Thanks for trying to help me Paleo Donk. Imparting information to me must seem like looking into the sleepy eyes of a doddery old mule to you. When it come to food and maths, I need really simple heuristics based on what I'm looking at - like 3 parts lean to 1 part fat.
(I understand why people are interested in the science and maths of nutrition, but I have no interest in it at all. Odd as it might seem for someone interested in diet, I have never ever counted calories, or done those calculations people do multiplying different parts by different amounts. It just doesn't appeal - I feel a warm fuzz descending over my brain just thinking about it. Maths and food don't mix for me.
I'm also a totally blank if someone says something like "you need bananas for potassium". I think for some people the "scientification" of it gives it greater authority, and helps them control their diet - for me, it makes it less authoritative, and makes me remember how often, and wildly, scientific recommendations have changed over my lifetime, and how wrong the mainstream nutritional scientific advice seems to be, even now. I suppose I believe that nutritional science is so incomplete (certainly my grasp of it is), that it's not much different to saying "bananas will halve your melancholic vapours".)
I will treat marrow mainly as fat, for my 3:1 rule of thumb.