Under the Microscope:Vitamins are essential to bodily health and if you don't have enough of a particular vitamin you will suffer from a deficiency disease. However, a normal healthy diet pretty much provides all the essential vitamins in sufficient amounts. There is little evidence that taking supplementary vitamins is good for your health. For example a 13-year follow-up of 10,758 participants in an American survey found that cancer and overall mortality were similar in regular users and non-users of supplements ( Nutrition Review, 52, 216-219, 1994).
The history of vitamins illustrates medical science at its best. In 1747 the Scottish naval surgeon James Lind was the first to test the notion that scurvy - swollen and bleeding gums, loose teeth and haemorrhaging under the skin - an eventually fatal condition, was caused by a deficiency in the diet. Scurvy often developed in sailors on long voyages. Lind did an experiment, one of the first in the history of medicine, in which he gave half the sailors lemons and denied lemons to the other half. Only the sailors who didn't eat lemons developed scurvy. We now know that the protective factor in the lemon, and other citrus fruits, is vitamin C. Gradually over the years each of the other vitamins was discovered and each, in very small amounts, was shown to prevent a deficiency disease such as rickets, pellagra, beriberi, anaemia, and so on.
Each of the vitamins plays an important role in bodily biochemistry. For example, several vitamins act as co-enzymes. A co-enzyme is a factor that is necessary for an enzyme to work and enzymes are the essential catalysts that speed up the chemical reaction that take place in our cells. Life would not be possible without enzymes.
Vitamins can be divided into two broad categories - water-soluble and fat-soluble vitamins. The water-soluble vitamins include vitamin B1 (Thiamine), vitamin B2 (niacin, nicotinic acid), vitamin B6 (Pyridoxine), vitamin B12, vitamin C (ascorbic acid) and folic acid. The fat-soluble vitamins include vitamin A, beta carotene (forerunner of vitamin A), vitamin D, vitamin E, vitamin K.
When taken in excess, water-soluble vitamins are immediately excreted in the urine. If you take large supplementary doses of these vitamins you will quickly pee your money down the toilet. With the exception of beta carotene, excessive doses of fat-soluble vitamins will be stored in body fat and slowly released into blood, causing high blood levels. Large doses of fat soluble vitamins should be avoided as they can be harmful.
Our cells generate necessary energy by "burning" food in oxygen (oxidation). Chemicals called free radicals are automatic by-products of this essential activity. Free radicals are dangerous chemicals that could damage the cell and, so, the cell has an efficient mechanism for neutralising them using its own natural anti-oxidants. Some vitamins are anti-oxidants, mainly vitamin C, vitamin A and vitamin E.
Antioxidants are present naturally in many foods, principally plant foods - dark green, yellow and orange vegetables, and in fruits. Several studies have shown that risk of cancer and heart disease is significantly reduced when people eat a diet high in fruits and vegetables. But what about taking supplementary vitamin pills?
Well, the latest large study, referenced at the start of this article, was designed to answer this question, at least for the antioxidant vitamins and selenium (a trace element that has anti-oxidant properties). This study was a meta-analysis, that is an analysis of a large number of previous studies that addressed this problem. They examined the effects of antioxidant supplements Beta Carotene, vitamins A, E and C and selenium on all-cause deaths of adults in prevention trials. Beta carotene, vitamin A and vitamin E were associated with 7 per cent, 16 per cent and 4 per cent increased risk of mortality respectively. No increased mortality risk was seen with vitamin C or selenium use, but there was no evidence that vitamin C increases longevity.
These findings clearly contradict claims that antioxidants improve health. The authors say: "Considering that 10 per cent to 20 per cent of the adult population (80-160 million) in North America and Europe may consume the assessed supplements, the public health consequences may be substantial".
There are several possible explanations for the negative effect of antioxidant supplements on mortality. One possible explanation is that supplementary anti-oxidant vitamins effectively neutralise almost all the naturally-arising free radicals. A certain level of these free radicals may be necessary to adequately stimulate the natural anti-oxidant defence mechanisms
in the cell. In the absence of sufficient stimulation the natural defence mechanism may become flaccid and inefficient, thereby causing a problem.
I think the bottom line is that the average person eating a balanced diet, with adequate fruit and vegetables, has no need of vitamin supplements. Pregnant women need to be careful that they are getting enough folic acid and should talk to their GP about this. Otherwise we needn't worry. The normal body has highly sophisticated mechanisms for looking after itself. Clumsy attempts to artificially help these mechanisms may do more harm than good.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC - understandingscience.ucc.ie