Can antioxidants prevent cancer or do they, to the contrary, promote it? What if you already have cancer? Here's what you should know

Normal body functions such as breathing or physical activity, as well as lifestyle habits such as smoking and drinking alcohol, all produce free radicals that attack healthy cells. When your cells are weakened by free radicals, they are more susceptible to developing certain types of cancers.

Antioxidants are a broad range of compounds that destroy free radicals. Many of these compounds are present in the diet as vitamins. The best known antioxidants are vitamin A, vitamin C and vitamin E.

Getting the right amount of antioxidant vitamins

The best way to get antioxidant vitamins is through healthy eating habits.

Fruits and vegetables contain essential vitamins, minerals, fibre and other nutrients including antioxidants. A good start to optimal health is to eat at least 5 servings of fruits and vegetables a day (you can think of a "serving" as about a fistful).

Individuals who have diets rich in fruits and vegetables may have a reduced risk of cancer compared with those who eat a much higher proportion of high-fat and processed foods.

Antioxidant vitamins in cancer prevention studies

Numerous studies have examined the association between dietary antioxidant intake and the risk of cancer. The results of recent studies are summarised below. As shown, a major surprise of these studies has been that antioxidant supplementation actually appears to be harmful in some disease groups.

Antioxidant supplementation and cancer prevention: Randomised trials

Experimental InterventionSample SizeDuration Of StudyCharacteristics Of ParticipantsStudy Outcome
B-carotene: 15mg/day Vit E: 30mg/day Selenium: 50mcg/day29,5846 yearsMalnourished Age: 40 - 69Lower cancer rate
B-carotene: 20mg/day compared to atocopherol: 50mg/day29,1335 - 8 years Average 6 yearsMale smokers Age: 50 - 69 B-carotene: 20mg/day associated with increase in lung cancer
B-carotene: 30mg/day and  retinyl palmitate: 25 000IU/day18,3144 yearsMale and female Age: 45 - 74 Smoker and/or occupational exposure to asbestosNo clear evidence  of benefit. Associated with more death
B-carotene: 50mg on alternate days22,071

11.6 - 14.2 years, average 12 years

Male Age: 40 - 84 Current/former or non-smokersNo clear evidence of benefit or harm related to cancer risk
B-carotene: 50mg on alternate days 39,876 0.00-2.72 years, median 2.1 yearsFemale Age: > = 45 No significance difference in incidence of cancer
 B-carotene:  25mg/day Vit C: 1000mg/day Vit E: 400IU/day8644 years

Adenoma diagnosed Good health

Age: < 80

No clear evidence of benefit
B-carotene:  50mg/day 1,805

5 years

Recent non-melanoma skin cancer

 No clear evidence of benefit

Ref: U11