Lactose is a sugar found naturally only in the milk of mammals. At birth, almost all mammals, including humans; produce an enzyme called lactase which splits lactose into sugars (glucose and galactose). Most mammals, including humans, stop producing lactase after weaning. If they consume milk, the lactose cannot be digested in the small intestine and passes to the large intestine where bacteria ferment it.
Studies show that most people with lactose intolerance can tolerate between 150 to 200ml of milk with out problems. Larger quantities may produce wind with bloating and pain. Many studies show that the production of lactase for milk digestion in adulthood is genetically related. For instance, while many with Anglo-Saxon background and from Africa continue to produce lactase throughout life and continue to consume milk and digest its lactose without difficulty, providing a source of calcium and other nutrients, others from backgrounds such as Asia and the Middle East may gradually stop producing lactase by puberty.
Yoghurt can be eaten by those who are lactase deficient because the bacteria in yoghurt changes some of the lactose to lactic acid. Nutritionally, yoghurt is like a concentrated milk product so is an excellent source of calcium, protein and Vitamin B2.
Be aware that many types of yoghurt (frozen, flavoured etc.) may contain large quantities of added sugar. These products are often marketed to children. A more healthy option would be to use unflavoured yoghurt and add fresh fruit (pureed, finely diced etc.) for a higher nutrient content and less sugar.
Cheese is also an excellent way for those who are lactase deficient to obtain nutrients from dairy. During the production of cheese most of the lactose is left in the whey, leaving a low level in the finished product. It takes about 10 litres of milk to make 1 kg of cheese, so the consumption of cheese is a concentrated source of nutrients found in milk. Note that as ricotta is largely made from whey it may produce problems for those with low levels of lactase.