Vitamin D is important but not the whole story (unfortunately)
Vitamin D clearly has a role in normal hair growth. But does supplementing vitamin D (in those with low levels) cause it to come back? No. I have never ever witnessed convincing evidence of such a direct link. It is absolutely true that low vitamin D is associated with some types of hair loss. This includes AGA (in some studies only) and alopecia areata.
Vitamin D supplementation does not promote hair growth for most
I do not believe that supplementing vitamin D alone will promote hair growth for the vast majority of people with the story you have just shared. Do I believe supplementing D is a good idea when levels are low? Yes. It has other health benefits and may help various hair growth treatments work better. That too has yet to be proven.
I appreciate this is a tough concept and doesn't always make a great deal of sense at first. If low vitamin D has a role in hair growth and my levels are low... it would only make sense that supplementing would help! Not quite.
A simple analogy
I often use a simple analogy. We all know that an engine is essential for a car to work. If you don't have an engine, you are not going to get too far. But consider the car that you see on an abandoned lot. It has problems with the engine sure. But there are problems with numerous aspects in the car. Replacing the engine is not going to allow you to drive the car off that lot. No matter how many engines you put in that car or how well engineered that engine is. Other things need to be addressed as well.
No one can argue that vitamin D is not important for hair. Babies who can't make vitamin D or who have problems with the Vitamin D receptor or signaling - have hair loss. So clearly it's important just like an engine in the car is important.
But giving back vitaminD to adults with hair loss or giving back a new engine to a car on an abandoned lot does not solve the main issue.
I always supplement vitamin D when low (and I would recommend fixing an engine on a car that does not work) but it is far too big of a jump to conclude that that alone is sufficient to grow hair (or get that car running again to drive off the lot).
Dr. Jeff Donovan is a Canadian and US board certified dermatologist specializing exclusively in hair loss. To schedule a consultation, please call the Vancouver office at 604.283.9299