As you get older there is a progressive micronutrient malabsorption issue creating downstream dysfunction. All of teh systems responsible for absorbing those micronutrients deteriorate with age. Stomach acid declines (↓ HCl → ↓ B12, iron, zinc, magnesium liberation from food). Parietal cells atrophy → intrinsic factor drops → B12 absorption tanks. Bile production slows → fat-soluble vitamins (A, D, E, K2) aren't absorbed properly. Pancreatic enzyme output drops → protein-bound minerals stay bound. But the energy side is also really interesting to me becauase nutrient absorption is an active process. Your enterocytes need ATP to run transport proteins like DMT1 for iron, calbindin-dependent transport for calcium, sodium-dependent carriers for B vitamins. As mitochondrial function declines with age (heteroplasmy accumulation, ↓ ETC efficiency), even with "adequate" intake your gut cells can't physically grab the nutrients. ↓ mito function → ↓ absorption → ↓ cofactors (B2, B3, CoQ10, iron, copper) → ↓ mito function further. Probably this explains a lot of the "unexplained" deterioration people experience after 50. So functional testing becomes WAY more important as you age. Metabolomix+, comprehensive bloods, functional markers like methylmalonic acid for B12 status. And delivery route matters too in many cases. Sublingual B12 might bypass the intrinsic factor problem. Dermal magnesium bypasses the gut. Interesting stuff to keep into account.