We avoid chicken and cow manure because those animals are given so many hormones and antibiotics. Instead, we use horse manure and rabbit manure when we can get it. The nutritional value of food varies not only with the composition of the soil but also, crucially, with the seed. That's why we use old heirloom seeds and cultivate open pollinated plants whenever we can. They're more able to pick up the trace minerals and hence end up producing more nutritional food.
Earthworms bring the soil alive by filling it with air and breaking down the minerals.
The lizards and birds keep the insect populations down. All in all, it's good to remember that gardening doesn't only promote health through good nutrition. It's also physically challenging to old bodies and fun for young minds. In this sense, gardening provides food not only for the body but also for the soul. It seems like a miracle to me every time I plant a seed and watch it grow into something beautiful and nourishing.