What it is
The nose-to-tail parts your hunter-gatherer ancestors ate first — marrow, heart, kidney, tripe — and modern eaters mostly skip.
Why your biology objects
The most nutrient-dense food there is: heart is a top source of CoQ10, kidney and sweetbreads load B12, selenium and copper, marrow brings fat and collagen — the original multivitamin, in the most bioavailable form. The one footnote is purines, which matter if you're prone to gout.