If you think you're cutting calories by eating diet or low-calorie versions of your favorite foods, think again. A new study by Canadian scientists published in the journal Obesity suggests that our bodies can't be fooled that easily.
Led by David Pierce, researchers at the University of Alberta studied the eating habits of young rats, and found that they tended to overeat when they were fed "diet" foods. Though the new study was conducted in animals, it adds to a growing body of research in humans that suggests a diet-foods paradox: the more low-calorie (or even zero-calorie) sodas and foods you consume, the more your body demands payback for the calories it was deprived.
Pierce and his team started with the assumption that animals, and young animals in particular, are adapted to crave high-calorie foods that are packed with fat and carbohydrates, the crucial biological fuel that rapidly growing juveniles need. Using classic Pavlovian conditioning techniques, Pierce trained his rats to associate low-calorie foods with a "diet" taste, and high-calorie foods with a different taste. So, when the rats were fed a high-calorie food that had been flavored with the diet taste, their brains assumed that their bodies were running low on calories. These animals then overate at their next meal in an effort to refuel and make up for the lost energy. "Animals have the ability to sense the caloric value of food they take in," says Pierce. "We found out that an animal can learn to use flavors to predict calories in an attempt to achieve energy balance."
This same phenomenon could explain similar results in recent studies of dieters, says Pierce. Two years ago, scientists at the University of Texas reported in an eight-year study that for every can of diet soda that a person drank, he raised his risk of being overweight by 41%, compared to a 30% increase in drinkers of regular, sugared drinks. Earlier this year, another study of diet-soda drinkers came to a similar conclusion, this time about metabolic syndrome, the dangerous constellation of risk factors, such as obesity, high cholesterol and insulin resistance, that increases the likelihood of heart disease. In this report, part of the 60-year-old Framingham Heart Study, researchers found that soda drinkers, regardless of whether they consumed diet or regular beverages, had a 48% higher risk of metabolic syndrome than non-soda drinkers.
At the time, even the study authors conceded that it was impossible to implicate diet drinks completely, since it's possible that those who drank low-calorie beverages were already overweight or at higher risk of metabolic syndrome, and chose the diet drinks in an effort to get healthier. But Pierce's work hints that a more basic, biological mechanism may be at work. The animals in his study were able to predict the amount of calories in a food based on taste, demonstrating that the body uses cues like taste and texture to make sure it's getting enough fuel. Just as Pierce's rats were fooled into thinking they hadn't absorbed enough calories after eating diet chow, people are preprogrammed to anticipate sugary, high-calorie fulfillment when drinking a soda or noshing on a sweet-tasting snack. So, the diet versions of these foods may leave them unsatisfied, driving them to eat more to make up the difference.
All of this emerging work could make the food and beverage industry, which has invested billions in diet and low-calorie versions of almost every food imaginable, a bit uneasy. "This study simply defies common sense," wrote Dr. Richard Adamson, scientific consultant to the American Beverage Association, in a prepared statement responding to the study. "To suggest that foods and beverages with zero calories contribute to weight gain contradicts the overwhelming body of scientific evidence that supports that they can help you reduce calories and maintain a healthy weight."
Dr. Ramachandran Vasan, lead author of the Framingham study, however, notes, "A zero-calorie drink could produce a metabolic response if it is sweet. It can condition you to develop a preference for sweet things, which can lead to weight gain or metabolic syndrome. So something that is sweet could produce a metabolic effect even if it doesn't have a whole lot of calories."
Of course, none of the studies has yet proved that diet foods or beverages actually cause weight gain or heart disease; they have merely found an intriguing association, which scientists are still trying to explain. Well, nobody ever said counting calories was easy.