Two new studies have reawakened a debate over the safety of artificial sweeteners, suggesting the chemicals may cause significant alterations in the make-up of out gut microbiome. Both studies took very different approaches to the research, and while they ultimately reached similar conclusions – that artificial sweeteners have a negative effect on gut bacteria – some experts are not convinced the research can be accurately applied to human subjects.

Debates over whether or not artificial sweeteners are entirely safe for human consumption have raged for decades, but as sugar has become a big health villain in recent years, more people are switching to products with artificial sweeteners every year. These two new studies are homing in on the effects of artificial sweeteners on one of the big medical research topics of the moment – the gut microbiome.

The first study, conducted by researchers at Ben-Gurion University of the Negev (BGU) in Israel and Nanyang Technological University in Singapore, set out to analyze the general toxicity of six popular artificial sweeteners on bacteria designed to be representative of the broad complex microbial system living inside our gut. The first step in this research was to develop a bacterial sensing model that could be used to observe the toxicity of individual compounds.

"We modified bioluminescent E. coli bacteria, which luminesce when they detect toxicants and act as a sensing model representative of the complex microbial system," explains Ariel Kushmaro, a researcher on the project.

The results showed that all six artificial sweeteners did indeed damage the bacteria but each individual compound had its own unique effects. Some compounds resulted in DNA damage to the bacteria, while others resulted in protein damage. The research was revealed to the world with a reasonably hyperbolically titled press release suggesting, "Artificial sweeteners have toxic effects on gut microbes."

This study, conducted in laboratory test tubes using concentrations of chemicals larger than a human would normally consume in a glass of soda, is fundamentally unclear as to what it means in relation to the health of everyday people.

Senior author of the study, Evgeni Eltzov, did even admit to Live Science that this research doesn't specifically mean that these artificial sweeteners are toxic to humans. Instead, the research suggests the chemicals simply have the potential to damage bacteria … in some way.

The second new study was presented recently at the annual meeting of the European Association for the Study of Diabetes. The research is yet to be published or peer-reviewed but presents a very interesting randomized, double-blind human trial investigating the effects of an artificial sweetener on glycemic control and the microbiome in non-diabetic, healthy adults.

This study, conducted by a team in Australia, took 29 healthy young adults and randomly administered an artificial sweetener combination to half of the cohort for two weeks, while the other half were given a placebo. A combination of sucralose and ace-k was administered, in capsule form, three times a day for two weeks. The researchers note the volume of artificial sweetener studied was equivalent to about 1.5 liters of diet soda a day.

After two weeks, the researchers discovered two major changes in the gut bacteria of those subjects consuming the artificial sweetener. A significant decrease in the abundance of a bacterium associated with positive gut health and an increase in 11 "opportunistic gut pathogens," not normally present in healthy individuals.

The study also found that after just two weeks of consistent artificial sweetener consumption the subjects displayed changes in how their bodies responded to glucose. In the presented study abstract the researchers write, "The observed decrease in fermentative bacteria populations and changes in the pathways used by bacteria to harvest energy predicted a deterioration in the body's ability to regulate glucose."

What this new study means for those prone to an occasional can of diet soda is still very unclear. Navel Sattar, a professor from the University of Glasgow who didn't work on this new study but is an expert in metabolic medicine, suggests the research is not convincing enough to stop consuming artificial sweeteners.

"This study, whilst well done, is not the same as taking one or two diet drinks (containing sweeteners) per day, more often than not with food, but rather is equivalent to almost five cans of diet drinks every day for two weeks – given in the form of tablets," says Sattar. "Also, we don't know what these gut marker changes really mean for health so it's all highly speculative, and there are no clear data in humans that sweeteners alter blood sugar levels in the way that is speculated by the authors."

Kevin Murphy, from Imperial College London, also questions whether this new study means a great deal in relation to human health. Murphy, an endocrinologist, notes the association between bacteria, microbial genes and glucose metabolism is compelling but may not be in any way causal.

"Further work is needed to confirm whether this altered bacterial activity is actually responsible for the impaired glucose control observed – the work as described cannot demonstrate a causal effect," says Murphy.

So the frustrating conclusion to all of this is still, we don't really know. Are artificial sweeteners more damaging than plain old sugar? It's certainly a situation of better-the-devil-you-know for some people, but the jury is still decidedly out on whether artificial sweeteners are fundamentally harmful to our gut microbiome and our general health.

The first study was published in the journal Molecules, while the second study was presented at European Association for the Study of Diabetes.