On Food and Cooking

age and continuing through adulthood.
The logic of this trend is obvious: it’s a
waste of its resources for the body to produce
an enzyme when it’s no longer needed; and
once most mammals are weaned, they never
encounter lactose in their food again. But if an
adult without much lactase activity does
ingest a substantial amount of milk, then the
lactose passes through the small intestine and
reaches the large intestine, where bacteria
metabolize it, and in the process produce
carbon dioxide, hydrogen, and methane: all
discomforting gases. Sugar also draws water
from the intestinal walls, and this causes a
bloated feeling or diarrhea.
Low lactase activity and its symptoms are
called lactose intolerance. It turns out that
adult lactose intolerance is the rule rather than
the exception: lactose-tolerant adults are a
distinct minority on the planet. Several
thousand years ago, peoples in northern
Europe and a few other regions underwent a