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Forty years ago, obesity was a serious but manageable concern to governments, health care professionals and individuals. Today, obesity rates have skyrocketed across the globe and treating obesity presents one of the most singular challenges to public health in recent history.

Nearly 14 percent of women in the world are considered obese, up from 7.9 percent in 1980. Among men, 10 percent are obese, up from 5 percent in 1980. The Centers for Disease Control and Prevention (CDC) predicts the obesity rate will jump from 36 percent in 2012 to 42 percent by 2030. Around the world, 10 percent of adults are obese, and an estimated 3 million deaths worldwide are caused every year by obesity-related illnesses. In the U.S., 70 percent of adults and 17 percent of teens and children are either overweight or obese.

One health study determined that humanity is 17 million tons (15 million metric tons) overweight. Exactly how big is that? Imagine an extra 242 million people of average body mass living on the planet.

The cost of treating obesity-related illnesses like heart disease and diabetes in the U.S. equals $147 billion every year -- that's almost one-fifth of the country's total health expenditures. The CDC defines adult obesity as having a body mass index (BMI) of 30 or higher. The CDC considers adults with a BMI of 18.5 to 24.9 a "healthy weight."

Marion Nestle, a professor of nutrition, food studies and public policy at New York University, and one of the leading nutritional experts in the nation, has been trying to change how people eat for years. She's written many books on the food industry, detailing how government policy has become intertwined with food choice and obesity. One of the arguments she makes in her latest book, "Why Calories Count," focuses on the impact Wall Street has made on food companies and ultimately what people consume. Obesity rates started to rise in the 1980s, she says in the accompanying video, largely because of demands Wall Street placed on food makers.

Wall Street "forced food companies to try and sell food in an extremely competitive environment," she says. Food manufacturers "had to look for ways to get people to buy more food. And they were really good at it. I blame Wall Street for insisting that corporations have to grow their profits every 90 days."

Traders and analysts may have shifted food companies' focus to producing profits over health, but changes to government policy also contributed to people's relationship with food, she notes. Large government subsidizes given to the corn, wheat, soybean and sugar industries allowed farmers to reap high returns on their crops. Farmers could grow these commodities cheaply and were encouraged by the food industry "to plant as much as they could. Food production increased, and so did calories in the food supply," Nestle writes in her book.