I think I’ll pretend I didn’t see the following story until another group of experts announces that the exact opposite is true:
"People who drink diet soft drinks don’t lose weight. In fact, they gain weight, a new study shows.
"The findings come from eight years of data collected by Sharon P. Fowler, MPH, and colleagues at the University of Texas Health Science Center, San Antonio. Fowler reported the data at the annual meeting of the American Diabetes Association in San Diego.
"’What didn’t surprise us was that total soft drink use was linked to overweight and obesity,’ Fowler tells WebMD. ‘What was surprising was when we looked at people only drinking diet soft drinks, their risk of obesity was even higher.’"
(Nod to the friend who sent me the link.)
Of course, this kind of thing is probably why the diet-soft-drink industry chooses advertising slogans such as "Just for the taste of it."
JIMMY ADDS: Actually, there are a couple of theories I’m aware of that may explain this. The first is that most folks who drink diet cokes think that they are saving more calories than they actually are and thus give themselves liberty for too much extra calorie intake, resulting in more net calories (e.g., "I didn’t drink that 180 calorie glass of Coke–I had Diet Coke instead–so it’s okay if I eat this 300 calorie piece of cake").
Another theory that may play a role is that most artificial sweeteners still have a very high glycemic index–higher even than sugar–and this may result in your brain thinking that you’ve just drunk a whole mess of sugar. The brain then gets ready to digest sugar and causes the insulin spike that inhibits weight loss.
My own conjecture is that the former conjecture may play a larger role than the latter. I lost a great deal of weight over a long period of time while drinking diet coke the whole time. The diff was that I had cut out almost all carbohydrates out of my diet and so wasn’t giving my body much to generate an insulin spike, even if the Nutrasweet in the diet coke was giving me a bit of one.