ARCHIVAL (NBC NEWS, 12-13-01):TOM BROKAW: News in depth tonight. Americans eating themselves to death.

ARCHIVAL (CBS NEWS, 1-8-10):KATIE COURIC: As we begin this new decade, the country is facing a dangerous weight problem.

NARRATION: For decades, Americans have heard the same message.

ARCHIVAL (ABC NEWS, 11-18-09):CHARLES GIBSON: Americans are heavier than ever, with the obesity epidemic only getting worse.

ARCHIVAL (NBC NEWS, 8-15-13):NEWS REPORT: Experts worry that will translate into declining life expectancies.

ARCHIVAL (ABC NEWS, 3-9-04):TOMMY THOMPSON (SECRETARY OF HEALTH AND HUMAN SERVICES): Were just too darn fat, ladies and gentlemen.

NARRATION: But what if the connection between weight and health was a lot more complicated than it seemed?

KATHERINE FLEGAL: I dont think you should rule out the idea that our whole perspective has been wrong about this.

ARCHIVAL:How much would you like to lose?

NARRATION: By the early 1990s, Americans obsession with weight was in full swing.

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NARRATION: C.D.C. epidemiologist Katherine Flegal decided to conduct a study to see whether the number of overweight Americans had actually shifted over the last few decades.

KATHERINE FLEGAL (FORMER SENIOR SCIENTIST, C.D.C.): I thought we should write an article, figuring it hadnt really changed for like 20 years. Much to our surprise, we found an increase. And that was really the point at which suddenly interest perked up.

ARCHIVAL (ABC NEWS, 10-26-99):NEWS REPORT: The increase was across the board and thats why researchers are mincing no words: In the 90s in the United States, we have had an epidemic of obesity.

NARRATION: After Flegals study, the research intensified as others examined the connections between weight and health, producing estimates that excess weight could be contributing to 300,000 to 500,000 premature deaths each year.

ARCHIVAL (CBS NEWS, 10-6-99):DAN RATHER: The new results out tonight from a study on obesity and health. They underscore what you may already know in your gut. Fat is a health risk across the board.

ARCHIVAL (ABC NEWS, 3-9-04):PETER JENNINGS: If something more isnt done to fight obesity, if Americans dont change the way they eat, obesity will soon be the greatest single cause of death in the country, greater than smoking.

NARRATION: Flegal was surprised by these rising numbers.

KATHERINE FLEGAL: Weve all been taught for years that smoking is very bad for you, smoking kills all these people, and suddenly the message was, well, obesity is so bad, maybe going to overtake smoking. That was really shocking to people. In adults, theres only like a little over 2 million deaths a year. So 500,000 is a very large number.

NARRATION: Flegal agreed that obesity wasnt without risk. After all, it was associated with conditions like type 2 diabetes, heart disease, and certain types of cancer. But, she had found issues with some mortality estimates, even outright miscalculations. And she worried that these inflated numbers might skew health policy away from other important factors while providing a justification for rushing weight loss drugs onto the market.

ARCHIVAL (CBS NEWS, 4-29-96):NEWS REPORT: This will be the first time that the government has allowed a drug to be used for maintenance of weight loss.

KATHERINE FLEGAL: There was a drug, Redux, that got approved by F.D.A., which was later withdrawn from the market for safety reasons. One of the reasons it was approved, and you read this in the approval, is because were not sure about this drug, but since obesity is so important, we should approve this drug. So we started to realize this wasnt just, you know sort of, a little methodological niche but it actually was an important topic.

NARRATION: Most of these population-level studies gauged risk using the Body Mass Index, or B.M.I. a crude calculation of weight versus height. But when Flegal completed her own large-scale analysis and broke it out into these categories underweight, overweight, and obesity she found something surprising: the number of premature deaths was far lower than what the public had been led to believe. And that wasnt the only twist.

KATHERINE FLEGAL: So the collective finding was that obesity was associated with about 112,000 deaths more than normal weight. But that was counterpoised by the finding for overweight, which that obesity was associated with about 80,000-90,000 fewer deaths.

ARCHIVAL (CBS NEWS, 3-20-05):BOB SCHIEFFER: Medical experts have told us repeatedly that being overweight can take years off our lives. Well get this: now a new government study says maybe not.

KATHERINE FLEGAL: It was not our intention to have a message, we just said, This is what we found, period. I think I should have prepared better for the onslaught of negative coverage.

NARRATION: Critics attacked Flegals work, claiming her results were skewed by looking at data that was overly broad. But as she re-analyzed her work in response, the findings stuck.

KATHERINE FLEGAL: We published a whole government report, we looked at all these issues of smoking and previous disease and weight stability and so on. And we found the same results, basically, no matter what we did.

NARRATION: Even today Flegal finds herself under attack, despite the fact that her results have long been accepted by the C.D.C.

KATHERINE FLEGAL: This narrative was being constructed, that our article was dangerous, threatening to science, as though any new information is intrinsically dangerous. What does that say? Science is supposed to have new findings.

NARRATION: Counterintuitive results actually shape our understanding of weight all the time, despite the public skepticism they can produce.

ARCHIVAL (FOX NEWS, 7-7-12):NEWS REPORT: A new study finds that heart failure patients carrying extra pounds have a lower risk of death and theyre calling it the obesity paradox.

ARCHIVAL (ABC NEWS, 4-9-14):DIANE SAWYER: Is it possible to be overweight and healthy? Are there times being overweight actually helps?

NARRATION: For decades, researchers have found that heavier patients with conditions from heart failure to lung cancer can sometimes do better than thinner ones.

These so-called paradoxes can upend standard protocols, as Bette Caan found as a cancer researcher over a decade ago. Back then, worries over weight caused doctors to counsel breast cancer patients not to gain pounds during treatment and even put them on diets. But when she tested that rationale, Caan got a very unexpected result.

BETTE CAAN (SENIOR RESEARCH SCIENTIST, KAISER PERMANENTE): In 2008, I published an article that demonstrated that in fact, those people who gained weight had no worse outcomes. Those people who lost weight however, did have worse outcomes. It was the first finding like this so I thought that theres something going on that I have to pursue. As a good scientist, you want to make sure that the finding is right.

NARRATION: The breakthrough finally came when C.T. scans showed that heavier patients also had more skeletal muscle, and that made all the difference.

BETTE CAAN: It took a while to get people used to that idea, because there was such a belief that no, it was weight gain and that was important.

I was part of a guidelines committee at that point from the American Cancer Society. In 2010, they werent willing to accept my study, but now, ten years later, theres been so many findings that theyre going to be included.

We have learned that overweight and even mild obesity is, for most cancers, associated with either an improved outcome, or no worse outcome than people who are in the so-called normal range.

NARRATION: While Caan focused on body composition, her fellow cancer researchers have discovered other seemingly paradoxical findings. Certain drugs, particularly in immunotherapy, are more effective on heavier patients, and being overweight can be correlated with less aggressive types of tumors.

BETTE CAAN: There are many answers to why the findings that we have are true. So I think that weve turned the tides to understand we have to study everything.

If I have to conclude with anything, its B.M.I. is just a number. Assuming that that captures whats really going on in the body is not true.

We should not be calling it a paradox. Its a finding, and we need to pursue the finding. If we didnt follow that, and we just assumed that it was wrong, we may not find anything new.

NARRATION: Obesity specialist Fatima Cody Stanford says that health is complicated, and by focusing on a weight chart instead of the science we may be missing a chance to help everyone live better and longer lives.

FATIMA CODY STANFORD (OBESITY MEDICINE PHYSICIAN, MASSACHUSETTS GENERAL HOSPITAL): Weve seen for so long that thinness equates to health because, I mean, the B.M.I. chart tells us that, right? And when you tell anyone anything differently, even if the data support it, oh, no. Leanness wins, always wins.

NARRATION: Dr. Stanford points out that for the majority of Americans, a healthy lifestyle should focus on factors like proper sleep, exercise and a nutritious diet instead of revolving around achieving the perfect weight or B.M.I.

FATIMA CODY STANFORD: There is this hyper focus on getting into these narrow, confined boxes that are often set up by us in both the medical and the public health community.

We dont listen to people because we make these judgments about how they look and how they must feel. So the quality of care for patients with obesity is often significantly lower. This leads to poor health outcomes. You see a large number of people that are avoiding care and we miss diagnoses.

For example, Ive had a patient that had significant hip pain for quite some time. And the surgeons were like, you know, its all because, you know, you just need to lose the weight and your hip will feel better. She lost 120 pounds. So what was the issue? She ended up having a big cancer in her hip that went unevaluated, not because she didnt complain about the fact that she had hip pain, but all you could see was her excess weight.

NARRATION: Kelly Peters had a similar experience. Her serious hormonal disorder went undetected for decades.

KELLY PETERS: I believed what doctors have been telling me for 20 years to just lose some weight. Just lose some weight, youll be fine. I was never tested for anything. Nobody ever even sent me for an ultrasound. Not once until I was 35.

As a fat person walking into a doctors office, youre both blamed for everything because of your weight and dismissed, and they wont look beyond your weight. So you exist on this line between youre dying an early death and theres nothing really wrong with you that losing 50 pounds wont solve.

FATIMA CODY STANFORD: We need to take a more holistic approach. Its not just the number on the scale. Its about other risk factors that can contribute to worsened health outcomes. So we have to look at the whole picture. Its not just one size fits all.