Evolution’s Sweet Tooth
Of all the indignant responses to Mayor Michael R. Bloomberg’s plan to ban the sale of giant servings of soft drinks in New York City, libertarian objections seem the most worthy of serious attention. People have certain rights, this argument goes, including the right to drink lots of soda, to eat junk food, to gain weight and to avoid exercise. If Mr. Bloomberg can ban the sale of sugar-laden soda of more than 16 ounces, will he next ban triple scoops of ice cream and large portions of French fries and limit sales of Big Macs to one per order? Why not ban obesity itself?
The obesity epidemic has many dimensions, but at heart it’s a biological problem. An evolutionary perspective helps explain why two-thirds of American adults are overweight or obese, and what to do about it. Lessons from evolutionary biology support the mayor’s plan: when it comes to limiting sugar in our food, some kinds of coercive action are not only necessary but also consistent with how we used to live.
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.
Since sugar is a basic form of energy in food, a sweet tooth was adaptive in ancient times, when food was limited. However, excessive sugar in the bloodstream is toxic, so our bodies also evolved to rapidly convert digested sugar in the bloodstream into fat. Our hunter-gatherer ancestors needed plenty of fat — more than other primates — to be active during periods of food scarcity and still pay for large, expensive brains and costly reproductive strategies (hunter-gatherer mothers could pump out babies twice as fast as their chimpanzee cousins).
Simply put, humans evolved to crave sugar, store it and then use it. For millions of years, our cravings and digestive systems were exquisitely balanced because sugar was rare. Apart from honey, most of the foods our hunter-gatherer ancestors ate were no sweeter than a carrot. The invention of farming made starchy foods more abundant, but it wasn’t until very recently that technology made pure sugar bountiful.
The food industry has made a fortune because we retain Stone Age bodies that crave sugar but live in a Space Age world in which sugar is cheap and plentiful. 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.
What should we do? One option is to do nothing, while hoping that scientists find better cures for obesity-related diseases like heart disease and Type 2 diabetes. I’m not holding my breath for such cures, and the costs of inaction, already staggering, would continue to mushroom.
A more popular option is to enhance public education to help us make better decisions about what to eat and how to be active. This is crucial but has so far yielded only modest improvements.
The final option is to collectively restore our diets to a more natural state through regulations. 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. Mr. Bloomberg’s paternalistic plan is not an aberrant form of coercion but a very small step toward restoring a natural part of our environment.
Though his big-soda ban would apply to all New Yorkers, I think we should focus paternalistic laws on children. Youngsters can’t make rational, informed decisions about their bodies, 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, have them immunized and make them wear seat belts. We require physical education 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?
Along these lines, we should ban all unhealthy food in school — soda, pizza, French fries — and insist that schools provide adequate daily physical education, which many fail to do.
Adults need help, too, and we should do more to regulate companies that exploit our deeply rooted appetites for sugar and other unhealthy foods. The mayor was right to ban trans fats, but we should also make the food industry honest about portion sizes. Like cigarettes, mass-marketed junk food should come with prominent health warning labels. It should be illegal to advertise highly fattening food as “fat free.” People have the right to be unhealthy, but we should make that choice more onerous and expensive by imposing taxes on soda and junk food.
We humans did not evolve to eat healthily and go to the gym; 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. We have evolved to need coercion.
Daniel E. Lieberman, a professor of human evolutionary biology at Harvard, is the author of “The Evolution of the Human Head.”
A version of this op-ed appeared in print on June 6, 2012, on page A27 of the New York Times with the headline: Evolution’s Sweet Tooth.