Breast milk is another term for human milk, the natural first food for infants. It contains proteins, fats including long-chain fatty acids, the milk sugar lactose, vitamins, minerals and water in proportions matched to a newborn’s needs, together with antibodies, especially secretory IgA, and oligosaccharides that help protect against infection and shape the infant gut microbiota. Its composition is not fixed: colostrum, the first milk, is rich in immune factors, and mature milk continues to adapt over the months of lactation. It remains the reference standard against which infant formulae are judged.