The film opens with a fetching red-head puffing away on a treadmill. She's perspiring, but she's smiling gamely into the camera. "It's not an average work-out, but I wasn't an average weight," she explains. "I have to do above and beyond what any of you guys would have to do. I have to try twice as hard, sometimes three times as hard - just to maintain this level of...chubbiness."

And she is right. She is chubby. By 21st century mainstream (and magazine) standards of beauty this young woman is probably 30 pounds overweight. The dimples, the pony-tail, the strawberries and cream complexion, and the undeniable on-camera charisma make her very appealing. But there is no doubt that most physicians would urge her to lose weight.

Later in the film, we learn that she exercises three hours a day. And when her mother was dying of cancer, this thirty-something nursed her and learned a great deal about nutrition. Dedicated and determined, she eats healthy meals and sticks to a strict exercise regime. Why, then, is she "chubby?"

Doctors don't know. That is one of the first things you learn in "Fat: What No One is Telling You," a 2007 documentary that is, by turns, entertaining, moving, and eye- opening. (The PBS home video, directed by Andrew Fredericks, can be rented on www.netflix.com or purchased on www.amazon.com).

The questions are endless, a narrator tells the audience. "Is it her genes, her childhood, a flaw in her character, stress, sadness, a lost love, processed food, television, seductive advertising, lack of sleep, a government that subsidizes corn, sugar and beef?"

All of the above may well contribute. But taken together, they still don't constitute an answer. Doctors cannot help the vast majority of obese people lose weight--and keep it off - because doctors don't know what causes obesity.

"If It Were That Simple..."

Although many physicians still "believe that obesity is caused by eating too much and not exercising enough, such thinking is too simplistic," says Dr Robert Lustig, of the Division of Pediatric Endocrinology at the University of California, San Francisco. An expert in the field, he knows that obesity is "a chronic condition." And we don't have a cure.

This is why, even when patients enter medically supervised weight-loss programs, and stick with the rules, Lustig explains, "95 percent" regain whatever pounds they lose.

"This is not simply 'energy in and energy out.' If it were we would have solved it a long time ago," says Harvard's Dr. Lee Kaplan, who heads the Weight Reduction Program at Mass General Hospital and has established a new, comprehensive basic and clinical research program.

"Obesity doesn't seem like a subtle disease," adds Kaplan, who appears in the documentary. "But it is. If something is off kilter by just 1 percent in your system that can lead to a 100 pound weight gain. More than 400 genes are involved in weight regulation. And that doesn't include the environmental factors."

"Fat" goes on to introduce us to a very bright, 300-pound 18-year-old who has sought medical help, researched obesity and, with the support of his doctor, is now planning on bariatric surgery (a.k.a. "stomach-stapling"). "They just haven't figured out this obesity thing," he says. "There is something haywire in your body.

"You become depressed when you realize that...you're going to die earlier. And when they bury you, they'll need twenty people to carry you rather than four." (I found hearing an 18-year-old worrying about how many people would be needed to carry his coffin to his grave incredibly sad.)

Obesity is "indescribably complex," he adds. In his own family, it is a mystery "My twin sister is skinny. I'm not. When we were born, we were the same weight and length. But many people in the family are overweight, which suggests a genetic component."

Another character in the film was not heavy as a child. "A tomboy, I was always very athletic," this young woman confides. And the pictures of her as an adolescent confirm that she was an extremely attractive female athlete. No fat - just great muscle tone. But then "I moved into the corporate world," she explains. "And since I've been at Microsoft, I've gained 125 pounds."

As an executive she travels frequently, and "If I'm traveling coach, it's really tight," she confides. "Last time, I was in a center seat, and kept apologizing to the people next to me. They were squished, but very nice about it," she says, still obviously terribly embarrassed by the memory.

Many young athletes move into the corporate world, and don't gain 125 pounds. Why would a young woman who loved sports suddenly find herself too heavy to play? This is the conundrum that medicine hasn't cracked. Meanwhile, this Microsoft executive would do anything to lose weight.

"I hate this," she says. "I can no longer do the things I love to do." She and her husband (who is also a very big man) haven't been able to get pregnant. To try to solve their problem, they have signed up for a comprehensive (and no doubt expensive) program that includes doctors, nutritionists, and trainers.

They have purchased health cook books, and admit that they really don't know how to cook. "I have no idea how to cook chicken," she acknowledges.

"What is chicken?" he asks, only half joking.

But, she tells her husband, "We'll just have to try these recipes. Some of them we'll hate - so we won't cook them again." She is hopeful that they will find others that they like.

In part 2 of this post, I'll disclose what happened to this couple, the boy who is hoping for bariatric surgery, and the young woman on the treadmill. I'll also expand on what doctors do know, at this point, about obesity and weight gain, cultural and ethnic differences, and why physicians say that people who come in larger shapes - and exercise - are healthier than very thin couch potatoes.

I'll also discuss why we have reason to hope that we are beginning to understand the neurological causes of unwanted weight as researchers investigate how the "lower brain" (in the gut) communicates with the "upper brain" (in the head).

Finally, I'll explore the role that society plays in obesity. On that score, I found the following Amazon review of the documentary telling:

"I happened upon this movie on PBS when I was on a business trip, and it essentially helped to kick-start my journey into a healthier lifestyle, where I have lost 40 lbs already in a 150lb long term goal," the reviewer wrote.

"I remember being deeply and personally moved by the stories, but it was also a game-changing flood of information about the latest biological research that did the trick for me. It allowed me to see the issue not in terms of will power and laziness (as is all too common in popular culture as well as years of medical haranguing), but in terms of physical compulsion akin to and even surpassing opiate addiction.

"Why was this new information so critical in my current success, where previously I had tried and failed? It is definitely that it shatters the myth that weight loss is as simple as consuming less than you expend - a glib and harmful misstatement of the problem as profound as saying that beating heroin addiction is as simple as going cold turkey. You'd think that learning exactly how hard it is, really, to lose weight would be discouraging, but it was exactly the opposite. After years of people, including my doctor and nutritionist, breezily tossing off advice and plans of action, I finally learned what I was up against. Then I declared war on it."

I would add this: Knowledge is power. Conventional wisdom is not. Right now, only 5 percent of those who fight the war win - and keep the weight off. Sadly, the way that both society and our healthcare system treat those struggling with this chronic disease can make their battle harder.