Drinking milk in adulthood is a recent genetic adaptation in evolution that only a minority of humans possess. According to recent studies, this lactose tolerance would however bring few significant benefits, because the consumption of milk does not reduce the risk of fractures and could even increase the risk of prostate cancer.
Milk has the characteristic of containing lactose, a sugar formed by the assembly of a molecule of glucose and a molecule of galactose. For young children, the presence of this lactose poses no problem, because they produce an enzyme (lactase) which breaks down this sugar and allows it to be absorbed by the intestine.
As they age, however, a large number of people stop producing this lactase and the undigested lactose is then fermented by intestinal bacteria, which can cause many inconveniences (bloating, flatulence, diarrhea).
65% of the world’s population unable to digest milk
This lactose intolerance is much more common than you might think: it is estimated that 65% of the world’s population is unable to digest milk from the age of seven or eight! From a biological point of view, milk is therefore a food primarily intended for children, which only a minority of adults can consume regularly.
In the northern regions of the world, however, the proportion of adults who digest milk is much higher (about 80% in Canada). This situation is explained by the fact that most of the inhabitants of the North are direct descendants of the peoples of Europe who were the first to practice cattle breeding.
Initially, all these farmers were lactose intolerant and could only consume milk in a fermented form (during the bacterial transformation of milk into yogurt, cheese or kefir, the lactose content of milk is greatly reduced). About 7500 years ago, however, a genetic mutation allowing sustained production of lactase in adulthood appeared in this population and allowed them to drink milk directly. At a time when finding enough food was a survival challenge, the impact of this mutation was extraordinary, and it quickly became embedded in the genetic makeup of the European and, later, North American population.
Lactose tolerance is therefore a rather rare example of selection induced not by biology, but by human behavior; it is therefore very recent, compared to the hundreds of millions of years of evolution that have shaped and selected the rest of our genes.
Increased risk of fracture and premature death
Unlike cheeses and yogurts, raw milk is quickly contaminated and has always been associated with many serious diseases, such as tuberculosis. It was therefore only after the introduction of pasteurization at the end of the 19th century that the consumption of milk really took off to become one of the pillars of the Western diet. In Europe, for example, food guides recommend that adults consume two to three servings of milk and alternatives every day, which corresponds to 750 ml of milk.
The reason most often given for drinking milk is its rich calcium content: since this mineral gives strength and resistance to the bones, it has been proposed that the abundant consumption of milk could improve the health of bone mass and prevent osteoporosis. and fractures. However, studies have shown that North American women, who drink milk and consume almost 1000 mg of calcium daily, are 10 times more affected by hip fractures than Asian women, who drink absolutely no milk. .
These observations have recently been confirmed by a large study of 100,000 men and women, which shows that the amount of milk consumed during adolescence has no impact on fracture risk in adulthood. Another study even reports that the consumption of two to three glasses of milk per day in adulthood increases the risk of fractures in women and is also associated with an increased risk of premature death.
Increased risk of certain cancers: prostate, lung, breast, ovary
A rigorous analysis of 32 studies on the impact of milk on the risk of prostate cancer indicates that men who consume a lot of milk (400 g per day) run a 7% higher risk of developing prostate cancer. the prostate, probably due to an increase in blood levels of a growth factor (IGF-1).
An association between milk consumption and the risk of cancer is also suggested by the observation that lactose intolerant people, who therefore do not drink milk, are significantly less at risk of being affected by certain cancers (lung, breast , ovary) than those who consume it regularly.
These observations therefore suggest that drinking milk in adulthood is not essential for the maintenance of good health and could even have certain harmful effects when it is consumed in excessive quantities. This is not very surprising considering that two-thirds of the inhabitants of the Earth never consume it because of their lactose intolerance, and this, without negative repercussions on their health. A diet rich in plants and low in red meats, combined with regular physical activity, remains the key to maintaining strong and robust bones as you age.
-Willett WC. Eat, drink and be healthy: The Harvard medical school guide to healthy eating. Free Press, New York, 2001, p. 163.
Feskanich D et al. Milk consumption during teenage years and risk of hip fractures in older adults. JAMA Pediatr, 2014; 168:54-60.
Michaelsson K et al. Milk intake and risk of mortality and fractures in women and men: cohort studies. BMJ, 2014; 349:g6015.
(5) JiJ et al. Lactose intolerance and risk of lung, breast and ovarian cancers: aetiological clues from a population-based study in Sweden. Br J Cancer, 2015; 112:149-52.