President Reagan's recent physical problems, tentative as they are, have drawn attention to a couple of inelegant but potentially dangerous conditions, both of which become increasingly common with advancing age.

And in both cases, there is the promise of near certain containment if they are caught in early stages.

They are colorectal cancer -- the second most lethal cancer in men (lung is first) and the third in women (behind lung and breast) -- and the ubiquitous diverticulosis, a gastro-intestinal disorder that affects up to 60 percent of the over-50 population.

The president's possible diverticulosis was diagnosed last May during the same examination in which the first of his intestinal polyps was discovered.

But, according to Dr. Arnold Levy, a Silver Spring gastroenterologist and vice president for education of the American Digestive Diseases Association, "it is not typical for a polyp as tiny as the president's doctors are describing to bleed, nor is this type of bleeding typical of diverticulosis.

"If someone has bleeding from diverticulosis, it is usually massive, with a lot of bright red blood, enough so you can see it easily, not the hidden, or, as we say, occult blood as found in the president's test."

So President Reagan is confronted with the kind of situation shared by thousands of Americans every year: Tests that, in themselves, suggest no more than the need for further examinations, and an unrelated condition in the same area that can be troublesome if it is neglected.

"Clearly," says Levy, "there is a concern because we know that carcinoma of the colon starts out in most situations as a benign polyp, and the larger the polyp, the greater the chance that it becomes cancerous." For example, he says, "any polyp that is greater than two centimeters in diameter has, statistically, a 10 percent chance that it already has a cancer in it -- pretty high odds."

But occult or hidden blood can come from many things, says Levy, "even bleeding gums or eating rare meat." Other foodstuffs -- turnips or horseradish, for instance -- can interact with the test and show false positive results, and megadoses of vitamin C, aspirin and other anti-inflammatory drugs can cause traces of blood to show up. Such things have been temporarily eliminated from the president's diet, pending further tests.

If the second round of stool tests -- in the president, or any of us who share his situation -- are still positive, there are further tests that may be done.

A so-called digital examination can detect some 6 percent of polyps.

A proctoscopic exam -- now done with an instrument "flexible as spaghetti" through which a physician can see portions of the colon -- can identify about 25 percent of the growths.

An X-ray of the colon in conjunction with a barium enema can reveal growths as shadows and, says Levy, "you're talking 85 to 90 percent diagnostic pickup rate."

Finally, says Levy, there is now a procedure that permits a gastroenterologist using a colonoscope, fitted with fiber-optic lenses and a steering mechanism, "to examine every square inch" of the colon.Using this instrument, a biopsy -- in which a crumb-sized slice of a suspicious bump, or the entire polyp, can be removed for microscopic study -- can easily be performed immediately. This procedure once required a hospital stay but now can be done on an outpatient basis.

"The key to controlling colon or rectal cancer is early detection," says Levy, "because once it has spread beyond the stalk of the polyp, it responds poorly to either chemotherapy -- only one agent works at all -- or to radiation. Surgery is not only the last resort, but virtually the only one. Yet if a cancer is removed still within the polyp, the patient is simply cured."

Early detection requires screening tests -- every year for everyone over 40, because, says Levy, only 1 percent of colorectal cancers show up in people under 40. Moreover, there needs to be appropriate follow-up. For example, he says, patients who have bleeding from hemorrhoids cannot simply assume that another positive test the next year is from the same disorder.

A number of home kits to detect occult blood are on the market, but Levy and other gastroenterologists believe that although the tests may be administered at home, they should be interpreted by a specialist because people tend to deny that anything is wrong and because they might misread a marginal result. "I worry," says Levy, "about someone missing a positive reading."

Scientists are beginning to believe that diet and environmental carcinogens like food additives may play a role in making colorectal cancers so common in industrialized areas -- the United States, Canada and Europe -- and so relatively rare in Asia, Africa and South America. The major dietary difference is fiber content -- high in the low cancer areas and low where cancer rates soar.

Whether or not a higher fiber diet can prevent polyps or cancer is uncertain, but researchers do believe that "the key thing in diverticular disease is prevention and the key thing in prevention is increased fiber in the diet."

A diverticulum forms when the colon lining herniates through the muscular wall of the colon forming a bulge on the outside and a pouch on the inside. It's rather like a blowout on a tire, where the outside lining has split and the inside lining bulges out through the wall itself, says Levy. "Obviously, the part that is bulging out is very thin."

These pouches can cause a great deal of trouble if they become plugged up, allowing infections and local abscesses to form. The inflamed pouches can cause gastrointestinal distress -- spasms, cramping that may appear very like irritable bowel syndrome.

The presence of these inflamed pouches is called diverticulitis. These pouches tend to bleed and if the walls are very thin, says Levy, "they can bleed a tremendous amount very rapidly, but they rarely bleed just a little here and a little there."

If the bulge should burst, emergency surgery will be required to prevent a wide-spread infection.

Diverticulosis is preventable and often manageable with a high fiber diet. (When diverticulitis exists, fiber must be temporarily removed from the diet to allow healing.) Diverticulosis is rarely seen in those under 30 but probably affects one third of the population over 50.

"The theory here," says Levy, "and it has been fairly well proven, is that people who have low fiber in their diets tend to have higher pressure generated inside the colon in the attempt to move food and waste materials, and it is probably the increased pressure that results eventually in a blowout." Resources

For more information write the American Digestive Diseases Society Inc. 7720 Wisconsin Ave. Bethesda, Md. 20814. On Tuesday nights call Gutline, 652-9293, to speak with a volunteer gastroenterologist.