Is Drinking Milk Natural?

Babies have high level of an enzyme called lactase in their small intestine.  This allows then to digest the lactose in milk.  The persistence of lactase in adults is genetically-determined and highly variable.

In cultures and geographical areas where milk drinking has a long history (Northern Europe, parts of Africa and Arabia) a high proportion of the population shows lactase persistence (Beja-Pereira et al, 2003; Hollox et al, 2001). This suggests that there has been an evolutionary shift towards lactase persistence paralleling the development of dairying approximately 8-9 thousand years ago.

Thus, whether or not you consider the drinking of cow's milk 'natural' is a matter of perspective.  A person with lactase persistence is able to drink milk in reasonable moderation and experience health benefits rather than deficits. 

Reference:

• Beja-Pereira, A. et al. (2003). Gene-culture coevolution between cattle milk protein genes and human lactase genes. Nature Genetics, 35, 311-313.
• Hollox, E.J. et al (2001). Lactase haplotype diversity in the old world. American Journal of Human Genetics, 68,160-172.