Vitamin B12

Vitamin B12 is the largest and most structurally complex of the vitamins, and it is the only one containing a metal atom at its centre. That atom is cobalt, held inside a ring called a corrin — chemically related to the ring that holds iron in haemoglobin and magnesium in chlorophyll. It is the only cobalt-containing molecule in human biochemistry.

It is also the only vitamin that no plant or animal can make. B12 is synthesised exclusively by bacteria and archaea; every animal that has it obtained it from microbial synthesis, directly or through the food chain. For a molecule this essential, that is a remarkable dependency.

Humans use it for exactly two enzymes, which is a very small number for something so consequential. One converts homocysteine back into methionine, keeping the cycle that supplies methyl groups turning. The other allows certain fatty acids and amino acids to enter the main energy-producing pathway. Both sit in metabolism rather than at its edges, which is why a shortage has effects out of proportion to the quantity involved.

• Two enzymes, no more: Methionine synthase and methylmalonyl-CoA mutase are the only human enzymes requiring it. A deficiency's consequences all trace back to one or other of those two reactions.

• Absorption is the difficult part: Taken by mouth it requires a carrier protein made in the stomach and a specific receptor in the intestine. That multi-step route is where absorption most often fails.

• Several chemical forms: Cyanocobalamin, hydroxocobalamin and methylcobalamin differ in what occupies one position on the cobalt atom, and they are handled differently by the body.

• Measurable in the blood: When the second enzyme lacks its cofactor, methylmalonic acid accumulates and can be measured — a direct biochemical readout rather than an inferred one.

A methyl group is a single carbon with three hydrogens attached, and the body transfers these constantly: onto DNA to control which genes are read, onto lipids to build membranes, onto dozens of other molecules. The system that moves them runs in a cycle, and the cycle only continues if what has given up its methyl group is restored.

Methionine is the principal carrier. Having donated, it becomes homocysteine, which must be converted back. Methionine synthase does that conversion and cannot function without B12 sitting in it. Without the cofactor the cycle stalls, methyl transfers slow across the board, and homocysteine accumulates — which is why it is measured as a marker of how the cycle is running.

The second enzyme handles an unrelated job. Odd-chain fatty acids and several amino acids are broken down to a compound that cannot enter the main energy pathway until it is rearranged, and methylmalonyl-CoA mutase performs that rearrangement using B12. When it is short, the intermediate accumulates and appears in blood and urine. Two enzymes, two quite separate functions, one cofactor holding a cobalt atom at the centre of both.

Lipo-C + Vitamin B12 combines it with methionine, inositol and choline, and the pairing is biochemically coherent: those components supply and use methyl groups, and this is the cofactor the cycle needs to keep turning.

LC-120 is the same lipotropic base without it, which makes the two a direct comparison of what the cofactor contributes.