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, even after James Lind's trial of 1747 showed that citrus cured it, until the navy issued citrus juice routinely from 1795; 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 loss that happened independently in guinea pigs, in bats and, around sixty million years ago, in the ancestor of monkeys and apes, and was harmless in animals that ate plenty of fresh plant food. 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.

In the early twentieth century a disease called pellagra, marked by skin rashes, diarrhoea and dementia, killed thousands of people a year in the southern United States. Many doctors thought it was an infection. The physician Joseph Goldberger showed otherwise: it struck orphanages and asylums where inmates ate a monotonous diet built on maize, but not the staff who ate more varied food, and it could be prevented by adding milk, meat or yeast. It was later traced to a lack of niacin, one of the B vitamins, and fortifying flour and meal largely ended it.

The puzzle was why the peoples of Mexico and Central America, who had eaten maize as a staple for thousands of years, rarely suffered from it. The answer lies in how they cooked it. Traditional preparation soaks and cooks the grain in an alkaline solution of lime or wood ash, a process called nixtamalisation, which frees niacin that is otherwise bound up and poorly absorbed. When maize spread to Europe, Africa and the American South without the technique, pellagra followed it. A kitchen method had been doing the work of a vitamin supplement for millennia.

The vitamin content printed in a food table is for the raw item, and what reaches your plate can be quite different. Water-soluble vitamins leak out of vegetables into cooking water, so boiling broccoli or cabbage for a long time and pouring the water away throws much of the vitamin C and folate down the sink. Heat also breaks some vitamins apart directly: vitamin C and thiamine are among the most fragile, while niacin and the fat-soluble vitamins survive ordinary cooking far better.

This is why steaming, microwaving with little water and shorter cooking times tend to keep more of the fragile vitamins, and why soups and stews, where the liquid is eaten, lose less overall. Light and air matter too. Riboflavin is broken down by light, which is one reason milk is often sold in opaque containers, and cut fruit slowly loses vitamin C as it is exposed to oxygen. None of this makes cooked food poor; cooking also releases some nutrients, such as the carotenoids in carrots and tomatoes, from tough plant cells so that the body can absorb them more easily.

Because the fat-soluble vitamins are stored rather than flushed out, they can build up to harmful levels, and the clearest examples come from the Arctic. Polar bear liver holds so much vitamin A that a single meal of it can poison a person, causing severe headache, drowsiness, vomiting and, days later, skin peeling away in sheets. Inuit and other Arctic peoples traditionally avoid eating it, and several European explorers who ignored the custom fell ill. The bear concentrates the vitamin from the seals it eats, which concentrate it from fish, so each step up the food chain stores more.

A grimmer episode may lie behind an Antarctic tragedy. In 1912 and 1913 Douglas Mawson and Xavier Mertz, stranded far from base after a companion fell into a crevasse with most of their food, survived by eating their sled dogs, livers included. Mertz died and Mawson lost the skin from the soles of his feet, and doctors later proposed vitamin A poisoning as a likely cause, though starvation and cold played their part. Vitamin D shows the opposite safeguard: you cannot overdose on it from sunlight, because the skin converts any excess into inactive compounds, so dangerous levels come almost only from supplements.

Two vitamins have a special connection to microbes. Vitamin B12 is made only by certain bacteria and archaea; no plant or animal can build it. Cattle and sheep obtain theirs from the microbes in their digestive systems, and it ends up in their meat and milk, which is why animal foods supply it and plant foods, unless fortified, do not. Its story began with a disease. Pernicious anaemia was once fatal, until doctors found in the 1920s that feeding patients large amounts of raw liver kept them alive, a discovery recognised with a Nobel Prize in 1934.

The active substance was finally isolated in 1948, and its structure, one of the largest and most intricate of any vitamin, built around a single atom of cobalt, was worked out by Dorothy Hodgkin using X-ray crystallography, work that contributed to her Nobel Prize in 1964. Vitamin K shows the other side of the partnership. Gut bacteria make a form of it, but a newborn baby starts life with almost no gut bacteria, and little vitamin K crosses the placenta or arrives in breast milk. That is why babies in many countries are offered a vitamin K injection at birth, which prevents a rare but dangerous bleeding disorder.

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.