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How Can Diet Sodas Make You Fat? Study May Explain It

Image: Various bottles of soda are displayed in a cooler at Marina Supermarket on July 22, in San Francisco, Calif.

A new study may help explain how artificial sweeteners may make some people gain weight. Justin Sullivan / Getty Images

Researchers trying to figure out whether artificial sweeteners really do make people fat think they’ve found a possible explanation — they may disrupt the bacteria in some people’s bodies.

Their findings may shed light on why studies often contradict one another, with some finding that people who drink lots of diet drinks are more likely to be obese, with others finding they may help people keep weight off.

Study Suggests Link Between Diet Soda and Heart Disease 2:03

Their answer: it may depend on what kind of gut bacteria you have to start with.

They found clear evidence that artificial sweeteners, including saccharine and sucralose, can affect gut bacteria, which in turn affect how food is digested and metabolized. Mice and a very few people given artificial sweeteners for the first time showed distinct changes in the way their bodies processed sugar.

It’s not a final answer, but the study, published in the journal Nature, may point research in a new direction. “Our findings suggest that non-caloric artificial sweeteners may have directly contributed to enhancing the exact epidemic that they themselves were intended to fight,” they wrote in their report.

“By no means do we believe that based on the results of this study are we prepared to make recommendations as to the use and the dose of artificial sweeteners,” said Eran Segal of the Weizmann Institute of Science in Rehovot, Israel, who worked on the study. But, he added, “In none of these experiments have we seen any beneficial effects.” He said the findings should at least prompt closer examination of the very widespread popularity or artificial sweeteners.

How could artificial sweeteners, which have no calories, affect metabolism? Segal pointed out that bacteria in the guts of both mice and people digest compounds that animals normally cannot, and they could thrive on chemicals that would not normally be used as food by people or animals.

It’s another example of how the microbiome — the population of microbes living in and on our bodies — can have huge effects on health.

Most of the study was done in mice. They fed the mice large amounts of sweeteners of all kinds and measured their gut bacteria and tested their metabolisms. Bacteria living in the intestines and colon are known to help digest food, and more and more studies are showing they can affect obesity and even appetite, as well as a tendency to disease.

Mice fed the sweeteners had definite changes in both gut bacteria and metabolism. Sugar did not have the same effect. To make sure it was the gut bacteria, the researchers removed bacteria from mice that had not eaten sweeteners, and grew them in lab dishes along with artificial sweeteners. They then put these sweetener-fed bacteria into new mice. The new mice began to show the same changes in metabolism as mice directly fed sweeteners.

The main flaws? Artificial sweeteners seemed to encourage a group of bacteria called Bacteroides and seemed to kill off another group called Clostridiales. Scientists are just beginning to understand what kinds of bacteria people have living inside their digestive systems and what balance might be healthy. But having too many Bacteroides and too few Clostridiales is a pattern sometimes seem in people with diabetes.

The researchers used mostly saccharine in their controlled experiments, but they said in early tests the mice responded the same whatever sweetener they used – saccharine, sucralose, aspartame or others. This baffled them, because the sweeteners are chemically very different from one another.

It’s worth more research, they said.

Mice are different from people, of course, but they tried the experiment in a small group. Seven people who did not normally use artificial sweeteners were given large amounts for a week. In four of them, their blood sugar shot up and they had other changes to metabolism associated with weight gain and pre-diabetes.

Larger studies have also suggested similar patterns – some people are adversely affected by sweeteners, while others are not. It may be a very individual thing, Segal said.

“We are identifying many foods which are considered as healthy food to have potential adverse effects for large subsets of individuals,” he told reporters in a telephone briefing. Genetic differences already demonstrate that some people can smoke tobacco with little effect, while most develop heart disease or cancer.

“What was super-striking and interesting to us was that we could predict ahead of time (who would be affected by the sweeteners),” Segal said. They profiled the microbiomes of their volunteers and found two distinct patterns. While everyone’s microbiome is different, there were larger overall patterns, and these predicted who would be affected by the sweeteners, Segal said.

“I think we must stress that by no means are sugary drinks healthy and that sugary drinks should be brought back as a healthy part of our nutrition,” added Eran Elinav, who led the research.

Researchers not involved in the study were skeptical, but most said it’s worth looking into more.

“The study is based primarily on mouse experiments and only seven human subjects were studied,” said endocrinologist Dr. Katarina Kos of the University of Exeter in Britain.

“Meanwhile, these findings support the widespread understanding that water is the healthiest drink option and that we should avoid sweet and sweetened drinks. Water is the best drink to control our blood sugar.”