In a brilliant new animated film "Belleville Rendezvous", director Sylvain Chomet shows the audience a New York-like city populated by grotesquely fat people who waddle around, their fat quivering as they move.

This sardonic observation is not far off the mark: currently 61 per cent of all Americans are overweight, with one in five being clinically obese, and five million qualifying for a surgical procedure that restricts the digestive function of the stomach. According to doctors, there is a long waiting list for this operation.

A long-retired director general of the Food and Agriculture Organization (FAO) once remarked: "In a world where there is so much hunger, there are too many fat people". If he is still alive today, he would be shocked by the expansion in waistlines that has taken place since he made that remark.

And yet it was not always so: for decades up till the early seventies, the number of overweight Americans was stable at 25 per cent. Over the last three decades, this number exploded to its current proportion of six in ten people. To investigate the reasons behind this epidemic, Greg Crister has written a bestselling book called (what else?) "Fat Nation".

Crister traces the origins of the current caloric crisis to two factors: the development in Japan of a sweetening agent called High Fructose Corn Syrup (HFCS), and the decision to allow the import of palm oil for the manufacture of cooking oil in the United States. The former allowed the processed food industry (including cold drink manufacturers) to take advantage of the huge over-production of corn in America, and free it from the vagaries of sugarcane harvests which were the mainstay of many Third World economies. Also, HFCS was six times cheaper than ordinary sugar. The downside is that fructose is much harder for the liver to break down than sucrose.

As for palm oil, this was developed by the British in the late nineteenth century as a cash crop in Malaysia which today exports it across the world. The problem with this oil is that although it sounds as innocuous as vegetable oil, in reality it is highly saturated, thereby clogging the arteries more effectively than even animal fat.

The parallel mushroom growth of fast-food chains like McDonald's and KFC put more and more HFCS and palm oil into more and more people. As this was a very competitive field and profit margins were low, McDonald's looked for ways to raise sales, but ran into the deeply ingrained inhibition against gluttony: people who would lick their plates clean were reluctant to order a second helping. But in the mid-seventies, a group of McDonald's franchises in Chicago discovered that if they offered customers a mix of items at what seemed like bargain prices, sales shot up. Welcome to the world of "value meals": suddenly people were eating more, not because they were hungry, but because they thought they were getting a good deal.

All these developments have lessons for countries like Pakistan. Although poverty has limited the sale and consumption of fast food and aerated sweet drinks, palm oil is widely used in kitchens across the country. Indeed, the Ghee Corporation of Pakistan imports huge amounts of palm oil for use in the many manufacturing units making cooking oil. In the cities among the middle class, heart disease and high blood pressure are rampant killers.

When I experienced a heart problem a few years ago, doctors did advise me to use as little fat as possible; however, they did not mention how much worse palm oil is than any other cooking medium, even butter. And while quantifying the caloric contents of various sweet drinks, books do not tell us that fructose is actually bad for the system.

In poor countries, the rich tend to be overweight, while the poor are skinny; in rich countries it is generally the other way around. The professional middle classes are more health- conscious; eat less fats and carbohydrates and more proteins; and exercise more. Studies have shown that there is a positive correlation between the amount of TV children watch and their weight. In America, black children living in ghettoes are often not allowed to play outside because their parents are concerned about security. And when both parents are working, it is much easier (and often cheaper) to buy a burger or a pizza than it is to cook a healthy balanced meal.

These concerns might seem remote in poor countries where the vast majority can only dream of a Big Mac, but sedentary lifestyles and rapid urbanization are putting a bigger proportion of the population at risk. Pakistani cuisine is high in fat and salt, both important factors in the growing numbers of patients suffering from high blood pressure that the Pakistan Hypertension Society regularly warns us about.

Dr Azhar Farooqi, executive director of Karachi's cardiac hospital and an indefatigable crusader for better heart care, has worked tirelessly to raise consciousness about the dangers of poor diet and no exercise. And yet the incidence of heart disease continues to rise remorselessly.

Mercifully, the trend is showing signs of reversing itself in the West. McDonald's has seen share prices and the number of its customers falling. When an American sued the firm for his weight-related health problems, he was the butt of ridicule in the media. However, the case caused tremors in the fast-food industry and many fast-food outlets have shut down in western cities. The 'slow food' movement which started in Italy a few years ago is gaining popularity, and governments concerned about the high social and economic cost of obesity are doing their best to encourage children to eat better and exercise more.

But ultimately, good health is about class: the well-off take better care of their bodies than the poor who are more concerned about survival than the quality of life. Those at the bottom of the socio-economic ladder have virtually no recourse to medical advice. The food industry, meanwhile, is more interested in profits than social welfare. Until their activities are more closely regulated by governments, they will continue making fat nations fatter.