In an editorial in the British Journal of Sports Medicine, three international experts said it was time to "bust the myth" about exercise.
They said while activity was a key part of staving off diseases such as diabetes, heart disease and dementia, its impact on obesity was minimal.
But others said it was risky to play down the role of exercise. Prof Mark Baker, of the National Institute of Health and Care Excellence, which recommends "well-balanced diets combined with physical activity", said it would be "idiotic" to rule out the importance of physical activity.
Ian Wright, director general at Food and Drink Federation, said: "The benefits of physical activity aren't food industry hype or conspiracy, as suggested. A healthy lifestyle will include both a balanced diet and exercise."