The United States is amid what one expert termed a "preventable epidemic" of colon cancer, and specialists hope that President Reagan's surgery will spur the public to learn more about how to escape this often-fatal disease.

Colon cancer is second only to lung cancer in frequency and as a cause of cancer death in the United States. An estimated 138,000 new cases will be diagnosed during 1985, and 60,000 Americans will die of the disorder.

A white male in this country has a 6.3 percent chance of developing colon or rectal cancer, and a white female a 6.8 percent chance. About half of those who get the disease die of it.

The large intestine is able to contain a slowly growing tumor within its walls for many years, making colon cancer one of the most curable of malignancies if it is detected before the growth breaches the intestinal wall. Yet, the intestine also hides tumors effectively.

"The colon is kind of a dumb organ. All it knows how to do, essentially, is to take in water and to store wastes. It doesn't have nerve endings that tell the rest of the body what is happening" when a tumor is growing within its walls, said Dr. Paul Sugarbaker, chief of colorectal surgery at the National Cancer Institute.

As a result, much research has centered on finding ways to reveal colon tumors early, when they are curable. But the perfect test still does not exist. The cheapest screening for the disease -- a chemical test for blood in the stool -- is unreliable, because so many other disorders produce bleeding. And the more accurate tests -- barium enema and colonoscopy -- are expensive, time-consuming, and uncomfortable for the patient. Bleeding is the commonest symptom that alerts someone with a colon or rectal tumor. But many tumors, and potentially precancerous growths like polyps, bleed only intermittently or in tiny amounts that may only darken a stool slightly or may be evident only with a chemical test. Sugarbaker said that by the time a patient notices bleeding or other symptoms, such as vague abdominal pain or a narrowing of the diameter of bowel movements, the cancer has already spread to lymph nodes.

Doctors vary in their opinions on how often adults without symptoms should be checked for tumors or polyps, and on what tests they should undergo.

The American Cancer Society recommends a digital rectal examination be performed annually on everyone over age 40, and that chemical tests to detect blood in the stool be done annually after 50. The society recommends an examination using a sigmoidoscope -- an optical device for peering into the last third of the 6-foot-long colon -- every 3 to 5 years after age 50, after two initial exams.

Some physicians forgo the sigmoidoscope exam unless chemical tests show blood in the stool. Others argue that even regular looks at the colon with a sigmoidoscope may be inadequate, because the instrument sees only the third of the large intestine closest to the rectum.

To view the other part of the colon, called the right, or ascending, colon, where Reagan's growth occurred, a patient must have either a barium enema (an X-ray procedure) or a colonoscopy (with a fiber-optic viewing tube that sees as it moves around the curves in the colon). A colonoscope was done on Reagan last Friday.

Other studies center on why colon cancer rates are highest in certain affluent countries -- the United States, Canada and Australia -- and lowest in Africa.

Colon and rectal cancer, perhaps more than any other malignancy besides lung cancer, appears to be an environmentally caused disease. It strikes city-dwellers more often than people in rural areas, attacks the rich somewhat more commonly than the poor and occurs most frequently in societies where the diet is high in fat and low in fiber.

Geographic differences exist even within the United States. Northerners have a significantly higher risk of colon cancer than southerners. And studies have established that people who immigrate from an area with a lower incidence of the disease, such as Asia, develop high "American" levels of colon cancer even after one generation.

National Cancer Institute epidemiologists also recently designed a study to find out what happened to colon cancer risk in northerners who move south. They examined the health histories of a group of New Yorkers who moved to the Miami area. The results were intriguing, according to Regina Ziegler, a cancer expert with the institute's Environmental Epidemiology Branch.

"There was some evidence that if you moved at a young enough age to Florida, you could acquire a reduced risk of colorectal cancer," she said.

Although clear geographic variations exist, Ziegler said no one knows what it is about life in Africa, or Florida, that makes a person less likely to develop colon malignancies.

She said the evidence is strongest that fiber is protective, suggesting that Americans should eat more of the foods richest in fiber -- vegetables, fruits, legumes (peas and beans), grains and nuts. No particular fiber source, such as bran, is considered better than others, nor do scientists understand what fiber's protective action is.

One theory holds that fiber, which is simply undigestible material, adds bulk to the stool and speeds its passage through the large intestine. But Ziegler said research has since shown that "transit time," or speed of passage, "wasn't that consistently related" to cancer risk.

Other evidence indicates that eating a diet high in fat raises risk of colon and rectal cancer, perhaps by stimulating the production of bile acids, important in digesting fat. These acids may be converted by colonic bacteria into cancer-promoting substances. If bile acids do contribute to cancer formation, fiber may counteract their influence by changing the population of bacteria that live in the colon, Ziegler said.

Protective substances in foods, those that may hinder the formation of cancer, include beta carotene (a source of vitamin A), vitamin C and "cruciferous vegetables" -- cabbage, broccoli, cauliflower and Brussels sprouts, she said.