09/22/2014 01:41 pm ET Updated Nov 22, 2014

Some Gut Microbes Love Diet Soda... and They're Not the Good Guys

Consumers are getting increasingly more worried about sugar. That's led to consumption of ever-growing amounts of non-caloric sweeteners such as Equal, Splenda, Sweet'N Low and Stevia. It's a whole-population uncontrolled experiment, long underway, with some mystifying outcomes: Diet drinks are associated with weight gain in several long-termlarge studies, even after one controls for the fact that heavier people are more likely to choose diet drinks. 

The new Dietary Guidelines for Americans diplomatically advises:

... replacing added sugars with non-caloric sweeteners may reduce calorie intake in the short term, yet questions remain about their effectiveness as a weight management strategy.

Increased appetite following calorie-free sweetness is one possible explanation, but now, a much discussed study from the Weitzman Institute in Israel, just published in Nature, has an intriguing and novel culprit for the diet soda obesity and diabetes link: our gut bacteria.

Bacteria outnumber our cells 10 to 1, and not only are most of them harmless, many of them actually play a critical role in our health, producing vitamins, aiding digestion and helping our immune system.

Unbeknownst to us until recently, each and every one of us has always been a universe, inhabited by hundreds of trillions of invisible one-cell beings. Much like invaders of unknown civilizations have learned the hard way (think Avatar), we're finding that interfering with the natives' inner workings -- with antibiotics and habitat change -- might cause havoc not just for them, but also for us.

The unintended consequences of artificial sweeteners 

The new study, led by Eran Elinav, fed mice several commercially available sweeteners (saccharin -- Sweet-n Low, sucralose -- Splenda, and aspartame -- Equal) and found that the mice developed glucose intolerance, a condition that is a harbinger of Type 2 diabetes. 

A series of further tests implicated an altered gut microbial flora as the reason. When these sweetener-treated mice were given wide spectrum antibiotics the glucose intolerance went away. When the fecal bacteria of the sweetener-treated glucose intolerant mice were transferred to normal mice, they, too, developed glucose intolerance. Furthermore, the gut bacteria that dominated the sweetener-treated mice's gut are associated with Type 2 diabetes in humans, and are involved in digesting and storing carbohydrates.

So far we're talking mice.

The researchers then looked at 381 non-diabetic people, and found that using non-caloric sweeteners was associated with glucose intolerance, high glucose measurements and other signs of the metabolic syndrome. The volunteers that reported using non-caloric sweeteners had the same patterns of altered gut bacteria populations seen in the sweetener-treated mice. 

Lastly, seven people who don't usually use non-caloric sweeteners were fed the maximum recommended amount of Sweet-n Low for seven days. In this very short time four of the seven developed glucose intolerance, and showed alteration in the gut microbial population. When the gut microbes of these four people were transferred to mice, those mice developed glucose intolerance, too.

This study shows quite nicely that non-caloric sweeteners can alter gut microbes in mice -- a change that has negative metabolic consequences -- and provides preliminary evidence that it can happen in humans too. This will have to be repeated in further, larger studies (and with other sweeteners). 

We're rightfully worried about our overconsumption of sugar, but unfortunately, there's mounting evidence that non-caloric sweeteners are also linked with obesity and diabetes. This study offers a fascinating mechanism that explains this link, and a serious suspicion that it's not just an association: those sweeteners might actually directly cause what we hoped they'd help fix.

Dr. Ayala 

Full disclosure: I'm vice president of product development for Herbal Water, where we make organic herb-infused waters that have zero calories and no sugar or artificial ingredients. I'm also a pediatrician and have been promoting good nutrition and healthy lifestyle for many years.