Devil's advocate here... don't run to the seaweed yet.
Following
how they eat is extremely more important than what they eat.
After staying in Japan for two weeks I noticed that their diet is a small part of the reason why their hair is long. Keep in mind that they do not eat as much sugar as we do. For instance, Japan's "Froot Loops" taste totally different from the American ones, as they have much less sugar content. Also many of their desserts are made straight from beans and rice: i.e., sweet bean breads. They also don't eat too much processed food. They eat out a lot, but the food that they eat is not the typical fast food: Misu soup, white rice, fatty meats and fish, pan seared stuff (instead of fried crap) at one sitting. They eat a lot.
They also get their beef from Australia because America puts too much crap in their meat-- steriods, hormones, milk fillers. Even the apple juice is different: comes straight from the apple-- not that crap in American Stores passing for apple juice.
Food out there is not something that you gorge on. They always present their food with flair (Eating is 90% visual, if the plate is pretty, chances are you'll enjoy it). Try eating at a table, without the TV on; make your plate a work of art.
I would change my whole mindset on what I eat before just running to eat what the Japanese eat.
Also, keep in mind that genetics and heredity plays the bigger role. AND they walk everywhere! Exercise helps in hair growth. Only rich people in Japan have cars so everyone walks... or runs to try to catch the subway
Thanks for reading my long ramble.