The world loves fat. It's what food industry executives say adds precious flavor and "mouth-feel" to our foods, and what doctors say adds detrimental calories to our diets. 91 million Americans are considered obese-defined as twenty percent above ideal body weight-and the incidence of obesity is rising around the globe. Today, inundated by tens of thousands of food ads each year, influenced by a standard of beauty built on being thin, and fighting a primal craving, we are losing the war on fat.

In "Fat," FRONTLINE travels the globe in search of the causes of the obesity crisis, examining how media and cultural ideals, as well as biology and genetics, influence our relationship with food.

This report opens with a look at modern life's impact on weight and health, as exemplified in a study done on the Pima Indians in Arizona whose diets and lifestyle today are similar to most Americans. The Pima are the fattest population group in the country and are plagued with obesity-related illnesses like diabetes, high blood pressure, and heart disease. But five hundred miles to the south in northern Mexico, another branch of the Pima is living a very different life, devoid of labor-saving devices and filled with hard physical exercise and a traditional diet. These Pimas are an average sixty pounds lighter than their American cousins.

While bad diet and lack of exercise account for much of the problem, many also blame the media. From food ads, to Hollywood, to the fashion industry, "Fat" lays out the pervasive, contradictory messages of high-fat, high calorie food advertising (much of it aggressively aimed at children) and the glamorous images of rail-thin models and celebrities exuding the message that to be thin is to be beautiful. The result, say many experts, is a high incidence of eating disorders like anorexia and bulimia and young people, especially girls, jeopardizing their health with quick fix diet programs and medications.

In this wide-ranging report, FRONTLINE looks at the pain and suffering of obese people who everyday must confront fat prejudice, and tells the story of some new scientific discoveries. Scientists and nutrition experts discuss the role of genes in obesity, biological factors in weight gain/loss, and why there's increasing support for the view that there's no such thing as an 'ideal weight.' In fact, they say, one can be healthy, fit, beautiful -and fat.

"Fat" concludes with the stories of morbidly obese patients who have had radical weight loss surgery and profiles of Americans who are fat by all clinical standards, yet also are leading active, healthy and happy lives.