This article is from the the official alt.spleen FAQ, by Andrew Stellman with numerous contributions by others.
Andrew says: NOTE: PLEASE DO NOT SEND ANY MAIL TO ME REQUESTING MEDICAL ADVICE! I AM NOT A DOCTOR, AND WILL NOT GIVE YOU ANY USEFUL INFORMATION.
As mentioned before, spam and haggis are important for spleens, and are high in nutrients required by the spleen. The spleen requires stomach-like and liver-like material, which is found in haggis (made from the innards of labs) and spam (made from rats and chickens and insects, all of which taste like stomach to a spleen). Also, striated meat (like USDA Grade C [spoiled] beef, shreded) is good spleen food.
Additionally, according to internist Dr. Jozxyqk (email@example.com), high-cellulose foods are beneficial:
"The consumption of copious amounts of LARD each and every day has been proven to significantly benefit your spleen. Honest."