Anti-Microbial Effects of Artificial Sweeteners in Humans - 2/3rds of a Can of Diet Coke May Have a Sign. Effect on the Gut Microbiome, but the Relevance is Questionable

2/3 of this can may suffice to make a difference. Whether this difference is (a) relevant or (b) irrelevant is yet as questionable as whether the changes the scientists observed will (i) have a negative (ii) a positive or (iii) no effect.
As a SuppVersity user you know that the whole craze about aspartam and sucralose is overblown. You will also know that any potential "pro-insulinogenic" effects occurred only in less than a handful of human studies. If they did, though, they occurred in response to the ingestion of artificial sweeteners and glucose or other insulinogenic carbohydrate sources (learn more). Against that background it's also not surprising that in clinical trials vs. observational bogus, artificial sweeteners have been shown to help people with weight problems lose body fat (learn more).

The one thing about the myth of the bad sweetener that has yet not been completely debunked, though, revolves around their negative effects on the human gut microbiome.
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You will remember from my previous post on this topic that - as (unfortunately) usual - all empirically valid data we have is based on rodent trials. In our mammalian cousins, the consumption of artificially sweetened products on top of an obesogenic diet has in fact been shown to have an additional effect on the modulation of the gut bacteria that appears to make turn an already "bad" diet into a nightmare (learn more). It is yet still very questionable, which part of the research can be translated to humans and to which subgroup of the population this would apply. Even if we assume a 1:1 translation from rodent to human, we would after all have to exclude most of you, because none of you will be consuming a hypercaloric, hyper-processed high fat + high carbohydrate diet (at least that's what I'd hope).

If we trust the results of a soon-to-be-published observational study from the George Mason University, though, our gut microbiomes could be in danger - although no one knows for sure.
Sweeteners, pre- and probiotics are not the only foods / supplements that can have a major impact on the bacterial ecosystem in your gut. Only recently scientists have found that the ergogenic effects of glutamine may also be mediated by the gut | more
Why do you have to care about the microbes in your gut? The good guys produce vitamins like vitamin K for you, they digest resistant starches and produce short-chain fatty acids which in are have beneficial effects on your intestine, your satiety and directly or indirectly even on your glucose control. There is unfortunately no way to really tell the good from the bad guys. At the moment, it would seem as if the lactobacilli and bifidobacteria would be the ones you want to have most. On the other hand, the number of bacteria cells in our gut surpasses the number of cells in our body and form a very complex and vurnerable ecosystem, where too much of one and too little of another species may be more of a problem than "having the wrong bacteria". For now, however, supplements containing various strains of the two aforementioned types of bacteria does in fact seem to be the most promising, but certainly not fully proven strategy to improve ones gastro-intestinal health.

Even if the good guys are unconditionally good, the ambiguous results of pertinent research clearly indicates that this doesn't imply that all of use will benefit from exogenous provisions of bacteria. There are for example both positive and negative associations for certain strains of bifidobacterium or lactobacillus species (Million. 2011), so that supplementation is mostly based on guessing which are good based on individual studies. This is also why I personally believe that tweaking the environment and thus steering the gut microbiome into the right direction with prebiotics, is a more viable and promising strategy than the ingestion of bazillions of preformed bacteria aka probiotics.
In their thorough, but small scale (N=31) analysis, Cara L. Frankenfeld and her colleagues analysed the fecal sample of their subjects using Multitag Pyrosequencing. This allowed them to compare the bacterial abundance and bacterial diversity across consumers and non-consumers of aspartame and acesulfame-K using non-parametric statistics and UniFrac analysis, respectively.

To predict some of the consequences of possible difference in the bacterial make-up Frankenfeld et al. applied a phylogenetic investigation of the communities by reconstruction of unobserved states (PICRUSt) in order to predict mean relative abundance of gene function.
Table 1: There were no sign. differences in BMI, energy intake, total carbohydrate and added sugar intake or the "quality" of the diet (as assessed on the Healthy Eating Index) between AS consumers and non-consumers (Frankenfeld. 2015)
Thus, the results of the gene function analysis must be met with a healthy degree of skepticism, because unlike the bacterial counts and diversity, which was also just estimated based on what "left" the subjects in form of feces, the gene assay is a model-based result... think of it like the weather forecast, one of which studies say it's relatively reliable (Langille. 2013).
Don't be a fool, stevia will mess with your microbiome just like if not even more than "unnatural" sweeteners.
Sweeteners could be a bad, but also a good thing. In pigs, SUCRAM® (a mixture of saccharin and neohesperidin dihydrochalcone) will significantly increase the abundance of the allegedly good Lactobacilli by more than 100% (Daly. 2014). In other studies, like the previously discussed study in rodents, saccharin has a negative effect. Whether a 100% increase in Lactobacilli as in Daly et al. (2014) or decreases in other bacteria as they have been observed in several rodent studies is something we'd want or not, is yet totally unknown. It's simply too early to predict the effect. For many of you that may be enough to avoid sweeteners, and I fully understand that.

I just want you to know that stevia is not the "healthy alternative", just because it's "natural". In fact, for stevia we even know that it will kill lactobacilli, i.e. those bacteria of which we think that they are the good guys (learn more about how stevia messes with the gut micriobome).
Among the seven aspartame consumers and seven acesulfame-K consumers (some consumed both), the researchers did indeed find some significant differences in the bacterial make-up compared to those subjects who had abstained from consuming sweeteners in the four days before the fecal samples were collected. I quote from the FT (Frankenfeld. 2015):
  • Bacteriodetes and Firmicutes had the highest median abundances and together accounted for the majority of the bacterial class representation in all individuals. 
  • The median Bacteriodetes:Firmcutes ratio did not significantly differ across aspartame non-consumers (0.96, range: 0.15-2.97) and consumers (1.08, range: 0.69-1.87), (median test p-value=0.60). 
  • There was no overall visual clustering of individuals by acesulfame-K consumption . 
  • Overall bacterial diversity evaluated with UniFrac analysis was different across consumers and non-consumers, but there were no significant differences in relative abundance of gene function across consumers and non-consumers (Figure 3).
  • There were no observable difference in the three individuals who consumed both aspartame and acesulfame-K (Supplemental Figure 1). 
So, let's briefly sum this up. In line with the overall hysteria about sweeteners, the changes were more significant in the aspartame group (p<0.01) than they were in the acesulfame-K group (p=0.03), but this could be explained by the simple fact that the subjects consumed significantly more aspartame (one can of diet coke contains 150mg, by the way) than acesulfam-K (5.3 mg/day to 112 mg/day vs. 1.7 mg/day to 33.2 mg/day).
Figure 1: Yes, there were differences in the bacterial make-up of the gut microbiome of the four groups, but no one can tell you what these or the vast individual differences in the groups mean for your health (Frankenfeld. 2015) !
Against that background, the "aspartame is the worst" hypothesis could neither be refuted nor supported based on the study at hand, even if the results would signify overall ill-health effects. With all three potentially important markers remaining unchanged, though, this is not the case:
  • the ratio of mostly "bad" gram-negative bacteriodetes and "good" gram positive bacteria remained the same - it's thus hard to argue that the subjects who consumed artificial sweeteners had an unhealthier gut microbiome
  • there was no general reduction in gut bacteria, which would indicate a general anti-microbial effect of artificial sweeteners as it occurs with antibiotics - it's thus hard to argue that the anti-microbial effects (which don't exist) of artificial sweeteners would leave you similarly defenseless and open to colonization with "bad" bacteria as antibiotics.
  • the gene essays say that despite the differences in the numbers of certain bacteria, the gene expression is the same - it's thus hard to argue that there was an epigenetic effect of artificial sweeteners that precipitates us to obesity or even makes us sick / diabetic / whatever
Against that background, there may be an urgent need for future research and technological development that would allow us to go beyond observing changes in the number and ratio of largely unknown gut bacteria that are (as of now) completely meaningless for us. 
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Don't be fooled! Scientists may understand the format of the graph in Figure 1 better, but if they were honest, they would have to admit that they have absolutely no clue what the changes they observed mean. Yes, they can use mathematics to tell you they are statistically significant, but they can't even tell you whether they're rather good or bad for you.

Eventually, the data in Figure 1 shows us only one thing: The gut microbiome is like a finger-print. It's different for all of us and despite changes due to artificial sweetener consumption, there's no clear pattern in any of the artificial sweetener groups that would allow us to predict negative or positive effects based on what we know now.

Against that background I would try not to freak out about the fact that aspartame and acesulfam-k can affect your gut microbiome. There is no, and I repeat, no convincing experimental evidence in humans that would remotely confirm that any potential changes of the gut microbiome that occur in response to the consumption of artificial sweeteners would entail ill health effects. The only thing we have,are observational and epidemiological studies that correlate obesity with artificial sweetener use and are abused by people who don't know or simply ignore the difference between associations and causation as "proof" that artificial sweeteners are (usually together with fructose) at the heart of the obesity epidemic. And even in the study at hand, the dietary control was not rigorous enough to exclude that the observed association was actually due to aspartame and acesulfam-K and not due to other agents in the artificially sweetened drinks or totally different foods, the subjects consumed during the four day lead-in, during the last months, or even chronically for years or decades | Comment on Facebook!
  • Daly, Kristian, et al. "Dietary supplementation with lactose or artificial sweetener enhances swine gut Lactobacillus population abundance." British Journal of Nutrition 111.S1 (2014): S30-S35.
  • Frankenfeld, Cara L., et al. "High-intensity sweetener consumption and gut microbiome content and predicted gene function in a cross-sectional study of adults in the United States." Annals of Epidemiology (2015).
  • Langille, Morgan GI, et al. "Predictive functional profiling of microbial communities using 16S rRNA marker gene sequences." Nature biotechnology 31.9 (2013): 814-821.
  • Million, M., et al. "Obesity-associated gut microbiota is enriched in Lactobacillus reuteri and depleted in Bifidobacterium animalis and Methanobrevibacter smithii." International journal of obesity 36.6 (2012): 817-825.
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