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Dairy Council Digest Archives
Dairy Food Sensitivity: Facts and Fallacies
Lactose Intolerance
Cause. To be absorbed and used by the body, lactose (milk sugar) must be hydrolyzed by the enzyme, lactase, at the intestinal brush border into glucose and galactose (8). Lactase activity of the fetus increases during late gestation and generally remains at a high level until weaning. Consequently, most infants and young children readily digest dietary lactose without symptoms.
There are primary, secondary, and congenital causes of low lactase levels. Congenital lactase deficiency is a very rare inborn error of metabolism observed in infants (9). This condition, unlike other conditions of lactase deficiency, requires complete avoidance of lactose. Secondary loss of lactase activity is a temporary condition resulting from diseases/conditions affecting the gastrointestinal tract such as Crohn's disease or parasitic infections, or from certain medications.
Primary delayed-onset lactase deficiency occurs at a variable time after weaning and is genetically determined. In the majority of the world's population, lactase activity declines beginning between 2 and 20 years of age as a result of an autosomal recessive single gene (9,10). In contrast to these lactose malabsorbers or lactase nonpersistent individuals, lactase activity remains high throughout life in a minority of persons. This ability to maintain lactase levels, or lactase persistence, is inherited through a single, autosomal dominant gene and is related to a long tradition of milk and dairy food consumption (11). Primary lactase deficiency is the most common and the form referred to in this Digest.
"People tend to overestimate the prevalence of reactions to cow's milk and are confused about lactose intolerance and milk protein allergy."
To better understand the effects of reduced lactase activity, terms such as lactose maldigestion, lactose intolerance, and milk intolerance need to be defined. Lactose maldigestion or malabsorption describes the incomplete hydrolysis or breakdown of lactose, as determined by a standard lactose tolerance test (9). Lactose intolerance is the occurrence of symptoms after persons with clinically diagnosed lactose maldigestion consume too much lactose relative to the amount of lactase necessary to break this sugar down. Most lactose maldigesters are not lactose intolerant and do not develop symptoms. Tolerance to lactose depends on a variety of biological, psychological, and dietary factors. Milk intolerance, the occurrence of abdominal symptoms following milk intake, may or may not be due to lactose maldigestion. Symptoms may be due to other gastrointestinal problems.
Prevalence. Worldwide, the prevalence of low lactase levels in adults ranges from less than 5% to more than 90%, depending on the population (10). Low lactase levels are least common among northern and western Europeans and their descendants and highest among populations in the Far East (10). In the United States, the prevalence of lactose malabsorption is about 15% in Caucasians, 53% in Mexican Americans, 80% in African Americans, and 90% in Asian-Americans (8,10). In all, an estimated 30 to 50 million Americans are lactose malabsorbers (8). The prevalence varies widely among different ethnic and racial population groups, and is influenced by age, at least before adulthood (8,12). Despite the high prevalence of lactose maldigestion, scientific evidence indicates that lactose intolerance is grossly overestimated.
Clinical Symptoms and Diagnosis. Low lactase levels per se do not give rise to symptoms or lactose intolerance (13–16). Most lactose maldigesters are unaware of their condition, possibly in part because their threshold for tolerance exceeds their intake (11). If clinical symptoms of lactose intolerance do occur, they may include diarrhea, flatulence, abdominal bloating, and abdominal pain/cramps (8). These symptoms are evident from about 30 minutes to several hours after consuming lactose-containing foods or beverages. The production of gases in the large intestine from the fermentation of undigested lactose by naturally occurring bacteria is responsible for the symptoms. In general, the symptoms are nonspecific, highly individual, and mild (12,13). The larger the amount of lactose consumed in relation to intestinal lactase and the faster the rate of gastric emptying, the more frequent and severe the symptoms (13). Psychological factors also influence the awareness of symptoms (12).
Because symptoms following lactose intake are nonspecific and can be caused by a variety of disorders, lactose intolerance cannot be diagnosed on the basis of symptoms alone (13,16,17). In one recent study, one-half of lactose malabsorbers reported symptoms after consuming lactose-free milk or more adverse symptoms after smaller than larger intakes of lactose (16).
A variety of tests, direct (e.g., measurement of lactase activity in the intestinal mucosa) and indirect (e.g., measurement of blood glucose, breath hydrogen levels, or stool acidity) are available to diagnose lactose maldigestion (7,17). Measurement of breath hydrogen excretion before and at several intervals following a standard dose of lactose is commonly used to diagnose lactose malabsorption (8). However, because this test often uses a large dose of lactose (i.e., 50g or the equivalent to the amount of lactose in a quart of milk), and the lactose is given in water without other foods, the test exaggerates the prevalence of lactose intolerance (18).
Well controlled studies have demonstrated that the vast majority of lactose malabsorbers can consume amounts of lactose found in usual servings of milk and other dairy foods without experiencing adverse symptoms (14,15,18). A diagnosis of lactose maldigestion does not mean that milk and other dairy foods need to be eliminated from the diet.
"Most lactose intolerant individuals can consume dairy foods as part of a nutritious diet by modifying the amounts and types of dairy foods consumed. Including milk in the diet on a regular basis may even improve tolerance to lactose."
Management. In most cases it is neither necessary nor nutritionally wise to consume a lactose-free diet (11,18,19). Avoiding dairy foods can cause inadequate intakes of calcium and many other essential nutrients. A deficiency of calcium increases the risk of developing osteoporosis, hypertension, and possibly some types of cancer (20). Fortunately, tolerance to lactose can be improved by adjusting the amount and types of dairy foods consumed (11). Further, regular exposure to lactose by lactose malabsorbers appears to increase tolerance to lactose, possibly by enhancing the ability of colonic bacteria to metabolize lactose (21,22).
The ability to tolerate lactose is not an "all or none" phenomenon and individuals who are lactose maldigesters need to determine their threshold and adjust lactose intake accordingly (8,11,14,15,23). Complete elimination of lactose is unnecessary. Even under severe conditions (i.e., lactose administered in water and fed under fasting conditions), lactose intolerant individuals can consume the amount of lactose in 1/2 cup of milk without experiencing symptoms (23). Further, individuals diagnosed with lactose intolerance can consume the amount of lactose in one to two cups of milk, especially when given with a meal, without experiencing adverse symptoms (15,16,24,25).
Development of lactose intolerance symptoms depends not only on the dose of lactose consumed, but also on the rate of gastric emptying and delivery of lactose to the colon. Consuming lactose with a meal or solid food slows gastric emptying and delivery of lactose to the colon, thereby allowing endogenous lactase more time to digest lactose (25–27). For this same reason, some types of dairy foods may be better tolerated than others by lactose malabsorbers (25). For example, whole milk appears to be better tolerated than lowfat milk or lactose in water (25). Likewise, chocolate milk may be better tolerated than unflavored milk by lactose maldigesters (25,28).
The semi-solid state of yogurt, as well as the release of lactase from the bacterial cultures (i.e., Streptococcus thermophilus and Lactobacillus bulgaricus) used to make yogurt, explain why yogurts with "live, active" cultures are well tolerated by lactose malabsorbers (27,29–33). Because of their high solids and/or fat content which slows gastric emptying, frozen yogurt and ice creams also may be tolerated by lactose maldigesters (34). Most cheeses, especially those aged six months or longer such as Cheddar and Swiss, are well tolerated because of their negligible lactose level and content of solids (11).
For individuals who have difficulty tolerating even small amounts of lactose, dairy foods with 70% to 100% of their lactose removed are available (8). Alternatively, commercially available lactase preparations can be directly added to milk to reduce its lactose content (8). Lactase enzyme tablets also can be consumed to help individuals digest lactose (8). For lactose intolerant individuals who do experience symptoms after consuming the amount of lactose in usual serving sizes of milk, tolerance can be improved by dietary changes, specifically by altering the amounts and types of milk and other dairy foods consumed (19).
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