The world is getting very fat, very fast and now the obesity epidemic has spread from rich countries to poor and developing countries it is likely 1 billion people will be obese by 2030.

Fat is being called the new tobacco.

Body weight is not just about vanity, it is about life and death, with obesity increasing the risk of heart disease, type-2 diabetes and some forms of cancer.

So how and why did the problem of obesity explode in places that, not so long ago, counted malnutrition and even famine as major health concerns?

Well it's about increasing wealth, changing diets, genetic programming and aggressive marketing by international food companies.

In our special examination of arguably the world's number one health issue, Globesity – Fat’s New Frontier, Foreign Correspondent visits the new obesity hotspots - Mexico, Brazil, China and India - where hundreds of millions are gripped by weight issues and associated diseases.

The most perplexing problem in emerging economies is how they are going to deal with a tidal wave of obesity with relatively scant health resources.

We've canvassed opinion from notable authorities on diet, nutrition and fat issues, and met people in these countries struggling with the consequences of obesity.

It's an eye-opening, sometimes shocking journey.

First stop: Mexico . The biggest killer here is diabetes. In the past 30 years this country has gone from dealing with widespread malnutrition to coping with two thirds of the population growing overweight or obese. One of the main culprits is the super-pervasive spread of soft drinks: Mexicans drink more carbonated beverages per head of population than anyone else in the world. In a country where running water isn't guaranteed and bottled water is expensive, these soft drinks have become a daily dietary staple for everyone from infants to the elderly.

First stop: . The biggest killer here is diabetes. In the past 30 years this country has gone from dealing with widespread malnutrition to coping with two thirds of the population growing overweight or obese. One of the main culprits is the super-pervasive spread of soft drinks: Mexicans drink more carbonated beverages per head of population than anyone else in the world. In a country where running water isn't guaranteed and bottled water is expensive, these soft drinks have become a daily dietary staple for everyone from infants to the elderly. Next is Brazil , where global food giants are moving into every corner of the country, from the big cities to the remote reaches of the Amazon to sell their highly processed products readily and cheaply. Profits and market share are soaring and so is obesity, with another 1 percentage point of Brazil’s population joining the ranks of the obese every year.

Next is , where global food giants are moving into every corner of the country, from the big cities to the remote reaches of the Amazon to sell their highly processed products readily and cheaply. Profits and market share are soaring and so is obesity, with another 1 percentage point of Brazil’s population joining the ranks of the obese every year. In India , as incomes have risen so too has weight. But the effects here are even more dramatic than elsewhere because the Indian body type magnifies the effects of obesity. Many have a genetic predisposition to diabetes and heart disease. The situation is so dire that health experts warn a staggering one in two babies born in India today will get type-2 diabetes, delivering future generations the real possibility of widespread disability and early death. India is bracing for an estimated 100 million type-2 diabetes patients.

In , as incomes have risen so too has weight. But the effects here are even more dramatic than elsewhere because the Indian body type magnifies the effects of obesity. Many have a genetic predisposition to diabetes and heart disease. The situation is so dire that health experts warn a staggering one in two babies born in India today will get type-2 diabetes, delivering future generations the real possibility of widespread disability and early death. India is bracing for an estimated 100 million type-2 diabetes patients. The final stop on our Globesity tour is China – where the rapid transformation of the economy has been mirrored by massive changes to the consumption of food. Not just the sort of things Chinese are eating but the way they eat. Snacking was a rarity not so long ago. Now it abounds. Thirty years ago the Chinese ate only small amounts of sugar and oil – today they are a big part of the diet and a big part of the reason more and more Chinese are getting bigger and bigger.