All mammals start out life able to digest milk. Later in life, almost all mammals lose this ability. However, somewhere between three and eight thousand years ago some groups of people developed the ability to continue to digest milk into adulthood.

You see milk contains a sugar called lactose and as babies we all have the genes that alow us to produce an enzyme called lactase that breaks lactose down into more digestible sugars. But at one time we were like most other mammals and would lose this ability as we grew.

This has been shown by research from England and Germany demonstrating that European adults several thousand years ago did not have the mutated gene that allowed them to digest milk.

The researchers Mark Thomas (University College, London, England) and Joachim Burger (Mainz University, Germany) have reported these results on line on February 26, 2007 in Nature.com and in the journal Proceedings of the National Academy of Sciences.

They stated that European adults did not possess the lactase gene possibly up to eight thousand years ago, which allows the human body to digest milk by breaking down lactose.

The scientists based their research on the study of DNA (deoxyribonucleic acid, which carries an organism’s genetic material) from several Neolithic skeletons found in Hungary, Lithuania, Germany, and Poland, which lived about 8,000 years ago. In this DNA, the gene that allows milk consumption was absent.

People with the non mutated gene lose the ability to digest milk during childhood, this occurs due to thier DNA producing a protein called a transcription factor which represses or switches off the lactase production.

At present we are not sure what causes the activation of the repressor transcription factor but it starts after being weaned.

Lactase persistence allows satisfactory digestion of significant quantities of fresh milk and is common in Northern Europeans and certain African and Arabian nomadic tribes. However research shows that even for those with lactase persistence levels of lactase decline with age and so they can become unable to digest milk later in life.

Lactase non-persistence, in which lactase is not present in adults, is responsible for adult-type hypolactasia, a condition which results in diarrhoea, flatulence and meteorism when significant quantities of fresh milk are drunk.

For small group of people there is another cause of problems with digesting milk. They are alergic to some of the proteins found in milk. This is more common in infants with between 2 to 5% having problems. Only about 1 - 2% of adults are likely to have trouble. Milk allergy can lead to Skin Reactions, Stomach and Intestinal Reactions, Nose, Throat and Lung Reactions.

More information on milk tolerance can be found here.