To what extent does milk satisfy the requirements of a balanced diet for a newborn mammal?
For an organism’s diet to be “balanced”, it must include the correct proportions of all the nutrients that are needed for that organism to function and grow. For humans this means lipids and carbohydrates for energy, mineral salts for growth, vitamins to prevent disease and protein to repair damaged tissues, with water to help transport them all around the body.
As mammals tend still to be undergoing important development for a long period after birth, many are unable to digest solid food by themselves, and so much of their diet is in the form of milk, which must supply the newborn with water and nutrients. Since it is the only thing which the offspring can consume, it must deliver everything that is needed in the animal’s diet.
The milk given to very young baby mammals is called colostrum. It is produced in smaller quantities than later milk, but with a higher concentration of the nutrient. Colostrum provides immune cells, including lymphocytes and multiple forms of immunoglobin, as well as many growth factors. The milk contains high quantities of Vitamin A, salt and proteins. It does not, however, contain much in the way of lipids, potassium and carbohydrate – all of which will appear in later milk.
Not all milk is the same. The milk given by most mammals is largely made of water, yet seals give more than half its volume to fat. Humans have more sugary milk than that of most other animals, but a much smaller proportion of protein. The composition of the liquid is, in all cases, designed to best suit the nutritional needs of the child in accordance with their native environment.
Humans and cows produce broadly similar milk, but there are noticeable differences. For example cows’ milk is 4% protein, whereas humans’ is only 1%. This is due to calves needing to grow faster and to a larger size than humans, with which protein assists. Humans, on the other hand, give greater priority to brain and nervous development, where protein is less important. Though both species have similar amounts of fat per given quantity of milk, cows’ is more saturated. This is again due to size versus brain: the human brain triples in size over the first year, requiring a good supply of monounsaturated fatty acids, such as Omega 3 and 6, whereas cows need saturated fat to grow larger bodies.