I recently did a deep-dive on low-calorie sweeteners with my rockstar RD coworkers (shoutout to Kathi and Becca!).  It's such an evolving field of research that it requires us to keep a pulse on what's emerging to ensure we are giving the best and most up-to-date recommendations. 

The downside to low-calorie sweeteners:

• There is some evidence that low-calorie sweeteners increase cravings for sweets in some people. 
• Low-calorie sweetened beverages (e.g. diet soda) have been correlated with increased risk of diabetes in some studies.  In other studies there is absolutely no correlation.  
• Some studies do show an increase in glucose and insulin secretion after consumption of low-calorie sweeteners.
• New research is showing a possible disruption to the gut bacteria by low-calorie sweeteners.  In fecal transplant studies (yes, you heard me correctly, and it's gross I know), the feces of mice that consumed low-calorie sweeteners was transplanted into healthy mice that had never consumed any.  The mice developed impaired glucose tolerance. 
• There are several limitations to this area of research – including consumption levels, sample sizes, study design, duration of study, lack of control groups, confounding factors, etc.  
• We don't have enough evidence from high-quality randomized control trials to make specific recommendations.  

The upside to low-calorie sweeteners:

• Low-calorie sweeteners are recognized as safe by the FDA
• Low-calorie sweeteners may reduce the risk of dental caries compared to sugar sweetened beverages.
• Low-calorie sweeteners may improve glycemic (i.e. blood sugar) control in people with diabetes. 
• Aspartame does NOT cause cancer.  In fact, Brigham Women's Hospital (a Harvard facility where the study was done) has apologized for promoting this study because the evidence was so weak. 

Bottom line…

There is still a LOT that we don't know, and therefore as an RD I can't give you specific amounts because it depends on way too many factors.  The issue is not black and white.  In general, as long as you're not going overboard with sugar or low-calorie sweetener then you will probably be fine.  If you have a health condition that warrants limitation you may need to do so.  Each person needs to make decisions that are best for themselves.