Dairy (Cow's Milk)

Dairy is a complex food category. The two components that most commonly cause symptoms are different: lactose (the milk sugar) and casein/whey (the milk proteins). Reactions to each have different mechanisms and different management, so identifying which one is the issue matters more than blanket dairy avoidance.

Lactose intolerance is the most common cause of dairy-related digestive symptoms in adults. Lactase, the enzyme that breaks down lactose, declines after childhood in most of the global population — rates of lactose persistence vary from above 90% in some Northern European ancestries to below 10% in some East Asian and West African groups. Symptoms include bloating, gas, diarrhea, and stomach pain hours after dairy. Many lactose-intolerant people can still tolerate hard cheeses (very low in lactose) and yogurt (partially fermented).

Milk protein reactions

Cow's milk protein allergy is distinct from lactose intolerance — it's an immune reaction to casein or whey, more common in young children but persisting in some adults. Non-IgE sensitivities to dairy proteins are also reported in conditions like eczema and chronic congestion, though the evidence base is mixed and supervised elimination-and-reintroduction is the most reliable way to test individual reactions.

Elimination and reintroduction

If you suspect dairy, a 2–4 week structured elimination followed by reintroduction is more informative than long-term avoidance. Try lactose-free dairy first to separate the sugar from the protein response. A registered dietitian can help — especially for children, during pregnancy, or for anyone with bone-health concerns, since dairy is a major source of calcium and vitamin D in most Western diets.