Nails that split, cuts that
won’t close. The honest story isn’t biotin.

The biotin question — what the evidence actually shows

Here’s the genuinely honest version of this story. A frequently cited 2007 review in Cutis examined biotin, vitamin E, vitamin C, vitamin A, silicon, zinc, iron, copper, selenium, and vitamin B12 for nail health specifically. Its conclusion was blunt: no evidence supports supplementation with any of these nutrients for improving nail health in well-nourished people — except biotin at 2.5mg daily and silicon at 10mg daily, which showed some benefit specifically for brittle nail syndrome. A 2018 review in the Journal of the American Academy of Dermatology reached a similar conclusion, adding an important detail: the studies showing biotin’s benefit were small, uncontrolled, and never confirmed whether participants were actually deficient beforehand — meaning even biotin’s positive evidence is thinner than the marketing built on top of it suggests.

The adequate intake for biotin is just 30 micrograms a day — an amount most people get comfortably from eggs, nuts, seeds, and a normal varied diet. Most commercial “hair, skin & nails” supplements contain 2,500 to 10,000 micrograms — 80 to 300 times that amount. True biotin deficiency does happen, but typically in specific situations: certain anti-seizure medications, long-term alcohol use, bariatric surgery, or rare genetic conditions. Outside those, reaching for more biotin when nails or skin aren’t cooperating is, honestly, treating a symptom with the wrong nutrient.

So if biotin’s evidence is this limited, what’s actually going on when nails split and cuts heal slowly? The more useful, more mechanistically grounded answer is this: nails and skin are both largely built from collagen, and three specific nutrients are directly, unambiguously required to build and stabilize it — vitamin C, zinc, and copper.

Vitamin C — the collagen story

Collagen is the single most abundant protein in your body, and it forms the structural scaffolding of skin, nail beds, blood vessels, and connective tissue everywhere. Building it correctly requires two specific enzymes — prolyl hydroxylase and lysyl hydroxylase — and both require vitamin C to function. These enzymes add hydroxyl groups to the amino acids proline and lysine within the collagen chain, which is what allows collagen to fold into its stable, load-bearing triple-helix shape. Without enough vitamin C, this step doesn’t happen properly, and the resulting collagen is structurally weaker — less able to hold a wound closed, less able to give a nail bed the support it needs.

This isn’t theoretical. It’s the entire mechanism behind scurvy, the historic disease of vitamin C deficiency, whose defining symptoms — bleeding gums, fragile skin, wounds that won’t close — are a direct, visible consequence of defective collagen. You don’t need to be anywhere near scurvy for a milder version of the same shortfall to show up as unusually slow healing.

Zinc — cell division and wound closure

Zinc’s role is different from vitamin C’s, and just as necessary. Zinc is a required cofactor for the enzymes involved in cell division — meaning the skin cells (keratinocytes) and connective tissue cells (fibroblasts) that need to multiply rapidly to close a wound or rebuild nail matrix tissue simply can’t do so efficiently without it. Zinc also enables zinc-dependent matrix metalloproteinases, enzymes that clear damaged tissue and remodel the wound bed during healing.

Worth being precise about the evidence here too: the clearest benefit shows up specifically in people who are zinc-deficient. A published meta-analysis found that zinc supplementation in zinc-deficient patients accelerated wound healing by roughly 9 days on average — a real, meaningful effect. In people who already have adequate zinc status, additional zinc hasn’t been shown to speed things up further. This is a genuinely honest way to think about zinc: it’s not a universal accelerant, it’s what happens when a real gap gets closed.

Copper — the cross-linking piece nobody mentions

Copper is a required cofactor for lysyl oxidase, the enzyme that cross-links collagen and elastin fibers into strong, stable connective tissue — the same mechanism we’ve covered elsewhere on this site in the context of cartilage and joints. It applies just as directly here: collagen that’s been correctly hydroxylated (vitamin C’s job) still needs to be properly cross-linked (copper’s job) to reach its full structural strength. This is part of why copper is deliberately paired with zinc in a properly balanced formula, rather than left out — the two minerals compete for the same absorption pathway, and sustained zinc intake without adequate copper is a well-documented way to quietly deplete it.

Where iron honestly fits in

Worth including transparently, even though it complicates a tidy story: iron is also a cofactor for prolyl hydroxylase, alongside vitamin C — meaning it plays a genuine, direct role in the same collagen-stabilizing reaction. Iron deficiency is also classically associated with koilonychia, the medical term for spoon-shaped nails that curve upward at the edges — a well-documented, specific clinical sign, distinct from ordinary brittleness.

This isn’t a case for adding iron to a multivitamin by default. Iron needs vary too individually — by diet, sex, and life stage — to assume into a universal formula, which is exactly why EVO HOMINUS’s multivitamin is deliberately iron-free. If your nails genuinely look spoon-shaped rather than just brittle, that’s a specific, real signal worth checking with an actual ferritin blood test — not a reason to start supplementing iron blind.

Other common, often more direct causes

When this is worth seeing a doctor about

What to actually do about it