I had heard that using artificial sweeteners can cause obesity, but hadn’t before seen an explanation. This article by Tom Philpott of Mother Jones describes the mechanism:

[A] slew of studies have shown that faux sugars may actually contribute to the very diet-related maladies they’re marketed to protect us from—type 2 diabetes, hypertension, metabolic syndrome, strokes, and heart attacks… [T]hose who drank at least one diet soda per day were 43 percent more likely to suffer strokes and heart attacks than people who drank none, even after controlling for such factors as weight, level of exercise, diabetes, high blood pressure, and intake of calories, cholesterol, and sodium. Another large population study, published in 2009, found that daily diet soda drinkers were 67 percent more likely to develop type 2 diabetes than people who shun them—again, even after adjusting for lifestyle and demographic factors. …

Purdue University behavioral neuroscientist Susan Swithers suggests that fake sweeteners do their dirty work by confusing our digestive systems’ Pavlovian response to sugar. When you smell food, she explains, you begin to salivate and your stomach begins to grumble; that’s your body preparing for what it assumes from experience is a hearty meal to come. Similarly, she says, a sweet taste is a “pretty good indication that sugar is going to arrive in your body”—that is, a blast of easily digestible calories. So, quaff Pepsi, and your body starts releasing digestive hormones and increases its metabolic rate, “because you have to expend energy to get energy out of your food,” she explains.

Fake sweeteners appear to subtly disrupt the trillions of microbes that live in our digestive tracts. But when you start ingesting sweet blasts that then don’t deliver the usual calorie blast, your body no longer knows what to expect. As a result, Swithers says, tests have found that “animals who have experience with artificial sweeteners don’t seem to be as good at regulating their blood sugar levels when they get real sugar”—hence the associations with diabetes and other metabolic troubles. And this mechanism would appear to be independent of the kind of low-calorie sweetener used.