Biological Determinants of Hair Quality: An Integrated Review of Nutritional, Molecular, and Environmental Factors Supporting Hair Longevity

Background: Hair quality reflects interactions among hair-shaft architecture, follicular cycling, nutritional status, scalp physiology, aging, and environmental exposure. Evidence specifically concerning healthy, non-alopecic individuals remains limited. Objective: We aimed to critically review biological, nutritional, environmental, lifestyle, and nutraceutical determinants of hair quality and propose an evidence-stratified framework for hair longevity. Methods: PubMed and Scopus were searched without date restrictions through July 2026, supplemented by backward reference screening. Systematic reviews and controlled human studies were prioritized for clinical claims, whereas observational, in vitro, ex vivo, and animal studies were used to evaluate associations and biological plausibility. Main Findings: Hair-shaft integrity depends on cuticular and cortical structure, keratin-associated proteins, and disulfide cross-links. Follicular cycling involves Wnt/β-catenin, BMP, TGF-β, VEGF, and neuroendocrine pathways. Correction of documented nutritional deficiencies is appropriate, but supplementation in nutritionally replete individuals is poorly supported. UV-related shaft damage is experimentally established, whereas evidence concerning stress, sleep, pollution, microbiome modulation, botanical agents, copper peptides, and nicotinamide remains predominantly preclinical or inconclusive. Conclusions: Hair longevity is a multidimensional research construct. Independent trials using objective, standardized outcomes in non-alopecic populations are required.