That happened with diabetes.

In 1996, the National Institutes of Health initiated a study involving thousands of people at risk of developing Type 2 diabetes. Researchers led these subjects through an intensive program of diet, counselling and exercise. It worked so well that the study was ended one year early. The intervention slashed people’s chances of developing diabetes by more than half.

The result was so consequential that Tommy Thompson, then the secretary of health and human services under president George W. Bush, joined the study’s principal investigator, Dr David Nathan, a Harvard diabetes researcher, to announce it in 2001, telling the nation that the study showed diabetes could be conquered.

“Just walk around the block, walk down the street 30 minutes each and every day, and we can lick this particular disease,” Thompson said at the time. He also said, “Prevention works.”

Until it doesn’t. Diabetes incidence has actually increased since those results were announced 23 years ago.

Permanent lifestyle changes, it turns out, are not easy.

“The problem, of course, is that people being people apparently find it difficult to maintain such changes over long periods,” Nathan said in a recent interview.

Christopher Gardner, a nutrition researcher at Stanford University, is a true believer in the power of a healthy diet. He has done study after study, funded by the National Institutes of Health, in which health professionals guided participants in their food choices or, in some cases, even delivered meals to them.

The studies succeeded – participants’ health improved. But when the studies ended, Gardner said, and when participants no longer had that guidance or food delivery, many, if not most, went back to their old eating habits and all those benefits of a healthy diet vanished.

“Yes, diet should be the answer,” said Gardner, who was paid five years ago by a purveyor of meat alternatives to study its products. But, he added, diet will be insufficient “unless some major changes are made to the whole food system in the United States.”

There’s also the problem of genetics, said Dr Jeffrey Friedman, an obesity researcher at Rockefeller University in New York. Some people have a genetic tendency to become obese, and they will gain weight as long as food is cheap and plentiful. Tweaks like changing portion sizes or food advertising are not enough – if popcorn were sold in smaller bags, many would simply eat two bags of popcorn, he said.

That has led Friedman, like Gardner and Nathan, to conclude that the only practical way to improve people’s health and allow them to lose weight is to rely on the new obesity drugs.

Dr Charles Burant, a professor of internal medicine at the University of Michigan, is sympathetic to Kennedy’s view that the problem is the food system.

He recalled an experience with one large food company that showed him how hard it could be to change what foods are marketed to Americans.

He had an idea for a palatable drink containing an amino acid that he thought might sate hunger.

So he went to the large food company and asked if it could make such a drink for him to test in a study. After a while, he received a reply: It was not in the company’s business plan to develop products that reduced appetites. It was not interested in helping.

“It wasn’t their business to sell less food,” Burant said.

“If you can rein in corporations or try to use pressure to modulate what they do, I think that’s great,” he said. “But in the meantime, until the population learns to eat good things, we need to do something to help people.”

Others, like Dr Kevin Volpp of the University of Pennsylvania, worry about promoting the widespread use of obesity drugs. He worries about the idea of tens of millions of Americans injecting themselves weekly with drugs to curb their appetites.

“There is not enough money in the system to suddenly fund drugs for that many people,” Volpp said.

He prefers to reserve the drugs for adults with the most serious diseases related to obesity.

Instead of handing out drugs to nearly all who qualify, Volpp said the country needed to consider other solutions, like marketing restrictions and taxes on sugary beverages.

Loading

He also would like health insurers to help subsidise the purchase of healthy food for those who can’t afford it and have chronic medical problems, like diabetes, that are related to diet. Eleven states have secured permission to test the use of state Medicaid programs for pilot studies along those lines, he said.

“All this is like pieces of a jigsaw puzzle,” Volpp said. “But we have to try everything that will help make our population healthier, given the rise in obesity from 12 per cent to 40 per cent in the past few decades.”

It’s not clear, though, how to put the jigsaw puzzle together or whether to even try.

For Dr Peter Lurie, the president of the Centre for Science in the Public Interest, the situation is dire enough to try everything all at once.

Loading

His organisation, he said, firmly believes that the food environment is an important factor driving high obesity rates, and he wants it to change.

He also describes himself as “something of a pharma sceptic”.

But “when the pharmaceutical industry comes up with strong data that affects important outcomes,” Lurie said, “it is irresponsible to turn your back on it.”

That is also the view of Dr Robert Califf, who has been serving as commissioner of the Food and Drug Administration under President Joe Biden. Before he joined the FDA in 2016, Califf, a cardiologist, ran a clinical trials centre at Duke University that received funding from drug companies as well as from the federal government.

“While I am totally in favour of helping Americans have a better diet, these drugs are highly effective” for people with obesity, Califf said. “Not just for losing weight,” but also for preventing heart disease deaths.

In the meantime, “we have to control the advertising,” he said, and “we have to gradually change the agricultural system,” and “we have to subsidise healthier foods.”

“You can either be a cynical old vet and say this is impossible, or you could be optimistic,” Califf said.