If obesity shortens one’s lifespan, why, over the last century, the increase in life expectancy occurred with an increase in obesity (i.e., life expectancy has increased as obesity has)?

Because it’s nearly twice as dangerous to be underweight as it is to be overweight to mildly obese. Higher mortality in “normal weight” adults: should the BMI range for normal weight be redefined? (illustration below screen-grabbed from that article).

The phenotype that allows a human to put on extra fat easily is due to a lot of factors, related to both metabolic and brain chemistry. Cultural behaviors (such as verbal and physical abuse of children) which encourage overeating have also conferred an evolutionary advantage for the 100,000 or so years humans have walked the Earth.

The same incredible changes of the Industrial Revolution and global capitalism that have allowed humans to build roomier, safer shelters, bathe regularly, drain sewage away from our safe and sanitary water supplies, control biting insects that spread disease, and access effective medicines when we need them, have also allowed a growing plurality of humans access to abundant calories every single day of our lives. Unfortunately, we still have the physiologies and cultures that evolved to survive in conditions of periodic drought, famine, and hard physical labor.

It’s really a blink of an eye, in historical terms, since there have been enough severely obese people around to even affect the average life expectancy of populations. It’s interesting to muse about how this might play out in the long run.