My prefix for gains
My suffix is bled
My infix in veins
My outfix is bed
What am I?
My guess:
beancod (or mostly bean-cod)
My prefix for gains
beancod; a variety of beans are good for muscle gain
My suffix is bled
beancod; cod is a type of fish and fishes are generally bled before cooking
My infix in veins
beancod; veins carry deoxygenated blood which also contains $CO_2$ back to the heart
My outfix is bed
beancod; bed is its outfix since it is split in two parts 'be' and 'd' around 'anco'