From [here][1]:

> The primary reason for requiring this washing is that in times of old (and again shortly when Mashiach arrives) before Kohanim were allowed to partake of Terumah they had to wash their hands to ensure they weren't impure. In order for them to get in the habit of doing so, the Chazal instituted this washing for all of Klal Yisroel, not just for Kohanim. 

According to [here][2], bread was the food most commonly made from Terumah grains.

[Another reason given][3] is that the Cohanim would wash their hands before performing the service in the Beis Hamikdash. Our sages say that our table is comparable to the Altar, and our bread like the offering brought on the Altar. Therefore we wash our hands before eating bread.

Does anybody have any more primary sources for these reasons? Talmud, Shulchan Aruch, etc.?


  [1]: http://halachafortoday.com/archivesnetilasyadayim.aspx
  [2]: http://www.jewishanswers.org/ask-the-rabbi-2881/washing-hands-before-eating-bread/
  [3]: http://www.jewishvirtuallibrary.org/jsource/Judaism/Hand_Washing.html