Choosing a diet soft drink over a regular, sugar-packed beverage may not be the best way to fight obesity, according to new research from Purdue University. But the researchers said this doesn't mean you should grab a regularly sweetened soft drink instead.
Professor Terry Davidson and associate professor Susan Swithers, both in the Department of Psychological Sciences, found that artificial sweeteners may disrupt the body's natural ability to "count" calories based on foods' sweetness. This finding may explain why increasing numbers of people in the United States lack the natural ability to regulate food intake and body weight. The researchers also found that thick liquids aren't as satisfying – calorie for calorie – as are more solid foods.
Based on the research, Davidson and Swithers suggest paying more attention to calories consumed and engaging in regular exercise to battle the bulge.
The Purdue's researchers' study, "A Pavlovian Approach to the Problem of Obesity," appears in the July issue of International Journal of Obesity. Davidson and Swithers, members of the Ingestive Behavior Research Center at Purdue, suggest that being able to automatically match caloric intake with caloric need depends on the body's ability to learn that the taste and feel of food by the mouth suggests the appropriate caloric intake. Much as Pavlov's dogs learned that the sound of a bell signaled food, people learn that both sweet tastes and dense, viscous foods signal high calories. This learning process begins very early in life and perhaps without conscious awareness, according to the researchers.
"The body's natural ability to regulate food intake and body weight may be weakened when this natural relationship is impaired by artificial sweeteners," said Davidson, an expert in behavioral neuroscience. "Without thinking about it, the body learns that it can use food characteristics such as sweetness and viscosity to gauge its caloric intake. The body may use this information to determine how much food is required to meet its caloric needs."
Over the past 25 years, there has been a dramatic increase in the consumption of artificially sweetened foods and low viscosity, high-calorie beverages, said Swithers, a developmental psychobiologist.
"Incidence of overweight and obesity has also increased markedly during this period," she said. "Our hypothesis is that experience with these foods interferes with the natural ability of the body to use sweet taste and viscosity to gauge caloric content of foods and beverages. When you substitute artificial sweetener for real sugar, however, the body learns it can no longer use its sense of taste to gauge calories. So, the body may be fooled into thinking a product sweetened with sugar has no calories and, therefore, people overeat."
Swithers said that the loss of the body's ability to gauge caloric intake contributes to increased food intake and weight gain, especially when people do not count calories on their own. A similar dynamic is at work with foods' texture and thickness.
"Historically, we knew that our body learns that if the food is thick, such as whole milk, it tends to have more calories than compared to a thinner liquid such as skim milk," Swithers said. "Now, our research reinforces this and takes it one step further, showing that our bodies translate this information about perceived calories into a gauge to tell us when to stop eating."
The researchers based their hypothesis on Pavlovian theory. Ivan Pavlov, known for his work in the early 20th century, is famous for his experiment in training dogs to associate food with the ringing of a bell. After being conditioned to the bell, the dogs salivated when they heard it – even when they did not see or smell food. Davidson and Swithers propose that rats learn a similar relationship between the taste or texture of a food and the calories it contains and may use this information to control food intake and body weight.
Davidson and Swithers' findings are based on two studies.
In the first study, two groups of rats were given two different sweet-flavored liquids. In the first group, both liquids were sweetened with natural high-calorie sweeteners so there was a consistent relationship between sweet taste and calories. For the second group, one of the flavored liquids was artificially sweetened with non-caloric saccharin so that the relationship between sweet taste and calories was inconsistent.
After 10 days of exposure to the flavors, the rats were allowed to eat a small amount of a sweet, high-calorie chocolate flavored snack. The researchers compared the two groups' ability to compensate for the calories contained in the chocolate snack. The rats that had experienced the inconsistent relationship between sweet taste and calories were less able to compensate for the calories contained in the snack and ate more than the rats that had experienced the consistent relationship between sweetness and caloric intake.
"This suggests that experience with the inconsistent relationship reduced the natural ability of the rats to use sweet taste to judge the caloric content of the snack," Swithers said.
In the second study, two groups of rats were given a high-calorie dietary supplement along with their regular food every day for 30 days. Although the supplements were identical in calories and nutritive content, they differed in viscosity. For one group the supplement had the consistency of thick chocolate pudding, whereas for the other group, the supplement was similar to chocolate milk. Davidson and Swithers found that over the course of the study, the rats given the milk-like supplement gained significantly more weight than the rats given the more viscous, pudding-like supplement.
"This finding indicates that rats are less able to estimate and compensate for the calories contained in liquids than in semi-solid foods," Davidson said. "If the body is less able to detect and compensate for calories contained in liquids, then intake of high-calorie beverages compared to semi-solid or solid foods could increase the tendency to gain weight."
The number of Americans consuming sugar-free products increased from less than 70 million in 1987 to more than 160 million in 2000. During the same period, the consumption of regular soft drinks increased by more than 15 gallons per capita annually.
"Increased consumption of artificial sweeteners and of high-calorie beverages is not the sole cause of obesity, but it may be a contributing factor," Swithers said. "It could become more of a factor as more people turn to artificial sweeteners as a means of weight control and, at the same time, others consume more high-calorie beverages to satisfy their cravings."
Davidson and Swithers are evaluating potential mechanisms that may produce the short- and long-term effects on food intake and body weight, as well as whether age or gender are contributing factors. Additional research also will need to evaluate if the body and brain can be retrained to naturally measure calories after consuming artificial sweeteners or high-calorie beverages.
The National Institute of Child Health and Development, National Institute of Digestive Diseases and Kidney Disorders, and Purdue School of Liberal Arts funded this research.