I gave up eating an impressive array of foods 15 years ago. Ever since then I haven’t had to cut the length of my hair because it just breaks off, when before I made the dietary change it was much longer. Does anyone study what actually makes hair strong, and what trace elements one needs to grow long hair? Do those hair-grow vitamins actually work?
Here is an excellent resource. Scientific rather than sales-y.
Essentially, like many building blocks in the body, a diet with a good balance of protein is the most important thing.
A balance of protein so that all essential amino acids are consumed: from fish, beef, chicken, nuts, beans.