Q: My doctor made a diagnosis of lactose intolerance when told that drinking milk gave me gas, stomach pains and diarrhea. Milk and dairy products are among my favorite foods. Can any tests be done to prove that his diagnosis is correct? Is it necessary for me to stop eating all dairy products?
A: Lactose intolerance is a common problem resulting from an inability to digest and absorb lactose, the major sugar contained in milk and dairy products. Lactose can only be absorbed from the intestine after it is broken down into the smaller sugars glucose and galactose by the digestive enzyme lactase. Although present in normal amounts in children, lactase production in the intestine declines to low levels in most adults, except those of northern European background. When lactose is not absorbed properly, it is broken down by bacteria in the large intestine. The products of lactose breakdown include lactic acid and hydrogen gas, which together cause symptoms of excess gas, crampy abdominal pain and diarrhea.
Two tests are used to confirm the diagnosis of lactose intolerance. In both, the subject first ingests a standard amount of lactose. In the lactose tolerance test, blood glucose levels are measured before and several times after lactose is administered. Poor absorption of lactose is indicated if the rise in blood glucose is quite small. The breath hydrogen test measures the amount of hydrogen in the breath after lactose ingestion. Large amounts of hydrogen are present in the breath when unabsorbed lactose is broken down by intestinal bacteria.
Unless your lactose intolerance is severe, you may be able to tolerate small to moderate amounts of milk and dairy products if they are taken as part of a meal. The fat in whole milk makes its lactose easier to digest than the lactose in skim milk. The lactose ingested with milk can be greatly reduced by purchasing lactase-treated milk, which contains only 30 percent of the original lactose, or by adding commercially available lactase drops to regular milk. Depending on the number of lactase drops added, the lactose content of milk can be reduced by as much as 99 percent during a 24-hour period of refrigeration.
If your lactose intolerance is severe, you can add the lactase drops to the lactase-treated milk obtained at the grocery store. Lactase-treated milk tastes sweeter than regular milk because glucose and galactose are sweeter than lactose. Hard cheeses, such as Cheddar and Parmesan, are low in lactate. To avoid or minimize symptoms from other dairy products, take one to three lactase caplets or tablets along with the meal. Yogurt that contains active cultures of bacteria is usually well tolerated because the lactose content of yogurt is less than milk and the bacteria digest much of the rest.
Many other foods contain small amounts of milk products, and as many as 30 percent of medications have a lactose filler. You may develop symptoms from these sources if your lactose intolerance is severe.
Dr. Margolis is professor of medicine and biological chemistry at the Johns Hopkins School of Medicine.