In principle, there are only 3 main factors affecting your physical fitness: nutrition, physical culture, lifestyle.
The factors are listed in order of influence on body shape and symmetry. Lifestyle are the least amenable to correction and change positively need, starting with the first two factors - diet and exercise.
The most powerful factor of power. You can achieve a slim body only using this factor. But then you will not have beautiful athletic forms.
With regular exercise, you can buy a good musculature and form, but without the correction of these forms of power are hidden fat layer.
On the other hand, if you combine a healthy diet and exercise you can find a beautiful slender body.
However, lifestyle also need to control and direct in a positive direction, otherwise there is a risk to stray from the path leading to the goal.