The permyz follow Joel Fuhrman’s Eat to Live regimen for all the reasons Gigi mentioned (Fuhrman doesn’t even describe the environmental benefits, but Francis Moore Lappe and Anna Lappe’s Hope’s Edge does). This diet seems especially well suited to home-grown food. In short, it just fits with permaculture.

By the way, the original crew of Biosphere 2 raised supposedly low-maintenance animals chosen specifically to eat agricultural by-products and not compete with human food production. But as Jane Poynter describes in The Human Experiment, the animals actually required lots of attention and provided only 150 calories a day  worth of food (less than a tenth of the bionauts’ actual diet). This suggests that ecologically speaking, animal food products should play a very minor role in our diet, and the medical data (Fuhrman) support this.