Some experts claim fat loss is 80-90% diet and 10-20% exercise, while others say it is 50/50, or even that you can eat anything you want if you are active enough. I know both are important, but where should I put most of my effort if I want to get the best results?
Diet is more important than exercise for fat loss since it has a greater effect on energy balance. You can’t put a percentage on it, though, because the relative proportions of the energy deficit due to restrictions on calorie intake and macronutrient ratios versus increased calorie expenditure from exercise and increases in lean body mass can vary tremendously between individuals. All else being roughly equal the percentage of fat loss due to diet will be much higher for someone who has a lot of fat to lose and is able and willing to restrict calories further than for someone who is only reducing calories slightly. For someone whose average calorie intake is about the same as their average expenditure before exercise the percentage of fat loss due to exercise would be higher (although relying on exercise to create a calorie deficit is very inefficient).
Diet and exercise (strength training) are equally important if your goal is to improve your body composition and not just lose fat. This is because strength training is necessary to maintain your muscle mass while fat is lost on a hypocaloric diet. If you don’t strength train you will lose muscle, which will negatively affect your body shape and tone and reduce your metabolic rate making it harder to lose fat and keep it off.
If your goal is to improve your body composition as much as possible the relative contribution of diet and exercise doesn’t really matter, though, because you should be putting one hundred percent effort into both of them, as well as all of the other factors that aid in muscle growth and fat loss.