"Clearly fat makes food taste good," Barbara Rolls says. "It gives food all those lovely textures: creamy, flaky, crunchy, crispy, smooth. It also carries the volatile odors to the nose and makes food more appealing. But we wondered if there were another reason people eat too much fat."
Gerald Lang
Researchers used to believe that people overeat fat because it was less filling than protein or carbohydrates, notes Rolls, a professor of nutrition at Penn State. "But as my students and I reviewed published studies, we noticed that when people were allowed to serve themselves, they took a consistent weight of food, regardless of the fat or calorie content. We started looking at energy density—the number of calories a food has per unit of mass." Water is heavy and it doesn't add calories, Rolls explains, so foods that contain a lot of water have low energy densities.
"As it turns out, the energy density of food has a very robust effect on food intake," Rolls says. She and her students performed a study in which the same foods—chicken and rice—could be served in three different forms: as a chicken rice casserole, as a chicken rice soup, and as a chicken rice casserole served with a 10-ounce glass of water. The soup-eaters ate 26 percent less during the subsequent meal than either of the other two groups, Rolls found, even though the types and amounts of ingredients in the soup matched those of the water-casserole combination exactly.
The discovery that low energy-density foods are effective in reducing caloric intake is the premise of Rolls' new book, Volumetrics: Feel Full on Fewer Calories—not a diet book, she says, but an eating plan: "This is a way to eat for life." The book uses diagrams of portion sizes to teach people how to lose weight by eating more. In order to take in the 100 calories in just ten jelly beans, for instance, Volumetrics shows that you have to eat nearly three cups of strawberries.
"Water in food is chemically different than water taken as a beverage," says Rolls. "It leaves the stomach more slowly." And added water may also help people feel fuller because they are conditioned to judge the "appropriateness" of portion sizes. "The serving of soup just looks bigger," Rolls says, "so the brain, as well as the stomach, is satisfied." Rolls substantiated this theory with another study. She and her students incorporated varying amounts of air into milkshakes by blending. The researchers found that the more air, the bigger the shake—and the less subjects ate during the following meal.
Fat has 9 calories per gram; carbohydrates, 4; protein, 4; and water, 0. Losing weight ought to be simple: Eat less fat. "But what happens as we change the food supply," Rolls explains, "is that now energy density doesn't vary predictably with fat content." She brought out a container of strawberry-banana flavored yogurt. Gray lettering across the top read, "65 percent less fat, 15 percent more calories than full-fat yogurt." "In a society that produces these delicious, low-fat, high-calorie foods," Rolls says, "we're eating too much of everything."
Nutritionists and healthcare professionals define obesity as a Body Mass Index (BMI) of at least 30. BMI is your weight, in kilograms, divided by the square of your height, in meters. The National Center for Health Statistics reports that, using this scale, 42.5 million Americans are obese. "For researchers, understanding the causes of obesity is hard," says Rolls. "The work is costly and labor intensive. But it is especially difficult because, when it comes to reporting what and how much they eat, people forget and they underestimate." For this reason, subjects must sometimes be confined to hospital-like rooms in the nutrition lab, where their caloric intake can be carefully monitored.
But in most cases, Rolls and her students conduct their research in cubicles. "When people are allowed to socialize with friends, they take in 50 percent more food," Rolls explains. The walls of the cubicles are blue-gray, and there are matching blue-gray floors, blue-gray chairs, and blue-gray curtains that subjects pull behind them as if they were voting. "Young children often take in a consistent number of calories no matter what is going on around them," Rolls says, "but adults get distracted easily."
On test days, subjects arrive at their cubicles, sample a bite of a test food, and fill out pre-meal questionnaires. "How hungry do you feel? How pleasant is the taste of this food? How much of this food do you think you could eat? How much fat do you think this food has?" The subjects' answers are tools Rolls and her students use to interpret the results of their studies.
The appetizers are served on cafeteria trays marked with colored dots that identify the subjects throughout the study. Then there are more questionnaires. "How hungry do you feel? How nauseated do you feel?" Palatability, the subjective measure of how a food tastes, is something Rolls monitors closely. "If people eat less because we're making them nauseated, we haven't accomplished anything."
For the main course, subjects serve themselves from an array of foods with different energy densities. When the subjects have finished eating, researchers determine how much the subjects ate, and the subjects answer more questions about their dining experiences.
A 20/20 segment comparing Rolls' plan to other currently popular diets provides graphic proof that you don't have to go hungry to lose weight. A typical day's food under Rolls' plan overflows the plates and fills the table. Daily allowances on other diets huddle in the center of the spare, seemingly huge white plates. Volumetrics is really about satiety—the subjective perception of fullness—says Rolls. "The bottom line is that in order to lose weight, you have to take in fewer calories than your body uses. But with low energy-density foods, sometimes you actually have to add fat to keep the volume of food down. Did you notice there was butter on our bread?"
Barbara Rolls, Ph.D., holds the Helen A. Guthrie Chair of Nutrition in the College of Health and Human Development, 226 Henderson Bldg., University Park, PA 16802; 814-863-8482; bjr4@psu.edu. Graduate students working on the energy density project are Elizabeth Bell, Fan Zhou, and Tanja Kral. The work is funded by the National Institutes of Health's Institute of Diabetes and Digestive and Kidney Diseases. Volumetrics: Feel Full on Fewer Calories was coauthored by Robert A. Barnett and published by Harper Collins in 2000. Writer Heather L. Fletcher is a graduate student in chemistry . The strawberry and jellybean images were created by Gerald Lang and Jennifer Tucker of the Penn State Digital Photography Studio. The Digital Photography Studio (illustrations) has received support from The Eastman Kodak Company, Calumet Photographic Inc., and Megavision.