Shio ("salt") ramen is probably the oldest of the four and, like the Chinese maotang (毛湯). It is the lightest ramen, a pale, clear, yellowish broth made with plenty of salt and any combination of chicken, vegetables, fish, and seaweed. Occasionally pork bones are also used, but they are not boiled as long as they are for tonkotsu ramen, so the soup remains light and clear. Shio is generally the healthiest kind of ramen; fat content tends to be low, and fresh vegetables like cabbage, leeks, onions, and bamboo shoots typically adorn the simple soup and curly noodles. Chāshū is sometimes swapped out for lean chicken meatballs, and pickled plums and kamaboko are popular toppings as well. Noodle texture and thickness varies among shio ramen, but they are usually straight rather than curly.credit: http://myjapanesefood.blogspot.com/2011/01/types-and-variations-of-ramen.html
