Lactose, or milk sugar, is a white crystalline disaccharide. It is formed in the mammary glands of all lactating animals and is present in their milk. Lactose yields D-galactose and D-glucose on hydrolysis by lactase (β-galactosidase), an enzyme found in gastric juice. People who lack this enzyme after childhood cannot digest milk and are said to be lactose intolerant. Common symptoms of lactose intolerance include nausea, cramps, gas and diarrhoea, which begin about 30 minutes to 2 hours after eating or drinking foods containing lactose. Between 30 and 50 million Americans are lactose intolerant, with certain ethnic and racial populations being more widely affected than others; as many as 75 percent of all African-Americans and Native Americans and 90 percent of Asian-Americans are lactose intolerant. The condition is least common among persons of northern European descent.