This is a huge topic, asking about the nutrition habits in Japan is like asking about the nutrition habits in the USA or anywhere else - it's a whole country with different people who eats differently in different areas and also there's a big difference between the tradition of Japanese cuisine and contemporary food in Japan. I think you''ll probably get better answers if you post a more specific questions. However, this is a little bit to go on:

a research about Japanese food: http://www.nmsu.edu/~ucomm/Releases/2006/july/dietetic_research.htm 

A survey of the Japanese nutrition and heath organization: http://www.nih.go.jp/eiken/english/index.html 

Traditional Japanese dishes: http://www.japan-guide.com/e/e2035.html

Food culture in Japan: http://72.14.209.104/search?q=cache:e6vXjKwmHR8J:web-japan.org/factsheet/pdf/JAPANESE_FOOD.pdf+traditional+japanese+food&hl=en&ct=clnk&cd=16&gl=us&client=firefox-a