 We've known that breastfeeding infants may protect against subsequent obesity for over 30 years, but why? Well, giving human infants formula based on cow's milk presents an unusual situation. Cow's milk is designed to put nearly two pounds a day onto a growing calf, 40 times the growth rate of human infants. The perfect food for humans finally tuned over millions of years is human breast milk. Remarkably, among all mammalian species, the protein content of human milk is actually the lowest. The thought is that it's the excessive protein content of cow's milk-based formula that sets the child up for obesity later in life. And then instead of just being weaned, we continue to drink milk. The question thus arises as to whether consumption of a growth-promoting substance from another species throughout childhood fundamentally alters processes of human growth and maturation. This study, for example, found evidence that greater milk intake is associated with increased risk of premature puberty, girls drinking a lot of milk starting to get their periods earlier. Thus, cross-species milk consumption and ingestion into childhood may trigger unintended consequences. In contrast to feeding artificial infant formula, only human milk allows appropriate metabolic programming and protects against diseases of civilization in later life. However, continued consumption of cow's milk and dairy products during adolescence and adulthood is an evolutionarily novel behavior that may have long-term adverse effects on human health. Teens exposed to dairy proteins, for example, casein, skim milk, or whey experience the significant increase in BMI and waist circumference compared to controls, whereas not a single study funded by the dairy industry found a result unfavorable to milk. The head of the Obesity Prevention Center at Boston Children's Hospital and the chair of Harvard's Nutrition Department wrote an editorial in the AMA's Pediatrics Journal questioning the role of cow's milk and human nutrition. We have no requirement for other animals' milk, obviously, and in fact dairy may play a role in certain cancers due to the high levels of reproductive hormones in the U.S. milk supply.