Infant can take lactose and lactase by drinking breast milk.

Lactase has producing in the small intestine from fetal life.

Lactose is divided glucose and galactose by adequate volume of lactase.

Glucose and galactose are absorbed from the small intestine.  They are the source of nutrition for infant.

Some lactose is transported to the large intestine from small intestine, and is fermented by lactobacilli.  Infant get the lactobacilli from mother.

By fermentation, stool frequency of infants is 2-5 times in a day.

 

Most of the Japanese people, to up to 5 years of age, lose 90% or more of lactase activity.

Thereby, most of the lactose is transported into the large intestine.

It causes osmotic diarrhea.

Large quantity lactose is causing over-fermentation. It leads a large amount of gas. In addition, by the short-chain fatty acids, stool become acidification. 
Thereby become severe intestinal peristalsis.  Abdominal pain, flatulence, and abdominal distention occur by them.