How is the stool pH affected in lactose intolerance?

In cases of lactose intolerance, the ability to properly digest lactose (a sugar found in milk and dairy products) is impaired due to a deficiency of lactase, the enzyme responsible for breaking down lactose into glucose and galactose. When lactose is not adequately digested, it passes into the large intestine where it is fermented by bacteria. This fermentation process produces short-chain fatty acids and gases, which can result in gastrointestinal symptoms such as bloating, diarrhea, and abdominal cramping.

The presence of undigested lactose in the bowel leads to an increase in the production of acidic metabolites during fermentation, specifically lactic acid and other organic acids. This accumulation of acids subsequently lowers the stool pH, making it more acidic. An acidic stool pH is a classic finding in lactose intolerance.

Therefore, the stool pH in lactose intolerance is typically acidic, reflecting the fermentation of undigested lactose by colonic bacteria. This mechanism underscores why patients with lactose intolerance may present with diarrhea that has a lower stool pH compared to those with normal lactose digestion.