Lactose

What Is Lactose?

Lactose is a disaccharide sugar found naturally in milk and dairy products. It is composed of one glucose molecule and one galactose molecule linked by a beta-1,4-glycosidic bond. Lactose accounts for approximately 2–8% of the weight of fresh milk, depending on the species of mammal. Human breast milk contains about 7% lactose, while cow's milk contains roughly 4.7%.

Lactose provides an important source of energy for infants, and its presence in breast milk promotes the growth of beneficial gut bacteria and aids in calcium absorption.

Lactose Intolerance

To digest lactose, the body requires the enzyme lactase (also called beta-galactosidase), produced by cells lining the small intestine. Lactase cleaves lactose into its component monosaccharides, which are then absorbed. Many people experience a decline in lactase production after weaning — a genetically determined trait. When lactase is insufficient, undigested lactose passes into the large intestine, where gut bacteria ferment it, producing gas, bloating, cramping, and diarrhea.

Lactose intolerance is common globally, affecting an estimated 65–70% of the world's adult population. Prevalence varies widely by ancestry:

• East Asian populations: up to 90–100%
• West African populations: 70–80%
• Northern European populations: 5–15%

Managing Lactose in the Diet

People with lactose intolerance have several management strategies:

• Lactase supplements: enzyme tablets taken before consuming dairy
• Lactose-free dairy products: pre-treated with lactase enzyme
• Fermented dairy: yogurt and aged cheeses have reduced lactose content
• Plant-based alternatives: soy, oat, almond, and other non-dairy milks

Lactose intolerance is distinct from a milk allergy, which involves an immune response to milk proteins (primarily casein and whey) rather than a digestive enzyme deficiency.