We are usually concerned with asanas when we talk about yoga. But asanas are just one part of Yoga. Asanas make our body strong. Swami Vivekananda said, "The world is the great gymnasium where we come to make ourselves strong".
In gym, mostly your muscles are involved but in yoga asanas, your internal organs such as liver, stomach, intestines would get nice massage in different different postures and due to this, the flow of the blood to these organs is improved and the blood carries nutrition to be absorbed by these organs for proper functioning. In gym or other physical activities, blood flow goes to the part which is involved. But we need to make ourselves body as well as mind. So, we should follow the whole yoga which includes asanas, pranayama, dharana and dhyana which makes our body and mind strong with values as contained in yama and niyama. Yama and Niyama are first two steps of Yoga. But still In asana, the cumulative effect of effort, concentration and balance forces us to live intensely in the present moment.
So, i think you should choose yoga or asanas for your holistic health.
As regards the diet, you should go for yogic diet or sattvic diet which adds fresh fruits and uncooked foods in your meals as much as possible.