The United States is challenging a strategy by the World Health Organization (WHO) to tackle obesity.
Weight-related deaths are likely to double in 20 years
Some scientists accuse President Bush's administration of planning to water down proposed junk food regulations, in order to protect big business.
But the US says there is no proof that fast food alone is to blame for the growing epidemic of obesity.
Obesity has reached epidemic levels in the world, and almost two-thirds of Americans are overweight or obese.
The WHO is pushing for lower recommended daily levels of sugar, salt and fat.
But in a document leaked to the BBC, the US Government has questioned the science behind the plan.
It criticises the focus on good and bad food, saying it has not yet been established what acceptable levels should be.
Correspondents say some scientists are worried that the strategy to reduce obesity is being sabotaged.
The WHO says that weight-related deaths globally are likely to double over the next 20 years.
And in the UK, the Food Standards Agency has warned that poor nutrition and lack of exercise means young people today on average are likely to live shorter lives than their parents - the first such reduction in more than a century.