What you eat and your hair: the honest version

Hair is a low-priority tissue. It is not required for survival, so when the body is short of something, hair is among the first places it economises. This is why hair often reflects a nutritional problem before anything else does.

The important consequence: correcting a genuine deficiency can help hair noticeably. Adding more of a nutrient you already have enough of does nothing at all. There is no bonus tier. Almost the entire hair supplement market depends on people not knowing this.

Protein

Hair is made of keratin, a protein. Sustained inadequate protein intake affects hair, and this shows up in people eating very restrictively or recovering from illness. Most people eating a normal mixed diet get enough.

Iron

The nutrient most consistently linked to hair shedding, and a common deficiency, particularly in women who menstruate and in people eating little or no meat. Low ferritin is associated with increased shedding, and correcting it often helps. This is worth a blood test rather than a guess, because iron supplementation without a deficiency is pointless and excess iron is harmful.

Zinc

Deficiency is associated with hair loss. Genuine deficiency is not common in a varied diet. Supplementing heavily without cause can interfere with copper absorption.

Vitamin D

Widely deficient, including in sunny countries, largely because of indoor life and covered skin. The link to hair is less direct than iron but deficiency is worth knowing about for reasons well beyond hair.

B vitamins, including biotin

Biotin is the headline ingredient in most hair supplements. Genuine biotin deficiency is rare, and where it exists, supplementation helps. Where it does not, evidence of benefit is weak. Biotin also interferes with several common blood tests, including thyroid panels, so mention it if you are having bloods taken.

A reasonable position: test before supplementing, supplement what is actually low, and treat combination hair vitamins with suspicion. They typically contain modest amounts of many things, most of which you were not short of, at a price that reflects marketing rather than contents.

Rapid weight loss is a well-recognised trigger for telogen effluvium, a temporary condition where a large number of follicles shift into the resting phase at once. Shedding usually becomes noticeable two to three months after the trigger, which is long enough that people rarely connect the two.

It typically recovers on its own once nutrition stabilises, though it takes months. The relevant lesson is preventative: very restrictive eating has a cost that arrives on a delay.

• Eat enough protein across the day rather than concentrating it in one meal.
• If you shed heavily and eat little red meat, ask for a ferritin test rather than a general iron test.
• Treat sudden diffuse shedding as a signal to look back two to three months for a trigger.
• Be sceptical of any supplement marketed primarily for hair.
• Remember the timeline. Nutritional correction shows in hair over months, not weeks.

None of this is medical advice, and deficiencies should be diagnosed and treated by a doctor rather than guessed at from an article.