In lactose intolerance the enzyme lactase is either completely absent or not present in sufficient quantities. Lactase is responsible for splitting lactose, a disaccharide, into two smaller sugars that can be easily absorbed by the body. If this splitting does not occur, lactose unable to be absorb, ferments causing excess gas, pain and diarrhea.

The primary causes for lactose intolerance is genetics and age related decreases in lactase secretion.  Rarely does this type of lactose intolerance affect children under the age of 5. This is because babies generally have high levels of lactase production while nursing, since lactase is responsible for the digestion of all types of milk not just cow’s milk. This does not mean that babies should be fed cow’s milk or cannot develop an allergy to milk, doctors still advise parents to not give their babies cow’s milk till they are at least one year old.

Normal declines in lactase production occur after the time of weaning and beyond as our need for products containing lactose declines, hence the reason why lactose intolerance is generally an adult condition.

Total loss of lactase production is rare but does happen, generally in lactose intolerant individuals lactase declines down to 10-30% of the normal level, this is why most individuals can tolerate small amounts of dairy. This type of lactose intolerance is considered permanent, and cannot be altered by eating differing amounts of dairy.

Secondary causes of lactose intolerance occur when damage occurs to the lining of the small intestine, making it impossible for the production of lactase to occur.  Damage of this type can occur for several reasons, including celiac disease, cancer, inflammatory bowel disease, or gastroenteritis. Also malnutrition and certain types of toxins can cause enough damage to the intestines for lactose intolerance to occur.

It is most common that when babies and small children develop lactose intolerance, it can be one of the first signs of celiac disease, or an infection of some kind in the small intestines.

This type of lactose intolerance is reversible, as soon as the primary issue is treated and the damage to the intestines has time to heal, usually 2-3 months, the lactase production will increase, and the lactose intolerance will disappear.