There are recent breakthroughs on the genetics of lactose intolerance and it is really fascinating. It turned out that the "intolerance" phenotype is the wild-type, and the ability to digest lactose evolved very recently in human history (in northern Europe and in Africa). The majority of Chinese are supposed to be lactose intolerant. However, intolerance only develops at around the time of weaning, so infants can digest their mom's milk without problem. Lactose intolerance is therefore thought to be nature's way to drive the toddlers away from their mother's breasts. Intolerant adults still retain a low level of lactase expression and some degree of ability to digest lactose, but won't be able to digest large amount of lactose. Undigested lactose is fermented by the gut bacteria, causing GI discomfort. I wonder if the goal of "每天一斤奶” is achieved, will we see 强壮中国人，or 拉稀中国人？