This is true, but I dont place too much importance on being the 'fittest' or even on 'physical survival'. The higher energy centers transcend the physical. All healing is a metaphysical event. The foods (or lack of) are just catalayst to open these doorways.
A weaker physical vehicle encourages use of the mind. You may notice many of the most advanced spiritually on this plane have much physical ailment. Much of this is pre-programmed catalyst before incarnation. Many of the most robust and healthy individuals are those polarizing negatively. One should observe this paradox and the illusion of the physical. Exactly what ails us is what speeds the teach/learnings of the lesson of love. This is more important than any principality. To love all as one and to forgive.
The metaphor of the coiled serpent being called upward by the light is quite appropriate and should be considered.