Vitamins: a handy overview

Vitamins are a group of organic compounds needed in minute quantities (less than 1 per cent of our food is composed of vitamins). There are thirteen known vitamins essential for health and growth, and nine of them are grouped together as the B group or 'B-complex' vitamins. These are thiamin (vitamin B1), riboflavin (B2), niacin, pyridoxine (B6), cyanocobalamin (B12), folate, biotin, pantothenic acid and a recent addition choline. Not all carry the number designation as some were discovered and numbered then later found to be other compounds - not vitamins.

Fat-soluble or water soluble?

Vitamins are usually classified as fat-soluble or water-soluble:

• Fat-soluble vitamins (A, D, E and K) are stored in the body, so excess of these vitamins can cause unpleasant side-effects. They occur in association with food fats and are fairly stable during cooking and processing.
• Water-soluble vitamins (B group and C) dissolve in body fluids and excess intakes of most are excreted in the urine. They are distributed in many foods - vegetables, grains, meat and milk - and are often sensitive to heat.

Why do I need them?

Vitamins regulate the body's metabolism, by speeding up chemical reactions, acting as antioxidants or forming an integral part of the structure of proteins, hormones and blood. The body cannot produce them so they must be supplied by food.

Vitamins are needed in minute amounts for health, growth and to prevent vitamin-deficiency diseases such as:

• scurvy (lack of vitamin C)
• beri-beri (vitamin B1)
• pellagra (vitamin B2) or
• rickets (vitamin D).

Vitamins - food or supplements?

Ideally, it is best to obtain all your vitamins from food, not pills. Food provides vitamins in the most biologically-available form, in the right amounts and combined with other complementary nutrients. Except under special circumstances, from food there is low risk of overdose.

Vitamins work best together. Too much of one can disturb the balance of others. Vitamin C increases the absorption of iron; vitamin E acts together with selenium and vitamin C as anti-oxidants; folate supplements can mask the symptoms of B12 deficiency.

Vitamin deficiencies do not appear immediately even on a totally depleted diet. For example, our livers store enough vitamin B12 to last three to five years; vitamin C reserves can last for thirty days.

In recent years, however, attitudes to supplements have changed, as numerous studies have emerged suggesting that supplements, especially the antioxidant supplements, are associated with better health outcomes .

While there are certain groups who could benefit from a supplement (such as smokers, fussy eaters or the elderly), be wary of the over-enthusiastic promises made by vitamin sellers. Often the scientific evidence is not reliable, consisting of anecdotes from 'satisfied' patients or the people treating them.

Not all the claims for supplements have been proven. An often-quoted examination of studies into vitamin C and the common cold showed that it could slightly shorten the duration of colds - but not prevent them entirely.

Not magic bullets

If you do take supplements, think of them only as a ‘top up' to your daily diet. Supplements aren't ‘magic bullets' - they won't supply everything we get from good food (as many beneficial phyto-chemicals are not found in any pill) nor can they counteract a bad diet with too much fat or salt.

Often they leave you with a false sense of security so that you think you don't need to worry about the junk you've eaten.

Overdose of vitamins

For some vitamins, there's the possibility of overdose. There are potential dangers from excess doses of vitamins. Examples:

• Excess vitamin A is toxic and causes loss of appetite, nausea and a dry itchy skin - one to watch during pregnancy. 
• Excess beta-carotene can turn the skin and whites of the eyes an orange-yellow (which slowly disappears once the excess stops).
• Excess vitamin D causes calcium to be deposited in soft tissues, as well as vomiting and diarrhoea.
• Excess niacin causes flushing and interferes with liver function. This is checked for when patients are given high doses for lowering cholesterol. 
• Overdose of vitamin B6 produces a loss of sensation in the body's extremities (due to damage to the sensory nerves).

Units for measuring vitamin intakes

References used

In the accompanying articles on individual vitamins in the Foodwatch website, you'll see figures for the recommended day's intake.

• These figures are for adults. Children generally need less than adults due to their lower body weight.
• Recommended intakes are taken from the Nutrient Reference Values (NRV) for Australia and New Zealand, NH&MRC 2006.
• The Recommended Dietary Intake (RDI) is the amount that is estimated to cover the needs of practically all healthy people, although individuals vary in their true requirements.
• Upper Limit figures are sourced from the NRVs as above. The Upper Limit is the highest quantity you can take from either food or supplements without having to worry about any side effects.

©  Catherine Saxelby