"Sadly, changing ways of life, such as reduced physical activity, are making people unhealthier and in turn prone to such diseases as cancer," Dr. Eduardo Cazap, president of the Union for International Cancer Control, said in a statement released by the World Health Organization.
For decades, health officials have worried about the impact of cigarette smoking -- another nasty habit common in industrialized countries -- on lung cancer deaths in developing countries.
But now, they say, it's becoming increasingly urgent that those nations also do something about overeating and poor health habits.
The WHO on Friday recommended 2 1/2 hours a week of moderate physical activity for reducing the risk of breast and colon cancers. Some scientists think increased levels of insulin and certain sex hormones in the obese may somehow trigger cancer growth.
Cancer is seen mostly in older people, and tends to be more common in societies without as much of the diseases, violence and other problems that kill people early in life.
Infectious diseases have dominated in less developed countries, and that's true even in the world of cancer. Cervical cancer, caused by a sexually transmitted virus, has been a leading cause of cancer deaths in women in many countries.
But in recent years, breast cancer has surpassed cervical cancer as a cause of death in some developing countries. And the number of new breast cancer cases has surpassed cervical cancer in places like Mumbai, India and Setif, Algeria, according to researchers at the American Cancer Society.
The cancer advocacy group released a report Friday showing that the developing world's share of cancer deaths worldwide is increasing. In 2008, there were 4.8 million cancer deaths in developing countries -- up from 4.7 million the previous year -- out of the 7.6 million deaths worldwide.
A separate report, meanwhile, estimates that 340,000 cancer cases could be prevented each year in the United States if more people ate better, kept their weight down, exercised and drank less alcohol. That estimate came from the American Institute for Cancer Research/World Cancer Research Fund.
The reports were released Friday to mark World Cancer Day.
Some cancer experts referred to a WHO prediction in 2007 that cancer would become the world's leading killer by 2010, replacing heart disease. But the WHO's definition doesn't include all the forms of heart disease. A WHO spokesman on Thursday noted there's disagreement about how to group heart diseases in such rankings. He added it's not clear whether the 2010 prediction had come true.