Lactose Intolerance

The FDA recommends that people drink three cups of milk a day for bone health and essential nutrients. This is a problem for a lot of people who can’t digest milk. Usually this is because of lactose intolerance, a condition where the gut doesn’t make the right enzymes to break down the sugar in milk, called lactose.

Lactose is a disaccharide meaning a two part sugar. It’s made out of two simple sugars that need to be broken apart before the body can use them. Lactose is broken down by an enzyme called lactase. People who are lactose tolerant produce the enzyme Lactase in their guts. When people who are lactose intolerant drink milk, their gut bacteria eat the lactose causing cramps and “intestinal distress.” (do you really wanna know?)

An infant needs the enzyme lactase to digest her mother’s milk. Although almost all children are born with the gene for lactase production, gene regulation begins turning the gene “off” as children mature, beginning around the age of two, when children are generally beginning to wean. Children are able to digest less and less milk until they become lactose intolerant around the age of six.

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Digression:
In the United States it’s very unusual for a child to be nursing by age 2. Most infants are weaned early here, but in other cultures children nurse for much longer and even older toddlers may nurse infrequently though they regularly eat solid food. Mother’s milk is not only an important source of calories and nutrients; it also offers immune protection and a safe source of liquid for children. In the United States and other developed nations we can afford to wean infants early because of our clean public water. In developing nations, however, mother’s milk keeps children alive in very harsh conditions: children’s immune systems are challenged by parasites and disease, and their water is unsafe to drink. A child would die of diarrhea except for a regular infusion from her mother’s immune system and safe hydration. When mothers in developing nations feed their children formula, they have to use dirty water to mix it and many children die of water borne diseases. UNICEF recommends all mothers “breastfeed exclusively for the first six months and continue to breastfeed for two years or more.” They estimate that exclusive breastfeeding would prevent 1.4 million infant deaths a year in the developing world.
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In some populations with a history of milk drinking, a mutation occurred allowing the lactase gene to stay “on”, resulting in lactose tolerant adults. The ability to digest milk would have been very helpful because milk allows us to obtain more nutrition from our domesticated animals (moo!) than just their meat. In places where sanitation is poor, cow’s milk is a safe source of water as well as calories and nutrients. People with a mutation that lets them to digest milk would have a genetic advantage over their peers, and they could produce more offspring. The mutation eventually became widespread in certain populations such as northern Europe and southern Africa. This is one of the most recent examples of natural selection acting on the human genome.

Dairy is an important source of calories in some areas but not in others; not all populations developed lactose tolerance. Although most people in the United States and Europe can digest milk, the majority of people on earth can’t. Lactose tolerance (the ability to digest milk) in adulthood is actually less common than lactose intolerance and it’s only a problem in areas where people drink milk. Lactose intolerance is quite variable with some people unable to digest even the slightest bit of lactose and others able to consume up to a glass of milk. In Japan, for instance, one study shows that most Japanese can digest up to 200ml of milk without digestive distress. They also found that people who drink milk habitually can tolerate much more milk. The researchers speculate that continuous exposure to milk can extend lactose tolerance into adulthood.