But to lose weight you need to expend more calories than you take in - be that eating less, exercising more, having a limb amputated?
Ah, that is the very point we are 'debating'. Whilst a surfeit of calories leads to weight gain, weight loss can be achieved by consuming the right amount of calories, you don't need to 'burn more than you take in'. The body doesn't really let you do this anyway. When you eat less than you burn, the body reduces the metabolic rate, so you don't end up with a calorie deficit anyway, and you are under-nourishing your body. This can lead to muscle wastage, and to people who are very skinny, but still with a fat belly, because the body will still be inclined to store fat whilst you are not supplying it's needs.
My assertion, is that to lose weight, you simply need to eat the right amount of food, in a healthy way. The right amount being what your body needs according to your metabolism and activity levels, and that food group, food quality, combinations, and timing of food are all key factors in achieving the right balance, and hence the advice of 'create a calorie deficit of 500 calories per day' is misleading at best.