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A QUICK, cheap test to identify people who can鈥檛 digest milk is now on the
cards. It would help hundreds of thousands of people with the condition, known
as lactose intolerance, who are never diagnosed because the current tests are
time-consuming and unreliable.

The test has been made possible by the discovery of the genetic basis for
lactose intolerance by Leena Peltonen鈥檚 team at the University of California,
Los Angeles. Her findings support the theory that retaining the ability to
digest milk into adulthood is an adaptation that only evolved recently in
milk-guzzling cattle herders.

For the majority of people in the world, especially southern Europeans,
Asians and Africans, lactose intolerance is the norm. It usually sets in at
weaning, when the body stops producing lactase鈥攖he enzyme needed to digest
the sugar lactose, which is a major ingredient of human and animal milk (but not
of fermented products like cheese).

Just the thought of a bowl of cornflakes makes you feel unwell, says Wendy
Stokes, who has suffered from lactose intolerance since childhood. At one point
she could not even have milk in her tea without experiencing constricted
breathing. 鈥淭esting was a long, long process,鈥� she says.

It usually involves giving people only lactose after they haven鈥檛 eaten for a
while, and then monitoring levels of glucose鈥攁 product of normal lactose
digestion鈥攊n the blood. Having a quick and easy test will help both
patients and doctors.

Without lactase, lactose passes through the stomach undigested and reaches
bacteria in the intestine. Bugs there feast on it, belching out by-products that
can leave people feeling gassy and nauseated, or worse.

Peltonen鈥檚 team studied nine extended Finnish families, as well as some
Germans, Italians and South Koreans. The researchers found two variations in the
human genome associated with lactose intolerance. One of these 鈥渟ingle
nucleotide polymorphisms鈥�, or SNPs, was present in all 236 people who were
lactose intolerant, while the other was found in 229. Both SNPs are near the
lactase gene, and probably affect proteins that regulate the expression of the
gene.

The fact that the same variations occur in distantly related populations
supports the theory that all humans were once lactose intolerant, and that
鈥渓actase persistence鈥� evolved only after people domesticated animals. Lactase
persistence also seems to be most common among peoples with a long tradition of
dairy farming, such as northern Europeans and Tutsis in central Africa. 鈥淚 find
it ironic that a so-called disease actually represents the original condition,鈥�
says Peltonen.

But Steve Jones, an evolutionary biologist at University College London, says
it is impossible to know whether culture selected the genes or vice versa.
Perhaps groups that happened to be lactose tolerant started dairy farming
because they didn鈥檛 get ill from milk, he suggests.

The prevalence of lactose intolerance was only recognised in the 1960s.
Before that, the dislike of milk in places like China was put down to culture,
and powdered milk for older children and adults was often sent as part of food
aid.