Vitamin C (ascorbic acid) is the world's best-selling supplement. It is also possibly the most controversial. An antioxidant, it fights free radicals, which can cause cancer, cardiovascular and other diseases.
Natural sources: Citrus fruits, strawberries, kiwi fruit, tomatoes, broccoli and Brussel sprouts.
Claims: Nobel prize-winning chemist Linus Pauling asserted in 1970 that Vitamin C can prevent and cure the common cold and cancer. He cited studies in which people taking 1,000mg daily had 45 per cent fewer colds than those who received no supplement. He insisted later that high doses of Vitamin C were effective against cancer and heart disease and that it slowed down the process of aging.
Evidence: More than 100 studies testing the efficacy of Vitamin C on the common cold have been contradictory. Some found no effect at all, while others found that, with patients experiencing acute physical stress, the incidence of colds was reduced by one half when they took Vitamin C; a few studies showed a one-third reduction among healthy subjects. Three controlled studies showed a dramatic reduction in pneumonia among people taking Vitamin C. But in research carried out on cancer patients, there was no difference in the outcomes between those receiving a daily dose of 10,000mg of Vitamin C and those given a placebo. But 10,000mg is a huge dose and, in excess, Vitamin C may act as a pro-oxidant and make the condition worse.
Conclusion: Taking a maximum of 1,000mg daily of Vitamin C is unlikely to do any harm and may boost the immune system against viral infections. But as yet there is no proof that it plays a part in reducing cardiovascular disease or cancer.
Vitamin E is a powerful fat-soluble antioxidant, protecting cell membranes from free radical attack. It also protects against heart disease.
Natural sources: Vegetable oils and soy beans.
Claims: It boosts the immune system, heals wounds, reduces the risk of cancer and heart disease and enhances male virility.
Evidence: There are eight different types of Vitamin E - the alpha-tocopherol form appears to be the most beneficial, according to research. High doses (up to 10 to 30 times the recommended daily allowance set by the US Food and Drug Administration) are shown to protect the heart, lungs and eyesight, and can prevent cancer.
Conclusion: It is impossible to consume from food alone, the high levels of Vitamin E necessary for health benefits. But there are many different kinds of supplement, and the beneficial effects can be reversed if one type is used together with another, as they have a tendency to cancel each other out. A study published in the medical journal The Lancet last year concluded that "our results do not support the routine use of vitamin E".
Calcium is essential for strong bones and teeth as well as for blood-clotting, maintaining normal blood pressure and for metabolic processes such as controlling hormone and enzyme production.
Natural sources: dairy products, broccoli, bokchoy, kale and sesame seeds.
Claims: It lowers blood pressure, strengthens bones and thereby helps to prevent osteoporosis, reduces risk of colon cancer, reduces premenstrual symptoms.
Evidence: Studies show that eating foods rich in calcium reduces the incidence of colon cancer. It is responsible for building bone, but cannot rebuild bone that has been destroyed by osteoporosis. Calcium's role in preventing osteoporosis, therefore, needs to start in childhood and children need to obtain peak bone mass with sufficient calcium intake and physical activity.
Conclusion: Calcium supplementation of around 1,000mg daily from 10 to 50 years of age is good for bones, teeth, blood pressure, blood clotting and hormone secretion, reduces the risk of colon cancer and helps alleviate premenstrual symptoms.
Omega-3 is a group of highly polyunsaturated fatty acids that regulate blood pressure, blood clotting and other functions of the heart and stimulate growth hormone secretion.
Natural sources: Oily or fatty fish such as salmon, mackerel, tuna (fresh, not tinned), sardines, walnuts, wheat germ oil and flaxseed.
Claims: It protects the heart, suppresses inflammation, promotes muscle growth.
Evidence: There is strong evidence that a diet rich in Omega-3 can protect against heart disease, colitis and other inflammatory diseases, kidney diseases, rheumatoid arthritis, pre-eclampsia in pregnancy and help in the treatment of attention deficit disorder. It also appears to control mood swings in depressive patients.
Conclusion: Eating oily fish just once a week is as effective as more frequent consumption, and confers cardiovascular protection.
Probiotics are so-called "good" bacteria that exist in the digestive tract.
They work with the immune system to ward off bugs such as salmonella, as well as to combat yeast infections and prevent cancer of the gastrointestinal tract. There are different types, the most well known being lactobacillus acidophilus, bifidus and bulgaricus. They have recently started appearing on supermarket shelves in the guise of "functional foods", contained in yoghurts and drinks.
Natural sources: Live cultured yoghurts.
Claims: They keep the digestive system healthy, boost resistance to illness, reduce cholesterol and prevent colon cancer.
Evidence: A significant improvement is found in resistance to viral and bacterial infections with regular use of probiotics in a number of animal and human studies. Bifidobacteria appear to be effective in fighting a number of food-poisoning bacteria, including Escherichia coli.
Lactobacillus GG is effective in reducing the severity of some kinds of diarrhoea. In addition, acidophilus has been shown to disable certain carcinogens in the digestive tract. There is no conclusive evidence that it prevents cancer.
Conclusion: As their beneficial effects are temporary, you need to consume probiotics daily for them to make a difference to the health of your digestive and immune system, whether in natural form such as live yoghurts or as functional foods.
The conclusion of all of this is that unless we have seriously depleted diets, we probably are not going to come to any harm if we do not supplement them. But whether we will enjoy optimum health - well, that depends on who you talk to, what you read and your own attitude.
Safe upper limits were set by the Food Standards Agency last year on eight supplements that could cause damage if taken over a long period of time in high quantities. Beta-carotene, manganese, zinc, niacin, phosphorus, calcium, Vitamin C, iron and chromium picolinate were singled out, based on extensive independent research carried out by the FSAExpert Group on Vitamins and Minerals. According to the Health Supplements Information Service, the limits set allow for the safe continuation of those supplements formulated to 100 per cent of the recommended daily allowance.
"These levels still fall considerably below the upper safe limits recommended by the Expert Group on Vitamins and Minerals for daily use."