The real problem with modern obesity is food abundance. If I were to give you a jumbo industrial role of toilet paper and allowed you to hold it while I unraveled it, we wound end up with a very long strand of toilet paper.
If I tore off the last square of toilet paper and gave you the entire rest of the strand, we could use your long strand of toilet paper to represent the length of human history where starvation was a real day to day threat. The single square in my hand would represent the length of human history where starvation was not much of a threat. Not since the end of the Great Depression and World War II has starvation not been a real possibility.
We have about 150,000 generations where efficient fat storage was essential for survival, and 3-4 generations where efficient fat storage can lead to obesity. The problem is not that we are inactive, the problem is that calories are so readily available to be consumed. An hour of jogging will burn only about 150 calories above your basal metabolic rate, but it only takes about 30 seconds to eat 150 calories of cookies.
We judge the value of our meals on the size of the portions we are given. When we go out to eat, we want to leave full. Studies show that there are about 1,000 Calories between being satisfied and feeling full. Even more frightening is that there are between 2,000 and 3,000 calories between feeling full and feeling stuffed. If you go out to an all-you-can-eat food bar and leave feeling stuffed, you may have consumed as many as 4,000 unneeded calories. When this happens we typically go out for a jog the next day to “burn off those calories”. But to burn off that many calories would require you to jog continuously for 27 hours.
The problem is not that we don’t burn enough calories, it’s that we put too many calories down our neck.