1. Sugar is harder to resist than any other food.
We need sugar to function—without it, we'd quickly power down. It's so important, in fact, that our bodies have the ability to synthesize it from fat, complex carbs and protein through processes that take place in the liver. But when presented with effort-free sugar on a platter (or in a drinking glass or a candy box), our bodies go wild. Sugar is even more compelling than fat, found a recent study in the American Journal of Clinical Nutrition. Researchers tracked brain activity in more than 100 high school students while they drank chocolate-flavored milkshakes and found that shakes that were high in sugar but low in fat fired up the brain's reward circuitry just as strongly as super-decadent shakes that paired sugar and fat in large quantities. Increasing the fat content of the shakes didn't have the same effect, affirming that sugar was what the students' brains really wanted.
2. ...In fact, sugar can be as hard to resist as drugs.
The idea of a "sugar addiction" isn't just a concept broadcast far and wide by your green-juice-obsessed neighbor. Experiments with both people and animals have shown that sugar affects the brain in the same way that morphine and other opioids do, and when sugar-addicted rodents were suddenly taken off the stuff, they went through withdrawal—complete with the shakes and anxiety. They had become chemically dependent, explains Nicole Avena, PhD, a neuroscientist with the New York Obesity Research Center at Columbia University who was involved in these studies and is the author of Why Diets Fail (Because You're Addicted to Sugar). Humans get hooked, too. The more sugar we eat, the more we want, Avena says, and we need increasing amounts of it to keep producing the same euphoric effects. That's one of the main reasons people who say they're just going to have a tiny scoop of ice cream (and really mean it) end up eating four scoops.