It may come as a shock, but your hair is dead -- as in physically dead, not dull and limp. Hair's main component is a particularly sturdy protein called keratin. Keratin cells have no nucleus -- no brain, you might say -- so they can't use nutrients the way living cells do. Products that supposedly feed your hair -- with vitamin E, for example -- might have other useful ingredients, but hair food they are not.
Hair's other essential component is a mixture of fats, protein and other substances called sebum. Sebum is secreted from glands near the hair root. It complements keratin by lubricating hair to help prevent breakage.
The scalp that produces hair and sebum, on the other hand, is alive; it's part of the skin. Like other organs, it's nourished from the inside by the foods you eat. The better you nourish your scalp, the better your hair will look.
All nutrients are needed for health, of course. The scalp relies especially on omega-3 fatty acids, which keep it supple and well-oiled. Oily fish such as salmon are excellent sources, as are walnuts and flaxseed. Sebum production requires vitamins A and C. You'll find both vitamins in citrus fruits and dark green, leafy vegetables. Iron, zinc and biotin also promote hair growth. Good sources are beans, beef and whole-grain foods. And hair is largely protein. Best bets for quality proteins include lean meats and poultry and soy foods.