Lactase (LCT) is a brush border glycosidase that hydrolyzes lactose, the primary disaccharide in mammalian milk, into glucose and galactose. The mature enzyme comprises two catalytic domains—one with preference for hydrophilic substrates (lactose and cellobiose) and another for hydrophobic substrates (phlorizin and glycosylceramides)—allowing it to process diverse β-glycosidic linkages. The protein is synthesized as a single-chain precursor that undergoes proteolytic cleavage in the endoplasmic reticulum and Golgi apparatus, with a membrane-anchoring domain enabling surface localization. Lactase activity typically declines after weaning; persistence into adulthood depends on genetic variation at the LCT locus. The C/T-13910 SNP promotes lactase persistence in Europeans 1, while distinct African variants (G/C-14010, T/G-13915, C/G-13907) independently enhance promoter activity, exemplifying convergent evolution driven by dairy domestication 1. Primary lactose intolerance—the gradual decline of lactase activity manifesting ages 1–5 years—correlates with homozygous loss-of-function genotypes; in children older than 6 years, LCT 13910CC and 22018GG polymorphisms significantly predict lactose malabsorption 2. Adults with lactase nonpersistence exhibit reduced milk and dairy consumption, leading to lower vitamin D and calcium levels, with increased osteoporosis and osteopenia risk 34. Congenital lactase deficiency represents the severe end of the LCT disease spectrum.