Lactose intolerance refers to the syndrome that occurs when patients suffer a decreased ability to digest lactose.  The enzyme beta-D-galactosidase is found in the small intestine and hydrolyzes lactose into glucose and galactose.  Decreased levels, or less commonly an absence, of this enzymatic activity results in the development of gastrointestinal symptoms 30 minutes to 12 hours after the ingestion of dairy products.  Symptoms include bloating, diarrhea, abdominal pain, and gas.  Asian, Native American, and African American populations are more commonly affected than are Caucasian patients.  An inborn error of metabolism may occur during infancy but is rare.  Most cases occur as an acquired disorder when lactase levels decline after weaning (primary disease).  Secondary causes of lactase deficiency include: HIV enteropathy, enteritis, celiac sprue, tropical sprue, Whipple’s disease, carcinoid syndrome, cystic fibrosis, diabetic gastropathy, kwashiorkor, Zollinger-Ellison syndrome, and radiation enteritis.  A transient deficiency of beta-D-galactosidase may develop after an acute episode of diarrhea and lead to prolonged symptoms.  Therefore, the first step in the work up of prolonged diarrhea should include instituting a lactose free diet to see if symptoms resolve.

     Various methods are available to establish the diagnosis.  The lactose tolerance test is performed by first obtaining a fasting serum glucose level.  The patient is then given 50 grams of lactose to ingest and then a repeat determination of the serum glucose is performed.  If the second glucose value fails to increase by at least 20 mg/dL over the baseline value, then lactose intolerance is diagnosed. The hydrogen breath test is performed by having patients ingest 25 grams of lactose after a 12 hour fast.  A baseline breath hydrogen concentration is obtained before the lactose load and then a second breath hydrogen concentration is obtained afterwards.  Diagnosis is suggested by an increase of breath hydrogen concentration of greater than 20 ppm over the baseline value. The most effective method of establishing the diagnosis is by biopsy of the jejunum.  However, this requires an invasive procedure.  Therefore, the lactose tolerance test and hydrogen breath tests are often preferred; however, a clinical response to either a lactose free diet or lactase supplementation therapy are probably the most widely used and easiest methods.

     Since most cases are not absolute, the vast majority of patients may be managed with dietary lactose restrictions by limiting milk intake to 240 mL or less per day.  Patients should be counseled to avoid hidden sources of dietary lactose to include: baked goods, processed cereals, instant potatoes, instant soups, instant breakfast drinks, margarine, nonkosher lunch meats, salad dressings, candies, and mixes for pancakes, biscuits, cakes, and cookies.  Lactose is also found as a carrier in prescription and over the counter medications and in dietary supplements.  When lactose restriction proves effective, the amount of daily milk intake may then be gradually increased as tolerated as symptoms often improve with controlled exposure over time.  Lactase enzyme capsules may be taken prior to dairy ingestion, and lactase enzyme drops may be added to milk 24 hours prior to ingestion.  Patients who use lactase drops should be instructed to refrigerate the milk after adding the enzyme and counseled that the milk will have a sweet taste secondary to the break down of lactase into the simple sugars glucose and galactose.