For many Americans milk - as the song says - is a "natural." So it is understandable that many people who had looked upon milk as an almost "perfect food" were shocked to learn that the majority of the world population, non-Caucasians in particular, have trouble digesting lactose, the main carbohydrate in milk.

As a result, many parents have been asking whether they should give their children milk. And indeed, some nutritionist and administrators who are responsible for large-scale feeding programs here and throughout the world have suggested that substitutes be found for milk in these programs.

It is hard to find a good substitute, however. Milk is inexpensive and plays an important role in the American diet, providing about 75 percent of our total calcium, 40 to 50 percent of the total riboflavin (vitamin B-2) 25 percent of total calories. Fortified milk is also our one certain dietary source of vitamin D.

While we have many substitutes for protein and riboflavin, there are not many for calcium, and vitamin D is especially important for children.

Fortunately, recent studies show that even those who have difficulty tolerating lactose usually can still consume some milk without problems. To understand this, you need to know what is involved in the digestion of lactose.

Lactose is a sugar that is broken down by the intestinal enzyme lactase into two simpler sugars, glucose and galactose, which are used by the body for fuel. Persons with lactose intolerance have a deficiency of lactase. So, instead of being broken down, the lactose remains in the intestine, where it is metabolized by bacteria, causing bloating, cramps, diarrhea nad nausea. At birth, all mammals that nurse from their mothers have high levels of lactase, and the same is true for human infants, except for a few with congenital lactase deficiency, who must be given special milk-free formulas.

As many children grow up, however, their lactase level goes down. We don't know whether this is an inherited deficiency that slowly manifests itself, or whether it results from decreased intake of milk, but the odds are in favor of an inherited condition. Western European populations with a long tradition of raising cattle and drinking milk have a low level of lactose intolerance, and some suggest that natural selection has evolved children there with high lactase levels. But between 60 to 80 percent of Greek Cypriots, Arabs and Ashkenazic Jews have lactose intolerance, and among African Bantus, Japanese, Thais, Formosans and Filipinos, the figure rises to 90 percent.

In the United States, it has been estimated that up to 55 percent of the entire population has a lactase deficiency, and 70 percent of all American blacks have lactose intolerance. In children, the deficiency is found in 10 per cent of whites and 35 percent of blacks.

To diagnose lactose malabsorption, a person is given up to 50 grams of lactose, the amount in about a quart of milk. It's given on an empty stomach, and the body is monitored for a rise in blood glucose. If the increase is less than 20 milligrams for each 100 milliliters of serum - which is the normal increase - the patient is not absorbing the lactose properly. In addition, 70 to 90 per cent of those patients with lactase deficiency will show the gastronintestinal symptoms of lactose intolerance.

But even if the results of the tests are positive, the person may still be able to drink a normal amount of milk without showing adverse effects. For example, a group of children and adults tested in India by Drs. V. Reddy and J. Pershad had a high rate of lactose intolerance, but all the children were able to drink about four-fifths of a cup of milk without difficulty. When Drs. C. Garza and N.S. Scrimshaw of M.L.T. tested about 100 white and black children, they found that over half of the 69 black children were lactose intolerant, but still had no trouble consuming a single cup of milk. And in still another study, it was found that adult volunteers who were lactose intolerant could still drink over a cup of milk or eat 15 grams of lactose.

Still, this does not mean that there are not children and adults who cannot tolerate milk. Obviously, there are. Dr. M.C. Latham and his colleagues at Cornell report that although the vast majority of children they studied liked milk, 13 percent reported stomach pains after drinking it. But such people still may be able to drink smaller portions - say, half a cup at a time - spaced throughout the day. Also, it is easier to tolerate milk when it is at room temperature.

Parents whose children simply cannot drink milk should be certain that adequate substitutes are offered. Cheese, which contains little or no lactose, and yogurt, in which bacteria convert lactose to lactic acid, which is easily absorbed by all, may be acceptable. And if you doubt that your child is getting enough calcium or vitamin D, consult your pediatrician or a dietitian recommended by your doctor.