First there was the bad news that dietary cholesterol, sodium and saturated fats contribute to heart disease. Now comes the word that extra body weight increases the risks even more.

For most Americans, figuring out how to lose weight, make permanent diet changes and still eat food that tastes good was a real challenge -- until a team of researchers at Baylor College of Medicine in Houston published "The Living Heart Diet" (Simon & Schuster, $19.95).

This is no fad diet. The "Living Heart" approach is based on extensive scientific research and a five-year track record. Dr. Michael DeBakey, the noted heart surgeon, and Dr. Antonio Gotto Jr., former president of the American Heart Association, wrote the book with Lynne W. Scott, chief dietitian at Baylor's Diet Modification Clinic, and John P. Foreyt, director of the diet clinic and a clinical psychologist.

"The Living Heart Diet" gives solid advice for anyone who wants to lose weight, follow doctor's orders to lower cholesterol, decrease blood pressure, or just cut down on dietary fat as a way of lowering the risk of heart disease. "This isn't just a diet," says Foreyt. "It's a way a person can live to maximize health or to become healthy if one is not."

Forget calories. The "Living Heart" approach uses a system of food groups and "exchanges" to meet the dietary guidelines recommended by the American Heart Association and more recently by a consensus conference sponsored by the National Institutes of Health. Dieters have several plans to choose from; average weight loss is a sensible 1 to 1 1/2 pounds a week.

Exchanges are servings of food, divided into major food groups. "Living Heart" participants learn how many exchanges of each group they need, and then choose the foods that are most appealing to them. For instance, during a given day, a dieter might be allowed to eat two exchanges of low-fat dairy products, three exchanges of grains and starchy vegetables, five exchanges of oils, margarines and other fats, three exchanges of fruits, and six exchanges (about 6 ounces) of lean meat, poultry or seafood.

And, as the term implies, they can be exchanged for other foods in the same group. One "bread exchange" could be a slice of whole wheat bread, 1/2 cup of cooked macaroni, 1/3 cup of corn, 1/4 cup of sherbet, or other quantities of starchy foods such as peas, beans, rice, crackers, pasta and so forth.

The "Living Heart" carefully balances the level of calories consumed in fats, carbohydrates and protein. It does not allow more than 300 milligrams of dietary cholesterol, and fat intake is limited to the recommended 30 percent of total calories, with proper proportions of saturated, polyunsaturated and monounsaturated fats.

Protein provides only 20 percent of the day's calories, and carbohydrates are raised to 50 percent.

"To the best of our knowledge," adds DeBakey, "these are the things that we can do now to help reduce the risk of heart disease . . . It's a healthy diet, very moderate. There's nothing radical about it."

The plan is also good for people with special dietary needs created by existing diseases, such as diabetes and hyperlipidemia -- in which high levels of fat build up in the blood, apparently leading to a significantly increased risk of cardiovascular disease. It has a special section -- including numerous recipes -- devoted to people who must limit sodium intake.

DeBakey and his coauthors wrote the "Living Heart Diet" to fill a void they found in nutrition and diet book market. "In reviewing most of the diet books, we found them unsatisfactory and couldn't recommend them to patients," DeBakey says. "We decided to write this book so that lay people can read and understand diet and its relationship to atherosclerosis [hardening of the arteries]."

The book accomplishes this with a simple, straightforward approach that limits scientific jargon and depends on practical, easy-to-implement diet advice. "I have tried to include very basic recipes that don't take a lot of special ingredients or a lot of time to prepare," says dietitian Scott. "It's not a temporary diet. We would rather help people develop healthy eating habits that they can use for the rest of their lives and for every member of their family."

Key to the "Living Heart" approach is a large dose of nutrition education. "Most popular diet books won't give you this," explains DeBakey, who follows the diet himself. "What we wanted to do was to educate the individual, so that he or she can control the diet themselves."

The book also teaches the important art of self-monitoring. Adherents to this approach chart their weight every day and learn to keep a food diary, one that has them record not just the amount of food eaten, but also the time, the place, who it was eaten with and the mood at the time of the meal. "The two most important strategies are charting the weight and writing down the food eaten," says Foreyt. "If you did just two things, those would be it."

Research indicates that the "Living Heart" approach works. Since 1976, some 5,000 people have gone through Baylor's program. About 73 percent lost weight and either kept it off for at least a year or were continuing to lose weight, Foreyt says. As for the one in four who didn't do as well, 10 percent gained back some of the weight lost, and 17 percent gained it all back.

Results of studies on the diet also show that it successfully lowers blood cholesterol and other blood fats. Even people with normal blood cholesterol levels (at least for Americans) were able to reduce blood cholesterol by about 9 percent.

What's particularly interesting is that follow-up studies show that older people -- those between 50 and 80 -- were the best at sticking with the diet over three years. "We were surprised at the data," says Foreyt. "They suggest that the more senior people can and are willing to change their health habits ; they just have to be given the opportunity."

Adds Gotto: "If there's one message to try to get across, it's for people to start the diet and use it before heart disease gets started."