Lactose intolerance is a deficiency in the enzyme, lactase.

Lactase is responsible for the degradation of lactose to galactose and glucose, in the small intestine.

When there is lactose intolerance, lactose moves into the large intestine where it is reacted with bacteria and is broken down to 2-Carbon metabolites, CO2, 3-Carbon metabolites, and H2. These lead to bloating, diarrhea, and dehydration.

H2, from the breakdown of lactose by bacteria in the large intestine can be measured in the breath.

- Lactose intolerance results from the absence of brush border lactase and, thus, the inability to hydrolyze lactose to glucose and galactose for absorption.  Nonabsorbed lactose and H2O remain in the lumen of the gastrointestinal tract and cause osmotic diarrhea.