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How do we explain studies that suggest overweight individuals live longer?

Martin Luther King Jr. warned that human progress is neither automatic nor inevitable, and the same may be true of the human lifespan. In the 1800s, life expectancy was less than 40 years, but it has been “advancing steadily” over the last two centuries, “increasing at approximately 2 years per decade”—until recently. Longevity gains appear “to be faltering or even being reversed.” Thanks to the obesity epidemic, we may now be raising the first generation to live shorter lives than their parents.

“This downward trend in longevity will almost certainly accelerate as the current generation of children—with higher body weights from earlier in life than ever before—reaches adulthood.” Current trends “potentially signal a looming social and economic catastrophe” if the obesity epidemic continues unchecked. In the coming decades, some predict we may lose two to five years or more of life expectancy in the United States. To put that into perspective, a miracle cure for all forms of cancer would only add three and a half years to the average U.S. life expectancy. In other words, reversing the obesity epidemic might save more lives than curing cancer.

The evidence that being overweight increases our risk for debilitating diseases like diabetes is considered “indisputable.” However, surprisingly, there is controversy surrounding body weight and overall mortality. In 2013, scientists with the Centers for Disease Control and Prevention (CDC) published a meta-analysis in the Journal of the American Medical Association suggesting that being overweight was advantageous. Grades 2 and 3 obesity, such as being an average height of 5’6″ and weighing about 216 pounds (98 kg) or more, were associated with living a shorter life, but grade 1 obesity, weighing about 185 to 215 pounds (84 to 97 kg) at that height, was not. Being overweight (about 155 to 185 pounds/70 to 83 kg at 5’6″) appeared to be protective compared to those who were a “normal weight” of 115 to 155 pounds (52 to 69 kg). The overweight individuals, those with a body mass index (BMI) of 25 to 30, appeared to live the longest.

Headline writers were giddy. “Being overweight can extend your life rather than shorten it,” read one. “Dreading your diet? Don’t worry—plump people live LONGER than their skinnier counterparts…” read another. “Extra pounds mean a lower chance of death.” Not surprisingly, the study ignited a firestorm of controversy in the public health community. The study was called “ludicrous,” “flawed,” and “misleading.” The chair of nutrition at Harvard lost his cool, calling the study “really a pile of rubbish” and fearing the food industry might exploit the study in the same way the petroleum industry misuses “controversy” over climate change. 

Public health advocates can’t just dismiss data they find inconvenient, though. Science is science. But how could being overweight increase the risk of life-threatening diseases, yet, at the same time, make you live longer? This became known as “The Obesity Paradox,” the subject of my video Is the Obesity Paradox Real or a Myth?.

The solution to the puzzle appears to lie with two major sources of bias, the first being “confounding by smoking.” The nicotine in tobacco can lead to weight loss. So, if you’re skinnier because you smoke, then no wonder you’d live a shorter life, albeit with a slimmer waist. The failure to control for the effect of smoking in studies purporting to show an “obesity paradox” leads to the dangers of obesity being “grossly underestimated.”

The second major source of bias is reverse causality. Instead of lower weight leading to life-threatening diseases, isn’t it more likely that life-threatening diseases lead to lower weight? Conditions such as hidden tumors, chronic heart or lung disease, alcoholism, and depression can all cause unintentional weight loss for months or even years before they are even diagnosed. It’s normal to be overweight in the United States, for example. So, people who are “abnormally” thin—in other words, at an ideal weight—may be taking care of themselves, but they may instead be “heavy smokers, frail and elderly, and seriously ill with weight loss due to their disease…” 

To put the obesity paradox to the test once and for all, The Global BMI Mortality Collaboration was formed, reviewing data from more than 10 million people from hundreds of studies in dozens of countries—the largest evaluation of BMI and mortality in history. To help eliminate bias, the researchers omitted smokers and those with known chronic disease, then excluded the first five years of follow-up to try to remove from the analysis those with undiagnosed conditions who lost weight due to an impending death. And? The results were clear: “This analysis has shown that both overweight and obesity (all grades) were associated with increased all-cause mortality”—a greater risk of dying prematurely. So, “adjusting for these biases leads to eliminating the obesity paradox.” In other words, the so-called obesity paradox appears to be “just a myth.”

Indeed, when intentional weight loss is put to the test, people live longer. There are bariatric surgery studies like the SOS trial that show that weight loss reduces long-term mortality, and randomizing study participants to weight loss through lifestyle changes shows the same thing. Losing a dozen pounds through diet and exercise was found to be associated with a 15 percent drop in overall mortality. Now, exercise alone may extend lifespan even without weight loss, but there appears to be a similar longevity benefit of weight loss through dietary means alone.

If you missed the previous blog posts in my series on the ABCs of obesity, see:

Check out the final two videos in this series: What’s the Ideal BMI? and What’s the Ideal Waist Size?.

I cover all of this and more at length in my book How Not to Diet, and its companion, The How Not to Diet Cookbook, has more than 100 delicious Green-Light recipes that incorporate some of my 21 Tweaks for the acceleration of body fat loss.

Why are people obese in the first place? I have a whole series of videos on that topic. Check the videos in the related posts.