How nails grow: the biology
Nails grow from the nail matrix — the tissue beneath the base of the nail under the cuticle. Fingernails grow approximately 3–4 mm per month; toenails grow more slowly (1–1.5 mm). Nail growth is not uniform: the dominant hand's nails grow faster, summer growth outpaces winter, and growth slows with age. The nail plate (visible nail) is composed almost entirely of keratin — a protein made of amino acids including cysteine, serine, and methionine. Nail hardness is determined by sulphur bridges between keratin chains. The cuticle (eponychium) is the sealed tissue between the nail and surrounding skin. It forms a waterproof barrier that prevents bacteria, yeast, and water from reaching the nail matrix. Damaging or removing the cuticle repeatedly increases infection risk and can disrupt nail plate formation, causing ridges and uneven growth.
Why nails become brittle, peeling, or slow-growing
The most common causes of brittle, peeling nails are not nutritional deficiencies — they are environmental. Frequent water exposure (dishwashing, handwashing, prolonged baths): nails absorb and release water repeatedly. This cyclic swelling and shrinking weakens the keratin bonds. Nail polish remover overuse: acetone strips oils and moisture from the nail plate. Using acetone-containing remover more than twice per week significantly dehydrates nails. Gel, acrylic, or SNS nails: physically damage the nail surface during removal. UV gel curing can also cause photo-onycholysis (nail plate separation) with prolonged exposure. Low thyroid function (hypothyroidism): brittle nails are a known symptom. Iron deficiency anaemia: nail changes (koilonychia — spoon-shaped nails) indicate severe deficiency. Nutritional deficiencies (biotin, iron, zinc, protein) are genuine causes, but are far less common than the environmental factors listed above. Before supplementing, consider environmental causes first.
Biotin for nail growth: what the evidence actually shows
Biotin (vitamin B7) is the most marketed supplement for nails. The evidence: A 2017 review in the Journal of Drugs in Dermatology found that biotin supplementation (2.5 mg/day) improved nail firmness in people with documented biotin deficiency or brittle nail syndrome. Improvement took 6 months. The key caveat: biotin deficiency is uncommon in people with a balanced diet. Most people taking biotin supplements do not have a deficiency, and supplementation does not meaningfully accelerate growth in non-deficient individuals. Biotin at 5,000–10,000 mcg (5–10 mg) doses is frequently marketed but the evidence does not support these high doses over standard 2.5 mg doses. If you eat a varied diet, biotin supplementation is unlikely to produce dramatic nail changes. If your nails are brittle despite good nutrition, investigate environmental causes first.
Topical nail strengtheners: how they work
Topical nail treatments work by depositing bonding agents on the nail surface that cross-link keratin chains, making the nail plate harder and more resistant to peeling. Active ingredients in nail strengtheners: Formaldehyde (or formalin): the original nail hardener. Highly effective but controversial — can cause skin sensitisation and nail brittleness with overuse. Now largely replaced by alternatives. Dimethyl urea: a formaldehyde-free keratin cross-linker. Effectively hardens the nail plate with less irritation risk. Keratin hydrolysates: small keratin fragments that bond to the nail surface, filling microcracks and improving tensile strength. Vitamin E acetate: antioxidant that protects keratin from oxidative damage and helps maintain flexibility. Calcium: deposited into the nail plate to reinforce mineral structure. For best results, apply a topical strengthener daily or every other day and avoid removing it between applications — continuous protection is more effective than intermittent use.
Cuticle care: the overlooked foundation
Cuticle health is the most underrated aspect of nail growth. Healthy cuticles protect the nail matrix from infection and produce uniform nail plates. Do not cut cuticles — push them back gently after a shower (when soft) using a wooden or rubber-tipped cuticle pusher. Cutting cuticles removes the waterproof seal and increases infection risk. Apply cuticle oil daily: jojoba oil most closely mimics the skin's sebum and penetrates the nail fold effectively. Vitamin E oil, sweet almond oil, and argan oil also work well. The goal of cuticle oil is to maintain flexibility and prevent dry, cracked cuticle skin that can harbour bacteria and disrupt nail matrix function. A quality cuticle oil applied twice daily shows measurable results in cuticle condition within 3–4 weeks.
Nail fungal infections: causes, treatment, and prevention
Nail fungal infections (onychomycosis) affect approximately 10% of the adult population and are more common with age. Symptoms: nail thickening, yellowing, brittleness, and separation from the nail bed. Dermatophytes (Trichophyton rubrum most commonly) are the usual cause; Candida and non-dermatophyte moulds cause some cases. Treatment: over-the-counter antifungal nail solutions (tea tree oil, tolnaftate, ciclopirox) can help mild cases but penetrate poorly. A 2015 study found tea tree oil (100% concentration applied twice daily for 6 months) improved appearance in 60% of participants — comparable to tolnaftate cream but inferior to prescription antifungals. Prescription options (oral terbinafine, itraconazole) are significantly more effective but require medical consultation. Prescription oral antifungals clear infection in 70–90% of cases. Prevention: keep nails dry, wear breathable footwear, avoid sharing nail tools, use antifungal powder in shoes, replace nail tools that contact infected nails.