If your hair seems thinner, you’re probably wondering what’s causing it.

Is it stress? Using hot tools too much? Your diet?

Well, those things can definitely lead to breakage and damage, which will make your hair look thinner. However, one of the most likely causes for the actual loss of hair strands is a vitamin deficiency.

Now, you may have already heard about vitamins for hair that include ingredients like biotin. And those are helpful. But biotin can’t help if the true cause of your hair loss is this common vitamin deficiency.

So what is it?

Vitamin D. And here’s why it’s so critical for hair.

Your body needs vitamin D to regulate hair growth and produce keratin. And keratin is responsible for keeping your hair and nails strong and healthy. So when your body doesn’t have enough vitamin D, it all falls apart. Your hair follicles have trouble regulating hair growth and shedding, and pretty soon, your hair is noticeably thinner.

In fact, vitamin D makes such a big difference that it’s no surprise that a deficiency is linked to several different types of hair loss. For instance, one scientific review found low levels of vitamin D in people who experience:

1. Excess hair shedding
2. Alopecia areata, where hair falls out in clumps
3. Female pattern hair loss

Another study found that vitamin D deficiencies could worsen hair loss over time. And a 2016 study found that among younger people with hair loss, women in particular showed greater vitamin D deficiency.

Is it reversible?

The good news is that if you’re deficient, taking vitamin D supplements can help thicken your hair. The best thing to do is to get your vitamin D levels tested, then work with your healthcare provider to get your level in the healthy range.

If you’re very low in vitamin D, your doctor can prescribe a high-dose vitamin D pill, which you’ll take over a short period of time to boost your levels.

If your levels are fine or just slightly slow, your doctor can advise you on the right amount of vitamin D to take. The amount should be based on your lifestyle, skin type, where you live, and health history.

Also, to optimize your levels of vitamin D, you need to make sure you’re getting enough magnesium. That’s because magnesium helps regulate levels of vitamin D.

In fact, one study even found that when participants were low on magnesium, they couldn’t raise their levels of vitamin D ... even when they were taking really high doses of vitamin D supplements!

So magnesium is key. And as it turns out, many people are magnesium-deficient, too. So along with vitamin D, you could take a magnesium supplement. Another option is to make sure you’re eating a lot of magnesium-rich foods, like dark leafy greens, beans, whole grains, dark chocolate, fatty fish like salmon, nuts, bananas, and avocados.

So give these tips a try, and let me know how it goes. Just don’t wait too long to get to the bottom of your hair loss. The sooner you address it, the more likely you can prevent permanent damage.