Millions of Americans pop multi-vitamin pills in a bid to stay in shape, although scientific evidence on whether they help protect against chronic illnesses remains inconclusive.

"The researchers found no association between multivitamin use and the risk of localized prostate cancer.

"But they did find an increased risk of advanced and fatal prostate cancer among men who used multivitamins more than seven times a week, compared with men who did not use multivitamins," the study said.

Some 295,343 men and their eating habits were followed in the study conducted at the National Cancer Institute in Bethesda, Maryland.

About a third said they took multi-vitamins on a daily basis, with five percent saying they took large quantities of tablets, more than seven pills a week.

In the five years since the start of the study, 10,241 of the volunteers were diagnosed with prostate cancer, 8,765 with a localized tumor and 1,476 with an advanced form of cancer. A total of 179 died from the disease.

The incidence of the illness was higher in those men who had a family history of prostate cancer and "men who also took selenium, beta-carotene, or zinc supplements," the study said.

A recent Danish survey cross-referencing 47 studies indicated that taking vitamins A and E or beta-carotene could shorten a person's life, and increase mortality rates by 5 percent.