We know eating organ meat, preferably from a good source is beneficial. Is just adding liver to your diet good enough? Is there a very good reason to go out of your way to get the kidney, heart, brain, intestines and so forth on a monthly basis?
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Liver is "good enough". But I think you should make a point of eating all of them if you can. They provide a diverse array of nutrients and you will be better off for it. Spleen provides way too much iron so I don't really think people should eat a lot of spleen, but everything else seems good. Especially brains. http://evolutionarypsychiatry.blogspot.com/2010/08/zombieland.html
From a nutritional perspective, liver is amazing, it is like a multi-vitamin. According to FitDay, 4oz of fried beef liver has about 6x your RDA of vitamin A, 15x of vitamin B12, 2x of riboflavin (vitamin B2), 8x RDA of copper, full RDA of niacin, and about half your RDA of a few other nutrients. That is just in 4 ounces. One serving nearly takes care of those nutrients for the week (though you could probably go over the RDA and be even healthier).
Brain, kidneys, and other organ meats have a lot of nutrients too, but liver has much higher concentrations. I like organ meats and eat them whenever I can, but from a nutritional perspective, I'd focus on liver. The same goes for chicken liver BTW -- not quite as dense nutritionally, but still very good for you.
I have always found ONE organ to be sufficient. However, it might be more fun to have two (or three). There are a lot of pretty girls on Paleohacks.