Summary

Low-calorie sweeteners have come in for some new scientific scrutiny recently. It appears that uncoupling sweetness from calorie content has some undesirable effects on weight control. Brain scans show sweeteners activate different pathways in the brain from sugar and may lead to people eating more.

Introduction

Swap sugar for a low or no cal sweetener and you'll lose weight. Makes sense, doesn't it? According to the Calorie Control Council ( www.caloriecontrol.org), the number of people consuming products containing sugar-free sweeteners went up from 70 million in 1987 to 160 million in 2000. And, at the same time, obesity rates went up from 15 percent to 30 percent. While we don't have evidence to blame sweeteners for this national weight gain, it's clear they are not doing the job they set out to. An article in the Journal of the American Medical Association explains why sweeteners, rather than being a weight loss aid, may even make you eat more.

Separating sweetness and calories - the downside

Humans have always sought out sweet tastes for the pleasure they give. But sucrose and fructose, the most common of the simple sugars, come loaded with calories. Sweeteners isolate the sweet taste and this, it turns out, messes up your ability to assess your calorie intake. Rats given saccharin-containing food were more likely to overeat than those given sugar-sweetened food. It is as if they had lost the ability to predict the 'consequences' of their eating. The saccharin-fed animals also experienced a smaller than expected rise in their core temperature when they were then fed naturally sweetened food. This blunting of thermal response made the animals eat more and use the calories they did eat less efficiently. We now need to now whether this behavior is reproduced in humans - but it could be a potent factor contributing to obesity.

The wrong brain pathway

Because sweeteners and sugar both activate the same taste receptors in the tongue, it had been assumed they both stimulated the same parts of the brain through the same pathways. Expectation and satisfaction should result from eating something sweet, whether it contains sugar or saccharin.

This turns out not to be so, according to functional magnetic resonance imaging studies carried out at the University of Pittsburgh. Sucrose leads to a stronger and more satisfying brain response, activating the all-important 'dopamine rush' which is classically linked to the feeling of reward and pleasure. Sucralose, the sweetener used in this study, did not activate dopamine and would thereby possibly trigger a desire for more sweetness until the craving could be satisfied. The brain knows the difference, but you may not consciously perceive it. The lack of the brain feedback that provides a feeling of satiety may be what drives those on sugar substitutes to eat more. Further human studies are needed, but maybe a little sugar for sweetness is not a bad thing after all.