Obesity rates in the US have more than doubled in the last 30 years, and rates of childhood obesity and extreme adult obesity have tripled. One third of US adults are considered obese, and another third overweight. This is the "obesity epidemic".
The obesity epidemic has coincided with significant changes in the US diet, which are clearly involved. However, there's another probable contributor that's often overlooked: declining smoking rates.
Here's a graph of cigarette consumption over the last century in the US (1):
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