Biotin (Vitamin B7) is the essential cofactor for the carboxylase enzymes that build keratin — the structural protein that forms approximately 95% of the hair shaft, the protective outer layer of skin, and the nail plate. It also supports fatty acid synthesis, which maintains the skin’s moisture barrier. When biotin is sufficient, hair grows thicker and more resistant to breakage, nails are stronger and smoother, and skin retains moisture more effectively. Clinical studies show biotin can increase nail plate thickness by up to 25%.