The fullness factor: It's not just about how much; it's about what you eat

One important factor is satiety -- that is, how full you feel while you eat and afterwards. The sensation of fullness occurs when your stomach and intestines send signals to the brain. If you're trying to lose weight, you should know that satiety is not just a matter of how much you eat, but also which foods you choose.

One expert on how to feel full on fewer calories is Dr. Barbara Rolls. Her book The Volumetrics Weight-Control Plan is based on a series of studies she conducted over the last few years at Penn State University in the Laboratory for the Study of Human Ingestive Behavior.

We like her advice. Much of it is just common sense, but that's in short supply in this age of supersized fast foods, hugely popular fad diets, and surging obesity rates.

Water everywhere
The key to weight control, according to Rolls, is to eat foods with a low energy density -- meaning relatively few calories per ounce -- so that you leave the table feeling full and satisfied without breaking the calorie bank.

Notable among these foods are fruits and vegetables and dishes that contain them (such as stews, pasta dishes, or smoothies), as well as soups. What these have in common are a high water content and usually lots of fiber. In contrast, foods with a high energy density -- that is, lots of calories per ounce -- typically have a low water content, and often are high in fat, which is the densest source of calories.

Its easy to follow Rollss plan. For example, to reduce the energy density of chili, use lean meat and add celery, extra tomatoes, and mushrooms. To bulk up a pasta salad and cut the calories in half, add zucchini, carrots, and other veggies -- fresh, canned, or frozen. Add lettuce, tomato, and pepper slices to a sandwich.

Snack on an apple instead of chips or pretzels, for example, and grapes instead of raisins. A 100-calorie serving of raisins is only one-quarter cup; but a 100-calorie serving of grapes is nearly two cups. It's obvious which is going to make you feel fuller.

Soups usually have a low energy density (except for those containing lots of butter or cream). In one Penn State study, women who had soup as a first course ate fewer calories overall during meals. Salads serve the same purpose, provided you use low-calorie dressing.

If you consume bulky water-rich foods, you don't have to eat less food when you diet. And such foods tend to be very nutritious.

Rolls also recommends whole-grain pasta, breads, and cereals; their fiber makes them more filling. Seafood, skinless poultry, lean meats, and low-fat or nonfat dairy products are also on the menu. Because of its protein content, milk, even nonfat, helps people feel full and thus eat less. Whole fruit is always preferable to juice.

Food that satisfies
Rolls is not the only researcher studying satiety. Australian researcher Dr. Susanne Holt at the University of Sydney has developed a Satiety Index based on how full people feel during the two hours after eating 240 calories? worth of various foods, which are all compared to white bread.

Bulky high-fiber foods such as fruits and vegetables rate high. High-fat foods rank low, since 240 calories? worth is a small portion. For instance, baked potatoes are more than three times as filling as white bread, but fatty croissants are only half as filling as the bread. Whole-grain bread is 50% more filling than white bread. Cakes, doughnuts, and cookies (high in fat and sugar) are among the least filling.

The more fiber, protein, and water a food contains, the longer it will satisfy.

Despite the claims made by advocates of high-fat diets, fat seems to have less of an effect on satiety than protein or carbohydrates. However, studies on carbohydrates and satiety have had inconsistent results, in part because foods containing them are so varied.

Clearly, some high-carbohydrate foods, such as fruits and vegetables, which are high in fiber, are more filling than others, such as white bread or pasta. Fiber boosts satiety in a number of ways.

And while insoluble fiber (abundant in whole wheat) increases fullness in the short term, soluble fiber (in oats, for instance) can produce a feeling of satiety many hours after a meal. A number of studies have shown that high-fiber foods consumed at breakfast or lunch can significantly reduce food intake at the next meal, compared to low-fiber foods.

And keep in mind: One way to eat more filling foods is to find more fruits, vegetables, and whole grains that you like. Most Americans have a very limited range. Try a new fruit or vegetable each week. And keep adding them to different dishes. It will never get boring.

Good Luck!