Northern European humans developed the ability to digest lactose in adulthood around 7000 years ago, according to some research projects. The reason for this may be that there is less sunlight in the north of Europe (especially in autumn and winter). Only when there is enough sunlight can the body produce sufficient amounts of Vitamin D which is also important for bone growth and regeneration (absorption of calcium). This task can also partly be fulfilled by lactose. If too little vitamin D is present, then the body can maintain the absorption of calcium with the help of lactose. This is not necessary for people who live in sunny regions (see next paragraph). This also means that lactose intolerance is not an illness, but in fact the norm. Those who are able to digest lactose at advanced age have a genetic variation.
The production of the enzyme lactase is – like all enzyme production – genetically controlled. During the course of evolution Northern European humans have gained the ability to also produce this enzyme at a later age. This development correlates with the domestication of cattle and concerns around ¼ (other sources say 1/3) of the world’s population.
There is a clear North-South divide. In northern countries nearly 90% of populations are able to digest lactose, while the rate is only around 30% in southern regions of Europe. Close to the equator and in Asia only 2% of the population are able to digest lactose. In Central Europe 80-90% are able to synthesise the enzyme lactase.
Cultural factors also play a role. 80% of the Japanese population, for example - mostly upper class – are able to digest lactose. Many of those who originate from populations that are normally lactose intolerant, but have grown up in a society where drinking milk is normal, will no longer be affected by lactose intolerance. This implies that cultural conditions can also determine lactose intolerance.
The African Tuareg and Massai tribes are also an anomaly because although they actually live in southern regions they still produce lactase. This is due to their traditional intensive cattle-farming and the resulting raised consumption of milk.
In South America around 50% of the population have lactose intolerance; in North America (USA) research was able to prove that 15% of white Americans, 53% of Mexican Americans and 80% of African-Americans have lactose intolerance. This may be for genetic reasons rather than for cultural reasons after all.
Some researchers have attempted to establish a distinction between males and females. Definite proof has, however, never been established, which means that lactose intolerance appears to be equally distributed between males and females.
Worldwide prevalence of lactose intolerance in recent populations. An average of all ethnic groups of the total population of a country is represented. Because of sometimes poor data situation partly extrapolated. Data were collected over the years in countless publications, and then evaluated and displayed graphically.
Densupsoontorn N, Jirapinyo P, Thamonsiri N, Chantaratin S, Wongarn R (2004): Lactose intolerance in Thai adults. J Med Assoc Thai. 2004 Dec;87(12):1501-5.
Scrimshaw NS, MurrayEB (1988): Prevalence of Lactose Maldigestion. Am J Clin Nutr 48 (Suppl): 1086-1098, 1988
Sahi T (1994): Genetics and epidemiology of adult-type hypolactasia. Scand J Gastroenterol 29 (Suppl 202): 7-20, 1994
Yoshida Y, Sasaki G, Goto S, Yanagiya S, Takashina K. (1975); Gastroenterol Jpn. 1975;10(1):29-34. "Studies on the etiology of milk intolerance in Japanese adults."