We hear every day about the perils of obesity and poor eating habits. Rates of obesity have tripled in the last several decades, while rates of diabetes, hypertension, and cardiovascular disease have also soared. Every week, it seems a new smoking gun—fat, sugar, caffeine, etc.—is identified, but each new culprit brings its own uncertainty. Are we sure this time that the result will hold up? And how should policymakers respond?
Our knowledge of obesity is in flux. At a basic level, we know that having a high body mass index is a predictor for a host of different medical problems. By some estimates, obesity is responsible for almost 3 million deaths globally each year and over $2 trillion in medical costs and lost productivity. We also know that obesity is deeply tied to what we eat and drink. But there is wide disagreement about which particular nutrients—fat, salt, sugar, etc.—are more harmful to us and contribute more to obesity. This disagreement matters, because it can deeply affect policy. Policy, in turn, can affect what people eat and drink.
In the early to mid-1980s, fat was blamed for the rise in obesity and cardiovascular disease. The reasons, at the time, were sensible. A large-scale study looking at associations between nutrition and cardiovascular disease across seven developed countries found an association between a high saturated fat diet and cardiovascular disease and obesity. Other studies supported, and to some extent, continue to support this conclusion. Policymakers, heeding the evidence, responded by recommending that consumption of saturated fat, and all fat, be reduced to combat the obesity epidemic. The Dietary Guidelines for Americans Committee, which issues guidance on nutritional consumption, recommended restricting fat intake: “Avoid too much fat, saturated fat, and cholesterol” was their third recommendation, after “eat a variety of foods” and “maintain ideal weight.”
The public seemed to respond. The percent of calories from fat consumed declined by about 10 percentage points between the early 1980s and the mid-1990s. But obesity rates did not decline; indeed, they increased. Fat wasn’t the smoking gun, and reducing fat, or at least reducing fat without making other changes to diet and lifestyle, was no silver bullet. Eventually, more scientific studies were conducted, showing that although an excess of fat could be harmful, fat on its own was not necessarily detrimental. Finally, the Dietary Guidelines committee abandoned recommendations to restrict all fats, replacing it with restrictions solely on saturated fat, and consumption of fat is once again on the rise.
As concerns about fat declined, fears about carbohydrates, and particularly sugar, surfaced. New studies, starting in the late 1990s and early 2000s, have emerged tying sugar consumption to obesity and diabetes. New diets argue for limited sugar consumption. Policymakers debate taxes on sugar and junk food. And although the 2015 Dietary Guidelines for Americans Committee report has not yet been released, there are indications that they will recommend new reductions in consumption of added sugars.
Are we right this time? The evidence against sugar is stronger than was the case against fat and continues to mount each day with more rigorous and more long-term studies. But there is also strong evidence that sugar is not the only culprit in the rise in obesity, just as fat alone was not to blame.
As research on obesity has proliferated, so have the factors associated with its rise. We now have evidence that gut flora, hormones, genetics, and epigenetics may all influence whether an individual becomes obese; external factors such as food environment, food prices, and the physical environment all have an impact as well. Obesity is overdetermined: there are multiple causes which can explain its rise but no one obvious fix.
That returns us to our question: what is a policymaker to do when faced with this uncertainty? Because so many factors influence obesity, it is unlikely that any individual policy will fix the problem. And the probability of interactions, spillover effects, and substitutions when policies such as taxes are introduced is high.
Instead of reactive policies, we need more science, not only on nutrition but also on the effects of interventions. We need to test the solutions put forth by nutritionists and others. Through new experiments, rigorously evaluated, we can add to the knowledge about obesity and work toward reducing it, tailoring each new experiment based on the knowledge gained from the previous intervention. It’s not dramatic policy; there is no silver bullet. But it offers a course to improve lives in a climate of uncertainty.