MILK is indisputedly the ideal food for infants and an excellent food for young children. But as people get older most lose their ability to digest it fully. This happens because intestinal production of the enzyme lactase declines with age in all but a small segment of the world's population. Lactase is needed to split the two-part milk sugar lactose into its component sugars, glucose and galactose, so that they can be absorbed through the intestinal wall.

Adult lactase deficiency is so common that it is considered the normal state of human affairs. Only in northern Europeans and their descendants and in two African tribes do most adults retain their childhood ability to digest lactose. An estimated 30 million Americans cannot digest lactose properly, and among certain ethnic groups - blacks, Asians, Mediterraneans, Jews, southern and central Europeans and American Indians - 70 percent or more have difficulty digesting lactose as adults.

The Problem. When someone with a lactase deficiency consumes large amounts of a lactose-containing food at one time, much of the sugar passes undigested through the stomach and into the intestines, where it absorbs a lot of water and becomes food for intestinal bacteria that form gases and acids. This often results in such symptoms as abdominal bloating, flatulence, cramps, loose stools and watery diarrhea.

Because a direct relationship between consumption of milk products and symptoms may be hard to detect, many people suffer silently for years, perhaps blaming various other foods for their discomfort and avoiding the suspected culprits. It's hard for many sufferers to consider that so innocent and wholesome a food as milk could be the cause of their misery.

Others see doctor after doctor seeking in vain an explanation for their discomfort. Sometimes they are misdiagnosed as suffering from bowel disease. Occasionally, when no disease is found, they are told they have an emotional disorder.

However, once the problem is properly identified, it can nearly always be kept under control, and usually without giving up the consumption of nutritious dairy products.

Milk and milk products are excellent sources of essential nutrients: phosphorus, potassium, riboflavin, vitamin B-12, and artificially added vitamin D and vitamin A. But the most crucial of milk's nutrients is calcium, the mineral necessary for strong bones and teeth.

Few popular foods rival dairy products as a calcium source. Two cups of milk supply three-fourths of the recommended daily amount of calcium for preteen-age children and nonpregnant adults. Only sardines eaten with the bones are a better source of calcium. Vitamin D is needed for the body to absorb calcium and nearly all milk is fortified with this vitamin. In the United States, milk and milk products supply three-fourths of all dietary calcium.

In part because Americans today drink much less milk than in the past, many people fall short of the recommended daily intake of calcium. In fact, many experts in bone disease say the daily recommended amount of calcium is too low; they suggest that adults should consume the calcium equivalent of a quart of milk a day, between 1,000 and 1,200 milligrams of calcium a day.

A chronic calcium shortage can result in a disease called osteoporosis, a weakening of the bones that is epidemic among older Americans and is responsible for the fractures that commonly afflict older people.

Not everyone with a detectable lactase deficiency gets symptoms of lactose intolerance, as the difficulty in digesting milk sugar is called. Most people who would react to a large dose of lactose can consume small amounts at a time, especially if it is part of a meal.

Thus, the milk added to coffee is not usually a problem, and many lactose-intolerant people can comfortably drink a glass of milk at the end of a meal.

Furthermore, some cases of lactose intolerance are not a permanent problem built into the genes. A temporary lactase deficiency can follow a diarrheal illness, radiation therapy to the abdominal area and treatment with certain anti-inflammatory drugs and antibiotics. Lactase deficiency can also result from stomach surgery and disorders like celiac disease, cystic fibrosis, tropical sprue, cholera, irritable colon, Crohn's disease and ulcerative colitis.

Diagnosis. There are many ways to diagnose lactose intolerance. One of the simplest is to see if symptoms disappear when all sources of lactose are eliminated from the diet, and whether they return when lactose is again consumed.

Dr. Norman S. Rosensweig, gastroenterologist at St. Luke's-Roosevelt Hospital Center in New York, suggests another easy test: consume two glasses of milk at one time in the morning on an empty stomach and take note of the resulting symptoms. Those with lactose intolerance are likely to react within an hour or two of drinking this amount of milk. The test is best done on a day when you can afford to feel unwell for a few hours.

A more sophisticated test involves measuring the amount of hydrogen in the breath following a dose of lactose. The hydrogen comes from digestion of the lactose by intestinal bacteria; the hydrogen gas is absorbed into the blood stream and then exhaled through the lungs.