Aseem Malhotra, writing in the Washington Post:
The idea that our obesity epidemic is caused by sedentary lifestyles has spread widely over the past few decades, spurring a multibillion-dollar industry that pitches gadgets and gimmicks promising to walk, run and kickbox you to a slim figure. But those pitches are based on a myth. Physical activity has a multitude of health benefits — it reduces the risk of heart disease, Type 2 diabetes, high blood pressure and possibly even cancer — but weight loss is not one of them.
Spoiler: eat less refined carbs. In particular sugar:
And no, artificial sweeteners are not a good substitute.