Why Diets Fail
The most immediate reason that diets don't work over the long term is that they promote a loss of the internal signals for hunger and fullness that are necessary for normal eating. This was the finding of a classic study conducted by Janet Polivy and Peter Herman at the University of Toronto, published in 1999. In this experiment, a group of dieters and a group of nondieters were given the task of comparing ice cream flavors. Participants in each group were divided into three subgroups. Before getting the ice cream, the first subgroup was asked to drink two milkshakes, the second subgroup was asked to drink one milkshake, and the third subgroup wasn't given any milkshakes. Next, the researchers offered the groups three flavors of ice cream and asked the participants to rate the flavors, eating as much ice cream as they desired.
The results revealed that the nondieters ate as you might expect: those who hadn't consumed any milkshakes ate the most ice cream, those who'd consumed one milkshake ate less ice cream, and those who'd consumed two milkshakes ate the least. The dieters, by contrast, reacted in the opposite way. Those who were offered no milkshakes before the taste test ate small amounts of ice cream, those who drank one shake ate more ice cream, and those who'd consumed two milkshakes ate the most ice cream!
The researchers termed what had happened to the dieters "disinhibition," which occurs as a result of a "diet-mentality." The milkshake preload had a different effect on dieters than on nondieters. Nondieters, eating in an unrestrained and normal manner, tend to regulate their food consumption according to internal physical cues of hunger and satiety. Therefore, in the experiment, nondieters regulated the amount of ice cream they ate based on perceived fullness. What could be more obvious and natural?
The dieters, however, reacted in the opposite way—the more milkshakes they consumed, the more ice cream they ate. Why did they lose the capacity to regulate their intake? According to the researchers, this "counterregulation" occurs because a milkshake preload disinhibits a dieter's usually inhibited or restrained eating, almost like a switch: "I've blown it anyway, so I might as well keep eating before I go back on my diet." This is an almost irresistible incentive to go on eating well past physical fullness.
Most of us have internalized cultural ideals about the body we'd like to have and how much we want to weigh, based on improbable models of perfection beckoning from just about every media site. But these images usually bear little resemblance to what's natural, healthy, and physically possible for our individual bodies, because genetics plays a major role in determining our size and shape.
Weight is the result of a complex combination of factors that aren't yet fully understood. Although the question of nature versus nurture has long been debated when it comes to weight, research shows that the weight of adopted children resembles that of their biological parents, not that of their adoptive parents. When researchers looked at identical twins raised apart, they found that their body-mass index was nearly identical, despite different environmental settings. This means that the influence of genetic inheritance has an enormous impact on what we weigh.