Hi!

There are lots of issues.

It depends on what specific sweetener you are referring to, and what you mean by "bad for us" or "contribute to obesity"

• As far as bad for us - sucralose can be a trigger for migraines

The paper Body & brain: No-cal sodas can trick the brain: Sugar-free sweeteners may contribute to obesity risk has lots of details. I'll try to summarize

>saccharin and other sugar-free sweeteners — key weapons in the war on obesity — may paradoxically foster overeating.

This was not a big data study, only 24 subjects.

>One strong link to higher diet soda consumption was reduced activation of the caudate head, an area associated with the food motivation and reward system. Green and Murphy note that decreased activation of this brain region has also been linked to higher risk of obesity.

It finds a link (but not proven causality) that the area of the brain lit up (fMRI studies) in response to diet soda was the same area that is associated with obesity.

in earlier studies

> Swithers’ group showed that rats that always received a saccha- rin-sweetened yogurt learned to modu- late their food intake to account for the sweetener’s failure to deliver calories. But rats that alternately got saccharin- and sugar-sweetened yogurts got fat.

If you are curious about the research, Bisphenol-A, found in BPA plastics has been linked to obesity. Childhood exposure causes people to want more sweet foods.