 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, but has been steadily increasing over the last two centuries, gaining about two years per decade—that is, until recently. Longevity gains have faltered, or even reversed. Thanks to the obesity epidemic, we may now be raising the first American generation to live shorter lives than their parents. The downward trend in longevity is expected to accelerate as the current younger generation, who started out heavier and earlier than ever before, ages into adulthood. If the obesity epidemic continues, unchecked current trends signal a potential looming social and economic catastrophe. In the coming decades, in particular, we may lose two to five or more years of life expectancy in the United States. I mean, to put that into perspective, a miracle cure for all forms of cancer would only add 3.5 years to the average American lifespan. In other words, reversing the obesity epidemic might save more lives than curing cancer. The evidence being overweight increases your risk for debilitating diseases like diabetes is considered indisputable. But surprisingly, there's controversy surrounding body weight and overall mortality. In 2013, CDC scientists published a meta-analysis in the Journal of the American Medical Association suggesting being overweight was actually advantageous. Yes, grade 2 or 3 obesity, like being average height, 5 foot 6, and weighing about 215 plus pounds, was associated with living a shorter life. But grade 1 obesity, between about 185 to 215 pounds at that height, was not, and just being overweight, 155 to 185 pounds appeared to be protective compared to those who were normal weight, 115 to 155. The overweight individuals, BMI 25 to 30, appeared to live the longest. Headline writers were giddy. Being overweight can extend your life. Dreading your diet? Don't worry, plump people live longer. Extra pounds might mean lower chance of death. Not surprisingly, this study ignited a firestorm of controversy in the public health community. The study was called ludicrous, flawed, misleading. The chair of nutrition at Harvard lost his cool, calling the study really a pile of rubbish. Fearing the food industry might exploit this 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. I mean, 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 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, well, then no wonder you'd live a shorter life with a slimmer waist. The failure to control for the effect of smoking in studies purporting this show in 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, is it more likely that life-threatening diseases are leading to lower weight? Conditions such as hidden tumors, chronic heart and lung disease, alcoholism, and depression can all cause unintentional weight loss months or even years before they're even diagnosed. It's normal to be overweight in this country, right? So people who are abnormally thin, in other words, ideal weight, I mean, couldn't actually be taking care of themselves, right? But maybe heavy smokers, elderly and frail, or seriously ill with weight loss from their disease. To put the obesity paradox issue to the test once and for all, the global BMI mortality collaboration was formed, reviewing data for more than 10 million people from hundreds of studies in dozens of countries, the largest evaluation of BMI mortality in history. To help eliminate bias, they omitted smokers and those with known chronic disease and 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, being overweight, and all grades of obesity were associated with a significantly greater risk of dying prematurely. So adjusting for these biases leads to eliminating the obesity paradox altogether. In other words, the so-called obesity paradox appears to be just a myth. Indeed, when intentional weight loss is actually put to the test, people live longer. There are bariatric surgery studies like the SOS trial that show weight loss reduces long-term mortality and randomized people 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% drop in overall mortality. Now, I mean, exercise alone may extend the lifespan even without weight loss, but there also appears to be a similar longevity benefit of weight loss through dietary means alone.