Rats get fat, but we may get lucky
Artificial sweeteners -- those diet-friendly ways to satisfy the sweet tooth -- recently got some bad press. In a study that has spurred discussion among scientists and on dieting blogs, researchers at Purdue University found that rats consuming saccharin-sweetened yogurt ate more food overall and put on more weight during a two-week period than rats consuming glucose-sweetened yogurt.
The rodent finding has led some to ask: Are artificial sweeteners really good for a diet? Or do they, in fact, undermine weight-loss efforts?
Some researchers, including authors of the rat study, say the answer is the latter. Zero- or very-low-calorie sweeteners such as saccharin and aspartame are charlatans, they say -- signaling sweetness without delivering the goods. As a result, the body’s Pavlovian association of “sweet” with “calories” -- is weakened, upsetting the ability to balance how many calories are eaten against how many are used up.
The result? Weight control becomes more difficult.
“There’s no reason to believe that humans don’t do the same thing” as the rats, says Susan Swithers, lead author of the rat study and an associate professor of psychology at Purdue University.
Other nutrition researchers aren’t convinced that the rat study applies to people and point to human studies with different results. They say that even if taste signals are weakened in humans consuming artificial sweeteners, any imbalance is likely to be dwarfed by other influences on eating -- including portion size, mindless munching and eating for self-comfort’s sake.
“We don’t quite know where this fits,” said Barry Popkin, a professor of nutrition and director of the Interdisciplinary Obesity Center at the University of North Carolina. “It’s another part of the puzzle, the long- and short-term human effects of all the sweeteners that have been added to our diet -- both the caloric and diet -- over the last 20 to 30 years.”
The rodent study, published last month in the journal Behavioral Neuroscience, manipulated the signal that sweet taste sends. Rats ate yogurt (some days it was sweetened and other days it wasn’t) in addition to their regular chow. Glucose was the sweetener in one group of rats, and saccharin was used in a second group.
The saccharin-eating rats ingested 5% to 10% more calories overall, gained 20% more weight and increased their percentage of body fat by more than 5%. Swithers and co-author Terry Davidson suggest that, by interfering with what sweet taste means, artificial sweeteners upset an ancient physiological system that evolved to regulate food intake and energy use.
In other words, just as artificial sweeteners trick our taste buds and satisfy our sweet tooth, they may confuse other systems involved in assessing calorie intake and controlling appetite.
Scientists who doubt the rat finding point to similar studies with human beings in which artificial sweeteners didn’t make people overeat.
In one of them, published in 1990 in the American Journal of Clinical Nutrition, researchers gave healthy U.S. adults 40 ounces of either aspartame-sweetened soda or high-fructose corn syrup-sweetened soda every day for three weeks. Then, after a three-week break, the volunteers drank the other test beverage for three weeks.
Under both conditions, subjects reduced their caloric intake of other foods, to the tune of about 200 calories per day. The aspartame group lost a little weight (the average was less than a pound) and the high-fructose corn syrup group, who took in 530 calories each from their soda, gained a little weight (on average 1.5 pounds).
In a 2002 Danish study also published in the American Journal of Clinical Nutrition, overweight subjects were given packages containing food and drink sweetened with either sucrose or a variety of artificial sweeteners (a mix of products was used, so a mix of artificial sweeteners was consumed). They were instructed to consume a certain amount of the foods provided each day, which resulted in consumption of about 600 extra calories daily in the sucrose group.
After 10 weeks the researchers found that both groups compensated elsewhere in their diet for consuming the sweetened foods; the noncaloric sweetener group lost a little weight and the sugar group gained a little weight (less than a pound either way).
Both these studies were too small (30 to 40 subjects) and too short (three to 10 weeks) to be considered definitive. However, many obesity experts say they are more comfortable with their findings than those of the animal study. “I’m a little taken aback, because people are getting all excited about this rat study,” says Mark Pereira, an associate professor of epidemiology at the University of Minnesota. And yet, he adds, these two human studies show that in terms of weight gain, “a calorie is a calorie.”
The two studies also showed that people don’t fully compensate for the calories they drink in sugar-sweetened beverages by reducing their intake of other calories by the same amount. Indeed, study subjects overshot their intake by about 70% of the amount of calories they drank with the beverages provided by the researchers. Many obesity researchers agree that liquid calories are less likely to be counted than solid ones -- and thus that quaffing sugary drinks is particularly likely to lead to a slow gain in weight.
“When you’re consuming liquid sugar, you’re probably going to end up with a higher caloric intake on a daily basis,” Pereira says.
Michael Tordoff, a faculty member at the Monell Chemical Senses Center in Philadelphia who conducted the U.S. study, says he can’t say for sure what would happen over months or years if someone drank only the diet version of all their soda. But based on his three-week study, he says, “I can say for certain that if you are a regular soda drinker and you switch to diet soda, that’s a good thing.”
As well as the small, controlled diet studies, there are also large population studies on the matter of weight gain and diet drinks. And here the findings differ, with several studies showing an association between diet beverages and weight gain and/or higher obesity rates.
Two recent long-term studies found a positive correlation between diet beverages and metabolic syndrome, which is a constellation of risk factors for diabetes and cardiovascular disease.
A 2007 study published in the journal Circulation looked at soft drink consumption in a group of 6,000 adults who were part of the famous Framingham Heart Study and found a 50% increased risk of metabolic syndrome over four years in participants who drank one or more sodas per day compared with those who drank less than one soda per day. Whether the soda was regular or diet made no difference.
Similarly, data collected from about 9,500 adults in another large prospective study called the Atherosclerosis Risk in Communities study showed that over nine years, participants who consumed one serving of diet soda a day had a 34% higher risk of metabolic syndrome compared with those who didn’t drink diet soda. (This study, published in Circulation earlier this year, also found a positive correlation with eating meat and fried foods, but not sugar-sweetened beverages.)
Although these kinds of studies have the advantage of monitoring changes over time, they still may suffer from a chicken-or-egg scenario: Is the diet soda causing metabolic syndrome, or are those high-risk individuals drinking diet soda because they’re counting carbs?
The bottom line on the artificial sweetener imbroglio: a knotty tangle of data that screams “more research needed.”
“I don’t think we have the answer, and I don’t think these authors are claiming that they have definitive evidence that this is causing the obesity epidemic in humans,” says Richard Mattes, professor of foods and nutrition at Purdue University.
But, he adds, “They are posing an interesting and testable question.”