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Ever since the first meal took place, about four billion years ago, probably the most powerful driver of the behaviors of all living things has been the need to find, use, and store energy. Of course, reproduction has always been a concern as well, but in order to successfully reproduce, every organism first and foremost needs to be well fed.

Throughout the history of life, most organisms, including humans, have known hunger. In fact, even today, other than humans and domestic animals, most organisms still experience scarcity. A lucky few have occasionally known abundance, but life has generally been characterized by struggle and by hardship—the next famine always just around the corner. Because of this, we evolved to be energy-getting-and-storing machines. We are very good at getting fat when the conditions are right.

Very recently (in evolutionary terms), we started growing our own food, and starvation became less of an issue. Then we developed machines that run on fossil fuels that do most of our work for us. This dramatic change in the way we live has led to a global obesity pandemic.

Today, the market has responded to the alarming increase in obesity rates, producing thousands of products that are less food-like—products that are meant to provide consumers with less energy. This goes against the psychology and physiology that has been honed over billions of years to help us seek out and conserve as much energy as possible. Just what is food, if it’s not fuel for doing the work of life? What happens to the human animal when it finds itself in such an energy-rich environment yet is admonished to practice restraint?

Duke University evolutionary biologist Greg Wray and Director of the Yale Rudd Center for Food Policy and Obesity, Kelly Brownell, help explain the tragic downfall of food. From the North Carolina State Fair, to the diet food aisle in your local grocery store, this audio piece tells the story of calories—how they went from being our most valuable, most sought out obsession, to being a dirty word.

Other sources include Karen Erikson, a dietician at University of North Carolina Weight Loss Research, and Patrick Harrington, owner of Velocity Bicycle Couriers.