According to the World Health Organisation, a sixth of the world's population is overweight - most of it due to the over-consumption of fast food and lack of exercise.
Americans spend over a hundred-billion US dollars on fast food every year. Globally, our consumption of burgers, pizzas, fried chicken and chips has also increased. Why are we eating so much fast food? And what does it do to us? Is it fast food that's bad for us or is it the way we live our lives? Today's Big Question with Emma Joseph is: Can fast food ever be healthy?
"By the end, my cholesterol has gone up by 65 points, my liver was so filled with fat that doctors compared it to pate and my blood pressure was off the charts", says film director Morgan Spurlock, who set himself the challenge of eating nothing but fast food for a whole month. Everyday all his meals would come from McDonalds, and he ended up gaining 11 kg. "I didn't know what to expect, so, when things started to fall apart in my body, I, as well as the doctors, were very, very shocked."
So how does McDonalds respond to criticism of its food ?
"I think what Mr Spurlock showed in that film was exactly why we should not overeat, and exactly why we should have the right amount of exercise. If you get a burger meal, it's a balanced meal", says Mike Love, McDonalds Europe vice-president, "You've got bread, meat, cheese, vegetables, and potatoes. Nothing that you wouldn't eat at home." But, he advises, it should be part of a balanced diet - combine fast food with other kinds of foods - and take some exercise.
So why has McDonalds introduced new items, like salads, fruit bags and porridge to its menu? "It's a matter of giving more options", says Mike Love. "We're listening to our customers and to what they want. And now we've become the biggest retailers of salad." But McDonalds still sells many more burgers than salads in its chain of 30.000 restaurants around the world.
So, why do we continue to eat fast food despite all the warnings? For nutritionist Tom Sanders, director of the Nutrition, Food and Health Research Centre, in King's College, London, it's all to do with taste.
"Food that is tasty quite often has a high fat content or is quite sugary." He believes that if you eat fast food occasionally (like once a week), it shouldn't do any harm. But, he says, the biggest problem is the availability and the quantity. "The way that you're encouraged to eat more than you need is particularly unhealthy. If you have a diet of fast food, you'll get fat, there is no doubt about it."
And it's not just a problem for the richest countries anymore.
Tim Lang, professor of Food Policy at City University in London, says the changes in diets around the world are contributing to a global public health problem. "One of the things I worry about is that as developing countries get richer, they start following westernised diets. That's bad news." Professor Lang warns of an epidemic of diet-related diseases, like diabetes and heart conditions. "The globalisation of this Western diet is a disaster. India cannot afford coronary bypass operations, it cannot afford the diabetes problems it's got at the moment."
So, if there are changes in what we're eating, is the way we eat also changing? In China, a country with thousands of years of tradition in cooking, fast food is becoming a popular choice, especially among youngsters. One of the consequences, is that 1 in 20 people are now obese - in the big cities this rises to one in five.
But convenience food has been around for a long time. Professor Felipe Fernandez-Armesto, author of Food: a History, reminds us that the population of ancient Rome ate more fast food per head of population than we do today in cities like London and New York.
"The difference then", says Professor Fernandez-Armesto, "is that the meal was probably cooked by a neighbour, somebody who could easily be tracked down, and the food was produced with fresh ingredients according to the local tradition."
"One of the mysteries of the world today is why do people want to eat junk food? It can't be just because it's fast. We've got no explanation, but it's a curse to the world."