The warning comes as more than 2,500 experts and health officials gathered in Sydney on Sunday for the International Congress on Obesity to discuss what organisers said was one of the most important global health issues.
Professor Paul Zimmet, a diabetes expert who is chairman of the meeting, said: "Obesity is now recognised by the World Health Organisation as an insidious killer and the major contributing cause of preventable diseases such as diabetes and heart disease."
The world now has more overweight people than those who are undernourished, and obesity is the single largest contributor to chronic disease, the organisers said in a statement.
The conference will hear research and papers from almost 400 experts and discuss issues including how to prevent people from becoming overweight, clinical treatment of the problem and links between how the brain works and obesity.
Among the top issues will be obesity among children, a problem organisers said was a mounting epidemic that could produce a generation with chronic health problems.
The children in this generation may be the first in history to die before their parents, warned Professor Kate Steinbeck, an expert in children's health at Sydney's Royal Prince Alfred hospital.
The WHO says more than 1 billion adults around the world are overweight and 300 million of them are obese, making the problem a global epidemic.
"There are now more overweight people in the world than undernourished"
Professor Ian Caterson, an organiser of the conference
The number of underweight people is estimated at around 800 million.
Professor Ian Caterson, another organiser of the meeting, said: "There are now more overweight people in the world than undernourished."
Overweight people are at much higher risk of diseases such as diabetes, heart problems, high blood pressure, stroke and some forms of cancer.
The main cause is people eating more fatty and sugary foods, and doing less exercise, the WHO says.