How did Kim Kardashian, a reality tv star, lose enough weight to fit into a slinky dress once worn by Marilyn Monroe? She has talked about a diet and exercise, but lots of her fans think it could also be thanks to new weight-loss drugs that many far-from-fat celebrities are said to be taking to keep their figures supremely svelte. There is no need to speculate about Elon Musk, a famed entrepreneur: he readily admits that one such drug, Wegovy (semaglutide), has helped him shed weight. In fact, social media are awash with pictures of delighted patients flaunting before-and-after snaps that prove just how effective these novel medicines are.

Investors and analysts are as excited as the gossip columnists. Some estimate that Novo Nordisk, the Danish pharmaceutical firm that makes Wegovy, will sell $3bn-4bn of it in America alone this year. The firm plans to launch the drug in many more countries in the coming months. Its share price is up by 40% over the past year and has doubled over the past two.

Eli Lilly, another pharmaceutical firm, hopes to start selling a similar treatment for obesity, called Mounjaro (tirzepatide), as early as this summer. In trials, recipients shed an astonishing 20% of their weight on average. UBS, a bank, thinks it could become the “biggest drug ever”. Jefferies, an investment bank, says that by 2031 the market for these drugs, collectively known as GLP-1 agonists, will exceed $150bn (see chart 1). That is on a par with all drugs to treat cancer, sales of which amounted to about $185bn in 2021.

Even such head-turning numbers do not fully capture the drugs’ potential, however. Obesity is a problem of staggering global proportions—and one that afflicts few celebrities, but legions of ordinary people. In 2023 the World Obesity Federation (wof), an NGO, says 1.1bn people aged older than five, or roughly 14% of all people in that age bracket, were obese. A further 1.6bn, or 24% of all the world’s over-fives, were overweight. In a report to be published on March 3rd, to mark World Obesity Day, the federation projects that 4bn people—half of everyone over five—are likely to be overweight or obese by 2035 (see chart 2, left-hand panel).

The report estimates that the annual cost of humanity’s growing paunch will reach $4trn in 2035, of 2.9% of global gdp, in the form both of spending on health care and of working time lost to illness and premature deaths (see chart 2, right-hand panel). That is the equivalent of another covid-19 pandemic every year.

Moreover, obesity is not just a first-world problem. The costs are growing faster in poor and middle-income countries than they are in rich ones. By 2035, the wof projects that 47% of Mexicans, 46% of Iranians and South Africans and 42% of Malaysians will be obese. Spiralling health-care costs in these countries will be a drag on economic growth. Any treatment that can reduce these numbers could potentially improve the health of billions, and also make the world wealthier.

Tummy trouble

Obesity is typically determined using a ratio of weight to height known as the body-mass index (BMI). A BMI over 25 is normally considered overweight; one over 30 is the standard definition of obesity. Thus an American man of average height (175cm or 5’9”) is considered overweight if he tips the scales at 77kg (170lbs) or more and obese if his weight is 92kg (203lbs) or more. (There are exceptions, such as bodybuilders, who weigh a lot thanks to copious muscles.)

Excess weight is not simply a matter of appearance. Obesity is associated with such grave illnesses as diabetes, strokes and heart disease. John Speakman of the University of Aberdeen notes that women with a BMI of 35 are 90 times more likely to have diabetes than those with a BMI of 23. For men, the risk is five times greater. Obesity also increases the risk of 13 types of cancer, including those of the breast, bowel, womb and oesophagus. More recently, it was found to raise the odds of a patient dying from covid-19. In these and other ways, it causes the deaths of around 4m people a year.

The solution might seem obvious: to eat less and exercise more. To those of a judgmental nature, the failure to lose weight reflects a lack of willpower. But Louise Baur, a professor at the University of Sydney and president of the wof, rejects the idea that the obesity epidemic represents “the moral failure of hundreds of millions of people”. Fatima Stanford, who studies obesity at Massachusetts General Hospital and Harvard Medical School, notes that attempts to lose weight through diet and exercise work for only 10-20% of the population: “For most people we don’t see a dramatic drop.”

Fat-linger error

The reason almost certainly lies in the evolutionary past. In a state of nature food is rarely plentiful, so Homo sapiens has evolved to hang on to fat. One adaptation is that, when a body’s caloric intake is reduced, its resting metabolism slows. Another is that it releases extra ghrelin, a hormone which signals to the brain to increase food intake.

Worse, from the point of view of the would-be diet