Artificial Sweeteners and Weight Gain: Mystery Solved?

New research may finally explain how zero-calorie sweeteners could cause you to gain weight. 

Monica Reinagel, MS, LD/N, CNS
4-minute read
Episode #303

What Are You Feeding Your Microbes?

The second piece of evidence is that zero-calorie sweeteners affect the makeup of your intestinal population. Although these sweeteners have no calories for us, they still provide a food source for your gut bacteria. Different bacteria appear to prefer different sweeteners. Lactobacillus and bifidobacteria, the  beneficial bacteria found in yogurt and other probiotic foods, enjoy fructose, for example. A less helpful bacteria called Streptococcus mutans has an affinity for saccharine and aspartame (Equal.) Streptococcus mutans has been associated with an impaired ability to process sugar...the first step toward Type 2 diabetes.

Whichever strains of bacteria you feed are going to proliferate in your gut, often at the expense of others.

This brings us to the latest study, published this month in the science journal, Nature. The researchers started with a series of animal studies. First, they determined that mice fed saccharine, aspartame, or sucrolose had higher blood sugar levels than mice fed sugar or no sweeteners. Furthermore, the blood sugar problems in the mice appeared to be related to changes in their intestinal bacteria. They then did a very small, but tantalizing human study.

Step Away from the Pink Packet

Researchers recruited 7 healthy, normal-weight volunteers who did not normally use artificial sweeteners. For 4 days, the subjects consumed foods and beverages sweetened with saccharine, taking in the equivalent of about 10 packets of Sweet-n-Low a day, divided between breakfast, lunch, and dinner. (This, by the way, is the maximum amount of saccharine considered safe for daily consumption!)

After 4 days, 4 of the subjects (slightly more than half) had signs of impaired glucose tolerance. They also had altered gut bacteria. 

So, what does this research tell us? Honestly, not a whole lot...but it's a start. This study needs to be repeated with a much larger group of subjects. We need to test a variety of artificial sweeteners, not just saccharine, which has been largely replaced by sucralose (Splenda) and aspartame (Equal). We need to test a variety of dosages to see whether the effect only kicks in at higher intake levels. And, if this small study is representative, we need to see if we can figure out what makes nearly half of the population immune to this effect.

Should You Stop Using Artificial Sweeteners?

But what are we supposed to do while we wait for all that research to be done? Should you swear off artificial sweeteners, just in case?

Even before this research emerged, I have advised against using artificial sweeteners as a free pass to unlimited consumption. Drinking artificially-sweetened sodas all day long is a good way to train a sweet tooth. Binging on artificially sweetened jello, pudding, and ice cream also isn't improving the nutritional quality of your diet. 

My recommendation is (and always has been) to consume artificial sweeteners with the same restraint as you would use with sugar.  One small regular soda, sweetened coffee, or dessert is about all the added sugar an average person should eat in a day. If you want to substitute one diet soda, artificially sweetened coffee, or dessert, that would probably be fine. What probably isn't fine is substituting 10 diet sodas, artificially sweetened coffees, and desserts.

For Now, Stick to Stevia

One last tip: Although the research on how various zero-calorie sweeteners affect gut bacteria is still pretty limited, there is some data to suggest that stevia-based sweeteners may be the best choice. Unlike Equal and Splenda, which encourage the growth of Streptococcus mutans, stevia appears to encourage the growth of a strain called Bacteroides, which, in combination with a diet rich in vegetables and fiber, appears to promote a healthy body weight.


Payne AN, Chassard C, Lacroix C. Gut microbial adaptation to dietary consumption of fructose, artificial sweeteners and sugar alcohols: implications for host-microbe interactions contributing to obesity. Obes Rev. 2012 Sep;13(9):799-809. Link to study

Ridaura VK, Faith JJ, et al. Gut microbiota from twins discordant for obesity modulate metabolism in mice. Science. 2013 Sep
6;341(6150):1241214. Link to study.

Suez J, Korem T, et al.  Artificial sweeteners induce glucose intolerance by altering the gut microbiota. Nature. 2014 Sep 17. Link to study.

Photos of pink packet and man sweetening his tea courtesy of Shutterstock.


About the Author

Monica Reinagel, MS, LD/N, CNS

Monica Reinagel is a board-certified licensed nutritionist, author, and the creator of one of iTunes' most highly ranked health and fitness podcasts. Her advice is regularly featured on the TODAY show, Dr. Oz, NPR, and in the nation's leading newspapers, magazines, and websites. Do you have a nutrition question? Call the Nutrition Diva listener line at 443-961-6206. Your question could be featured on the show.