Glasses of juice may go down easier and quicker than bowls of fruit, but if you drink them, beware. Your body is less likely to register the calories they contain, and you may end up overindulging.
That's the conclusion of researchers Richard Mattes and Wayne Campbell, professors of foods and nutrition at Purdue University in West Lafayette, Ind., in a paper just published in the International Journal of Obesity. Having a liquid form of a food instead of a solid one, they found, results in more calories consumed during the day.
The study adds to a body of research on the effects of high-calorie beverages on diet, but the new research takes the science further by comparing not just high-carbohydrate but also high-fat and high-protein drinks with solid food equivalents.
In the three-day study, 120 men and women ate a specially prepared "test lunch" and later ate a dinner of their choosing. They answered questions about their feelings of hunger after the test meal and every hour thereafter until bed. They also kept a record of what they ate.
The test lunches began with a meal of chicken sandwiches accompanied by water, given as a control on the first day. On the second and third days, a liquid or a solid food sample accompanied the sandwiches. In one group, the sample was either high-protein milk or cheese (a nutritionally equivalent solid). In another, it was either high-carbohydrate watermelon or watermelon juice; in a third, it was either high-fat coconut or coconut milk.
After consuming the samples, subjects ate as many of the sandwiches as they wanted, and later ate dinner until they felt full.
The results: Compared with the sandwich-and-water control, subjects who ate solid test foods consumed fewer calories after their lunches, but subjects who drank their test foods ate more. This was true whether the sample calories came from sugary, fatty or protein-rich sources.
Overall, the subjects in the solid test food groups gave similar answers regarding hunger and fullness as those in the liquid test food groups. They also consumed about the same number of sandwiches at that meal. But key differences later emerged.
All three groups consumed the most total calories on days when a meal supplement was liquid (be it milk, watermelon juice or coconut milk), consuming 12% to 20% more calories than on solid food days.
Campbell says many prior studies have measured people's feelings of hunger and it's been "a leap of faith to believe that feelings of hunger correspond to the amount of calories consumed."
This experiment stands out, he says, because calorie intake was directly measured -- revealing that people drinking liquid foods later consumed more calories even though they had reported feeling just as full.
Dr. Zhaoping Li, an associate professor of clinical medicine at the UCLA Center for Human Nutrition, says the study is more elegant and controlled than previous studies that compared solids and liquids, such as Coca-Cola versus doughnuts, that were nutritionally very different.
Many signals set off by food but not beverages could lead to people feeling more satisfied, says Sai Das, an assistant professor at the Friedman School of Nutrition at Tufts University, such as sight, smell, chewing and pressure of food in the stomach.
Barry Popkin, the director of the interdisciplinary obesity program at the University of North Carolina says the study highlights how a broad array of liquid foods do not fill us up the way solid foods do. He says the reason may be evolutionary. Today, people may get 20% to 50% of their calories from beverages -- but for most of human history, the only beverage was water.
The idea that liquids contain nutrients, he says, is one our bodies aren't yet aware of.