What Are Vitamins? The Thirteen Molecules the Body Cannot Make

The idea came from the deficiency diseases. Scurvy killed more British sailors in the eighteenth century than combat did until James Lind's trial of 1747 showed that citrus cured it; beriberi, which paralysed rice-eating populations in Asia, was traced in the 1890s to polished rice that had lost its husk; and rickets, which bowed the legs of children in industrial cities, was cured by cod liver oil and sunlight. In 1912 the Polish biochemist Casimir Funk proposed that each of these was caused by the lack of a specific substance in the diet, which he called a vital amine, and the word vitamin stuck even after most of them turned out not to be amines. They were isolated one by one over the following forty years and given letters in the order of discovery, with gaps where a supposed vitamin turned out to be several or none.

The list is divided by what they dissolve in, since that governs how they are stored:

• •Fat-soluble, stored in the liver and fat, so that both deficiency and overdose take time: A for vision and the immune system; D for absorbing calcium and building bone; E as an antioxidant protecting cell membranes; K for blood clotting
• •Water-soluble, not stored and needed regularly, with excess passed in urine: C for building collagen and absorbing iron; and the eight B vitamins, thiamine, riboflavin, niacin, pantothenic acid, B6, biotin, folate and B12, most of which help enzymes extract energy from food, with folate and B12 also needed to make DNA and red blood cells

Most vitamins are cofactors: small molecules that an enzyme must hold in order to work, in the way a key must be in a lock. The B vitamins are the clearest case, each one becoming part of an enzyme that moves electrons or carbon fragments in the reactions that burn sugar and fat, so that without thiamine the nervous system cannot get energy from glucose and beriberi follows. Vitamin C is needed by the enzyme that cross-links collagen, the protein of skin, blood vessels and gums, which is why scurvy opens old scars. Vitamin A becomes the pigment in the retina that absorbs light, and its lack causes night blindness. Vitamin D is really a hormone, made in the skin by sunlight and only necessary in food when the sun is short, which is why rickets appeared in the smoky cities of the north. Because vitamins are tools rather than materials, the amounts needed are tiny, and a varied diet supplies all of them.

Almost every mammal makes its own vitamin C from glucose in the liver, and does not need to eat it. Humans, apes, guinea pigs and fruit bats cannot, because the gene for the last enzyme in the pathway is broken in all of them, a mutation that happened in a common ancestor some sixty million years ago and was harmless in animals that ate fruit all day. It only became a problem on ships. The same accident of history explains why vitamin deficiency is mostly a disease of restricted diets, of sailors, of prisoners, of the very poor and of people on the same few foods for months, and why it was nearly unknown among hunters and farmers eating a wide range of things.

For a person eating a reasonable diet, the trials say mostly no. Large studies giving multivitamins to healthy adults find no reduction in heart disease, cancer or death, and high doses of some, vitamin E and beta-carotene among them, have slightly raised risks in smokers. The clear exceptions are specific: folic acid before and during early pregnancy cuts the risk of spina bifida by about seventy percent and is recommended for everyone who may conceive; vitamin D in winter for people at high latitudes, with dark skin or who stay indoors; B12 for vegans and for older people, whose stomachs absorb it poorly; and vitamin A in the parts of the world where its deficiency blinds children. The pills are cheap and mostly harmless, but the evidence that they do anything for the well-fed majority who buy them is thin, and the doses in some products exceed the levels at which the fat-soluble vitamins do harm.

Vitamins are thirteen small organic compounds that the body needs in minute amounts as tools for its enzymes and cannot make for itself, discovered through the diseases their absence causes: scurvy, beriberi, rickets and the rest. Four dissolve in fat and are stored, nine dissolve in water and must be eaten regularly, a varied diet supplies all of them, and supplements help in specific cases such as pregnancy, winter sunlight and vegan diets rather than as general insurance.