Diet can affect the hair. Some people have lost their hair from lack of food.
A diet that is unbalanced in protein will tend to affect the hair as hair is mostly protean. This does not mean one has to eat meat there are other sources of protean like nuts and even some vegetables may have adequate amounts of protean.
Silica is a very important mineral for hair.This mineral can be taken as a supplement to improve hair heath and growth.
Stress will affect the health of your hair. Stress affects hormones, hormones affect so many things including sleep this in turn becomes even more stress and our hair will suffer.