Iron, vitamin D, zinc, B12: deficiencies and hair

In the pharmacy aisle, the “hair” boxes line up: iron, zinc, biotin, every vitamin you can think of. Standing there, one question nags at you: what if my hair is falling out because I’m missing something? It’s a fair question. Low biotin, iron, protein or zinc can cause noticeable hair loss¹. But there’s a big gap between “it can happen” and “a capsule will fix it”.

Here’s what’s established for each nutrient, and why the blood test comes before the box.

Can a deficiency make your hair fall out?

Yes, and the most typical form is diffuse shedding all over the head, called telogen effluvium. Dermatologists list deficiencies among its triggers, alongside stress, illness or an overly strict diet⁴.

Iron is the first suspect. It’s the most widespread deficiency in the world, and it contributes to this type of hair loss³. For the other nutrients, studies often contradict each other, and that’s where the promises on the box go beyond what we actually know.

Iron

Low iron is common in women and can cause diffuse shedding. It often comes from heavy periods, pregnancies close together or a diet low in iron. Less often, it points to hidden bleeding that needs looking into². That’s one more reason not to fix it on your own: your doctor also looks for why it’s low.

The main measure is ferritin, your body’s iron stores. And this is where it gets complicated. A team at Duke University in the US compared 381 women being followed for hair loss with 76 women with no hair loss at all⁵.

The authors conclude that iron deficiency is common in women, but no more common in those losing their hair⁵. The study has limits: a single center, white women only, and a fairly small group without hair loss. Above all, it doesn’t say whether correcting iron makes hair grow back; the authors say so themselves⁵.

A 2019 review of vitamins and minerals reaches the same point: the link between low ferritin and hair loss has been debated for years, and there’s little evidence on what iron supplements do for this kind of shedding³. That doesn’t mean you should ignore a deficiency: iron-related shedding can come back as long as the deficiency isn’t corrected⁶. So dermatologists recommend correcting low iron when a blood test shows it⁴.

Vitamin D

Low vitamin D is often found in people with alopecia areata, a disease that makes hair fall out in patches. For diffuse shedding and female pattern hair loss, the data conflict³. Most authors agree on one thing: give vitamin D to people who are losing hair and are low in it³. It’s one of the tests the French Society of Dermatology lists for a checkup².

Zinc

A proven zinc deficiency makes hair fall out, and it grows back once the deficiency is corrected³. But in people with diffuse shedding or pattern hair loss, the zinc results don’t all point the same way, and testing it routinely isn’t recommended³. In a US study of 115 people with telogen effluvium, 9.6% were low in zinc; the study had no comparison group³.

Vitamin B12 and folate

In that same study of 115 people, 2.6% were low in vitamin B12 and none were low in folate³. The data are too thin to recommend taking them for hair loss³. B12 is still a useful test: low iron or B12 can make shedding worse, including in a woman with pattern hair loss⁷.

Biotin

It’s the star of hair supplements. A real biotin deficiency makes hair fall out, but it’s rare: food provides enough, and severe deficiency has never been described in a healthy person eating a normal diet³. Nothing shows that taking it without a deficiency makes hair grow³.

It also has a little-known downside: it skews some blood tests, up or down, including thyroid and hormone tests. The US Food and Drug Administration (FDA) reported a patient whose heart test, skewed by biotin, came back falsely reassuring; his heart attack was missed and he died³. If you take it, tell your doctor before a blood test.

Too much is a problem too

Supplements aren’t risk-free for your hair either. Taking large amounts of vitamin A or selenium is toxic and can make your hair fall out¹.

Selenium is a striking example. A liquid supplement that contained 200 times the dose on its label caused severe hair loss in most of the people who took it³. The European Food Safety Authority (EFSA) actually used hair loss as the marker to set the upper limit not to exceed: it’s one of the first signs of too much⁸. According to EFSA, adults are unlikely to go over it unless they regularly take high-dose selenium supplements or often eat Brazil nuts⁸.

Stacking supplements without knowing what you’re missing means risking missing the real cause, or even adding a new one.

Blood test first

If you’re losing hair and wondering about this, the right step is an appointment with your doctor. A blood test can look for low iron, low vitamin D and B12, and a thyroid problem, which also makes hair fall out². British dermatologists list iron and the thyroid among the causes to rule out⁶.

Your doctor picks the tests that make sense for you. If one shows a deficiency, they’ll tell you how to correct it and for how long. Without a deficiency, nothing shows that a supplement helps. As for food, the British leaflet simply recommends a balanced diet, with foods rich in iron and protein, plus fruit and vegetables⁶.

Tracking the effect of a correction

If a deficiency is corrected, regrowth doesn’t show in a few days. After diffuse shedding, it can take many months for your hair to get its volume back⁶. In the meantime, the mirror won’t help: the light changes, the angle changes, and you end up seeing only what’s missing.

The most reliable way is to take the same photo every week: same room, same light, same distance, hair dry. Aim at the crown and the part, where diffuse shedding shows best.

That’s what Calvap does: your last photo shows up as a see-through overlay so you can line up in the same spot, and your photos sit on a timeline. Note the date of your blood test and when the correction started in the journal. Over the months, you’ll see whether the shedding is slowing down, without relying on a feeling. Your photos stay on your phone.