Food and diet

Sweeteners’ link to obesity

Artificial sweeteners may be making slimmers put on weight, reports The Times . Drinking “low calorie drinks may increase the risk of putting on weight”, the newspaper said. Other news sources also report the story, some suggesting that the rise in obesity may be linked to the use of sweeteners.

The research behind the news stories is a study that looked at behaviour and weight gain in rats fed low-fat yoghurt sweetened with either artificial sweeteners or natural sugar. The scientists carried out three separate experiments, in a small number of rats. However, rat and human metabolism are different, so the results are unlikely to be directly applicable.

Obesity is a complex disorder. Many behavioural, environmental and metabolic factors interact. It is highly unlikely that artificial sweeteners alone are responsible for its increasing prevalence. Until further research is carried out in humans, it is not possible to identify what contribution sweeteners make, if any.

Where did the story come from?

Drs Susan Swithers and Terry Davidson carried out this research. The study was funded by grants from the US National Institutes of Health and the Purdue Research Foundation. It was published in the peer-reviewed medical journal: Behavioural Neuroscience .

What kind of scientific study was this?

This was a laboratory study in rats where researchers conducted three separate experiments to explore the effect of sweet tastes on metabolism and nutrition. In the first experiment, three groups of rats were compared to see if there was any effect of diet on fat and lean (muscle) body content. The first group received plain, unsweetened yoghurt for three of six days per week, then yoghurt sweetened with natural sugar (glucose) for the remaining three days. In the second group, the rats were fed unsweetened yoghurt for three of six days per week, then yoghurt sweetened with artificial sweetener (saccharin) for the remaining three days. A third group of rats were given a normal diet to begin with and glucose-sweetened yoghurt for the last three days of the week. All rats were allowed free access to ordinary food and water on the seventh day of the week. The rats received this diet for five weeks and all rats who consumed at least 70% of their yoghurt were compared in terms of their fat and lean mass.

In the second experiment, the researchers were interested in comparing rats that associated a sweet taste with high-energy, high-calorie food with rats that did not expect many calories from a sweet taste. Using a Pavlov-dog style experiment, they trained the rats to expect calories after a sweet taste, by giving  them unsweetened yoghurt for seven of 14 days (not necessarily consecutive days) followed by glucose-sweetened yoghurt (i.e. high calorie). Another group were trained to expect no calories after a sweet taste, by giving them unsweetened yoghurt followed by saccharin-sweetened yoghurt (i.e. low calorie) for the same period of time.

All the rats – a total of 20 – were then given normal food and water for one day and then half of the rats in each group were offered a high-calorie, sweet meal followed by normal food again. The other half in each group were not given this extra sweet meal. After three days of ordinary food and water, the groups were reversed so that the rats who received the final sweet meal in the first round didn’t get it again.

In the third experiment, 16 rats were implanted with a device to measure their core body temperatures. As in the previous experiment, they were fed either glucose-sweetened yoghurt or saccharin-sweetened yoghurt. They were then all offered a super-sweet meal. Fourteen rats were analysed; researchers compared the temperature and activity between the groups from different time points: before they ate the yoghurt, and during both yoghurt consumption and consumption of the super-sweet meal.

What were the results of the study?

In the first experiment, there were no differences between the rats at the beginning. However, the rats who received yoghurt sweetened with saccharin had higher body fat content at the end of the study.

In the second experiment, the researchers found that the rats who were trained to expect no calories with sweet taste (those that ate saccharin-sweetened yoghurt), consumed more calories overall than the other rats. Body weights did not differ between the groups at the beginning, but those eating saccharin-sweetened yoghurt gained more weight than the other rats. Although there was no difference between the two groups in the quantity of final super-sweet meal they ate, the rats that had been trained to expect high calories with the sweet taste consumed less ordinary food the day after.

In the third experiment, the researchers found that rats that did not expect high calories after a sweet taste (those that ate saccharin-sweetened yoghurt) had a smaller change in core body temperature.

What interpretations did the researchers draw from these results?

The researchers say that their results suggest that consumption of products containing artificial sweeteners may lead to increased body weight and obesity by interfering with fundamental physiological (natural) processes.

What does the NHS Knowledge Service make of this study?

The study uses standard ways of exploring the effects of saccharin-rich diets on metabolism in rats. The findings will be of interest to scientists who may explore further what effects saccharin has on human health. Though the researchers are keen to draw parallels between the effects of these highly-controlled diets in rats and human diets, they do say that “the generality of findings obtained with rats in the laboratory to humans in their much more complex food environments can and should be questioned”. Further research into the effects of saccharin on human metabolism is needed before we can estimate what effects, if any, artificial sweeteners have on human weight gain.

The complex interplay of many factors, such as the environment in which we live, our genetic and biological makeups, as well as the social and behavioural pressures in normal human life make a single, “sweet tooth”, explanation for rising obesity levels unlikely.

Sir Muir Gray adds...

Tough on rats who wants to lose weight; but for humans stick to the sweeteners and wait for more research before switching back to sugar. Of course, another option is to give up all sweet drinks and walk an extra 30 minutes a day.

NHS Attribution