Give me a thorough answer and a scientific reason with proof of why.

if lactaid passes through the stomach (ph 2) then it will digest lactose when it reaches the small intestine

Lactaid is the brand form of the intestinal enzyme lactase or β-galactosidase. Lactase hydrolyses lactose into galactose and glucose in the small intestines.

Explanation:

Scientific reason:

The optimal pH of the enzyme lactase to digest lactose is around 6; however, it can function even at an acidic pH of about 2-7 which is similar to the pH of small intestines in humans. Thus lactase acts upon the undigested lactose molecules and hydrolyses it in the small intestines.

Since the pH matches with that of the small intestines, supplementing with lactase products like Lactaid will help correct lactose intolerance among children and adults.