Fat is killing us, or so we’re told. Americans’ sugar-saturated diets and couch-bound lifestyles have apparently produced an epidemic of heart disease and diabetes, cancer and infertility, depression and insomnia. Decades of historic public health advances are being wiped out by a tidal wave of blubber.

A 2005 study by demographer and epidemiologist Jay Olshansky predicted that, after a century in which American life spans had lengthened by more than half, life expectancy is now on track to level off or even start to sink by 2050, thanks to obesity. A 2009 report from RTI International declared that Americans would save $147 billion in medical spending every year if only they could figure out how to keep a healthy weight.

But on a closer look, it turns out that our knowledge about fat is surprisingly thin. Researchers don’t know exactly what makes us obese, nor how obesity affects our bodies, let alone our economy. In fact, several studies published in recent years are casting doubt on some of our most widely held ideas about the problem.

1

Chubbiness might not be so deadly

In 2005, Katherine Flegal, an epidemiologist with the Centers for Disease Control and Prevention, released a study showing that excess weight might not be as dangerous as everybody thought. Using data going back to 1971, she found that obesity led to excess deaths, though not as many as had been previously proposed. Stranger was her finding that being merely overweight produced the opposite effect: A little extra flab somehow saved 86,094 lives.

Many in the field disputed Flegal’s findings, but earlier this year she and her colleagues produced an omnibus review to prove their points. Combining data from 97 studies, the team looked at 2.9 million case histories and determined once again that modest excess weight may actually extend your life. They also showed that Grade 1 obesity (a body mass index between 30 and 35) does not appear to elevate the risk of death. Only the most obese Americans (a BMI more than 35) face a mortal danger.

—Association of All-Cause Mortality With Overweight and Obesity Using Standard Body Mass Index Categories, by Katherine M. Flegal et al, January 2013

2

Fat may not be what makes fat people sick

Obesity tends to be associated with an underlying metabolic syndrome, which can have terrible effects on human health. But a growing body of research suggests that prejudice against those with excess weight can be just as damaging. Obese women, for example, are more susceptible than their smaller peers to breast and cervical cancers. That might be taken as a sign that body fat and tumors have some direct, physiological connection.

But a 2000 study of more than 8,000 women found evidence that there could be an entirely different cause: Fat patients were less likely than normal-sized ones to have received a Pap smear or a mammogram in the preceding two or three years. (The researchers controlled for socioeconomic and insurance status.) This may be because, as surveys have found, many health-care providers consider the obese lazy and weak-willed, and therefore consider treating them to be a waste of time. Further work has shown that overweight women make less money and are less likely to get married than their slimmer peers. These factors are strongly correlated with chronic disease.

—Screening for Cervical and Breast Cancer: Is Obesity an Unrecognized Barrier to Preventive Care?, by Christina C. Wee et al, May 2000

3

Overweight citizens won’t bankrupt the nation

One of the key assumptions behind Obamacare is that preventive medicine can improve the nation’s health while also saving money. In theory, interventions targeting obesity could save billions of dollars in doctors’ bills that could be given instead to programs such as Medicaid.

But going on a national diet might not be so thrifty. In 2008, a Dutch research team modeled costs for lifetime treatment of three types of people: healthy-size smokers, healthy-size non-smokers, and obese non-smokers. Though caring for members of the last group would cost more until age 56, after that their medical bills would plummet. That’s because both smokers and the very fat die at younger ages, while the slender and clean-living are more likely to end up dying of slow, degenerative, and costly illnesses. Of course, the premature death of smokers and the obese is nothing to celebrate. The goal of public health policies is to save lives, not money. But these cold figures do complicate the widely touted notion that losing weight will save the health care system’s finances.

—Lifetime Medical Costs of Obesity: Prevention No Cure for Increasing Health Expenditure, by Pieter H. M. van Baal et al, February 2008

4

Obesity is not our genetic destiny

If obesity is so bad for you, how did humans get so prone to overeating? Fifty years ago, geneticist James V. Neel explained this evolutionary mystery with what became known as the thrifty gene hypothesis. Neel argued that our tendency to put on weight dates to a time when food was scarce and famines weeded out the skinny. Having genes for getting fat helped our ancestors survive. But this venerable theory doesn’t tell us why so many people—including two-thirds of America’s adults—don’t become obese, despite living in the land of chocolate-covered peanut-butter pretzels.

Biologist John Speakman recently offered up a new idea: In times gone by, our genes may have kept us slim and nimble, so as to scamper free of predators. Once humans began to make weapons and build fires, the danger of being eaten diminished, freeing our bodies to fluctuate in size. That’s why we see a range of builds today: Instead of “thrifty genes,” we have “drifty genes.”

—Thrifty Genes for Obesity, an Attractive but Flawed Idea, and an Alternative Perspective: The “Drifty Gene” Hypothesis, by J. R. Speakman, October 2008

5

Overeating may not be the problem

Most people blame our lifestyle for the obesity epidemic: We eat oversized portions of too-sweet meals and not enough fruits and vegetables; we spend too much time sitting at desks or watching television and not enough exercising. But there’s an additional possibility—unproven, but intriguing. Studies suggest that obesity can be induced by viral infections, or by endocrine-disrupting chemicals in the environment.

A team of researchers reasoned that if these infections and pollutants are the problem, they might be affecting non-human species, too. Sure enough, when they looked at body-size data from captive and feral monkeys, chimps, dogs, cats, mice, and rats, they found that the fat epidemic spans taxonomic borders. In about half the populations studied, the researchers found a significant tendency toward heavier bodies from one decade to the next. That’s despite the fact that many of these animals haven’t changed the way they eat and exercise. Could they be getting fat from viruses instead, or environmental toxins? And if so—could we be as well?

—Canaries in the Coal Mine: A Cross-Species Analysis of the Plurality of Obesity Epidemics, by Yann C. Klimentidis et al, November 2010