The researchers at Johns Hopkins University in Baltimore reached this conclusion after considering 20 published studies along with data from national surveys of weight and behaviour.

They say if people continue to gain weight at the current rate, by 2015 fat will be the norm.

The study leader Dr. Youfa Wang says obesity is a public health crisis and by 2015 almost 24 percent of U.S. children and adolescents will also be overweight or obese.

The researchers used the standard medical definition called body mass index (BMI) as a definition of adult overweight and obesity.

People with a BMI of 25 or above are considered overweight, while those with BMIs of 30 or above are obese and at serious risk of heart disease, diabetes and some cancers.

Previous research has shown that 66 percent of U.S. adults were overweight or obese in 2003 and 2004 and that 80 percent of black women aged 40 or over are overweight and 50 percent are obese.

According to federal government estimates 16 percent of U.S. children and adolescents are overweight and 34 percent are at risk of becoming overweight and Dr. Wang says every group is steadily getting heavier.

The researchers say their analysis showed patterns of obesity or overweight for various groups of Americans and that obesity is likely to continue to increase.

Unless something is done, it will soon become the leading preventable cause of death in the United States.

The study is published in the journal "Epidemiologic Reviews".

They were also more active than normal mice, and after eating, their brains had higher levels of a compound called superoxide dismutase, an antioxidant that protects cells from damage.

White says diet, exercise and lower weight keep the peripheral tissues sensitive to insulin, which means the body needs to make less insulin.

The researchers said the engineered mice lived longer because the diseases that kill them, such as cancer and cardiovascular disease, were being postponed due to reduced insulin signaling in the brain, even though circulating levels of insulin were high.

They suggest that in the future, it may be possible to design drugs to reduce IRS2 activity to reproduce the same effect, although they would have to be specific to the brain.

The team is now planning research into possible links between IRS2 signaling and dementia, which research has shown is associated with obesity and high insulin levels.

The research is published in the journal Science.