Traditional diet advice over the last fifty years, still espoused by most health authorities around the world, holds that if you want to lose weight, you need to cut down your fat intake. In the last few decades, a number of alternative diets have sprung up claiming that you should instead be cutting down your carbohydrate intake. These include the LCHF, Atkins, paleo, and more recently the ketogenic diets. But what do the randomized controlled trials say? Should you cut down on fat or carbs if you want to lose weight?