Psychologists at Purdue University's Ingestive Behavior Research Center reported that, relative to rats that ate yogurt sweetened with glucose (a simple sugar with 15 calories/teaspoon, the same as table sugar), rats given yogurt sweetened with zero-calorie saccharin later consumed more calories, gained more weight, put on more body fat, and did not make up for it by cutting back later, all at levels of statistical significance.
Authors Susan Swithers, PhD, and Terry Davidson, PhD, surmised that by breaking the connection between a sweet sensation and high-calorie food, the use of saccharin changes the body's ability to regulate intake. That change depends on experience. Problems with self-regulation might explain in part why obesity has risen in parallel with the use of artificial sweeteners. It also might explain why, says Swithers, scientific consensus on human use of artificial sweeteners is inconclusive, with various studies finding evidence of weight loss, weight gain or little effect. Because people may have different experiences with artificial and natural sweeteners, human studies that do not take into account prior consumption may produce a variety of outcomes.
Three different experiments explored whether saccharin changed lab animals' ability to regulate their intake, using different assessments -- the most obvious being caloric intake, weight gain, and compensating by cutting back.
The experimenters also measured changes in core body temperature, a physiological assessment. Normally, when we prepare to eat, the metabolic engine revs up. However, rats that had been trained to respond using saccharin (which broke the link between sweetness and calories), relative to rats trained on glucose, showed a smaller rise in core body temperate after eating a novel, sweet-tasting, high-calorie meal. The authors think this blunted response both led to overeating and made it harder to burn off sweet-tasting calories.
"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," the authors wrote.
The authors acknowledge that this outcome may seem counterintuitive and might not come as welcome news to human clinical researchers and health-care practitioners, who have long recommended low- or no-calorie sweeteners. What is more, the data come from rats, not humans. However, they noted that their findings match emerging evidence that people who drink more diet drinks are at higher risk for obesity and metabolic syndrome, a collection of medical problems such as abdominal fat, high blood pressure and insulin resistance that put people at risk for heart disease and diabetes.
Why would a sugar substitute backfire? Swithers and Davidson wrote that sweet foods provide a "salient orosensory stimulus" that strongly predicts someone is about to take in a lot of calories. Ingestive and digestive reflexes gear up for that intake but when false sweetness is not followed by lots of calories, the system gets confused. Thus, people may eat more or expend less energy than they otherwise would.
The good news, Swithers says, is that people can still count calories to regulate intake and body weight. However, she sympathizes with the dieter's lament that counting calories requires more conscious effort than consuming low-calorie foods.
Swithers adds that based on the lab's hypothesis, other artificial sweeteners such as aspartame, sucralose and acesulfame K, which also taste sweet but do not predict the delivery of calories, could have similar effects. Finally, although the results are consistent with the idea that humans would show similar effects, human study is required for further demonstration.
Meanwhile, a recent review published in the European Journal of Clinical Nutrition concluded that low-calorie (or no-calorie) sweeteners may be of help in resolving the obesity problem. Although they are not magic bullets, low-calorie sweeteners in beverages and foods can help people reduce their calorie (energy) intakes. "Low-calorie sweeteners reduce the energy of most beverages to zero and lower the energy density of many foods," said study co-author, Dr. Adam Drewnowski, director, Center for Public Health Nutrition at the University of Washington. "Every dietary guideline these days tells us to bulk up, hydrate, and consume foods with fewer calories but more volume."
Preabsorptive ("cephalic phase") insulin release, the body's supposed reaction to non-caloric sweet taste, is cited as the potential mechanism for overeating. A problem is that there is no cephalic phase insulin release in humans following the ingestion of aspartame, as demonstrated by Abdallah et al. (1997) and not referenced here.
A recent study, also based on rats, showed that any flavor associated with a lack of calories led to overeating -- even salt. However, that effect was observed only in very young rats (four weeks) and disappeared four weeks later.
Findings in animal (e.g., rat) studies are not necessarily applicable to humans. Generally, clinical studies with humans follow animal studies. The present study went backwards.
Some have blamed "sweet tooth" and sugar calories for rising obesity rates. Others now blame "sweet tooth" and the absence of sugar calories for rising obesity rates. The human desire for sweet taste is an innate reflex that is present at birth: it is not learned; it is not acquired; it is not conditioned, and it is not going away anytime soon.
"This study oversimplifies the causes of obesity," said Beth Hubrich, a dietitian with the Calorie Control Council, an association representing low- and reduced-calorie foods and beverages. "The causes of obesity are multi-factorial. Although surveys have shown that there has been an increase in the use of 'sugar-free ' foods over the years, portion sizes of foods have also increased, physical activity has decreased and overall calorie intake has increased," she added.
Several studies conducted in humans have shown that low-calorie sweeteners and the products that contain them can be useful tools in weight control. For example, a study conducted by Dr. George Blackburn and published in the American Journal of Clinical Nutrition investigated whether the addition of aspartame to a multidisciplinary weight control program would improve weight loss and long-term control of body weight in obese women. Some 168 obese women aged 20 to 60 years were studied over a two-year period. The researchers found that participation in this multidisciplinary weight control program including the use of aspartame-sweetened foods and beverages not only facilitated weight loss, but long-term maintenance of a reduced body weight.
A 2007 study published in Pediatrics found that using sucralose or sucralose sweetened beverages, as well as increasing activity, helped maintain and lower body mass index for children participating in the "Families on the Move" program. Additionally, a study published in the Journal of Food Science found that people who use reduced-calorie products (containing low-calorie sweeteners) not only had a better quality diet but also were more likely to consume fewer calories than those who did not use reduced-calorie products.
"Rising obesity rates have now been linked to the presence of sugars in the food supply and to the absence of sugars from the food supply," noted Drewnowski. "Consumers find it difficult to know who to believe. In the final analysis, all health experts agree that weight loss is best achieved by a combination of reducing caloric intake, lowering energy density of the diet, and increasing physical activity. By all accounts, low-calorie sweeteners do help. Suggesting that low-calorie sweeteners actually cause people to gain weight is an irresponsible direct application of rat models to dietary counseling and to public health."
From the March 3, 2008, Prepared Foods e-Flash