Nature, by way of genetics and epigenetics, has given each of us our own body type. We have to work with what we’ve got.
Some people stay thin no matter what they do. At first glance, this might be enviable. It certainly has advantages for the ice-cream addicted. However, just being thin is not the same as being fit. Thin people may lack the motivation to build muscle mass, which may impact their bone density. They may not see the point in doing cardio exercise, or any exercise. For thin people, the focus should be on ensuring that body composition is appropriately lean and on functional fitness for quality of life.
The rest of us may find our motivation dropping when all our exercise hard work does not enable us to eat cake with impunity. People who tend to put on pounds need exercise to bump up metabolism and burn calories. Changing body composition by increasing muscle mass improves health, mood, and appearance.
No matter who we are and what nature has given us, we can benefit from fitness.