By Ray Dunne
BBC News Online health staff
Around 300m people around the world are obese and that figure is predicted to rise sharply in the years ahead. Who, if anyone, is to blame?
Fast food is often high in calories
This week, the world's largest fast food chain announced plans to introduce healthier menus in many of its restaurants.
Out go super-sized fries and drinks and in come chicken salads, yoghurts and chopped fruit.
But what should have been a public relations triumph for McDonald's quickly turned sour when it emerged that its salads may contain more fat than its burgers.
The subsequent headlines put the company back in the firing line over obesity.
The corporate giant, like many others in the food industry, has been accused of contributing to rising rates of obesity in many western countries, something it strongly denies.
"The obesity debate is a very complex one and is widely recognised to include considerations not just of diet but also individual responsibility and increasingly sedentary lifestyles," the company said in a statement.
Around 300m people worldwide are obese
One in three American adults is obese
One in four Britons is obese
Obesity is estimated to cost the US $75bn annually
The courts appear to agree. Last year, a group of New York teenagers tried to take McDonald's to court claiming its food was addictive and had made them obese.
The case was quickly dismissed. The presiding judge Robert Sweet said: "It's not the place of the law to protect them against their own excesses."
Certainly, there has been a growing trend, in the United States at least, towards shifting blame away from the fast food sector.
It's a view shared by many American politicians. This week, the US House of Representatives voted in favour of legislation which would make it illegal for anyone to sue fast food companies claiming they made them obese.
The so-called Cheeseburger Bill needs the backing of the US Senate if it is to become law, but reports suggest it already has the support of the White House.
Nevertheless, many scientists insist fast food is contributing to rising rates of obesity.
Last year, researchers at Princeton University published a study which suggested fast food may be as addictive as heroin.
They said rats fed a diet of high-sugar, high-fat foods suffered withdrawal symptoms when they went without.
The health risks
Being obese increases the risk of diabetes, heart disease and cancer
Obesity can reduce life expectancy by up to nine years
Poor diet and a lack of exercise killed 400,000 Americans in 2000
In October, British scientists warned that the human body simply wasn't designed to eat the high-calorie food being sold in fast food outlets and supermarkets.
"The systems regulating human appetite have evolved for the low-energy diet still being consumed in rural areas of the developing world where obesity is almost non-existent," said Professor Andrew Prentice of the London School of Hygiene and Tropical Medicine.
"Our bodies were never designed to cope with the very energy-dense foods consumed in the West and this is contributing to a major rise in obesity."
Most experts maintain that the causes of obesity are complex.
"We don't think there is one actor to blame in this," said David Porter of the World Health Organization. "It is a consequence of a much greater availability of calories and less physical activity."
The facts are that high-energy or high-calorie food is much more widely available than it was just a generation ago.
Lifestyles have also changed. Cars, computers and television have all contributed to making people less active.
"People like to be able to point the finger at one particular thing," said Dr Toni Steer, a nutritionist at the UK's Medical Research Council.
"But I don't think it is constructive or helpful. We need to stop trying to find someone to blame. We need to start recognising the solutions."
Certainly, there is growing pressure in many countries for decisive action to defuse what some experts describe as the obesity time bomb.
In the UK, there have been calls for a tax on fatty foods and a ban on junk food ads, particularly those targeted at children.
So far, the government appears to be shying away from going down this road, although ministers insist that obesity is top of their agenda.
The Department of Health is in the middle of a major consultation exercise aimed at canvassing the views of the public on how best to improve the nation's health.
The results are expected to be fed into a new white paper on public health to be published later this year.
The issue is also expected to dominate this year's meeting of the World Health Organization assembly in May.
In the meantime, ministers and scientists will no doubt continue to encourage companies like McDonald's to explore healthier ways of making money.
"The decision by McDonald's to introduce healthier foods in its restaurants will not by itself solve the problem of obesity," said Dr Steer.
"But they are the market leaders and if it starts a trend and others follow, it could help."