One thing I know for sure is that malnutrition can cause hair loss, but I am not sure if better nutrition helps recover from hair loss. I was on a severe diet for 9 months and lost over 100 pounds of fat. My typical daily meals included:
Breakfast - half a bowl of rice + minimal meat | Lunch - protein bar + apple | Dinner - half of an apple
Yes, I ate like this for 9 months straight, and my hair got very soft and thinner. I definitely think people with bad eating habits are more predisposed to hair loss. Of course, there are some people who can eat whatever they want and still keep full thick hair, but chances are, if you are on this forum, you aren't one of them.
Similarly, eating healthy CAN help with hair loss, but there is no guarantee on this. Some people see great benefits while some do not see any result.