You are what you eat, according to an old saying. Modern science has proven this saying true. A healthy body requires a healthful diet.

But what is a healthful diet? It's not exactly easy to find out. Bookstore shelves are crammed with weight-loss books, each proclaiming that it alone has the true solution to the problem of excess pounds. And diet advice aplenty circulates among friends and on the Internet.

Two organizations recently took a step toward clearing some of the murk. The Physicians Committee for Responsible Medicine (PCRM) released two brief reports, each of which compared the nutritional soundness of various popular diet books. And the United States Department of Agriculture (USDA) published a report comparing three types of diets: low-carbohydrate, high-fat diets; moderate-fat diets for losing weight; and low-fat and very-low-fat diets.

PCRM Diet-Book Ratings

PCRM has twice assessed popular diet books. In its May 2000 issue of Nutrition Action newsletter, it compared 10 books and rated them as acceptable or unacceptable. You can find the full report on the Web at www.cspinet.org/nah/5_00/diet.htm. On Jan. 9, 2001, PCRM issued a press release in which it rated 11 diet books with stars, from zero stars (unsafe) to five stars (outstanding). You can find this press release at www.pcrm.org/news/health010109. html.

In general, PCRM rated low-fat vegetarian diets as the healthiest, and high-protein, low-carbohydrate diets as the least healthy. The latter were found to be low in fiber, calcium, fruits, and vegetables and high in cholesterol, fat, and saturated fat and were deemed unsafe to follow. Mid-ranked diets frequently had too little fiber and calcium and too much cholesterol.

The table on page 66 shows how the individual books stacked up. Star ratings are from PCRM's May 2000 report, and ratings of "acceptable" and "unacceptable" are from PCRM's January 2001 press release. Book titles and authors are given as they were cited in these reports; current editions may not be identical in title or authorship.

PCRM's press release rated each diet based on five questions:

* Does it supply at least 25 grams of fiber a day?

* Does it include five or more servings of fruits and vegetables per day?

* Does it supply less than 50 milligrams of cholesterol a day?

* Do fewer than 30 percent of the calories come from fats?

* Do fewer than 10 percent of the calories come from saturated fats? A diet that can answer "yes" to all five questions reduces a person's chance of various illnesses, including heart disease, obesity, and cancer.

USDA Examination Of Diets

The USDA issued its report as a 40-page supplement to the March 2001 issue of the journal Obesity Research. Written in technical jargon, this report looks at the claims made for three different types of diets and the scientific evidence for or against each claim.

USDA divided popular diets into three groups:

* High-fat, low-carbohydrate diets, which get 55 to 65 percent of their calories from fat, fewer than 20 percent from carbohydrates, and 25 to 30 percent from protein. The USDA listed as examples the books Dr. Atkins New Diet Revolution, Protein Power, The Carbohydrate Addict's Diet, Dr. Bernstein's Diabetes Solution, and Life Without Bread.

* Moderate-fat, balanced nutrient reduction diets, which get 20 to 30 percent of their calories from fat, 55 to 60 percent from carbohydrates, and 15 to 20 percent from protein. The USDA listed as examples the Weight Watchers, Jenny Craig, and Nutri-Systems programs and--when calories are reduced--the USDA Food Guide Pyramid, the National Cholesterol Education Program Step I and Step II Diets, and the DASH diet of the National Heart, Lung and Blood Institute. Although USDA does not cite it, the American Diabetes Association's book The Commonsense Guide To Weight Loss For People With Diabetes by Barbara Caleen Hansen and Shauna S. Roberts also falls into this category.

* Low-fat and very-low-fat diets, which get fewer than 10 to 19 percent of their calories from fat, more than 65 percent from carbohydrates, and 10 to 20 percent from protein. The USDA listed as examples Eat More, Weigh Less and The New Pritikin Program.

The USDA report concluded, among other things, that:

* The amounts of weight and fat people lose when dieting depend on how much they cut calories and increase their activity, not on the proportions of fats, proteins, and carbohydrates they eat.

* Decreases in blood sugar levels and blood pressure depend on the reduction of calories, not on the proportions of fats, proteins, and carbohydrates.

* All popular diets result in weight loss and fat loss. But they vary in healthfulness and how well the weight stays off.

* It's possible to eat a nutrient-poor diet with all three kinds of diets. However, it's easiest to eat an adequate diet with a moderate-fat, balanced nutrition reduction diet. In contrast, high-fat, low-carbohydrate diets are low in several important nutrients.

* The benefits and long-term efficacy of high-fat, low-carbohydrate diets are not supported by scientific evidence, whereas the health benefits of moderate-fat, balanced nutrient diets are backed up by strong scientific evidence. Most of the research on low-fat diets has focused on their good effects on heart health, not on weight loss.

* The best diet for weight loss consists of 1,400 to 1,500 calories per day.

* It's not yet clear whether it's better for people who want to lose weight to focus on limiting calories or on limiting fat.

* For maintaining weight loss, low-fat, low-calorie diets are best.

Reconciling The Ratings

The PCRM and USDA ratings may seem to have very little in common. PCRM focuses on books and summarizes their good and bad points briefly and in easy-to-read language. USDA plows through the nitty-gritty scientific details of three different styles of food plans, with little to say about the pluses and minuses of specific books or diet plans.

But both PCRM and USDA use similar criteria for judging diets. They look at whether the diets provide the minimum amount of vitamins and minerals a person needs each day. They also look at whether the diets emphasize foods associated with lower risk of disease and limit the foods associated with increased risk of disease. So both give higher marks to diets that are:

* Low in saturated fats (found in meat, whole milk, and other animal foods)

* Low in cholesterol

* High in fruits, vegetables, and beans

* High in grains, especially whole grains

One small difference between the two sets of ratings is their evaluation of very-low-fat diets. Both organizations recognized that such diets tend to be low in certain nutrients found in high-fat foods, such as vitamins E and B-12, calcium, and zinc. Even so, PCRM gave two such diets its highest ratings because they are high in produce and fiber and low in cholesterol and fats.

Choosing A Diet

Which of the popular diet plans should you choose? Clearly, there are plenty of healthful ones to pick from. As long as you avoid plans that are low in fiber or plant foods or high in saturated fat or cholesterol, many diets are good choices. Your decision should be based on the answers to these questions:

* Is this diet easy for me to follow?

* Is this diet compatible with my family's lifestyle?

* IS this diet something I can stick to the rest of my life?

* Is this diet compatible with my health problems? For example, if you have kidney disease, your doctor may want you to restrict protein; if you are overweight, you'll want a diet that restricts calories; if you have type 1 diabetes, a very-low-fat diet may not be suitable.

* Is my dietitian comfortable with my following this diet? There's no reason not to combine useful features of several plans to create your own custom weight-loss plan. The better a diet suits you and your life, the easier it will be to stick with it.