Vitamin supplements and cancer risk
Dietary supplements such as vitamin and mineral tablets have become widely available in Australia and may be taken for a variety of reasons. Some people are advised by their doctor or dietitian to take dietary supplements if they have certain medical conditions, are pregnant or have a restricted dietary intake. However, the majority of people do not need dietary supplements if they eat a wide variety of nutritious foods.
Cancer Council recommends people eat a variety of nutritious food especially fruit and vegetables.
Supplements versus whole foods
Getting nutrients from tablets is not as good as getting them naturally from food. Foods like fruit and vegetables contain lots of vitamins, but also many other beneficial substances such as fibre, minerals and antioxidants. Dietary fibre can help to ensure a healthier digestive system and reduce the risk of bowel cancer, while antioxidants help protect against the damaging effects of free radicals in the body. Therefore eating whole foods is best to obtain maximum benefits.
In addition, research has shown that some vitamins that are taken as supplements may not have the same effect as when they’re consumed in food. This is because the effect of the vitamin may be due to interactions between different food compounds that don’t occur when vitamins are isolated in a supplement.
Eating nutritious foods like fruit and vegetables can also help you to maintain a healthy body weight.
Supplements and cancer risk
Studies suggest that people who eat a diet high in plant foods such as fruit, vegetables and wholegrain cereals have a lower risk for some types of cancer.
However in most cases it is not known which compounds (or combination of compounds) is best when it comes to lowering the risk of cancer. There are likely to be important, but as yet unidentified, components of whole food that are not included in supplements.
Currently there is no evidence to suggest that supplements (e.g. tablets, capsules) can reduce cancer risk. The few clinical trials testing whether supplements can reduce cancer risk in humans have had disappointing results. Any link between diet and cancer is far more complex than simply adding vitamin supplements to your existing diet.
Some studies suggest that certain antioxidant supplements such as beta-carotene are not protective, and may in fact increase overall mortality.
Beta-carotene is the orange colour pigment found in vegetables and fruit and is converted into vitamin A in the body. Normal levels of beta-carotene from foods such as dark-green leafy vegetables, carrots, sweet potato, mango and dried apricots may help protect against cancer.
However studies have shown that there is a convincing link between beta-carotene supplements and the risk of lung cancer in people who smoke cigarettes.
Therefore it is possible that the protective effect of beta-carotene at dietary intake amounts is lost or reversed with dietary supplementation and the higher levels that this can supply.
Cancer Council recommends people avoid taking high doses (>18 mg) of beta-carotene supplements, especially if they smoke.
Vitamin A (retinol) is obtained from foods in two ways: preformed from animal food sources and derived from beta-carotene in plant foods. Vitamin A supplements, whether in the form of beta-carotene or retinol, have not been shown to lower cancer risk, and high-dose supplements may be toxic. Long-term use of high dose supplements increases the risk for lung cancer, particularly amongst people at high risk such as smokers.
Vitamin C is found in many vegetables and fruits. Many experimental studies have linked consumption of foods rich in vitamin C to a reduced risk for cancer. However the few studies in which vitamin C has been given as a supplement have not shown a reduced risk for cancer. High doses of vitamin C (more than 2000mg/day) can cause a number of side effects such as headaches, diarrhoea, nausea, heartburn, stomach cramps and possibly kidney stones.
Vitamin E is an antioxidant found in foods such as whole grains, seeds, nuts and vegetable oils. Studies of vitamin E supplementation and cancer risk have shown inconsistent results (positive effects and no effects). High doses of vitamin E supplements can cause certain side effects and should be avoided.