At his Longevity Center on the boardwalk here, Nathan Pritikin is taking on heart and artery disease, diabetes, hypertension, arthritis - and most of the medical profession.

Just one year ago, Pritikin moved his Center into the 125-room hotel formerly owned by Synanon. He brought in a staff of physicans, exercise equipment, sophisticated monitors, a recipe file containing almost nothing but vegetarian dishes, and a highly controversial program for drastically overhauling the life styles of very sick people.

Some of the patients can barely walk a few hundred feet when they arrive. They pay $4,000 for 26 days in the Pritikin program which is based on the belief that diet and exercise not only can reverse the course of many degenerative diseases by cleaning the fat and cholesterol out of the arteries, but prevent disease as well.

The nutritionist has recently described the plan in his very popular book The Pritikin Program for Diet & Exercise written with Patrick McGrady Jr. (Grosset & Dunlap, $12.95) in which an additional 20 or so diseases, including obesity and cancer, are listed as possible candidates for reversal.

The problem is that Pritikin's views are not accepted by many doctors - largely because conclusive scientific proof of the effects has never been conducted. Two years ago, when the Longevity Center was still located in a motel on the beach in Santa Barbara, the slim founder, whom most people take for 10 years younger than his 63 years, was not so critical of traditional medical practice, in particular the National Heart Lung Blood Institute (NHLBI).

At that time Pritikin still entertained the idea that the Heart Institute's director, Dr. Robert Levy, could be won over to his theories and would fund research to prove them scientifically. Today, Pritikin is openly hostile: "Levy is our enemy," says the ordinarily mild-mannered man who was once a candidate for a heart attack. So Pritikin will undertake to fund the research himself. He says it will cost him $1 million to compare the results of his diet with those of the American Heart Association diet soon to be tried on 1,000 heart patients.

Pritikin's diet permits only 5 to 10 percent of the total calorie intake from fat, 10 to 15 percent from protein and 80 percent from complex carbohydrates. The Heart Association diet allows 30 percent fat. The traditional American diet is more than 45 percent fat.

Dr. Levy contends that even though Pritikin's diet "could produce changes, the bottom line is there is no followup." As head of a government agency which controls millions of dollars a year in research funds, Levy says: "We can't invest in this unless we know we'll get enough compliance and follow-through. One of the biggest problems is dropout. [Pritikin hasn't] demonstrated research and longevity."

And before the government invests federal funds it wants research data. According to a March 8, 1978, memorandum of a visit to the Longevity Center by group of NHLBI consultants, "The critical problem (and short-coming) related to this program involves the apparent absence of followup and maintenance strategies."

"The whole system is excellent for 30 days in a hospital," says Levy, "but they did not get these people ready for a lifetime of this."

Pritikin's diet permits only minute amounts of fat and salt. Sweeteners and beverages containing caffeine or alcohol are forbidden. Only 1 1/2 pounds of lean meat per week are allowed on the maintenance diet; far less at the Center. A vigorous exercise program is tailored to the physical ability of the patient.

Most of them are suffering from advanced stages of cardiovascular disease; often they are candidates for coronary bypass surgery. Usually they go to the Center as a last resort, not at all convinced that the course of their disease can be reversed without an operation or that they might go home well enough to stop their medication.

In the preface to his book, which is now on every major best-seller list, Pritikin admits: "This is not an easy diet. It requires a reversal of life-long bad habits and nutritional fallacies." On the other hand, he says: "The diet revives old food friends, unjustly scorned by fad diets: breads, cereals, pastas, fruits, vegetables, soups, rice, potatoes . . . With some ingenuity you can produce gourmet delights in every meal category."

To stick with it, especially outside the home, requires a strong commitment because most restaurant, air-plane and dinner party food is forbidden. How many people, not faced with imminent death or disability, are likely to give up soft drinks, steaks, butter, cookies and cakes because it might lengthen their lives?

Pritikin says there are already people coming to the Center for prevention rather than cure. "No one is immune to damage from high fat and cholesterol," he says. While he acknowledges that there are genetic differences which make some more susceptible to heart disease than others, "You don't find anyone anymore in this country that is free of artery closure at 20 years old. There are no exceptions."

The goal of the Center is to get serum cholesterol levels below 160. "Anything above 160 is going to close your arteries," Pritikin says. "The average cholesterol level of the guy who drops dead of a heart disease is 237."

Dr. Arthur Ulene , a physician who offers medical information on both network and local television news programs, disagrees that everyone needs the diet. "Pritikin discounts genetic differences that exist. Some of us appear to be genetically lucky. So the blanket advice he has for everyone is inappropriate. People who can eat eggs should be able to."

And even some of those who feel they must stay on the diet have trouble with it. Jim Guthrie, a civil engineer from College Park, had a heart attack in 1971 at the age of 33 and bypass surgery in 1976. He went to the Center this winter, unable to walk more than a block and taking two to 12 nitrogylcerin pills a day. After the Center he was walking 10 miles a day. He lost 18 pounds. But he still misses sweets. "The food is filling and you don't feel hungry. Yet I don't feel satisfied."

Guthrie has slipped off the diet and says he can't find the time to exercise. Some of his pain has returned and he's taking one nitro a day. He "doesn't like only allowed certain things." But he says "I can see it's helped me, and if I can stick with it I feel I can get rid of my chest pains completely."

Levy agrees that Pritikin's diet may be responsible for cardiovascular improvement. "Regression has been shown in animals," he says, "and has been suggested in man." Levy has modified the position he took in April 1977, when he said: "If Pritikin wants to claim people feel better and can walk further and lose weight in the 30 days, I have no objection, but if he wants to claim he has reversed atherosclerosis [a thickening and loss of elasticity in the arterial walls] or has a cure-all that can help all of America, that's not true."

At that time Pritikin backed off his claims for reversal. "We do not talk of reversal, we talk about return to normal function. We have too few cases of [anatomical] reversal but tremendous functional reversal."

Since then, regression of atherosclerosis in man has been demonstrated and there have been several studies showing regression in animals.

Some other physicians are a little less cautious than Levy. According to Ulene, Pritikin's program "is extremely similar to one used several years ago at the University of Southern California and proved objectively to reverse atherosclerosis."

Like Levy, Ulene feels that "the diet is difficult to maintain. But that's not a reason to criticize it."

In a telephone survey, conducted by two University of Southern California psychologists, 40 Longevity Center graduates were asked how they felt about the diet: 85 percent said good; 15 percent said bad. Asked about compliance, 60 percent said they complied at least 70 percent of the time; 20 percent said they complied less than 50 percent of the time. "These responses," says the survey, "make it clear that some form of active support system is needed to help the graduates adhere to the program."

There are other criticisms of the Center. Dr. William Castelli, director of laboratories for the famous Framingham heart study, said, "The trouble with Pritikin's place is it's too highly commercialized." But he added, "It's probably one of the places you can get a proper diet. In some instances they do a very good job."

Pritikin agrees the Center is not for everyone. "How many people can we influence through the Center - 5,000? 10,000? The book is our means of education." And for those who cannot afford either Santa Monica or the branch in Bal Harbor, Fla., Pritikin has started a better-health program. It is a course of weekly two-hour lectures, demonstrations and testing. It lasts 12 weeks and costs $625.

Pritikin believes so strongly in the power of education, especially through the media, that he thinks he could convince "50 percent of the people in this country to change their diets" if he could have eight half-hour segments on network television. "If American women can be talked into wearing cowboy boots and high heels, they can be talked into changing their life styles."

What those sympathetic to the Pritikin program can't understand is why, as one of them said, "the government won't take some of those millions they spend on health care every year to help the center conduct the research and collect the data." CAPTION: Picture, Nathan Pritikin has filled his Longevity Center with physicians, exercise equipment and sophisticated monitors, photo by John Barr for The Washington Post