Nah. The problem is, we pack in a bunch of calories and don’t move very much. They did those old studies, even the ones in the 80s when we complained about not exercising enough, but people still moved around more than they do today. More food is delivered, available via drive-thru, we’re on the computer instead of out doing yard work or working on the car, etc.
Maybe not individuals, but people in general. Video games, Internet, pda/phones, and so on, it’s easy to entertain ourselves without moving. The 2000 calorie diet is not only chewed through in a fast food meal or two before we even get to the soda or snacks, one calculation I saw had a 180lb man that spent most of his time in a chair (desk at work, car for the drive home, restaurants and recliner at home) could skim by with only 1200 calories a day. Maybe the calculation is ideal, but that’s basically a large fast-food meal. Once a day. Nutrients are an extra 50 cents…
Eating less and better might help, but I bet moving (not necessarily “exercise”, but little stuff, remember how (great-)grandma always seemed to be doing something, cooking, cleaning, gardening, and (great-)granddad always had some strange project he was working on? How many folks are like that today? I won’t even start with their own parents’ energy output.
Could the sweeteners cause a problem? Probably. But I think the only reason we’d notice it today is because it’s the only difference in our metabolism. A 10,000 calorie athlete probably wouldn’t even notice the effect.