growth is not straightforward, with animal research indicating that the vitamin D receptor matters for hair follicle cycling, while supplementation alone has not been shown to reverse hair loss conditions.

The sentiment within the community towards vitamin D and its impact on hair loss is that it's probably only relevant if you have a deficiency. Deficiency is more common in people who get little sun (Holick, 2007, PMID 17634462), but only a blood test for 25-hydroxyvitamin D can show whether you have it. While some individuals report improvements in hair quality and density with vitamin D supplementation, especially in cases of identified deficiency, others remain skeptical of its efficacy as a standalone treatment for hair loss. Discussions often emphasize the importance of a comprehensive approach to hair loss treatment, which may include vitamin D supplementation among other therapies like finasteride and minoxidil. The community generally agrees on the value of testing for and addressing vitamin D deficiency, not only for potential benefits to hair health but also for overall well-being.

In summary, vitamin D is needed for various bodily functions, and it may play a part in the regulation of hair growth. Its precise role in alopecia and hair loss is still under investigation, and a large review of vitamins and hair loss found no trial evidence yet that supplements help hair in people who are not deficient (Almohanna et al., 2019, PMID 30547302). Too much vitamin D can raise blood calcium (hypercalcaemia), which can cause serious harm (Marcinowska-Suchowierska et al., 2018, PMID 30294301). The US upper limit for adults is 4,000 IU a day (MedlinePlus, U.S. National Library of Medicine). Do not take high doses without a blood test and medical advice.