It is odd that the one natural product we use the most in our day-to-day diet is one we are naturally intolerant to. I'm talking about milk. The human infant is born with the enzyme lactase which helps aid digestion of breast milk during the first several months of life, lactase being the enzyme that reduces the protein lactose found in milk to the easily digestible glucose and galactose. After that, intake of milk reduces and the body no longer has need for the enzyme and natural production reduces (lactase nonpersistence). The human milk contains nutrients that the infant needs for growth. The proteins are less complex when compared to that of bovine forms of milk, which is the milk most people use in current times. The composition of cow's/buffalo's milk is vastly different with bovine forms containing much higher calcium content and more complex proteins. If you compare the sizes and development of calves and the human infant, these differences are justified. Hence the human baby would find it harder to digest bovine milk even with higher lactase production. So would the human adult, whose lactase production is naturally lower.
I came across an interesting article regarding evolution of lactase in the human in which was stated that early humans could not digest milk ("Early man couldn't stomach milk."). The comparatively increased production of lactase in human intestines now seen, whereby many people are able to digest milk without any problems while the rest of the population is lactose intolerant, is considered a genetic mutation. Evolutionary changes take a long time to manifest so it might be some centuries more before lactose intolerance disappears completely.
Plant sources of rich protein and calcium, to say nothing of other essential minerals and vitamins, are abundant. So milk, though touted as absolutely necessary for bone growth and muscle mass, is not absolutely essential; certainly not the large quantities consumed daily by us.
Though taking milk directly might not be entirely a good thing, the more beneficial milk products need not be ignored. Curds or yoghurt is a more easily assimilated form of milk. Here bacteria do the initial digesting for us by breaking down the complex lactose in the milk, during the curd-forming process. These are the lactic acid bacilli which are also present in the human intestinal flora. So these are gut-friendly bacteria. This predigested form is the best way to take milk.
Paneer ( cottage cheese ) too is a good alternative. In this form, complex protein are left behind in the liquid whey that is drained out of cottage cheese during preparation. Paneer is a more easily digestible form of cheese and can be used to partially replace heavier cheese like cheddar in any meal.Paneer is available commercially but is much more tastier when prepared fresh at home. It is easy to prepare and has more flavor when compared to the kind available in supermarkets.