Why they can affect you: According to the U.S. Department of Health and Human Services, an estimated 30 to 50 million American adults are lactose intolerant. "We are the only species that continues to drink milk after weaning, and many people lose the ability to digest lactose -- the milk sugar -- as they grow into adulthood," said Teitelbaum. Lactose intolerance is caused by a deficiency of the enzyme lactase, which is produced by the cells lining the small intestine, stated the National Digestive Diseases Information Clearinghouse, a service of the National Institute of Diabetes and Digestive and Kidney Diseases. "So any lactose you don't digest becomes party food for the gut bacteria, and they say 'thank you' by making a lot of gas," said Teitelbaum. Substituting "real" milk, ice cream and cheese for dairy products labeled lactose free should ease this problem, along with choosing milk that contains lactase and hard cheese, since it naturally contains less lactose than soft cheese.