Nutrient
Vitamin B12
Water-soluble vitamin required for nerve function and red blood cell formation; absorption declines with age.
Function
B12 is needed to make DNA and red blood cells, and to maintain the myelin sheath around nerves. Unlike most vitamins, deficiency is more often caused by poor absorption than low intake, especially past age 50.
Food sources
Found almost exclusively in animal products (meat, fish, dairy, eggs). Plant-based diets typically require a fortified food or supplement.
Deficiency
Relevant populations
Elderly — reduced stomach acid impairs B12 absorption from food even when intake is adequate.
Citations
[1] NIH Office of Dietary Supplements — Vitamin B12 fact sheet.