Nails are primarily composed of keratin, a fibrous protein that accounts for more than 90% of their structure. This protein is formed from a sequence of essential amino acids, among which cysteine plays a critical role. Cysteine’s sulfur-containing side chain enables the formation of disulfide bonds, which give keratin its rigidity and resilience. This molecular architecture provides nails with their strength, durability, and resistance to external stress. Maintaining optimal keratin synthesis is therefore essential for nail integrity, growth, and overall appearance, making it a key target in nutritional strategies for nail health.