Several studies presented at a meeting of the American Association for Cancer Research show that supplements - ranging from ordinary multivitamins to specially formulated vegetable-based capsules - can have a positive impact in combating the disease.

In one study, Dr Omar Kucuk and colleagues at the Barbara Ann Karmanos Cancer Institute in Detroit found tomato supplements helped slow prostate cancer in men diagnosed with the disease.

The supplements, made by LycoRed Natural Products in Beersheva, Israel, contained lycopene, the chemical that makes tomatoes red.

Tomatoes contain a cancer-fighting substance

Lycopene, which survives cooking and is especially concentrated in tomato sauce and tomato paste, is a known antioxidant and a member of the carotenoid family of nutrients that include beta-carotene and vitamin A.

It cancels out the effects of free radicals, charged particles which damage the body's genetic material and can lead to cancer.

Dr Kucuk's team studied 30 men aged 60 to 80 who had prostate cancer and were waiting to have surgery.

Half received 30 mg of the supplements a day, and half received dummy pills.

Their tumours were then examined after surgery for levels of prostate-specific antigen (PSA), a chemical that is produced by prostate cancer cells.

PSA is not only a marker for prostate cancer but can also predict its severity.

The men who took lycopene had lower PSA levels in their tumours, which were also smaller than those in men who took the dummy pills.

Dr Kucuk said: "This shows that lycopene may not only be possibly preventive for prostate cancer but may in the future play a role in treatment."

Dr Kucuk said other tomato products in the supplements may also have acted on the tumours.

Head and neck cancer

In a second study of 135 patients Bruce Trock and colleagues at Georgetown University Medical Centre in Washington, found that men who took vitamin supplements and later developed head and neck cancer experienced a less agressive form of the disease.

Dr Trock said previous studies had shown 60% of men with head and neck cancers - which affect heavy smokers and drinkers and especially African-American men - have mutations in a gene called p53, which is known to help control the mechanisms that lead to cancer.

"Those patients who had been regular users of vitamin supplements prior to the development of disease had a three-fold reduction in the mutation of p53," Dr Trock said.

When doctors checked to see which patients had taken known antioxidants, such as vitamins A, C and E, the effect was even stronger, with a four-fold reduction in the mutations.

Dr Trock said having fewer genetic mutations could translate into a milder version of cancer.