The study is the work of Drs Susan Swithers and Terry Davidson, two psychologists based at the Ingestive Behavior Research Center at Purdue University, West Lafayette, Indiana, and is to be published in the February 2008 issue of Behavioral Neuroscience, a journal of the American Psychological Association (APA).
The authors suggest that a sweet taste may cause animals to anticipate the calorie content of food, and eating artificial sweeteners with little or no calories undermines this connection, leading to energy imbalance by increasing food intake or reducing energy expenditure.
They conducted three sets of experiments on adult male laboratory rats who were put in two groups. One group was given yogurt sweetened with glucose (equivalent to table sugar, containing 15 calories a teaspoon), and the other group was given yogurt sweetened with zero-calorie saccharin.
The rats that had the saccharin-sweetened yogurt consumed more calories, put on more weight, gained more body fat, and did not cut back on their calorie consumption in the longer term.
All these results were statistically significant, said the authors, who argued that by breaking the link between the sweet taste and the anticipated high calorie food, the saccharin changed the body's ability to control food intake.
They also suggested that the change depends on experience, which might explain why the obesity epidemic in humans has gone up in line with increased use of artificial sweeteners, and why scientists fail to agree on the effect of artificial sweeteners on humans: some research shows weight loss, others show weight gain or no effect at all. Swithers said it could be because those studies did not take into account prior consumption and that people have different experiences with artificial and natural sweeteners.
The authors also measured changes in the core body temperature of the rats. Usually, when the body of an animal gets ready to eat, the "metabolic engine" revs up, which raises the core temperature of the body. But when they gave the rats fed on saccharin sweetened yogurt a new, sweet tasting, high calorie meal, their core body temperature did not go up as much as that of the rats who had been fed on yogurt sweetened with glucose.
Swithers and Davidson argued this was because the saccharin fed rats had a blunted response that had the double effect of making them eat more and making it harder for them to burn off calories. As they explained in their paper:
"The data clearly indicate that consuming a food sweetened with no-calorie saccharin can lead to greater body-weight gain and adiposity than would consuming the same food sweetened with a higher-calorie sugar."
Although they recognized that these results may be contrary to expectations, and indeed the news may not be well received by clinicians and health professionals who support the use of low and zero calorie sweeteners as a way to lose weight, and this data is based on rats and not humans, the authors pointed out their findings are in line with increasing similar evidence. More and more studies are showing that people who consume more articially sweetened diet drinks are at higher risk of obesity and metabolic syndrome.
Metabolic syndrome is a cluster of health problems that increases risk of heart disease and diabetes, and includes high abdominal fat, high blood pressure and insulin resistance.
The authors suggest that other artificial sweeteners like aspartame, sucralose and acesulfame K, probably have a similar effect as saccharin. They also said that although they anticipate the results on the rats would be similar in humans, this it is yet to be demonstrated with human subjects.
Swithers and Davidson pointed out that it is not all doom and gloom. Although it takes more conscious effort, counting calories is still a good way to keep control of weight.
"A Role for Sweet Taste: Calorie Predictive Relations in Energy Regulation by Rats."
Susan E. Swithers and Terry L. Davidson.
Behavioral Neuroscience, Vol. 122, No. 1, February 2008.
Sources: American Psychological Association press release.