Hair and nutrition
Hair growth depends on an adequate intake of protein, iron and zinc, and responds to deficiencies with a delay of several months.
Hair is made of keratin, a protein rich in sulphur-containing amino acids. Producing it is one of the most costly processes in the body in terms of energy and materials, which is why it is among the first things sacrificed in cases of restriction or deficiency.
Three nutrients stand out. Protein first: an insufficient intake directly weakens the hair fibre. Then iron, whose deficiency is one of the best-documented causes of diffuse hair loss, particularly in women. And finally zinc, which is involved in keratinisation.
The time lag is the most bewildering point. Hair follows a cycle: a growth phase lasting several years, then a resting and shedding phase. A trigger (a restrictive diet, childbirth, a high fever, a shock) tips hairs into the shedding phase early, but this only becomes visible two to four months later. So people look for the cause at the wrong time.
A word on biotin (vitamin B7), found everywhere in hair supplements: deficiency is exceptional in someone who eats normally, and supplementation has a proven effect only where a deficiency is confirmed. It does, however, interfere with several blood tests, including thyroid tests, so mention it before a blood test.