I am not a nutritional zealot. I'm not pushing a bunch of supplements at you from all sides. In fact, based on the current nutritional and natural foods marketplace, I'm hesitant about advocating on any nutritional topic. In the marketplace, there are too many claims of dubious value, and I don't want to be associated with them.
Even before medical school, I have always had a sense that there was something wrong with human health, in a deep way. It didn't make sense that so many people would have so many odd diseases, or malaise, or general weakness. Over time I began to see life itself not so much as an "on-off" condition, not as a binary state, but as a quantity. Some people are more full of life than other people. Some situations make people more full of life than other situations. And this "life" that people have in them, it is not imaginary. It is a real thing, though you can't touch it, or measure it. Still, you can sense its relative presence or absence.
Looking back on my education, I see now that I have been watching for something in the human body that would explain this variation in quantity of life, and the relative absence of life in some people that leads to sickness, malase, and weakness. I have considered every organ system, every physiologic process, and every mental state looking for this answer.
What I have found, and what seems to be the problem, is widespread vitamin D deficiency.