Dear Dr. K:  I’m pretty sure I’m allergic to milk because it always gives me gas and diarrhea, but my allergy test to milk was negative.  What gives?

In all likelihood you have lactose intolerance due to lactase deficiency.  Lactose is the carbohydrate milk sugar which is a disaccharide “two-sugar”.  It consists of the monosaccharide glucose bound to the mono-saccharide galactose.  Unlike the bovine intestinal track which can absorb disaccharides, the human intestines can only absorb monosaccharides. 

Lactase is the enzyme found on the brush border in the intestines where the two sugars split, and lactose intolerance occurs when there is a complete or partial absence of the lactase enzyme.

Later symptoms will occur when the amount of lactose ingested exceeds the amount of enzyme present. 

When sugars are not broken down and absorbed in the intestine they hit the bacteria-filled colon where they are fermented.  This process of fermentation leads to gas and intestinal irritability.

In most people lactase deficiency is partial and therefore they can tolerate modest amounts of lactose.  Also, lactase enzyme is available as an over-the-counter, self-help remedy to be taken with any cow milk product.  To be successful, the dose of “lactaid” has to be titrated to the lactose load present in the meal.