How to Deal with Humans’ Built-In Sweet Tooth

From The Marshall Memo #440

“An evolutionary perspective helps explain why two-thirds of American adults are overweight or obese, and what to do about it,” says Harvard human evolutionary biology professor Daniel Lieberman in this New York Times Op Ed article. “… Obesity’s fundamental cause is long-term energy imbalance – ingesting more calories than you spend over weeks, months, and years. Of the many contributors to energy imbalance today, plentiful sugar may be the worst.” 

Here’s the three-step evolutionary process that set up modern humans’ tendency toward obesity: 

• In ancient times, when food was scarce, a sweet tooth was adaptive – it drove early humans to seek out a vital form of energy that was relatively scarce. Except for honey, most foods were not sweeter than a carrot.
• Excessive sugar in the bloodstream is toxic, and natural selection favored humans with the ability to rapidly convert digested sugar into stored fat.
• This was adaptive to remaining active during periods when food was scarce. Human brains burn lots of energy, so storing fat was particularly important to maintaining the advantages that large brains conferred.

“Simply put, humans evolved to crave sugar, store it, and then use it,” says Lieberman. “For millions of years, our cravings and digestive system were exquisitely balanced because sugar was rare… Until recently, all humans had no choice but to eat a healthy diet with modest portions of food that were low in sugar, saturated fat, and salt, but high in fiber. They also had no choice but to walk and sometimes run an average of 5 to 10 miles a day.”

But the beginning of agriculture made starchy foods more plentiful, and when humans learned how to refine sugar, they were able to ingest much more than they were burning. “Sip by sip and nibble by nibble, more of us gain weight because we can’t control normal, deeply rooted urges for a valuable, tasty, and once limited resource,” says Lieberman. 

So what is to be done? One option is to do nothing, based on the libertarian belief that people have a God-given right to drink mega-portions of soda, eat junk food, avoid exercise, and get fat. A second option is to hope that scientists will develop a pill to control obesity. A third is to educate people, especially children, about the dangers of too much sugar consumption. And a fourth is to introduce some form of coercion – along the lines of New York City Mayor Michael Bloomberg’s recent proposal to ban large servings of sugary soft drinks, which has stirred up vehement resistance, including a New York Times editorial saying Bloomberg has gone too far in the direction of the “nanny state.”

Lieberman comes out in favor of paternalistic coercion – but only for children. “Youngsters can’t make rational, informed decisions about their bodies,” he argues, “and our society agrees that parents don’t have the right to make disastrous decisions on their behalf. Accordingly, we require parents to enroll their children in school, and we don’t let children buy alcohol or cigarettes. If these are acceptable forms of coercion, how is restricting unhealthy doses of sugary drinks that slowly contribute to disease any different?”

Lieberman also suggests banning all unhealthy food in school – soda, pizza, French fries – and requiring daily physical education. 

“We humans did not evolve to eat healthily and go to the gym,” he concludes. “Until recently, we didn’t have to make such choices. But we did evolve to cooperate to help one another survive and thrive. Circumstances have changed, but we still need one another’s help as much as we ever did. For this reason, we need government on our side, not on the side of those who wish to make money by stoking our cravings and profiting from them.”

“Evolution’s Sweet Tooth” by Daniel Lieberman in The New York Times, June 6, 2012 (p. A27), http://www.nytimes.com/2012/06/06/opinion/evolutions-sweet-tooth.html?_r=1