I think some of it is genetics. Here's a great example in my family:
My sister eats nothing but deep fried food or food fried in animal fats. Mostly meat and potato. No fish. Veggies are usually out of a can if she eats them at all. Her hair has always been ~BSL. She takes after my mom with thick, course strands. Nothing much phases it.
Me? I eat fresh fruit and veg smoothies almost daily. Fish. A healthy range of meats. I rarely eat deep fried. Mostly I sautee in EVOO or broil. I buy 75% organic foods and eat salads like it's going out of style. Lots of grains, fruits, and seeds. Unfortunately, I take after my dad with very very fine, brittle strands. It breaks, splits, and SKs like nobody's business. I've hovered between EL and NL all my life. I had almost full SL once, maybe twice.
Water does help keep it moist internally and the overall quality of my hair is decent. It's the genetics that slips me up. Also, like a PP said, said lots of times it's from improper handling too (chemicals, dyes, rough combing).