Back in 2014 a study was published, titled: Diet Drink Consumption and the Risk of Cardiovascular Events.  A copy of the study can be found here.  It's nothing daunting--a mere 6 pages long.   Though once published, it's pretty clear that the media didn't bother spending any time reading, let alone analyzing the study.  Instead, they likely skipped to the last paragraph and read the following:

"In conclusion, this study suggests an association between consumption of two or more diet drinks per day and adverse CVD (Cardiovascular Disease) events, as well as increased mortality."

That was enough to produce headlines like these:

Diet Drinks Raise Heart Concern in Postmenopausal Women
Diet Drinks Linked With Heart Disease, Death
The End of Diet Soda? Huge Study Links Aspartame to These Major Health Problems
Daily Soda Intake Increases Heart Attack Risk

It's worth noting that for the 3rd link above, the study makes no mention of what type of artificial sweetener was in the 'diet drinks' consumed, whether Aspartame or otherwise.  That last link is particularly amusing, since this study was strictly among postmenopausal women and yet this is posted on Men's Fitness. Well, let's take a minute and dig into the science behind and the findings of this study, shall we?

The method of the study was, very briefly, to round up nearly 60,000 women ages 50-79 who did not have pre-existing cardiovascular disease (e.g. coronary heart disease, heart failure, myocardial infarction, etc.) and have them submit a self-reporting questionnaire annually relating to their diet soda consumption.  This was continued for several years, and the resulting data was broken down into four categories of diet soda consumption: 0-3 per month, 1-4 per week, 5-7 per week, and >2 per day.  Researchers looked at all kinds of outcomes of these women over those years, primarily focused on the prevalence of 'cardiovascular events' (i.e. the onset of heart failure) and mortality.  Before we analyze the data presented by the study, let's first identify a few problems with how the study was structured:

1) self-reported studies are notoriously unreliable and inaccurate.

2) The grouping of the diet soda consumption groups is odd.  The questionnaire had 9 consumption groupings, yet the study reports on 4 groups--were some groups combined? or ignored?  By creating a "0-3 per month" group, researchers are assuming and asserting that consuming up to 3 diet sodas a month is negligible and equivalent to never consuming diet soda.  Also, the first three groups pretty well cover the full spectrum of consumption possibilities from 0 times a month all the way through 7 times a week (i.e. one a day).  Why, then, is there such a large gap between the consumption of group 3 and 4 up to "2 or more per day"?  What about all the people who consume 7-14 per week?  The reduction in groups from the original 9 down to the 4 reported-on gives license to manipulate the output data to artificially inflate the results you're interested in.  It appears that this highest consumption group was intentionally biased away from the remaining groups to help stratify the data and produce more stark results.

3) The annual questionnaire asked participants to self-report on consumption activities (from memory) for the most recent 3 months.  What about the other 9 months of the year?

Now, as far as the data and conclusions are concerned: yes-- the "2+ a day" group did show higher rates of both CVD and mortality.  But here are some other equally valid conclusions that could be made using the very same flawed dataset:

1) Consuming 1-7 diet sodas per week reduces mortality up to 20%.
2) Consuming 1-7 diet sodas per week reduces cardiac death by 20%.
  
There you have it-- MODERATE DIET SODA CONSUMPTION SAVES LIVES.  Why didn't we see these in the headlines?

Another glaring issue with this study is the authors own observation of the following:

"There was increased prevalence of diabetes and hypertension in the women who consumed the most diet drinks, and they had a higher BMI (Body Mass Index), a greater proportion of smokers and higher calibrated energy intake."

It seems pretty scientifically irresponsible to suggest an association of CVD events and mortality with the high consumption of soda when that same consumption group has noticeably higher rates of smoking, obesity, calorie consumption, etc.  Perhaps a more appropriate conclusion for this study would have been:

Women with higher BMI and higher rates of diabetes, hypertension, and smoking are associated with adverse CVD events as well as increased mortality.  Oh, and they tend to consume more diet soda, too (maybe because they are trying to manage their weight?).

This is a prime example of what is most likely a spurious relationship:  Where higher BMI, diabetes, hypertension, smoking is present, both increased mortality and increased diet soda consumption are observed: soda consumption is the lurking variable.  To the author's credit, he does state the need for further studies to confirm or disprove the link between diet soda consumption and CVD events or death and also to evaluate whether there is a causal relationship between diet soda consumption and CVD risk.  In other words, this study of over 60,000 women didn't really prove anything and, as the favorite saying goes, correlation does not imply causation.  But the media didn't bother to report "Diet Soda Consumption: More Studies Needed."

Just for kicks, if we just want to look at data correlations within the study, here's a few more "conclusions" (for entertainment purposes):

1) Lower diet soda consumption correlates with higher college graduate rates (headline: Diet Soda Linked To College Dropouts!)
2) Diet soda consumption rates correlate with higher income (headline: Drink Diet Soda, Earn More Money!)
3) Diet soda consumption rates correlate with lower alcohol consumption (headline: Drinking Diet Soda Helps Quitting Alcohol)

This study is yet another example of borderline junk science getting gobbled up by the media and converted into unsubstantiated headlines that people base their life decisions around.