LONDON - Tackling just five health factors could prevent millions of premature deaths and increase global life expectancy by almost 5 years, the United Nations World Health Organization (WHO) said Tuesday.

Poor childhood nutrition, unsafe sex, alcohol, bad sanitation and hygiene, and high blood pressure are to blame for around a quarter of the 60 million premature deaths around the world each year, the WHO said in a report.

But while not having enough nutritious food is a big health risk for those in poorer countries, obesity and being overweight pose yet bigger risks in richer nations -- leading to a situation in which obesity and being overweight causes more deaths worldwide than being underweight.

"The world faces some large, widespread and certain risks to health," the WHO said in its Global Health Risks report. It examined 24 major health risks, and said recognizing and assessing them would help policy makers draw up strategies to improve health in the broadest and most cost-effective ways.

"As health improves, gains can multiply," it said. "Reducing the burden of disease in the poor may raise income levels, which in turn will further help to reduce health inequalities."

The report warned that although some major health risk factors, such as smoking, obesity and being overweight, were usually associated with high-income countries, more than three-quarters of the total global burden of diseases they cause now occurs in poor and developing countries.

"Health risks are in transition: populations are aging owing to successes against infectious diseases; at the same time, patterns of physical activity and food, alcohol and tobacco consumption are changing," it said.

"Understanding the role of these risk factors is important for developing clear and effective strategies for improving global health."

The Geneva-based U.N. health agency listed the world's top mortality risks as high blood pressure (responsible for 13 percent of deaths globally), tobacco use (9 percent), high blood glucose (6 percent), physical inactivity (6 percent), and obesity or being overweight (5 percent).

These factors raised the risk of chronic diseases and some of the biggest killers such as heart disease, diabetes and cancers, and affected "countries across all income groups -- high, middle and low," it said.

The WHO said its study, which used data from 2004 -- the latest available -- showed how health was becoming "globalised" and warned that developing countries now increasingly face a double burden of risks to health.

"The poorest countries still face a high and concentrated burden from poverty, undernutrition, unsafe sex, unsafe water and sanitation," it said. "At the same time, dietary risk factors for high blood pressure, cholesterol and obesity, coupled with insufficient physical activity, are responsible for an increasing proportion of the total disease burden."

The WHO added that if the risks in its report had not existed, life expectancy would have been on average almost a decade longer in 2004 for the entire global population.