(Luke Sharrett/Bloomberg)

Robert Paarlberg is an adjunct professor of public policy at the Harvard Kennedy School of Government and author of “The United States of Excess: Gluttony and the Dark Side of American Exceptionalism.”

The most recent official data on obesity were a shock. According to the Centers for Disease Control and Prevention, roughly 38 percent of American adults are technically obese (with a body mass index over 30), up from 35 percent in 2011-2012 and 32 percent in 2003-2004. Why can’t we get this crisis under control?

It sometimes seems as though we’ve tried everything: steps by food producers and restaurant chains to reformulate their offerings, self-restraint by companies regarding junk food and soda ads, calorie counts on menus, more nutritious lunches at schools and a high-profile advocacy campaign led by a popular first lady, Michelle Obama.

But, in fact, we haven’t tried everything. In the United States, we consistently stop short of using our most powerful policy instruments: taxes and regulations.

In Europe these measures are widely employed, and obesity rates are often less than half as high as ours. Hungary imposed a tax on manufactured foods with high sugar contents, and Finland has ratcheted up its tax on soft drinks. Both sugar and chocolate are taxed in Norway. In 2010, Denmark increased by 25 percent its excise tax on chocolate, ice cream, sugary drinks and confectionery products, and two years later France imposed a tax on drinks with added sugars. Regulations in Britain now restrict both the content and timing of ads for foods high in fat, salt or sugar. Greece, Denmark and Belgium also restrict food ads aimed at children. Sweden and Norway simply ban such ads.

Using such taxes or regulations to fight obesity has been harder for the United States. Many Americans side with food and beverage companies in opposing junk-food taxes, with the strongest opposition often coming from low-income and minority communities — even though these communities bear the largest health burdens as a result of their sale and consumption. One survey found that fewer than 1 in 4 Americans favored soda taxes, which helps to explain why a Democratic Congress never seriously considered imposing such taxes to help pay for Obamacare in 2009. Political leaders quickly learn that if they support such taxes, they will be accused of placing a regressive tax on the poor or disrespecting the minority communities that are heavy consumers of the products that would be taxed. In 2012, when New York Mayor Michael Bloomberg tried to restrict the sale of soda in containers larger than 16 ounces, strong objections arose from the City Council’s Black, Latino and Asian Caucus.

Regulating food ads is also more difficult in the United States because our courts treat such ads as “commercial speech” constitutionally protected under the First Amendment.

Because of these unique barriers to a strong obesity-prevention policy, the United States has drifted toward a second-best policy of acceptance and treatment. Rather than imposing the taxes or regulations needed to discourage excess calorie consumption, we try to live with that excess by providing physical accommodations and ramping up spending on medical treatments for the health risks associated with obesity. Relying on costly treatments (including prescription drugs and surgeries) rather than prevention is a typically American approach to health care. By some calculations, 70 percent of illnesses being treated in the United States are preventable. The result of this approach, however, is higher costs and increased inequality.

Providing treatment is now the United States’ primary policy response to the increase in obesity. Between 1990 and 2010, when the number of U.S. diabetes cases more than tripled, our response was to develop and deliver improved medical interventions to control blood sugar, blood pressure and cholesterol. As a result, diabetes treatments in the United States now cost $176 billion a year. This can sometimes seem like a winning strategy, because the treatments — when delivered — get gratifying results. Thanks to new interventions, plus a decline in smoking, the percentage of people with diabetes who will eventually die of the disease has declined from about 41 percent to 17 percent.

Yet responding with treatment alone fails badly on equity grounds. The groups in the United States most likely to become obese are racial minorities, those without any education beyond high school and members of single-parent households; these are also the Americans least able to access quality medical care. Obesity rates are twice as high among children whose parents lack a high-school diploma, and such children are frequently off the medical grid. Treatment can be a good second-best option for upper-income and college-educated Americans, since most have ready access to medical insurance and services. But an approach that emphasizes treatment over prevention will work far less well for Americans with fewer resources.

If the United States settles for a second-best policy of adaptation and treatment, then a social as well as a financial price will be paid. We will have become, in yet another arena, a nation of haves and have-nots.