The Centers for Disease Control's NHANES surveys documented a massive increase in obesity in the United States between the 1960-62 and 2007-2008 survey periods (1). In 1960, 13 percent of US adults were obese, while in 2008 that number had risen to 34 percent. The prevalence of extreme obesity increased from 0.9 to 6.0 percent over the same time period!
Something has changed, but what? Well, the most parsimonious explanation is that we're simply eating more. Here is a graph I created of our calorie intake (green) overlaid on a graph of obesity prevalence (blue) between 1970 and 2008:
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