Vitamin B12

Grade A for correcting deficiency, which causes irreversible neurological damage if missed. Grade D for the energy, mood, and cognition claims marketed to people with normal levels.

B12 is the vitamin whose deficiency you most want to catch early, because part of the damage does not reverse.

It is made by bacteria, concentrated in animal tissue, and essentially absent from plants — which is why strict vegans need a supplement, not as a precaution but as a requirement. Studies of vegan populations find deficiency rates that are frankly alarming when supplementation is inconsistent.

The second at-risk group is larger and less aware of it. Absorbing B12 requires stomach acid to liberate it from food and intrinsic factor to carry it across the ileum. Both decline with age, and both are suppressed by long-term proton pump inhibitors and by metformin. Somewhere around one in ten adults over sixty is deficient, and many of them are eating plenty of meat.

What deficiency does is the reason for urgency. The anaemia is reversible. The neurological damage — numbness and tingling in the hands and feet, unsteady gait, memory and mood disturbance — may not be, if it goes on long enough. And folic acid supplementation can mask the anaemia while the nerve damage continues quietly, which is a genuine clinical trap.

Against that, the marketing. B12 is the headline ingredient in energy shots and the most-requested vitamin injection in wellness clinics, on the strength of a real association: deficient people are exhausted, and correcting them restores energy dramatically. That says nothing about people with normal levels, in whom trials show precisely nothing. Your body stores several years of B12 in the liver and excretes the excess.

Practically, if you eat animal foods regularly, are under fifty, and take no acid-suppressing medication, you almost certainly do not need this. If you are vegan, over sixty, on metformin or a PPI, or have had gastric or ileal surgery, test and supplement — 250-1,000 mcg daily orally is enough for most, since passive diffusion bypasses the intrinsic-factor bottleneck. Form matters little; the argument for methylcobalamin over cheap cyanocobalamin is mostly theoretical.

How it works

B12 is a cofactor for two enzymes: methionine synthase, which regenerates methionine and keeps folate metabolism and DNA synthesis running, and methylmalonyl-CoA mutase. Failure of the first causes megaloblastic anaemia; failure of the second damages the myelin sheath, producing the neurological signs.