Unfortunately this is a cause from something so simple that it's overlooked.........diet.
I spent a good 6 years of my life studying human health and healing, and the final conclusion of all that studying......we've been taught, "marketed to", to eat things that the human body really isn't supposed to be eating. We can eat those things and survive as can be seen, but our health and the health of future generations has to pay the piper for it, "this is very evident in our times".
The food pyramid taught in schools is actually nothing more than marketing from large industries who have lobbied to the government to tell us this is "healthy" and we need it.
Studying the animal species that most closely resembles us will give you insight into what our bodies run optimally on. But beyond just studying this subject......once you experience it you will absolutely know without a doubt what your body is supposed to use as a fuel source.
I'll make it easy on ya..... Chimpanzee ---