Well, if you limit your thoughts on this to just a categorical perspective, then you'll see "diet vs exercise" and your internal comparative evaluator will put the emotional tags appropriately: i.e, diet=less-effort=good, exercise=lots-of-effort=not-so-good, so conclusion=diet changes only!!!, yeah!
...but no so fast. There's another valuable perspective: human biology (the ecology of human physiology). From an ecosystem viewpoint, diet and exercise coordinate and enhance each other to get you all the neurotransmitters, hormones and enzymes needed to get those glycogen stores moving out of the muscles so that the fats and glucose product can be used properly as fuel and so the brain maintains proper levels of everything it needs to be healthy and develop like a muscle with the plasticity it's capable of.
Systems in the body that're not fed properly can start robbing and borrowing from each other, operating in a context of forced famine and accumulated maltreatment of the entire ecosystem can manifest as ills of various sorts: mental and physical.
By exercise, I mean increasing activity levels, just moving the body and moving so that, over time, the concepts of sustained action come into play and until the concept of endurance becomes applicable. Much research has already demonstrated decreases in symptoms of depression and anxiety by people who just make a practice of walking around the block once a day, for example.
The people who wrote the books we read were on the go, hunting and gathering - their research subjects as well as the data itself. Gurdjieff was on the move. Don Juan, the master of self-importance lessons was always on the move. Wise Native Americans and the Sufi masters of the Middle East were people on the move... :)
There's even emerging viewpoints in the neurosciences suggesting that the cerebellum and its motor cortex, once thought of as strictly confined to physical concerns, actually has a role in the timing and sequencing of synaptic activity involved in cognition!
My conclusion is that humans are biologically engineered to move and to move in certain ways and that most people will likely reach optimal health by incorporating this assumption into their daily activities and noticing the benefits.
That wasn't too "preachy" was it?
