Most adults with lactose intolerance can tolerate the amount of lactose in a cup of milk
Research Activities, November 2010, No. 363
Most adults with presumed lactose intolerance can tolerate 12-15 grams of lactose, the amount in a cup of milk, concludes a review of studies on the topic. Doses of 24 grams or more of lactose, the primary carbohydrate in milk, could produce gastrointestinal symptoms, such as diarrhea, bloating, flatulence, and abdominal discomfort. Individuals with lactose malabsorption have reduced levels of the enzyme lactase, which breaks down lactose in the digestive system. Levels of this enzyme decline dramatically after infancy in humans and can result in lactase deficiency. While both adult lactase deficiency and lactose malabsorption can be measured directly, a diagnosis of lactose intolerance depends on the patient's self-reported symptoms after consuming foods containing lactose.
The review authors analyzed 36 studies on lactose intolerance and ways to reduce its symptoms. The trials, which were of generally poor quality, failed to find significant reduction of lactose intolerance in individuals given lactose-reduced milk or lactase supplements with 0-2 grams of lactose. Evidence was also insufficient to show reduced lactose intolerance with probiotics such as bacterial yogurt cultures, attempts to produce colon adaptation through increasing lactose doses, and other agents. The review authors searched a number of health- and food-related databases for articles that dealt with either the maximum tolerated lactose intake in patients diagnosed as having lactose intolerance or evaluated strategies to manage lactose intolerance in individuals.
Most of the studies used evidence of lactose malabsorption, rather than of actual lactose intolerance. The study was funded in part by the Agency for Healthcare Research and Quality (Contract No. 290-07-10064).
More details are in "Systematic review: Effective management strategies for lactose intolerance," by Aasma Shaukat, M.D., M.P.H., Michael D. Levitt, M.D., Brent C. Taylor, Ph.D., M.P.H., and others in the June 15, 2010, Annals of Internal Medicine 152(12), pp. 797-803.