D is for cancer, C is for colds, B is for behaviour... As claims about supplements proliferate, Jeremy Laurance offers a guide to the alphabet soup of key vitamins

VITAMIN A

Health of the skin and mucous membranes (in the nose, for example). Boosts immunity from infections and helps night vision. Is also known as retinol.

SOURCES

Cheese, eggs, oily fish, milk, fortified margarine and yogurt.

HOW MUCH YOU NEED

0.7mg a day for men and 0.6mg for women. Being fat-soluble, you don't need it every day, as it can be stored in the body.

CAN YOU OVERDOSE?

Yes. Some research suggests more than 1.5mg a day over many years may weaken the bones. In pregnant women, large amounts may also harm their unborn baby. People who eat liver more than once a week or take supplements containing vitamin A should beware.

VITAMIN B6

ESSENTIAL FOR

Formation of haemoglobin, the constituent of red blood cells that carries oxygen around the body. Also allows the body to use and store energy from protein and carbohydrate.

SOURCES

Many foods including pork, chicken, cod, bread, eggs, vegetables, milk, potatoes and fortified breakfast cereals.

HOW MUCH YOU NEED

1.4mg a day for men and 1.2mg for women. It is water-soluble, so it is needed in the diet every day as it cannot be stored.

CAN YOU OVERDOSE?

Large amounts - more than 200mg a day - cause loss of feeling in the arms and legs, known as peripheral neuropathy. In some cases this has proved irreversible. The Food Standards Agency advises against taking more than 10mg a day.

VITAMIN B12

ESSENTIAL FOR

Making red blood cells, keeping the nervous system healthy, releasing energy from food and processing folic acid (see below). Some experts controversially say that a lack of B12 may explain behavioural problems in children. Injections are given to elderly people suffering from pernicious anaemia, which affects absorption of the vitamin.

SOURCES

Virtually all meat products and certain algae such as seaweed. Also cod, salmon, milk, cheese and eggs.

HOW MUCH YOU NEED

0.0015mg a day for adults. It is water soluble, so is needed every day. It is not present in vegetables so vegans risk becoming deficient.

CAN YOU OVERDOSE?

The effects of large doses are unknown. The FSA says that a dose of up to 2mg a day is unlikely to cause harm.

VITAMIN C

ESSENTIAL FOR

Protecting cells and keeping them healthy, and helping the body to absorb iron from food. Some claim that megadoses of 1g (1,000mg) or more can ward off colds, but this is unsubstantiated.

SOURCES

Fruit and vegetables. Good sources include broccoli, peppers, Brussels sprouts, sweet potatoes, oranges and kiwi fruit.

HOW MUCH YOU NEED

40mg a day for adults. It is water-soluble so is needed every day. Excess to the body's needs is excreted. Critics of winter vitamin C supplements say they only create "expensive urine".

CAN YOU OVERDOSE?

Large amounts taken as supplements can cause stomach pain, flatulence and diarrhoea. Up to 1,000mg a day is unlikely to do harm, according to the FSA.

VITAMIN D

ESSENTIAL FOR

Regulating the amount of calcium and phosphates in the body (needed for healthy bones and teeth). Lack of vitamin D is linked to certain cancers, heart disease, diabetes, multiple sclerosis and schizophrenia.

SOURCES

Ninety per cent of the body's supply comes from the action of sunlight on the skin. Good food sources of it are oily fish and eggs.

HOW MUCH YOU NEED

0.01mg a day for pregnant women, the elderly, Asians, and people who avoid the sun. 0.025mg may halve the risk of cancer of the breast, colon and ovary.

CAN YOU OVERDOSE?

Very large doses may interfere with the absorption of calcium and weaken the bones. The FSA says supplements the provide up to 0.025mg a day are unlikely to cause harm.

VITAMIN E

ESSENTIAL FOR

Protecting cell membranes by acting as a powerful antioxidant. Prevents degenerative conditions including heart disease, arthritis and possibly cancers.

SOURCES

Found in a variety of foods. Good sources are avocado, corn, olive and soya oils, nuts and seeds and wheatgerm found in breakfast cereals.

HOW MUCH YOU NEED

4mg a day for men, 3mg a day for women. It is fat soluble so can be stored.

CAN YOU OVERDOSE?

Few toxic effects in large doses but the long-term outcome is unknown. Research suggested high-dose supplements might cut the risk of heart disease but studies have proved disappointing. The FSA says supplements up to 540mg a day are unlikely to cause harm.

VITAMIN K

ESSENTIAL FOR

Blood clotting and helping wounds to heal, building strong bones. Babies are given an injection of vitamin K to prevent bleeding.

SOURCES

Green leafy vegetables (broccoli and spinach), vegetable oils and cereals.

HOW MUCH YOU NEED

0.001mg a day per kg of body weight for adults. It is fat soluble, so is not required every day.

CAN YOU OVERDOSE?

Little is known about the effect of large doses. The FSA advises against taking more than 1mg of vitamin K a day.

VITAMIN Biotin

ESSENTIAL FOR

Turning food into energy. It is one of the B group of vitamins.

SOURCES

Meat (especially kidney and liver), eggs and some fruit and vegetables, especially dried mixed fruit.

HOW MUCH YOU NEED

Up to 0.2mg a day for adults. It is water -soluble, so is required every day.

CAN YOU OVERDOSE?

Little is known about the effect of large doses. The FSA advises against taking more than 0.9mg a day.

VITAMIN Folic Acid

ESSENTIAL FOR

Forming red blood cells (with vitamin B12). Also reduces the risk of neural tube defects (spina bifida) in unborn babies. It is one of the B group vitamins.

SOURCES

Broccoli and Brussels sprouts, peas, yeast extract, brown rice and some fruit.

HOW MUCH YOU NEED

0.2mg a day. It is water-soluble. Pregnant women should take 0.4mg from the time they stop using contraception to the 12th week of pregnancy.

CAN YOU OVERDOSE?

High doses can mask vitamin B12 deficiency which may eventually damage the nervous system if it goes unnoticed. The FSA advises against taking more than 1mg a day.

VITAMIN Niacin

ESSENTIAL FOR

Breaking down fat, protein and carbohydrate in the diet. Keeping both the digestive and nervous systems healthy.

SOURCES

Beef, pork, chicken, wheatflour, cornflour, eggs and milk.

HOW MUCH YOU NEED

17mg a day for men, 13mg a day for women. It is water-soluble, so it is needed every day.

CAN YOU OVERDOSE?

High doses can cause skin flushes in the short term and liver damage in the long term. The FSA advises against taking more than 17mg a day of nicotinic acid supplements (or 500mg of nicotinamide supplements).

VITAMIN Pantothenic Acid

ESSENTIAL FOR

Helping to release energy from food, like the other B vitamins.

SOURCES

Virtually all meat and vegetables, and especially beef, chicken, potatoes, porridge, tomatoes and eggs.

HOW MUCH YOU NEED

No set recommended amount but adults should be able to get all they need from their daily diet. It is water soluble so is needed every day.

CAN YOU OVERDOSE?

There is no good evidence that high doses are harmful. The FSA advises against taking more than 200mg a day.

VITAMIN Riboflavin

ESSENTIAL FOR

Keeping the skin, eyes, nervous system and mucous membranes healthy, producing red blood cells, and helping the body to absorb iron. Also known as vitamin B2.

SOURCES

Milk, eggs, rice, mushrooms and fortified breakfast cereals.

HOW MUCH YOU NEED

1.3mg a day for men, and 1.1mg a day for women. It is water-soluble so is needed every day.

CAN YOU OVERDOSE?

No good evidence that high doses are harmful. The FSA advises against taking more than 40mg a day.

VITAMIN Thiamin

ESSENTIAL FOR

Keeping nerves and muscle tissue healthy, and breaking down protein, fat and carbohydrate.

SOURCES

Pork, vegetables, milk, cheese, fruit, wholegrain bread and fortified breakfast cereals.

HOW MUCH YOU NEED

1mg a day for men and 0.8mg a day for women. It is water soluble, so is required every day.

CAN YOU OVERDOSE?

No good evidence that high doses are harmful. The FSA advises against taking more than 100mg a day.

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