The world has a weight problem. Within the past decade obesity rates have shot up by 50 percent, rising from 200 million people in 1995 to 300 million in 2003. Even places more familiar with famine than fat are starting to worry about their waistlines. Africa now has weight-loss clinics. In a recent survey in India--home to half the world's undernourished population--55 percent of women between 20 and 69 were found to be overweight. Twenty percent of Chinese adults are overweight. Brazil's childhood-obesity rate has jumped 239 percent in a generation--four times the growth rate for youth in the United States. This March, the International Obesity Task Force (IOTF) revealed that 1.7 billion of the planet's 6 billion people were overweight (with a weight-to-height ratio--known as body mass index--over 25) or obese (with a BMI greater than 30).

How did we all get so fat? The problem stems from the collision of a number of modern trends: rising affluence and urbanization; the proliferation of conveniences like cars, computers, fast food and television; and the 21st-century work culture, with its desk jobs and long hours. In rich countries, the familiar mantra of a low-fat diet and exercise is routinely ignored. In the developing world, where health education is frequently nonexistent, people are particularly vulnerable to the glamour of American burgers and the comforts of the couch. Rural migrants to cities are suddenly confronted with market-stall snacks and McDonald's. In newly industrialized countries, packaged foods and fast-food chains are "promoted as an index of affluence and of being part of the international jet set," notes Philip James, chair of the IOTF.

The democratization of foods high in fat, sugar and salt--combined with increasingly sedentary lifestyles--is creating what health experts call a "nutrition transition." And it's killing us. "We have brilliantly built an environment guaranteed to make people obese," says James. "We've designed a society that's guaranteed to make people unfit. And so the world is dying of fat-related illnesses."

Diet-linked diseases--chiefly heart disease, cancer, diabetes and hypertension--now account for more than half of all deaths in Arab countries. In Barbados, more than 60 percent of hospital and drug-service budgets go toward coping with obesity-related illnesses. Sheng Hongguang, a Shanghai expert in weight-related diseases, has seen the number of patients in his ward increase by 30 percent. "In the past, people would only come to the hospital because they had infectious diseases," says Sheng. "Now we are seeing a lot more 'rich people's' diseases."

The problem is so pervasive that the World Health Organization (WHO) has declared obesity a global epidemic. It is currently preparing guidelines for nutrition aimed at preventing diet-induced disease. Though the final report is not due out until next spring, it is already sparking controversy. Representatives from restaurant associations and vending-machine trade groups have informally questioned its medical basis. Their reaction underscores a key difficulty in combating obesity: it pits health advocates, who argue that it's tough to eat well in today's fast-food world, against a multibillion-dollar food industry, which insists that diet is a matter of personal choice.

The debate is made all the more critical by the fact that the fat epidemic is spreading from richer to poorer parts of the world. Biology, as well as culture and the environment, is to blame, say nutritionists. According to the Barker Hypothesis, low-birth-weight babies and malnourished children are actually more prone to gaining abdominal fat, which is the most important risk factor for developing type 2 diabetes. In many places, fruits and vegetables are far more expensive than fats, sugar and meat. The average Czech, who earns $550 a month, is more likely to opt for traditional meat and dumplings--at a cost of about 50 cents--than something with fresh vegetables, which would run double that. Coca-Cola is cheaper than bottled water in most of the world. "With the wealthy and well educated having the luxury of workouts and lean diets, the obese population will increasingly be a poor one," says Barry Popkin, professor of nutrition at the University of North Carolina at Chapel Hill.

Nothing puts weight on a nation like the shift from an agricultural to an urban economy--a trend currently reshaping Africa and Asia. A 1999 United Nations survey found that the number of overweight Chinese had jumped from 10 to 15 percent (an increase of 64 million people) in just three years. Chen Linnan, 57, recalls her days of hard labor. As a farmer, she spent all day rotating soil, plucking green beans and digging potatoes. Life was tough, but "I was healthy," she says. "I never had to see a doctor before the age of 45." In the '80s, as China began shifting to a market economy, Chen became a factory supervisor. Her rising income, combined with her husband's, allowed the couple to buy modern appliances. Rather than wringing clothes by hand, Chen got a washing machine. A refrigerator cut out the need for her daily walk to the shops. Television and air conditioning meant she spent hours lazing on the sofa, devouring sunflower seeds. Her farming weight--45 kilos--spiked to 84. Today she spends a month each year in the hospital, under observation for diabetes and dangerously high blood pressure.

City living certainly makes exercise more difficult. Cars take the place of biking or walking. Development--particularly when it's unplanned, as in many African and Asian cities--gobbles up green space. In highly polluted areas, adults who want to exercise have to buy gym memberships; fears of crime and traffic keep kids in their yards. No wonder urban obesity rates are skyrocketing. "Mandela's Children," a University of the Witwatersrand study of 3,200 kids in Johannesburg and Soweto, found that the poorer the child, the less exercise he gets, and the weaker and heavier he is. According to the study, white children played on average two hours of sports each week, while black children averaged only 29 minutes--with 55 percent of them not playing at all.

Cities are also havens of fast food. As Mediterraneans have moved en masse to urban areas, they have abandoned their famously healthy diet of olive oil, fish and fruit, prompting the Italian and Greek governments to launch anti-junk-food campaigns. In Egypt, female rural migrants to the cities are particularly prone to weight gain, says Wafaa Mehelba, an Alexandria-based nutritionist. "Women used to work on the farm, but now they just sit in their small houses in the city," she says. "It's just a few steps to the souk, so there's no chance to move." In Brazil the poor used to fill up on beans, which provided iron and protein. But mass movement to the cities has meant that people eat fewer meals at home, says Augusto Bozza, general secretary of the Brazilian Society of Cardiology; beans and rice have given way to burgers and ice cream. "Obesity is an epidemic of progress," he says.

In some parts of the world, this "progress" has been so rapid that people have leapfrogged from famine to fat in a single generation. In South Africa "women progressed from undernutrition to overnutrition, passing proper nutrition along the way," says Dr. Krisela Steyn, director of the South African Medical Research Council's Chronic Diseases of Lifestyle Unit. In South African women, plumpness has always signified health, wealth and fertility. Over the last generation, fears of looking like a victim of AIDS--commonly known as "the slimming disease"--have reinforced the traditional respect for fat. Today about half of South African women--59 percent of black women--are overweight or obese.

The theory of evolution may even play a part in the obesity epidemic. Historically, nomadic peoples ate whenever food was available because they didn't know where their next meal was coming from. According to the "thrifty gene" theory of weight gain, the body, which learns to cope with minimal calories and nutrition in lean times, can quickly become obese when exposed to unlimited food. "Their bodies were brought up to harness or conserve food," says David Porter, a WHO nutrition official, referring to Latin Americans who migrate to the United States. "Suddenly it is overwhelmed with food. They gain weight." Especially when that food is high in fat and low in nutrition: in poor urban areas of South Africa, people commonly buy stall snacks of fried bread or low-grade scraps of meat fried in cheap oil.

Indeed, the growing trend toward snacking--rather than sitting down to a formal meal--means that people are simply eating more. In the Philippines, fast-food snacks come in addition to--not instead of--three regular meals, and often consist of deep-fried spring rolls or squid balls. Grazing, says Anna Jung, director general of the European Food Information Council, is itself a recent American import to Europe, which has always been a diehard three-meal-a-day zone. "In the U.S., eating is totally destructured," she says. "Americans always have something in their hand. It's permanent eating with-out getting the pleasure out of it." In France, experts say, this penchant for snacking--not the steak frites for lunch--has helped double the rate of morbid obesity (45 kilos or more overweight) over the past six years.

As family life and work hours around the world start to look more American, eating habits are following suit. Portion sizes are ballooning; according to a study published in the Journal of the American Dietetic Association, the average size of an order of french fries has nearly tripled from 75 grams in 1955 to 220 grams today. Family meals are increasingly quaint traditions rather than daily habits. "When your parents aren't around, it's much easier to order pizzas and save the effort of cooking," says New Delhi student Madhavi Narayan, 16. In Japan, the fattest tranche of the population is middle-aged men, so stressed and overworked that they often break for dinner only near midnight.

And while American franchises like Burger King and Kentucky Fried Chicken are most often blamed for the world's bulging bellies, local ethnic fast foods can prove just as damaging. The shawarma, the massive meat sandwich beloved by Middle Easterners, can have just as much fat as a burger or fried chicken, says Abdulrahman Musaigner, director of the Arab Task Force on Obesity and Physical Activity. In Russia, restaurant chains have begun offering "Russified" fast food. "People are always going on about how unhealthy McDonald's is," says Moscow teacher Sergei Yurkov, 24. "Then they go and buy a greasy Russian pirozhok"--or meat pie--"from the street."

Many post communist countries can blame the West for one thing--its economic system. A report out this year from the Harvard Institute of Economic Research argues that regulated markets are thinner than free ones: "Countries that are more regulated and that support traditional agriculture and delivery systems have lower rates of obesity." In China, the old pre-'80s ration system limited each person's consumption of oil, rice and meat. Now "you can use all the oil and meat that you want," says Hu Pin, a Shanghai nutritionist. Russian doctors point to the spoils of the free market--shorter queues, less manual labor and 8 percent annual growth in car ownership--as key reasons for their country's weight gain.

The Czech Republic has one of the highest obesity rates in Europe: a quarter of women and 22 percent of men--a 25 percent and 37.5 percent increase for women and men since 1989. Better picture quality and uncensored programming mean Czechs spend far more time in front of the television than they did during the communist era, says Dr. Marie Kunesova, an obesity specialist in Prague. Under communism, Czech women finished work at 2, so they had time to buy fresh ingredients for the family supper. "Now [the Czech woman] rushes home at 7 p.m. and shoves a pizza from the freezer in the oven," notes Miroslav Pesek, manager of the Kometa buffet diner in Prague. "We kept all our own bad eating habits and gained all the Western ones on top."

In many places, obesity--and related illnesses such as diabetes (sidebar)--are growing fastest among children and teenagers. That may be because those groups are more vulnerable to the excesses of their newfound liberation. A generation ago, eating out was rare in developing regions like India and the Middle East. Now, particularly in richer states like the United Arab Emirates and Saudi Arabia, teenagers have cars and can go to the mall with their friends, where high-fat fast food beckons. The Arab Task Force on Obesity's Musaigner found that what Arab teens valued was not necessarily the food but the freedom: "They said they enjoyed the social environment and the chance to select their own food," he says. "At home, they told me, 'My mother chooses it'."

Nutritionists also blame food marketing--and the proliferation of supermarkets--for the rise in childhood obesity. The rapid spread of supermarkets over the past decade has radically changed the way kids eat. In Latin America in 1990, only 16 percent of all food came from supermarkets; a decade later 60 percent did. Unlike market stalls, stores need foods with long shelf lives--namely, processed items, which are --high in salt, sugar and fat. The Italian Society for Obesity partly blames the spread of supermarkets--with their aisles full of brightly packaged goods aimed at kids--for the scary rise in childhood obesity in Italy, where 20 percent of adolescents are clinically obese (up from 10 percent a decade ago).

Some countries are starting to look at muscular new legislation to curtail food producers. Rio de Janeiro's 1,035 public schools have banned junk food, building a lunch program around old-fashioned beans and vegetables. Both the British and Australian medical associations are pushing for a tax on fatty foods. EU health commissioner David Byrne recently proposed legislation that would make it more difficult for companies to claim health benefits for their foods. At a recent conference on obesity in Europe, IOTF experts recommended following the example of Sweden--where it's illegal to advertise on television to kids under 12--by adopting new laws to protect children from aggressive food-ad campaigns.

The tough new climate has spooked food producers. Some are adding "healthy" items like water, juice and fresh vegetables to their portfolios. Last month Kraft announced a "global initiative to respond to obesity," promising a reduction in sugar, fat and serving sizes and the elimination of all in-school marketing. McDonald's has added salads and fresh fruit to its menus.

It doesn't hurt that food companies are increasingly being held accountable for the damage their products cause. In the United States last year, a public-interest-law professor and a group of students won a $12.5 million settlement against McDonald's for failing to disclose that french fries contained beef fat. In Brazil a consumer-rights attorney has filed suits against Coca-Cola and the soft-drink maker Ambev to restrict aggressive marketing and force them to print health warnings on their labels. An April report from JPMorgan warned food manufacturers of the risk of increased regulation and litigation. But the report tried to be encouraging, too. "Obesity concerns create a growth opportunity," it said, for players that can reposition themselves in healthy products. At this rate, perhaps one day we'll be able to order spinach on a whole-wheat bun at the drive-through window.