Iron, vitamin D, biotin: which deficiencies actually cause shedding

The supplement aisle has a hair section, and every bottle in it is sold on the same logic: hair is made of protein and needs nutrients, so more nutrients means more hair. The logic fails at the second step. A follicle that has what it needs does not grow faster or thicker when given more. Only a follicle that is short of something responds to being given that thing.

So the useful question is not which vitamins are good for hair. It is which deficiencies cause shedding, how common they are, and how to find out whether you have one. Two 2017 and 2019 reviews in dermatology journals went through the evidence nutrient by nutrient. This article follows them.

Iron: the one with real evidence

Iron deficiency is the most common nutritional deficiency in the world, and it is particularly common in menstruating women, in pregnancy, and after childbirth. It is also the deficiency with the strongest association to diffuse shedding in the literature, though the reviews are careful to say the relationship is an association and the mechanism is not fully settled.

The marker that matters is ferritin, which measures stored iron, rather than haemoglobin. A woman can have normal haemoglobin and low ferritin, and it is low ferritin that dermatologists look at in the context of shedding. A 2006 review in the Journal of the American Academy of Dermatology proposed that iron stores below a certain threshold may contribute to shedding, and many clinicians aim for ferritin well above the bottom of the laboratory reference range when a patient is shedding, though the exact target is debated.

Vitamin D: plausible, under study

The vitamin D receptor is present in the follicle and is involved in the hair cycle; mice engineered without it develop alopecia. In humans, several studies have found lower vitamin D levels in people with telogen effluvium and with alopecia areata than in controls. That is a correlation. The trials that would show whether correcting a low level improves shedding are small and inconsistent.

The reviews conclude that screening for vitamin D is reasonable in a person who is shedding, and that correcting a documented deficiency is sensible on general health grounds regardless of what it does for hair. They do not conclude that vitamin D supplementation grows hair in someone whose level is normal. Vitamin D deficiency is common in northern latitudes, in winter, and in people with darker skin, so the test is often worth doing.

Biotin: the supplement almost nobody needs

Biotin is the most heavily marketed hair supplement, and its evidence base is the thinnest of the three. A 2017 review found that every published case of biotin improving hair or nails was in a person with a documented deficiency or an underlying condition that causes one. There is no evidence that biotin improves hair in healthy people, and biotin deficiency in the general population is rare because the vitamin is in a wide range of ordinary foods and is produced by gut bacteria.

There is also a downside that the bottles do not mention. High-dose biotin interferes with a range of laboratory tests, including some thyroid and cardiac markers, and has caused misdiagnoses. If you take it and are having blood drawn, tell the person drawing it.

The rest of the aisle, briefly

The last two rows deserve emphasis. Several multi-ingredient hair supplements contain vitamin A and selenium at doses that, taken daily for months on top of a normal diet, approach the levels associated with toxicity. Both cause hair loss in excess. It is possible to buy a hair supplement and shed because of it.

What to ask for

If you are shedding and want to rule out a nutritional cause, a single blood draw covers it. Ask for ferritin, a full blood count, vitamin D, and thyroid function (TSH, and free T4 if TSH is abnormal). Zinc if your diet is restrictive. The thyroid tests are not nutritional, but thyroid disturbance is a common cause of shedding that presents identically, and it is cheap to include.

If everything is normal, you have learned something valuable: your shedding is not nutritional, and no supplement will change it. Stop buying them and look elsewhere for the cause. If something is low, correct it under guidance, and then wait. Hair that responds to a corrected deficiency responds on the hair cycle's timeline, which means three to six months before you can judge.

Where topical care fits

A nutrient applied to the scalp does not reach the follicle in any meaningful amount, and the follicle is fed from the blood, not from above. A shampoo with biotin in it is a shampoo. The supplement question is a systemic one, answered by a blood test and, if necessary, by something you swallow. Scalp care addresses the surface; it is a different job, and it should be described as one.