Vitamins

Thirteen vitamins are essential, meaning the body cannot make them, or cannot make enough. They are nothing alike: the most abundant is counted in hundreds of milligrams a day, the rarest in micrograms, a thousand times less.

The figures come from EFSA, the European authority, as for Minerals. B1 and B3 are the exception: their need follows energy expenditure, 0.1 mg of B1 and 1.6 mg of B3 per megajoule, hence a range rather than a single figure.

Vitamins fall into two families, and that split explains almost everything else. Vitamins A, D, E and K dissolve in fat: they need a meal with some fat to cross the gut wall, and the body puts them in store, in the liver for A and D, in fat tissue for E.

Vitamin C and the eight B vitamins dissolve in water. The body keeps almost none, the surplus leaves in the urine, and they have to be eaten again often. They also leach into cooking water and take heat badly: a vegetable boiled for a long time loses a good share of them, steamed much less.

B12 is the exception that matters: water-soluble, yet stored in the liver for several years. That is why a deficiency takes so long to show, and why it is well established by the time it does.

Vitamin A is for seeing, first of all: in the retina, it is the moving part of the pigment that turns light into a nerve signal. It also directs the renewal of skin and mucous membranes, and much of immune defense, which makes it, worldwide, the deficiency that blinds the most children.

It comes in two forms. Retinol, in liver, butter, egg yolk and cheese, is usable as is. The carotenoids of plants, which color carrot, sweet potato, apricot and melon, have to be converted, which the body does at its own pace and only if the meal holds a little fat. Grated carrots without oil therefore give far less than their figure suggests.

Vitamin D commands the absorption of calcium in the gut. Without it, most of the calcium swallowed passes straight through, and bone loses its mineral. It is also the only vitamin the body makes itself, in the skin, from cholesterol and the ultraviolet in sunlight.

That manufacture stops in winter. At our latitudes, the sun from November to February is too low for its ultraviolet to get through the atmosphere: the skin makes nothing at all, however long you stay outside. You then live on your summer reserve, and it does not last until spring.

Food fills the gap badly, because very few foods contain any: oily fish, salmon, herring, sardine and mackerel, egg yolk, and cod liver oil. The 15 µg of the reference intake, incidentally, assume little or no manufacture by the skin.

Vitamin E protects fats from oxidation. It sits in the membrane of cells, which is made of fragile fats, and absorbs there the attacks that would otherwise turn the membrane rancid from the inside.

Hence a simple rule: the need follows the amount of polyunsaturated fat eaten, and it is precisely the foods richest in it, vegetable oils, seeds and nuts, that supply vitamin E. The fuel comes with its own extinguisher. Deficiency is therefore rare outside diseases that prevent fat absorption.

The K comes from Koagulation: this vitamin switches on the proteins that clot blood. It switches on others too, among them osteocalcin, which binds calcium into bone. It is found in green leaves, spinach, cabbage, broccoli, parsley, and in another form in fermented foods and animal products; gut bacteria make some of it.

Deficiency is rare in adults. One case calls for attention: anticoagulants of the vitamin K antagonist family work by blocking it, and an intake that changes abruptly unbalances the treatment. The instruction is not to avoid green vegetables, but to eat a steady amount of them.

Vitamin C is used to build collagen, the mesh that holds skin, blood vessels, gums and bone together. That is the whole story of scurvy at sea: without it, new collagen is no longer assembled, the old comes apart, gums give way and old wounds reopen. Ten or so milligrams a day are enough to avoid that, far below the 110 mg advised.

It has a second role, more useful day to day: taken in the same meal, it multiplies the absorption of plant iron, the kind that is poorly taken up. And it is fragile, destroyed by heat and carried off by cooking water.

The orange is not the champion it is taken for. Blackcurrant, red pepper and kiwi hold more, and broccoli does just as well, raw or lightly steamed.

B12 does two things: build red blood cells, and maintain the sheath that insulates nerves. When it is missing, red cells leave the marrow too large and badly finished, and nerves are damaged. The first effect can be corrected, the second not always.

It is made by bacteria, and turns up only in animal products: offal, meat, fish, shellfish, eggs, dairy. No plant contains any, neither seaweed nor yeast, whatever the labels claim. A vegan diet therefore requires a supplement, and that is not up for debate.

Its absorption is complicated: it requires a protein made by the stomach, intrinsic factor. With age the stomach makes less, and some common drugs, antacids and metformin, reduce its uptake. A deficiency is therefore possible while eating meat.

Folate is used to build DNA, and so to make new cells. The tissues that renew fastest depend on it first: bone marrow, gut, and the embryo. The name comes from folium, leaf, because it was first isolated from spinach.

It is the only vitamin for which a supplement is advised to all women who may become pregnant, and before they are: the neural tube, which becomes the brain and spinal cord, closes in the first four weeks, often before the pregnancy is known. The reference intake then goes from 330 to 600 µg.

Green leaves, legumes, avocado and liver are the best sources. Like vitamin C, folate takes heat badly and leaves in the cooking water.

They work together, in the same reactions, and largely come from the same foods: offal, meat, eggs, dairy, seeds, legumes and whole grains. Refining grains removes them along with the bran and the germ, which is why white bread and white rice are enriched in many countries.

B1, thiamin. It opens the door through which glucose enters the energy circuit, and nerves, being heavy glucose users, are hit first when it is short. Its deficiency, beriberi, appeared with polished rice, and is seen today mostly in heavy drinkers.

B2, riboflavin. It carries electrons in cell respiration. Milk and eggs are the main sources, and it is destroyed by light: that is why milk is no longer sold in clear bottles.

B3, niacin. It forms NAD, the carrier involved in nearly every energy reaction. The body can make it from the tryptophan in protein, at 1 mg for 60 mg, so that a decent protein intake already covers a good share of the need. Its deficiency, pellagra, struck populations fed on maize.

B5. It forms coenzyme A, through which both fat and carbs pass. Its name, pantothenic acid, comes from the Greek pantothen, from everywhere: it is in everything, and its deficiency practically does not exist.

B6. It is the tool of the reactions that transform amino acids, so the one whose need rises with protein. It is also the only water-soluble vitamin that is genuinely toxic at high doses: supplements taken for a long time at several hundred milligrams damage the nerves of the hands and feet.

B8, biotin. It attaches carbon in the making of fat and glucose. Raw egg white contains a protein that traps it, avidin, which cooking destroys. At very high doses, as a supplement, it distorts several blood tests, including thyroid ones.

Vitamin D in winter. The skin makes none from November to February, and food fills the gap badly. It is the vitamin where supplementing is genuinely discussed.

B12 without animal products. No plant contains any. Without meat, fish, eggs or dairy, the supplement is not optional.

Folate before pregnancy, not during: the neural tube closes in the first four weeks.

Cooking is expensive for vitamin C and the B vitamins, which leave in the water and dislike heat. Steaming keeps far more than boiling.

The fat in the meal is part of the calculation for vitamins A, D, E and K: vegetables with no fat give up less of them.

Excess comes from supplements, never from the plate, with two exceptions worth knowing: liver, very rich in vitamin A, and B6 at high doses.