Hair Health from the Inside Out: Nutrition, Deficiencies and What Actually Grows Your Hair

How Hair Growth Works

Hair grows from follicles in the scalp. Each follicle cycles through three phases: anagen (active growth), catagen (transition) and telogen (resting and shedding). At any given time, 85% to 90% of your hair is in the anagen phase, which lasts 2 to 7 years depending on genetics.

Nutritional deficiencies disrupt this cycle. When the body detects insufficient resources, it prioritises vital organs over hair follicles. Hair growth slows, telogen shedding increases and the anagen phase shortens.

The significant point: nutritional hair loss takes 3 to 6 months to show up after the deficiency begins, and takes an equal or longer period to reverse after the deficiency is corrected.

Iron Deficiency: The Most Common Nutritional Cause of Hair Loss

Iron deficiency is the leading nutritional cause of hair loss in women globally. A 2006 study in the Journal of the American Academy of Dermatology found iron deficiency present in 72% of women with hair loss.

How to check: Request a serum ferritin test from your doctor. Ferritin measures your stored iron. A ferritin level below 30 ng/mL is associated with increased hair shedding in several studies, even when haemoglobin levels (the standard anaemia marker) remain normal.

Foods high in iron:

• Red meat and liver (haem iron; highest absorption rate)
• Oysters and mussels
• Lentils and kidney beans (non-haem iron; combine with vitamin C to improve absorption)
• Dark leafy greens (spinach, Swiss chard)
• Fortified breakfast cereals

Supplementation: Only supplement iron after confirming a deficiency through testing. Excess iron is harmful. Standard supplementation doses for confirmed deficiency: 65mg elemental iron daily. Results on hair density take 3 to 6 months from when levels normalise.

Biotin: What the Evidence Actually Shows

Biotin (vitamin B7) supplements dominate the hair growth market. The evidence for biotin supplementation is less robust than marketing suggests.

Biotin deficiency is uncommon in the general population. Studies showing biotin's effect on hair growth are largely conducted on people who were already biotin-deficient. For people with adequate biotin levels, supplementation produces no measurable increase in hair growth rate.

When biotin supplementation is relevant:

• Confirmed biotin deficiency (rare; associated with prolonged antibiotic use, raw egg white consumption or certain genetic conditions)
• Brittle nail and hair condition associated with a diagnosed deficiency

If your hair loss has a nutritional cause, check iron, vitamin D and zinc before attributing it to biotin deficiency.

Vitamin D and Hair Follicle Function

Vitamin D receptors are present in hair follicle cells. Low vitamin D is associated with alopecia areata (patchy hair loss) and general diffuse shedding in multiple studies.

Optimal serum vitamin D level for hair health: Above 50 nmol/L (20 ng/mL) is the minimum; above 75 nmol/L (30 ng/mL) is associated with better follicle function.

Sources:

• Sun exposure (15 to 30 minutes of midday sun on exposed skin daily)
• Oily fish (salmon, mackerel, sardines)
• Fortified dairy and plant milks
• Egg yolks
• Supplementation: 1,000 to 2,000 IU daily for general maintenance; higher doses for confirmed deficiency under medical guidance

Protein: The Building Block of Keratin

Hair is 95% keratin, a protein. Inadequate dietary protein reduces hair shaft diameter and slows growth rate.

Protein requirements for adults: The standard recommendation is 0.8g per kg of body weight daily. For active people or those recovering from deficiency, 1.0 to 1.2g per kg is more supportive of hair health.

Best protein sources for hair: Animal proteins (contain all essential amino acids including cysteine and methionine, which are direct keratin precursors). Plant proteins are adequate when combined correctly (rice and legumes together provide a complete amino acid profile).

Signs of protein deficiency in hair: Reduced diameter of individual strands, increased shedding during combing, a shift from glossy to dull surface texture over several months.

Zinc: Scalp Regulation and Follicle Repair

Zinc supports the oil glands surrounding hair follicles and plays a role in follicle cell repair and growth. Zinc deficiency is associated with both hair loss and dandruff.

Dietary sources:

• Oysters (highest zinc content of any food)
• Beef and lamb
• Pumpkin seeds
• Chickpeas and lentils

Supplementation note: Zinc supplementation above 40mg daily long-term interferes with copper absorption, which itself affects hair health. If supplementing, stay below 40mg daily and supplement with copper if exceeding 3 months.

The Supplement Summary