O RLY? I wonder if butter works like this too.

Sugar.  Life magazine and elsewhere, 1969-1971    Noted in this comm two years ago: HERE.

from the shadowy "Sugar Information" at a General Post Office Box. ??? 

Whether it's a Diet tip, hint, device, or dodge, I'm not buying it.

Fuller coverage and examples HERE at Go Retro!.
"Sugar just might cure baldness. Sugar just might be what Attila the Hun ate for breakfast before every battle. Sugar just might be the natural predator of indecisiveness. Sugar just might know tonight's winning lottery numbers. Sugar just might be a edible rocket fuel. Sugar just might be crystallized fairy dreams.

It doesn't, it isn't, and it won't, but wouldn't that be something? SUGAR. EAT MORE OF IT."
Not wrong, but misleading. If you are dieting, you have to remember to eat, not fast and torture yourself. But what you eat still matters and a soda or candy bar before a full meal isn't helping you diet, it's just shifting your (empty sugar) calorie intake and glycemic index up and the nutritional value down.
Not by much, and if this ad ran nowadays, they wouldn't make a distinction between cane sugar and high-fructose corn syrup.
"Hi! We keep changing facts about dietary needs every 5 years or so, to sell whatever it is we need to sell at the time. Right now it's candy and soda pop. Sugar is good for you!!!!"
Oh, mysterious Sugar Information people, no need to sell me on how delightful sugar is.
You would have to time it just right. In a little more than an hour your temporal blood sugar spike should have lowered under the starting level, making you ravenously hungry. Also, even a small piece of candy or a small bottle of soda will probably hold more calories than what you refrain from eating during the real meal.

So did people lack basic knowledge about human physiology, or were they dumb? *goes to develop diet candy, should be a bestseller even today*