Given that different people have different diets and absorb vitamin and minerals at different rates it is perhaps not surprising that different people experience different outturns from the same circumstances. One size definitely does not fit all. However - I hope there is enough here to draw out some general principles for those seeking to understand why B12 deficiency adversely affects energy levels without even without anaemia. As Sally Pacholok cogently pointed out - it is a primordial molecule at the very root of many biochemical processes. This is but one.
Ref 2 Porth 2005 7th Ed Pathophysiology: Concepts of Altered Health States
Ref 3 Bayliss and Tunbridge 1998 3rd Ed Thyroid Disease: The Facts
Ref 4 Madrid 2009 Vitamin D Prescription: The Healing Power Of the Sun And How It Can Save Your Life
Ref 5 Harvey 2008 We Want Real Food: The Local Food Lovers Bible