What's causing the metallic aftertaste in my Nutraloaf?
It sounds like most of those ingredients came out of metal cans, yes? Certainly the spinach and carrots, and I imagine the beans, tomato paste, and possibly the apple sauce? Canned foods do sometimes have a slight metallic taste, especially if you don't use them all at once and continue to store them in the can after opening it: (about.com)

However, I definitely wouldn't rely on this (or any other single recipe) as your sole source of nutrition. Even if it does include every one of the hundreds (thousands?) of nutrients your body needs in reasonable ratios (which is questionable), that still doesn't mean it's good for you. 

For example: if every meal you eat has lots of vitamin C and zinc, your body won't be able to absorb the trace amounts of copper it needs, since they interfere with each other. To be healthy, you need to mix up the nutrient profile of your meals.