Goh et al 2011 report that soy sauce may help to replace 33% up to 50% of the salt content of foods without reducing the palatability. The authors noted that percentage of salt reduction achievable may be higher in a population which already use soy sauce as flavouring. 
This method may be used in industrial formulations as well in home made foods, say scientists from the National University of Singapore in collaboration with soy sauce manufacturer Kikkoman.
Another study by Kremer, Mojet and Shimojo 2009 attained salt reduction of 50% in salad dressings, 17% in tomato soup and 29% in stir-fried pork, without decrease in consumer acceptance. With increased popularity of Asian foods the reduction of salt in traditional Wester European formulations becomes possible. 
The umami taste of soy sauce 
The fermentation of soy bean soy sauce production, gives a distinct delicious taste used as an umami seasoning. Umami is one of the five basic tastes together with sweet, sour, bitter, and salty. Umami is a Japanese word meaning "pleasant savoury taste".
According to Lioe, Selamat and Yasuda 2010 the Japanese and Indonesian soy sauces are rich in umami components with molecular weights lower than 500 Da. Free amino acids, add to the taste in combination with salt and small peptides, which, however, are less active as flavour..
 Goh FXW, Itohiya Y, Shimojo R, Sato T, Hasegawa K and Leong LP:Using naturally brewed Soy Sauce to Reduce Salt in Selected Foods. Journal of Sensory Studies. Volume 26, Issue 6, pages 429–435, December 2011. Doi: 10.1111/j.1745-459X.2011.00357.x
 Kremer S, Mojet J, Shimojo R: Salt reduction in foods using naturally brewed soy sauce. J Food Sci. 2009 Aug;74(6):S255-62.
 Lioe HN, Selamat J, Yasuda M: Soy sauce and its umami taste: a link from the past to current situation. J Food Sci. 2010 Apr;75(3):R71-6.