Now there is some science that says if you don’t eat before you exercise - i.e you work out in a fasted state, your blood sugar levels are lower, which forces your body to burn more calories from fat and less from carbohydrates.
Personally, I sometimes do run ‘fasted’ (before breakfast for me), not for fat loss reasons but to help train my body to switch more easily between burning carbohydrate and fat as fuel for my distance runs. When people ‘hit the wall’ in the marathon and you see the wobbly legs, this quite often when they’ve run out of readily available carbohydrates and the body is making a shift to using fat for fuel - which involves a slightly more complex process in the body - hence why it prefers more readily available carbohydrate during intense exercise.
So whilst it is true to a degree that you can burn more fat if you exercise on an empty stomach, exercising with low blood sugar can cause you to fatigue quicker which reduces your performance, endurance and therefore overall calorie burn. I can not strength train fasted - I go all weak and wobbly and then have a rubbish workout. So I can do it, but it just means the effectiveness of my workout is reduced.
Fasted training also means that you might be so hungry after your workout that you overeat to compensate. I like to suggest that if you are doing anything more intense than say yoga, Pilates or low intensity steady state cardio, you should eat before you workout. You want to have the energy and fuel to get the best of out of your session - you can’t train well if you’re feeling weak and wobbly from a lack of food.
And lastly, when it comes to fat loss, what matters the most is overall energy balance - which means consistently consuming fewer calories than you burn throughout the day. You will burn more through a properly fuelled workout - not just in the workout itself but my improving your muscle mass you will be increasing your metabolic rate and burn more energy that way.
So please do think about your fuelling based on what you're trying to achieve. Although in theory you can burn more fat by working out on an empty stomach... it really isn't as straightforward as that, and could be detrimental to your goals.