Here’s why obesity grew so quickly worldwide, and where that’s starting to change

This is part of a series about new obesity drugs that are transforming patients’ lives, dividing medical experts, and spurring one of the biggest business battles in years. Read more about The Obesity Revolution.

“Soaring obesity rates.” “An alarming surge in BMI.”

We’ve grown accustomed to the obesity trend stories over the last few decades, since prevalence rates started to rise. And rise they did: From 1976 to 1980, some 15% of U.S. adults qualified as having obesity while less than 5% of people in most of the world’s high-density regions were considered as such. Today, the U.S. number is 42% — a near tripling. And if global trends continue, a recent report from the World Obesity Federation suggested, over half the world’s population will either have overweight or obesity by 2035.

A closer look at the latest global obesity data tells a more nuanced story than monolithic surging. It’s true that, worldwide, obesity continues to rise. But in high-income countries, such as the U.S., the rate of increase in body mass index, or BMI, has actually been slowing, even beginning to level off. In these places, “the real takeoff was in the ’80s, ’90s, early 2000s,” said Boyd Swinburn, a University of Auckland professor who has been tracking global obesity trends, “and then it started to plateau.” Nowadays, most of the global growth is being driven by the sharp increases in low- and middle-income countries.

The idea that growth in average BMI has been slowing down in the U.S. may seem surprising given headlines about the obesity crisis or the inescapable advertisements for new weight-loss-inducing medications such as Ozempic, Wegovy, and Mounjaro. But the trend doesn’t mean there’s nothing to worry about. More Americans than ever are reaching the territory of severe obesity. It also doesn’t mean countries that saw their obesity rates increase first are going to plateau at the same levels. Swinburn and other researchers find that different countries follow different obesity trajectories based on unique sets of social, policy, economic, and cultural factors. In American Samoa, roughly 60% of adults now have obesity — while many countries see their growth stagnating before reaching such heights. In France, for example, the obesity rate has long hovered around 20%. The U.S., in the none-too-healthy middle, remains one of the countries with the highest obesity prevalence in the world.

Dissecting the latest data from the Centers for Disease Control and Prevention, the NCD Risk Factor Collaboration, and the World Obesity Federation, some exclusively shared with STAT, takes us a long way to understanding what’s going on with obesity in the U.S. and worldwide — and how we got here.

As countries gain wealth, obesity rates rise

For a while, humanity’s increasing body size was a good news story. Over a century ago, malnutrition and deficiency diseases such as goiter, rickets, and pellagra were common, stunting human growth — both height and body weight — as a result. As countries gained wealth, and more and more people got access to a diversity of nutrient-rich food, humans began to realize their potential in terms of body size, explained Stephen O’Rahilly, director of the MRC Metabolic Diseases Unit at the University of Cambridge.

As a species, we’re much taller today compared to a century ago. Body weight also increased, while the proportion of people who classify as underweight shrunk.

In the case of both height and weight, O’Rahilly summed up, “Genetics determine where you are on [the normal] distribution. And then the environment shifts the whole distribution.” You can see the shift in the U.S. here — more and more people were classified as having obesity after 1980. But unlike height, body weight followed a different trajectory. Not only were people gradually getting bigger, the normal distribution skewed: the tail on the right-hand side of the BMI distribution grew long. (BMI, while a flawed tool for assessing an individual’s body fat, is widely used to study population trends.)

Severe obesity is rising

This means more people have been living with more severe forms of obesity — a problem that’s set to increase dramatically in some countries, including places where the overall growth in BMI is slowing down.

Take the U.S. According to data from the World Obesity Federation, exclusively shared with STAT, severe obesity — classified in this case as a BMI 35 or greater — is currently projected at 20% of the population, and expected to rise to 36% by 2035, if current trends continue. Overall, the prevalence rate of severe obesity will double in high-income countries in the same period, from roughly 10% to 20%, overtaking the growth rate of obesity (BMI ≥30). According to the federation, low- and middle-income countries will also see this doubling in severe obesity.

This gets at the idea that “obesity begets obesity,” Swinburn said. “People end up caught in cycles,” and the result is “a shift of the whole curve distribution to the right, but it’s also skewed.” He called this “an underappreciated issue with big consequences” since the relative risks of many diseases, especially diabetes, increase exponentially when BMI is beyond 30.

The shift to ‘obesogenic’ food environments drove obesity

How did we get here? This is a matter of some debate among obesity researchers. One obvious culprit is that we’re more sedentary, and therefore burning fewer calories, especially on the job — though recreational physical activity may have increased over time. Another explanation researchers are actively probing is that thousands of chemicals — including fertilizers, insecticides, plastics, and additives — have entered our food supply, and may be interfering with human metabolism. The food supply changed another way: Calories are now plentiful. And there’s pretty strong consensus that increases in calorie intake can account for the increases in body mass — more so than any decrease in physical activity.

In particular, the rise of ultra-processed foods — that is relatively cheap, easy to consume, calorie-dense, and often shelf-stable goods — tracks neatly with growth in obesity levels.

Some of the world’s longest-term data on food consumption patterns, processed and otherwise, comes from Canada. Focused on calories available in the household and the share of the budget spent on different alimentary categories, a team of researchers at Université de Montréal and the University of São Paulo found that around the late 1940s, a quarter of calories available at home came from ultra-processed foods. Back then, the category featured mainly breads, spreads, and sauces, and spending on these goods accounted for less than a third of the family food budget. By the early 1980s, the calories available jumped to 47% — and obesity rates were rising in suit. Sugar- and fat-laden snacks and sweetened products, rather than foods like breads or sauces, were the major sources of ultra-processed calories.

Today, it’s not just that half of the available household calories are ultra-processed; roughly half of all the calories Americans consume are. The category is still rising globally, fastest in low- and middle-income countries. As Phil Baker, a researcher at Deakin University in Australia, said, you can now find cans of Spam in the remotest corners of the Pacific Islands, where obesity is an urgent problem. All the while, spending and consumption of unprocessed or minimally processed foods has plummeted. So has time spent cooking at home.

A recent preprint from a National Institutes of Health researcher describes how ultra-processed foods proliferated, along with obesity — another good news story turned bad for health. In the quest to feed humanity, and prevent the deficiency diseases that used to curb growth and development, humans revolutionized agricultural practices, boosted public investments in infrastructure, and improved manufacturing methods, processing to “‘add value’ to their products” and sell more. Meanwhile, government subsidies supported commodity crop production — foods like soy and corn that were used to feed animals and had to be heavily processed to feed humans.

“In other words, society has incentivized surplus agricultural production (relying on cheap fossil fuels) to provide low-cost inputs to food and beverage industries that produce heavily marketed, convenient, rewarding, timesaving, and relatively inexpensive ultra-processed foods in great excess of consumption needs of the population, albeit with large inequities in food distribution and nutrition security,” the paper reads.

Global food manufacturers also refined their techniques for successfully marketing their processed products. “They really focus from birth onward,” said Barry Popkin, professor of nutrition at the UNC Gillings School of Global Public Health at the University of North Carolina, pushing formulas to replace mother’s milk, and cartoon-filled advertising for foods targeting children.

It’s no wonder childhood obesity rates are also rising globally, and no country has turned the trend around:

This is all part of the ‘obesity transition’

Building on the work of Popkin, who described the “nutrition transition” — how diets, physical activity patterns, and causes of disease shift as countries move out of poverty — Swinburn and colleagues came up with the idea of the “obesity transition,” or four stages countries move through as obesity prevalence rises.

At stage one, when obesity rates are still low, populations are too poor, maybe even too war-torn, to get enough food — but obesity prevalence starts increasing among wealthy people, especially middle-aged women. (Middle-age is the peak obesity prevalence in all populations, and wealthy people tend to access calorie-dense, ultra-processed foods first, Swinburn said, but women may transition before men for physiological reasons – they have higher levels of fat for any BMI.) In stage two, as countries get wealthier, obesity prevalence rates continue to rise, and men start to catch up to women, as do lower socioeconomic groups. At stage three, the gaps between the sexes narrow and a flip occurs in the socio-economic gradient — obesity rates continue to rise but predominantly among lower-income people. By then, obesity has taken off. Eventually, rates in growth settle, to be followed by stage four — declines in prevalence. But again, we’re not quite there yet: No country is declining or projected to decline, according to the World Obesity Federation.

What’s behind the slowdown in BMI growth in wealthier countries? For Popkin, it’s all about market penetration. “Higher-income countries like the U.S. have had a high consumption [of ultra-processed foods] for 20 years, 30 years,” and consumption isn’t going to change much, whereas it’s only starting to take off in low- and middle-income countries. Swinburn thinks one driver might be that the messages about obesity prevention have started to get through, even in the absence of government policy, at least in some groups. “Some countries are showing reductions amongst preschool children, especially from well-off households,” he said. “So they’re going to be the first group that come out of this epidemic and start to show decreases in prevalence.”

What’s clear, they said, is that policy action is needed urgently given the health and economic costs of obesity. But even with strong evidence that the food environment is driving obesity rates up, policymakers — stifled by pressure from the food industry, and perhaps a lack of political will — have made little headway.

Low- and middle-income countries are now following the lead rich countries took in the epidemic, and experiencing the steepest increases in their obesity rates. Nine of the 10 countries poised to see the greatest increases in obesity prevalence are considered low- and middle-income, in Asia and Africa, according to the World Obesity Federation.

Some argue that since it’s food, and it’s everywhere, it’s difficult to regulate. But Swinburn pointed out that smoking was once common, even in doctor offices as it was on airplanes. After taxes increased the cost of cigarettes and regulations curtailed smoking in public places and tobacco marketing, social norms shifted, and consumption rates dropped. “When people say, well, [food is] not the same as tobacco, because we don’t need to smoke — we don’t actually need ultra-processed foods either. And so taxing, restricting marketing, keeping them out of schools, all of those sorts of policies, they’d go a long way.”

Other parts of this series examine the new message pharma companies are promoting about obesity; attempts to personalize obesity treatment; the flawed origins the body mass index; the debate over new childhood obesity guidelines; weight stigma even within treatment for eating disorders; and drugs that are trying to leapfrog Wegovy and Ozempic. Read more about The Obesity Revolution.

STAT’s coverage of chronic health issues is supported by a grant from Bloomberg Philanthropies. Our financial supporters are not involved in any decisions about our journalism