Most people hoping to shed a few pounds head straight for the gym.
But the miles they clock up on the treadmill may not actually help them lose any weight, scientists now claim.
Doing more exercise increases a person's appetite, and they tend to eat more food as a result, the researchers said.
Therefore controlling calories – with or without increasing physical activity - is the key to maintaining or losing weight, they concluded.
Dr Richard Cooper and Dr Amy Luke, both from Loyola University Chicago Stritch School of Medicine, have been studying the link between exercise and obesity for years.
Writing in the International Journal of Epidemiology, they said: 'Physical activity is crucially important for improving overall health and fitness levels.
'But there is limited evidence to suggest that it can blunt the surge in obesity.
'This crucial part of the public health message is not appreciated in recommendations to be more active, walk up stairs and eat more fruits and vegetables.
The prescription needs to be precise: There is only one effective way to lose weight - eat fewer calories.'
Numerous clinical trials have found that exercising as well as restricting calories achieves virtually the same weight loss calorie-restriction alone, they said.
And other studies show no link between the energy someone expends (through exercise) and subsequent changes to their weight.
They added that only 'extremely small' proportions of the US population do enough physical activity to affect their long term balance of energy.
Therefore, they argue that that physical activity does not influence obesity, they said,
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