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A recent study about the effect of artificial sweeteners suggests they may increase the risk of developing type 2 diabetes. This has been widely reported in the media, for example the Mail Online claims:

Artificial sweeteners may help cut calories but the sugar replacement could raise the risk of a person being diagnosed with type 2 diabetes, experts have warned.

Is this a credible claim?

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Summary: The Daily Mail misreported the findings. We do not know if there is a causal link.

The study they reported on is this one:

The study did not attempt to show causation, and could not do so, as it was based on a survey, not a randomised controlled experiment. The conclusions they drew were limited to correlations, not causation. This is suggestive of further research directions:

e.g.

consumption of aspartame is associated with greater obesity-related impairments in glucose tolerance.

[...]

In conclusion, aspartame artificial sweetener consumption may be associated with greater glucose intolerance, particularly for those with obesity. Future research is needed to determine if from an obesity and diabetes perspective, it may be prudent to limit all sweetener consumption.

The Daily Mail overstepped when it made claims like:

But, new study has shown they increase a person's risk of type 2 diabetes

It is not difficult to come up with plausible confounding variables, which make it impossible to soundly make the causation claim. For example, they do not appear to have considered whether the subjects have already been diagnosed with Type II diabetes, and have modified their diets to include aspartame to help manage their sugar levels.

Similarly, it seems like that obese people (who are more likely to have greater obesity-related impairments) are more likely to be dieting with aspartame, pushing the causation in the opposite direction. Indeed, given that there is more social pressure on women to reduce weight than men, this passage from the study is suggestive of that:

Individuals consuming artificial sweeteners (aspartame or saccharin) had a subtly higher BMI (28 vs. 27 kg/m2), and were more likely to be female (Table 1, P < 0.05).

Another observation against suggests the causation link may be the other way around:

We observe that aspartame was related to significantly greater impairments in glucose tolerance for individuals with obesity, but not lean individuals. In fact, our results suggest a beneficial effect of aspartame in lean individuals. However, as there are very few lean individuals in the population who reported consuming aspartame, this requires further investigation.

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  • I've seen a study in rats showing a causal link.
    – Peter Shor
    Jun 6, 2016 at 14:45
  • @PeterShor: Great. Answer the question and don't forget to provide some references to support your claims. Until then, I remain skeptical.
    – Oddthinking ♦
    Jun 6, 2016 at 16:57
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    I posted my answer, although my memory was wrong – it was mice.
    – Peter Shor
    Jun 6, 2016 at 17:25
  • And now that I look, the study you cite references the mouse study from my answer—and the results are entirely consistent with this effect being present in humans (they needn't have been), although they don't prove it.
    – Peter Shor
    Jun 7, 2016 at 20:18
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There has been a study in mice showing that consumption of artificial sweeteners changes the composition of microbiota in their guts, and seems to change it in a way that makes the mice more susceptible to diabetes.

See the 2014 Nature article Artificial sweeteners induce glucose intolerance by altering the gut microbiota.

From the abstract:

Here we demonstrate that consumption of commonly used NAS [non-caloric artificial sweetener] formulations drives the development of glucose intolerance through induction of compositional and functional alterations to the intestinal microbiota.

This at least gives a possible pathway linking artificial sweeteners and diabetes, thus allowing for the possibility that the correlation discovered by the study cited by the Daily Mail arises because artificial sweeteners help cause diabetes in humans.

It might also explain why this effect doesn't depend on the composition of the artificial sweeteners. The effect in the mice was present for all three artificial sweeteners tested—saccharin, sucralose and aspartame.

One possible mechanism for this effect (my speculation) is that it could be caused by the stomach signaling the gut microbiota that calories are coming, and then not actually producing any calories. If this is the mechanism, then non-caloric natural sweeteners might also be correlated with diabetes.

As far as I know, there isn't any definitive evidence for this in humans yet.

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