Low iron stores, low vitamin D and too little protein or calories are the nutritional deficiencies most often linked to hair loss in both men and women. They usually show up as diffuse shedding a few months after the shortfall begins, and the hair tends to recover once the deficiency is found and corrected. Supplements help when a blood test shows you are low; extra nutrients on top of normal levels do not add growth.
Key takeaways
- Iron (measured as ferritin), vitamin D, protein and, less often, zinc are the deficiencies with the most research linking them to shedding.
- Deficiency-related shedding typically starts about 2–3 months after the shortfall, because follicles pushed into rest hold their hair for that long before releasing it.
- Studies have defined iron deficiency anywhere from a ferritin of 15 to 70 ng/mL, and some hair specialists aim for above 40 ng/mL in people who are shedding.
- Biotin helps hair only in people who are deficient, which is rare, and high-dose biotin supplements can distort some lab tests, including thyroid and troponin.
- Too much vitamin A or selenium can itself cause hair loss, so stacking "hair vitamins" on a normal diet is not risk-free.
Why is hair so sensitive to nutrition?
Because a growing hair follicle is one of the fastest-dividing tissues in the body. The cells at the base of the follicle build keratin around the clock, and that takes a steady supply of amino acids, iron for the enzymes involved in cell division, zinc for protein synthesis and vitamin D for the signaling that keeps the follicle cycling.
Hair is also a low priority: when supply runs short, the body protects essential organs first. The review by Almohanna and colleagues (2019, Dermatology and Therapy) describes micronutrients as major players in the follicle cycle and deficiency as a possible modifiable risk factor for non-scarring hair loss.
Timing matters too. A deficiency pushes growing hairs into the resting phase, and they shed about 2–3 months later. That pattern is called telogen effluvium, and it means today's shedding usually reflects what your body was short of a season ago.
Which deficiencies are linked to hair loss?
Four come up again and again in the research. Here is how each one connects to hair, where it comes from in food, and when supplementing makes sense.
| Nutrient | Link to hair | Good food sources | Supplement when |
|---|---|---|---|
| Iron (ferritin) | Most studied; low stores linked to diffuse shedding and pattern loss in some studies | Red meat, poultry, fish, lentils, beans, fortified cereals (with vitamin C for absorption) | Ferritin is low on a blood test and the cause has been looked into |
| Vitamin D | Receptors in the follicle; lower levels found in some people with shedding | Sunlight, oily fish, eggs, fortified milk | 25-hydroxyvitamin D is low |
| Protein and calories | Severe restriction triggers shedding 2–3 months later | Meat, fish, eggs, dairy, soy, legumes | Diet has been restricted, or after rapid weight loss |
| Zinc | True deficiency causes hair loss; data in common shedding are mixed | Shellfish, meat, seeds, nuts, whole grains | A deficiency is measured, or absorption is impaired |
To find out which applies to you, the core blood panel for shedding is ferritin with a full blood count, thyroid function (TSH) and 25-hydroxyvitamin D, plus zinc if your diet or absorption is a concern. Our guide to blood tests for hair loss covers what each one finds and how to read the results.
Iron and ferritin: what level do hair specialists look for?
Many hair specialists want ferritin above about 40 ng/mL in someone who is shedding, and some aim for 70 ng/mL. Ferritin measures stored iron, and stores run down before hemoglobin drops, so a standard blood count can look fine while ferritin is low.
The Almohanna review lays out the range: studies have defined iron deficiency (without anemia) anywhere from a ferritin of 15 to 70 ng/mL, and authors treating telogen effluvium have recommended keeping ferritin above 40 or 70 ng/mL. The review also notes that the lower reference ranges for women have been questioned, because the populations used to set them included many iron-deficient women. A lab will often flag ferritin as low only near the anemia cut-off, so ask for the number, not just "normal."
The cause matters as much as the level. In women before menopause, heavy periods and pregnancy are the usual reasons. In men and in women after menopause, the review by Trost and colleagues (2006, Journal of the American Academy of Dermatology) points out that the most common causes are blood loss from the gut and poor absorption, which a doctor should investigate. That is why low iron in a man always deserves a look at why.
Does vitamin D deficiency cause hair loss?
It is linked to hair loss, and correcting a deficiency is recommended. Vitamin D works through a receptor in the follicle, and people born with faulty versions of that receptor lose their hair, so the mechanism is real.
For everyday shedding and pattern loss, the Almohanna review describes conflicting results: some studies found lower vitamin D in people with telogen effluvium or pattern hair loss, others found no difference. Most authors still agree on testing 25-hydroxyvitamin D and supplementing when it is low, which is worthwhile for overall health too.
What do crash diets and low protein do to hair?
A sharp drop in protein or calories can trigger heavy shedding about 2–3 months later. Hair is made of protein, and when intake falls hard enough, the body diverts amino acids away from the follicle.
The classic picture is shedding a few months after a very low-calorie diet, an illness that stopped you eating, bariatric surgery, or rapid weight loss on a weight-loss medication. The follicles are intact, so once intake recovers the shedding settles over the following months. Rapid weight loss can also drain iron and zinc at the same time.
How much protein is enough? The US recommended dietary allowance for adults is 0.8 g per kilogram of body weight a day. For adults over 65, the PROT-AGE study group (Bauer 2013) recommends at least 1.0–1.2 g/kg a day, and more for people who are active or recovering from illness. If shedding started around three months into a weight-loss plan, protein is the first thing to check.
Do zinc and biotin help hair loss?
Only when you are deficient. The evidence supports correcting a measured shortfall, not topping up normal levels.
Zinc
Zinc deficiency causes hair loss along with nail changes and slow wound healing, and it is more likely with a restricted diet, gut conditions or after weight-loss surgery. In common shedding, the Almohanna review found the data inconsistent and does not recommend routine zinc screening. Long-term high-dose zinc can lower copper levels, according to the NIH Office of Dietary Supplements, so it is best taken for a reason and for a set period.
Biotin
Biotin is the most marketed hair supplement, and it helps people who are deficient. True deficiency is rare: biotin is plentiful in ordinary food, and the Almohanna review notes that severe deficiency has never been reported in a healthy person eating a normal diet. The cases where biotin improved hair all involved an underlying cause of deficiency.
The bigger practical issue is lab testing. Many blood tests use a biotin-based chemistry, and high-dose biotin in the blood can push results falsely high or low. In a small study by Li and colleagues (2017, JAMA), six healthy adults took a typical supplement dose for a week, and 9 of 23 biotin-based assays gave distorted results, including thyroid tests. The FDA has also warned about falsely low troponin results, a heart test. If you take a hair supplement, tell your doctor and ask whether to pause it before blood tests.
When do supplements help, and how long until hair recovers?
Supplements help when a test shows you are low and you correct it: iron for low ferritin, vitamin D or zinc for a documented deficiency, more protein for a diet that has been short of it. Expect shedding to slow first, then new growth that takes months to see, because hair grows about 1 cm (½ inch) a month.
A fair window to judge a corrected deficiency is three to six months. More of a nutrient you already have enough of does not make follicles work harder, and the Almohanna review notes that excess vitamin A and excess selenium are themselves causes of hair loss. If shedding coincided with a stressful period rather than a dietary change, our article on whether stress can cause hair loss covers that trigger, and is hair loss reversible? sorts out which causes recover fully.
Supporting your scalp while hair regrows
Fix the deficiency first; a scalp routine then supports the scalp while new hair comes through. If shedding continues after levels are back to normal, and the thinning is concentrated at the temples and crown in men or along the part in women, the cause is more likely pattern hair loss, which has its own evidence base.
A simple daily step during recovery is the Folliq Rosemary & Batana Hair Oil, a 3-in-1 blend of black castor seed, batana kernel and rosemary leaf oils. About 3 drops for each of five scalp areas, massaged in with your fingertips, keeps the routine light and non-greasy, and our guide on how to build a scalp care routine shows where it fits. Take photos in the same light each month so you can see the change as it happens.
FAQ
Can low iron cause hair loss even without anemia?
It can. Ferritin, the stored form of iron, drops before hemoglobin does, and several studies have linked low ferritin without anemia to diffuse shedding. Whether to treat iron deficiency without anemia is a clinical judgment, so discuss your ferritin number with your doctor.
What vitamin deficiency causes hair loss in men?
The same ones as in women: iron, vitamin D, protein and zinc. Iron deficiency is less common in men, so when it does turn up, doctors look for a cause such as blood loss from the gut. Rapid weight loss and restrictive diets are common triggers in both sexes.
Will hair grow back after correcting a deficiency?
Usually, yes. Deficiency-related shedding leaves the follicles intact, so once levels recover, the hair cycle restarts. Shedding tends to settle first, and visible regrowth follows over several months.
Should I take a hair supplement just in case?
It is better to test first. Supplements help when you are low, but extra vitamin A, selenium, zinc or iron on top of normal levels can cause problems of their own. High-dose biotin can also distort blood tests, so tell your doctor if you take one.
How long does it take for hair to recover after iron deficiency?
Expect three to six months before you can judge the change. Iron stores take time to rebuild, shed hairs need a new growth cycle to start, and new hair grows about 1 cm a month.
When to see a doctor
- Shedding that is sudden, patchy, or leaves smooth bald areas. That is not a typical deficiency picture.
- Thinning alongside tiredness, feeling cold, weight change, heavy periods or palpitations, which can point to a thyroid or iron problem.
- Low ferritin that does not rise on supplements, or keeps falling.
- Any iron deficiency in a man or a woman after menopause, which needs its cause investigated.
- Hair loss after bariatric surgery or during rapid medical weight loss.
- Before starting iron on your own; in someone who is not deficient, it can build up.
Folliq products are cosmetic scalp-care products, not medical devices or medicines, and are not intended to diagnose, treat, cure or prevent any disease. The studies discussed here used clinical devices and preparations, which is why every Folliq product comes with 90 days to try it for yourself. If your hair loss is sudden, patchy or painful, see a dermatologist.
References
- Almohanna HM, Ahmed AA, Tsatalis JP, Tosti A. The role of vitamins and minerals in hair loss: a review. Dermatology and Therapy. 2019;9(1):51–70. PubMed 30547302
- Trost LB, Bergfeld WF, Calogeras E. The diagnosis and treatment of iron deficiency and its potential relationship to hair loss. Journal of the American Academy of Dermatology. 2006;54(5):824–844. PubMed 16635664
- Bauer J, Biolo G, Cederholm T, et al. Evidence-based recommendations for optimal dietary protein intake in older people: a position paper from the PROT-AGE Study Group. Journal of the American Medical Directors Association. 2013;14(8):542–559. PubMed 23867520
- Li D, Radulescu A, Shrestha RT, et al. Association of biotin ingestion with performance of hormone and nonhormone assays in healthy adults. JAMA. 2017;318(12):1150–1160. PubMed 28973622
- US Food and Drug Administration. Biotin interference with troponin lab tests: assays subject to biotin interference. fda.gov
- NIH Office of Dietary Supplements. Zinc: fact sheet for health professionals. ods.od.nih.gov