"We can't guarantee that you'll lose weight and keep it off," reads a sentence near the beginning of "Volumetrics" (HarperCollins), a new diet book that has just hit store shelves. The book also cautions that "changing your eating habits is very difficult." And right up front, on page 7, it tells readers, "if your overeating is rooted in depression or has other emotional causes, you will need to address these issues, perhaps with a therapist, before you are ready to adopt the eating style" here.

Hardly the stuff of blockbuster weight-loss plans, which promise to "melt the pounds away quickly," help you "lose weight permanently," and let you "eat your way to dynamic weight loss." But that's exactly what makes "Volumetrics" such a refreshing entry into the crowded weight-loss market. It's honest--and honestly motivating.

Most other diet books push their approaches as if they're being sold with manic fast talk off the back of a truck that plans to make a quick getaway after you hand over your money. This book treats its readers respectfully, never promising more than it can deliver--but delivering plenty nonetheless.

Written by Penn State obesity researcher Barbara Rolls and veteran health and nutrition writer Robert Barnett, "Volumetrics" weaves its pound-shedding strategy around research that indicates that people tend to eat roughly the same weight of food every day, regardless of the number of calories it contains. Lower the calorie content but not the food's weight, the book argues, and you will lose excess pounds without going hungry, since you won't be eating less food than you typically do.

How do you subtract calories from meals without subtracting weight or bulk? Take out some items that have a lot of calories for their weight, and add back foods of equal or greater weight--and volume--but with fewer calories. The extra heft of the replacement foods comes from water and fiber, both of which weigh a lot but are calorie-free.

Foods that are high in water and fiber? Vegetables, fruits and whole grains. Foods that have a lot of calories for their weight, or have a high energy density, to use "Volumetrics" parlance? Fatty ones, along with those that have a lot of sugar.

There's nothing new, of course, in recommendations to add fruits and vegetables to the diet and go easier on high-fat, high-sugar items. And the authors don't argue that there is. In fact, they suggest that people who want social support during their efforts might want to join plans such as Weight Watchers, Jenny Craig or TOPS (Take Off Pounds Sensibly), all of which follow similar guidelines.

But the authors delve into the concepts behind high-energy-density foods and low-energy-density choices in newly informative ways, both enhancing knowledge and making healthful eating for weight loss come across as more appealing and intriguing than it often does.

Consider, for instance, that while adding water to the diet helps you eat less, the water has to be part of food, not a beverage that's drunk separately. Rolls made the point in a Penn State study in which women were fed an appetizer of either chicken-rice casserole, the casserole plus a 10-ounce glass of water, or the casserole with the water added to it to make chicken-rice soup. All three versions contained 270 calories, but only the soup reduced the number of calories the women ate during the rest of the meal (by about 100 calories). The reason appears to be that water by itself leaves the stomach too quickly to register as food, but when water and solids are combined, the food leaves the gut more slowly, creating greater satiety.

Also helpful is the information that just because a food is low-fat doesn't mean it's low in energy density. Some cases in point: pretzels, crackers, fat-free cookies and Melba toast. While they're virtually fat-free, they also have a very low water content, so their calories are not "diluted." Indeed, the authors point out, Melba toast has the same energy density as cheddar cheese, which hardly makes it the "diet" food many people assume it to be.

To help people see at a glance which foods are high in energy density and which are low, the book gives the energy-density values for more than 600 items. Were you aware, for example, that seemingly innocuous reduced-fat Wheat Thins have a "high" energy density rating of 4.1, while Italian bread has a "medium" energy density of 2.7, meaning that an ounce of Italian bread contains fewer calories than an ounce of the Wheat Thins? Or that a McDonald's Quarter Pounder has almost twice the energy density of KFC's Tender Roast chicken breast (without skin)?

The book also puts the concept of energy density to work with menus for 1,600- and 2,000-calorie diets that can be adapted for people who want to follow a 1,400-, 1,800- or 2,200-calorie plan. The choices include 12 breakfasts of 400 calories, 10 lunches of 500 calories and 25 dinners of 500 calories, along with dozens of snacks containing fewer than 100 to 200 calories.

Recipes for many of the meals are included, too. And what you give up in Melba toast you gain in lots of flavor. There's Pork with Sweet Potatoes and Apples; Baked Red Snapper Provencale; Four-Cheese Vegetable Lasagna; and desserts such as Carrot Spice Cupcakes with Cream Cheese Frosting and Fudge Cake Brownies. There's even a Great American Volumetric Burger made with bulgur and grated carrot to reduce the energy density of the recipe's ground beef. (Don't worry--seasonings such as Worcestershire sauce and garlic beef up the taste.)

Finally, the book offers lots of down-to-earth, practical advice for how to work out deals with your body so that your mouth remains satisfied while you change your eating habits. One example: Don't try to swear off high-energy-density pretzels if you like them. But rather than eating several ounces at once (an ounce has 100 calories), have one ounce paired with a low-energy-density tomato drizzled with a little balsamic vinegar and sprinkled with fresh black pepper and salt.

Then, too, while the book waxes positively rhapsodic about the wonders of soup for satisfying the appetite, it's not so rigid that it doesn't remind readers, "if you don't like soup, start your meal with a salad, a piece of fruit, a glass of vegetable juice, or any first course that's low in energy density."

Lack of rigidity is one of the book's most appealing aspects, serving as the backbone behind the authors' recommendation that people should try to lose weight slowly and plateau on purpose after six months to see if they can sustain the initial weight loss before trying to drop more pounds. The authors also stress that weight shedders should plan to include favorite foods in the diet, whether those foods fit easily or not.

That's not to say the advice isn't firm where necessary. The book reminds readers not to play mind games with themselves by rationalizing that it's okay to "stuff yourself" with a so-called safe, low-fat version of a food (which can still contribute plenty of calories) or to allow yourself a heavy snack that you wouldn't have otherwise had just because you've exercised that day.

But the tone overall is sympathetic, not chiding. Perhaps the best evidence of that comes near the end, where the book says, "don't feel bad if you eat when you are not hungry sometimes, or continue to eat when you're full. . . . Everyone does."

Who can't relate to a realistic, fair-minded approach like that?