Why diet drinks with aspartame may actually help make you fatter

Aspartame is used as a sweetener in many diet drinks

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By Andy Coghlan

It may sound like a healthy switch, but sometimes people who drink diet soft drinks put on more weight and develop chronic disorders like diabetes. This has puzzled nutritionists, but experiments in mice now suggest that in some cases, this could partly be down to the artificial sweetener aspartame.

Artificial sweeteners that contain no calories are synthetic alternatives to sugar that can taste up to 20,000 times sweeter. They are often used in products like low or zero-calorie drinks and sugar-free desserts, and are sometimes recommended for people who have type 2 diabetes.

But mouse experiments now suggest that when aspartame breaks down in the gut, it may disrupt processes that are vital for neutralising harmful toxins from the bacteria that live there. By interfering with a crucial enzyme, these toxins seem to build up, irritating the gut lining and causing the kinds of low-level inflammation that can ultimately cause chronic diseases.

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“Our results are providing a mechanism for why aspartame may not always work to keep people thin, or even cause problems like obesity, heart disease, diabetes and metabolic syndrome,” says Richard Hodin at Massachusetts General Hospital in Boston.

Irritating bacteria

Aspartame is used around the globe and many reviews have found it safe to consume. “Decades of scientific research, including human clinical trials, show that low-calorie sweeteners such as those in diet drinks, have been found to help consumers manage their calorie intake when part of an overall healthy diet,” said Gavin Partington, of industry body the British Soft Drinks Association. “These [latest] claims are being made by a study conducted on mice, and run contrary to the overwhelming body of scientific evidence.”

The enzyme in question is called intestinal alkaline phosphatase (IAP). “There’s a lot of it in our gut and it seems to protect us, enabling us to live symbiotically with bacteria,” says Hodin.

IAP works by neutralising lipopolysaccharides, bacterial toxins that can irritate the gut lining. But when Hodin’s team mixed IAP with drinks that contain aspartame in the lab, it blunted the enzyme’s activity. This didn’t happen when it was mixed with sugary drinks instead.

When the team injected aspartame into segments of mouse intestine, levels of IAP plummeted by 50 per cent.

Weight gain

The team also found evidence that, when consumed in combination with a fatty diet, aspartame may lead to greater weight gain in mice. When the group fed mice a high-fat diet for 18 weeks, those that were also given aspartame put on more weight than those that weren’t.

Mice fed aspartame also had higher blood sugar levels between meals. Failure to soak up excess blood sugar is an early sign of diabetes, and this was seen in aspartame-fed mice that received a normal-fat diet too.

Aspartame was also associated with higher levels of inflammation in the mice. “It adds another mechanism suggesting some artificial sweeteners might not be as inert in the human host as once believed,” says Eran Elinav, at the Weizmann Institute of Science in Rehovot, Israel. His team has previously found evidence that some artificial sweeteners can change the balance and functioning of gut bacteria communities, leading to problems with glucose control.

“This research questions the effectiveness of sweeteners for weight reduction and adds to a growing body of evidence suggesting they might actually lead to weight gain,” says Katherine Jenner of UK campaign group, Action on Sugar.

Human tests needed

Hodin says his team’s findings may only apply to aspartame and not other sweeteners, because they don’t produce the same IAP-blocking chemical when they break down.

Researchers also caution that what has been observed in mice might not apply in people. “Validation of these various mechanisms and their possible effects on human health merit further clinical studies,” says Elinav.

The results do, however, also hint at a possible treatment for chronic diseases like metabolic syndrome and diabetes. Hodin and his colleagues say it might be possible to give people extra IAP, perhaps as a pill or supplement. In a 2013 study, they found that giving IAP to mice could prevent metabolic disease developing in mice given high-fat diets, and ease the symptoms of those already affected.

Journal reference: Applied Physiology, Nutrition and Metabolism, DOI: 10.1139/apnm-2016-0346

Read more: Sugar on trial: What you really need to know