Vitamin C, or ascorbic acid, is an essential nutrient with a well-established role in collagen synthesis, which is a key structural protein in the body, including in the skin, hair, and nails. The scientific basis for vitamin C's role in nail health primarily centers on its involvement in the formation and maintenance of healthy connective tissue. Collagen provides structural integrity to the nail bed and surrounding skin. Deficiency in vitamin C can lead to brittle nails, slow growth, and other nail abnormalities as seen in scurvy, a disease caused by severe vitamin C deficiency.

However, while the link between vitamin C and general tissue health is strong, there is limited direct clinical evidence specifically evaluating vitamin C supplementation solely for improving nail strength or growth in otherwise healthy individuals. Most available support is extrapolated from its effect on collagen production and connective tissue health. Some observational studies and case reports suggest that adequate vitamin C is necessary for maintaining normal nail appearance and strength, but robust clinical trials focused specifically on nails are lacking.

In summary, vitamin C’s use for supporting nail health is scientifically plausible due to its role in collagen synthesis and observed effects in deficiency states. Nevertheless, there is a lack of targeted research confirming that supplementation improves nail health in those without deficiency, so the evidence is rated as modest (2/5).