In one case report published in Annals of Dermatology, a 7-year-old boy with reduced vitamin D receptor (VDR) expression in alopecia areata had new hair growth after six weeks2 of applying a topical vitamin D analog (calcipotriol) daily. Complete regrowth was found after three months of application and no hair loss was observed for the next six months.

“Vitamin D3 directly interacts with hair follicles, and when D3 levels aren’t optimal, it can decrease the volume of hair on the scalp and throughout the body,” FWTS-certified trichologist and founder of Advanced Trichology William Gaunitz previously told mindbodygreen.

Considering 41% of U.S. adults3 are insufficient in vitamin D and approximately 29%3 are straight-up deficient in the sunshine vitamin, a large chunk of the population would benefit from getting more vitamin D if they’re looking to grow out their locks (especially following periods of hair shedding).