Monday, September 10, 2012


Have you ever wondered why some people are lactose intolerant and others aren’t? I have. So I was grateful to come across Harold McGee’s On Food and Cooking, the Science and Lore of the Kitchen with a great explanation. Here’s a slightly paraphrased excerpt:

People who drink milk after infancy are the exception within the human species (no other mammals drink milk as adults). The obstacle is the milk sugar lactose, which can’t be absorbed and used by the body as is; it must first be broken down by the digestive enzyme lactase in the small intestine. 

Lactase reaches its maximum level in the human body shortly after birth and then slowly declines to a minimum level at between two and five years of age -- probably because It’s a waste for the body to produce an enzyme when it’s no longer needed and, once weaned, most mammals never encounter lactose again. 

If your body doesn’t produce lactase and you consume a lot of lactose, the lactose will pass through the small intestine and reach the large intestine where bacteria will metabolize it and in the process produce carbon dioxide, hydrogen, and methane, causing a bloated feeling or diarrhea. 

Low lactase activity and its symptoms are called lactose intolerance.

Lactose intolerance is the rule for adults, not the exception. However, several thousand years ago, peoples in northern Europe and a few other regions underwent a genetic change that allowed them to produce the enzyme lactase throughout life, probably because milk was an exceptionally important resource in colder climates. 

About 98% of Scandinavians are lactose-tolerant, 90% of French and Germans, but only 50% of southern Europeans and North Africans, and 30% of Aftrican Americans. 

Fortunately, even lactase-less adults can consume about a cup of milk per day without severe symptoms, and even more of other dairy products. Cheese contains little or no lactose (most of it is drawn off in the whey, and what little remains in the curd is fermented by bacteria and molds), and the bacteria in yogurt generate lactose-digesting enzymes that remain active in the small intestine and work for us there. 

Wondering about the calcium-from-milk-for-bone-health issue? McGee has an answer for that too.

Various aspects of modern eating increase calcium excretion. A high intake of salt is one, and another is a high intake of animal protein, the metabolism of which acidifies our urine and pulls calcium salts from bone. The best insurance against osteoporosis appears to be frequent exercise of the bones we want to keep strong and a diet moderate in salt and meat but rich in dried beans, nuts, corn tortillas  and tofu (both processed with calcium salts), and several greens -- kale, collards, mustard greens.

These days I’m exercising my bones  carrying my huge copy of On Food and Cooking around. It’s just too fascinating to put down!

You can find Harold McGee at

12/28/12 update: Just found this new article about how lactose tolerance came about:

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.