Humans are the only species that drink milk from another species. Other mammals stop producing the enzyme lactase – which breaks down milk sugar lactose into simpler sugars – when they have finished breast feeding.
Humans and lactase
Humans, however, can continue producing lactase and drink milk also when they reach adulthood. About 65% of humans haven’t developed this physiological function, therefore are lactose intolerant. The condition is more common among non-Caucasians. Also, some groups of people have adapted better to milk due to their lifestyle and diet.
What is normal?
It is our natural state to be lactose intolerant. Evolutionary speaking, the ability to handle milk later in life is a more recent adaptation of the human body. Lactase’s only function is to transform lactose from milk into simpler components (galactose and glucose), so it is normal to stop producing it after breastfeeding, and to stop ingesting milk at that point too.
Some groups of people, especially those that enjoy a lot of milk, have developed a so called ‘lactase persistence’, which enables them to feed on a diet rich in milk and dairy products throughout their lives. This is actually a genetic mutation that is observed only in some people. In cultures where the use of milk is not so prevalent, this genetic adaptation hasn’t occurred.
Statistics shows that about 40 million Americans are lactose intolerant, and 75% of all adults have a decrease in lactose activity.
- In some Asian countries, 90% of the population can’t digest milk.
It usually takes about 30 minutes to develop the side effects of lactose intolerance, so many people might not connect their discomfort with the glass of milk they’ve just had.