Will Sugar Substitutes Lead to Weight Gain?

Sugar Substitutes Linked to Weight Gain

Tracy Hampton, PhD

JAMA. 2008;299(18):2137-2138.

Although low-calorie sweeteners are a dietary staple for many individuals trying to maintain or lose weight, an emerging body of evidence suggests these substances offer little help to dieters and may even help promote weight gain.

A 2007 review found that laboratory, epidemiological, and clinical studies examining effects of low-calorie sweeteners presented an unclear picture of their usefulness (Bellisle F and Drewnowski A. Eur J Clin Nutr. 2007;61[6]:691-700). Because the scientific findings are mixed, there is currently no official recommendation about using artificial sweeteners as a tool for weight control.

Recent studies reveal that consuming artificial sweeteners may not help, and may even thwart, efforts to lose weight.

However, some studies have suggested that low-calorie sweeteners may actually increase appetite for sweet foods, promote overeating, and lead to weight gain. Recent findings lend further support to this theory, demonstrating that artificial sweeteners blunt the body's energy expenditure mechanisms and activate taste pathways differently than sucrose?-findings that might prompt dieters to rethink their weight-loss strategies.

HOW SWEET IT IS


According to the Calorie Control Council, the number of US residents who consume products containing sugar-free sweeteners grew from fewer than 70 million in 1987 to about 160 million in 2000 (http://www.caloriecontrol.org). National survey data indicate that the incidence of obesity increased over that time from approximately 15% to 30% (Flegal KM. Physiol Behav. 2005;86[5]:599-602).

Scientists agree that while intensely sweet low-calorie sugar substitutes help lower the energy density of beverages and foods, these products also uncouple sweetness and energy, which may disrupt the body's ability to accurately assess caloric intake.

Investigators at Purdue University in West Lafayette, Ind, recently showed that rats fed saccharin-containing food took in more calories and gained more weight than rats fed sugar-sweetened food (Swithers SE and Davidson TL. Behav Neurosci. 2008;122[1]:161-173). The findings add to their previous discovery that rats fed low-calorie artificial sweeteners fail to predict the caloric consequences of eating sugar-sweetened food later on. In other words, rats fed low-calorie sweeteners did not reduce their intake of sugar-sweetened food in a subsequent feeding, unlike rats originally fed sugar-sweetened food (Davidson TL and Swithers SE. Int J Obes Relat Metab Disord. 2004;28[7]:933-935).

"Sweet things usually are predictive of high calories, and the taste generates a lot of responses that help the animal utilize calories efficiently," explained principal investigator Terry Davidson, PhD, of Purdue's Department of Psychological Sciences. "If we start giving sweet tastes that don't have calories with them, then those responses aren't going to be as strong, and they'll get extinguished," he added.

The researchers also noted in the new study that, compared with sugar-fed rats, saccharine-fed animals had a smaller rise in core body temperature following consumption of a naturally sweetened high-calorie food. This blunted thermic response caused the animals to eat more and to burn calories less efficiently.

This research suggests that individuals trying to lose weight "may just not be able to switch from their caloric sweeteners to noncaloric sweeteners and have everything magically get better," said first author Susan Swithers, PhD, also of Purdue's Department of Psychological Sciences.

Additional research is needed to determine whether the Purdue findings apply to humans who consume artificial sweeteners. "We hope that this model represents the key features of humans, but the control of feeding behavior is probably a lot more complex in humans than in rats," said Davidson.


TASTE AND THE BRAIN

Another recent study, in humans, revealed that sugar is more potent than low-calorie sweeteners in stimulating brain areas related to expectation and satisfaction, thereby turning off the desire for more sweetness (Frank GK et al. Neuroimage. 2008;39[4]:1559-1569). "We thought that sugar and artificial sweeteners, which both activate sweet receptors in the tongue, would both activate the same pathways in the brain," said principal investigator Walter Kaye, MD, of the University of Pittsburgh's Department of Psychiatry. "They turned out to not be the same," he added.

The researchers used functional magnetic resonance imaging to compare brain activity in women eating sugar with those eating the artificial sweetener sucralose. Their studies found that compared with sucralose, sucrose elicits a stronger brain response in the anterior insula, frontal operculum, striatum, and anterior cingulate and that only sucrose stimulation engages the dopaminergic midbrain areas that respond to food rewards.

"Thus, brain response distinguishes the caloric from the non-caloric sweetener, although the conscious mind could not," the authors wrote. "This could have important implications on how effective artificial sweeteners are in their ability to substitute sugar intake," they added.

The investigators propose that because sucralose does not provide calories that act as a natural feedback mechanism that results in satiety, other means?-such as additional eating?-will likely be needed to provide satiety. Their findings also indicate that certain regions of the brain, such as the anterior insula, may be attractive targets for modulating food-reward sensations.