Domestic cattle (Bos taurus). Photo © Muhammad Mahdi Kari · Wikimedia Commons · GFDL 1.2
Domestic goat (Capra hircus). Photo © Fir0002 · Wikimedia Commons · GFDL 1.2
Although accepted as a normal part of the human diet in affluent countries, milk is ambivalent in its nutritional value.
This is because it contains excessive calcium relative to other mineral nutrients such as boron, and excessive proportions of the sugar galactose, which is mildly toxic for adults despite its essential requirement in infants.
The main supplementary value of milk in an otherwise balanced diet is in a component of butterfat: butyric acid, which is poor in almost all other foods and qualifies as an essential fatty acid or even as a vitamin in its own right.
Robin and the Honey Badger, 8 January 2017