Why does drinking milk on an empty stomach in the morning cause diarrhea?

Experiencing diarrhea after drinking milk on an empty stomach in the morning is commonly linked to lactose intolerance. Lactose, the natural sugar found in milk and dairy products, requires the enzyme lactase—produced in the small intestine—to be properly digested. In individuals with lactose intolerance, lactase production is reduced or absent, leading to undigested lactose passing into the large intestine. There, gut bacteria ferment it, producing gas, short-chain fatty acids, and osmotically active byproducts that draw water into the colon—resulting in bloating, cramping, flatulence, and diarrhea.

Drinking milk on an empty stomach may exacerbate symptoms because gastric emptying is faster in the fasting state, delivering lactose more rapidly to the small intestine where insufficient lactase is available. Additionally, without other foods present to slow digestion or buffer gastric acidity, lactose reaches the colon more quickly and in higher concentration, intensifying fermentation and osmotic effects.

It’s important to distinguish lactose intolerance from a milk allergy, which involves an IgE-mediated immune response to milk proteins (e.g., casein or whey) and can cause hives, wheezing, vomiting, or even anaphylaxis—not just gastrointestinal symptoms. A healthcare provider can confirm lactose intolerance through clinical evaluation, hydrogen breath testing, lactose tolerance blood testing, or a supervised lactose elimination challenge.

Management strategies include reducing or eliminating lactose-containing dairy, choosing lactose-free milk or plant-based alternatives (e.g., almond, soy, or oat milk), using over-the-counter lactase enzyme supplements before consuming dairy, and ensuring adequate calcium and vitamin D intake through fortified foods or supplementation if dietary restrictions are long-term.