The lactose entering your body by eating yogurt, milk, cheese or quark is normally digested by the enzyme lactose. If this does not happen, the lactose remains undigested and is fermented in the large intestine. This produces gases, which in turn can lead to flatulence, heartburn, diarrhea, hypoglycemia or nausea.
In the case of lactose intolerance, there is a deficiency of the enzyme lactase and the above-mentioned symptoms occur.