With these caveats, one of the few reasonably reliable facts about the obesity epidemic is that it started around the early 1980's. According to Katherine Flegal, an epidemiologist at the National Center for Health Statistics, the percentage of obese Americans stayed relatively constant through the 1960's and 1970's at 13 percent to 14 percent and then shot up by 8 percentage points in the 1980's. By the end of that decade, nearly one in four Americans was obese. That steep rise, which is consistent through all segments of American society and which continued unabated through the 1990's, is the singular feature of the epidemic. Any theory that tries to explain obesity in America has to account for that. Meanwhile, overweight children nearly tripled in number. And for the first time, physicians began diagnosing Type 2 diabetes in adolescents. Type 2 diabetes often accompanies obesity. It used to be called adult-onset diabetes and now, for the obvious reason, is not.
So how did this happen? The orthodox and ubiquitous explanation is that we live in what Kelly Brownell, a Yale psychologist, has called a ''toxic food environment'' of cheap fatty food, large portions, pervasive food advertising and sedentary lives. By this theory, we are at the Pavlovian mercy of the food industry, which spends nearly $10 billion a year advertising unwholesome junk food and fast food. And because these foods, especially fast food, are so filled with fat, they are both irresistible and uniquely fattening. On top of this, so the theory goes, our modern society has successfully eliminated physical activity from our daily lives. We no longer exercise or walk up stairs, nor do our children bike to school or play outside, because they would prefer to play video games and watch television. And because some of us are obviously predisposed to gain weight while others are not, this explanation also has a genetic component -- the thrifty gene. It suggests that storing extra calories as fat was an evolutionary advantage to our Paleolithic ancestors, who had to survive frequent famine. We then inherited these ''thrifty'' genes, despite their liability in today's toxic environment.
This theory makes perfect sense and plays to our puritanical prejudice that fat, fast food and television are innately damaging to our humanity. But there are two catches. First, to buy this logic is to accept that the copious negative reinforcement that accompanies obesity -- both socially and physically -- is easily overcome by the constant bombardment of food advertising and the lure of a supersize bargain meal. And second, as Flegal points out, little data exist to support any of this. Certainly none of it explains what changed so significantly to start the epidemic. Fast-food consumption, for example, continued to grow steadily through the 70's and 80's, but it did not take a sudden leap, as obesity did.
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