People who use artificial sweeteners are just as fat as people who don't. Nobody has ever tried to read very much into this finding, though, because it's pretty obvious that some people are using artificial sweeteners because they are fat and worried about it. But anyway it has long been clear that saccharin and aspartame are not the cure for obesity.
Now, Israeli scientists have been feeding artificial sweeteners to mice, and they have found that in mice (at least), saccharin can actually cause metabolic problems, apparently by interfering with gut bacteria. So it seems possible that artificial sweeteners actually make some people fatter by changing the population of bacteria in their guts. After all, as we have seen so many times on this blog, gut bacteria control everything, so messing with them can be a very bad thing to do. (See here, here, here, here, here, and here. No, I'm not obsessed. Why?)