A large scale US study of postmenopausal women found no convincing evidence that long term use of multivitamins changes the risk of developing common cancers, cardiovascular disease or premature death.

The study was led by researchers from the Fred Hutchinson Cancer Research Center in Seattle, Washington and is published in the 9 February online issue of the Archives of Internal Medicine.

The authors wrote that millions of postmenopausal women take multivitamins, many of them in the hope that they will prevent chronic diseases like cancer and cardiovascular disease. They decided to investigate whether there was any evidence to support this.

In a separate statement, lead author Dr Marian L Neuhouser, an associate member of the Public Health Sciences Division at the Hutchinson Center, said:

"Dietary supplements are used by more than half of all Americans, who spend more than 20 billion dollars on these products each year. However, scientific data are lacking on the long-term health benefits of supplements."

For the study, Neuhouser and colleagues included 161,808 women taking part in the Women's Health Initiative clinical trials: just over 40 per cent were in overlapping trials of hormone therapy, dietary interventions, and calcium and vitamin D supplements, and just under 60 per cent of them were taking part in an observational study (ie they were going about their normal day to day lives with no particular interventions, and those who took multivitamins did so from personal choice).

The Women's Health Initiative is one of the largest US prevention studies of its kind and is looking at the most common causes of death, disability and impairments to quality of life in postmenopausal women.

The researchers collected detailed data on all participants at the start of the study (baseline) and at intervals throughout the study period. The women enrolled between 1993 and 1998, and were followed for a median of 8 years in the clinical trials and 7.9 years in the observational study.

Incidence of mortality and disease were noted up to 2005, including invasive breast cancers, cancers of the colon/rectum, kidney, bladder, stomach, ovary, and lung. The researchers also noted cases of cardiovascular disease, such as myocardial infarction (heart attack), stroke, and venous thromboembolism (blood clot).

The results revealed:

* 41.5 per cent of all the women in the study used multivitamins.

* Multivitamin users were more likely to be white, reside in the western United States, have a lower BMI (body mass index), be more physically active and have a higher level of educational attainment compared to non-users.

* Compared to non users, multivitamin users were also more likely to drink alcohol and less likely to smoke, and they reported eating more fruits and vegetables and eating less fat than non-users.

* After the follow up period (median of 8.0 years in the clinical trial and 7.9 years in the observational study), there were: 9,619 cases of cancer (breast, colorectal, endometrial, kidney, bladder, stomach, lung and ovarian); 8,751 cardiovascular events; and 9,865 deaths.

* A statistical tool called multivariate adjusted analysis showed no link between multivitamin use and risk of cancer, cardiovascular disease and death (none of the hazard ratios were very far from 1.00, with 95 per cent confidence intervals ranging from 0.85 to 1.29 for venous thromboembolism and 0.91 to 1.05 for breast cancer).

The authors concluded that:

"After a median follow-up of 8.0 and 7.9 years in the clinical trial and observational study cohorts, respectively, the Women's Health Initiative study provided convincing evidence that multivitamin use has little or no influence on the risk of common cancers, CVD [cardiovascular disease], or total mortality in postmenopausal women."

Neuhouser said:

"To our surprise, we found that multivitamins did not lower the risk of the most common cancers and also had no impact on heart disease."

The results are consistent with those of other studies that also found no health benefits from using multivitamins. However this Women's Health Initiative study is more definitive said Neuhouser, because:

"The Women's Health Initiative is one of the largest studies ever done on diet and health. Thus, because we have such a large and diverse sample size, including women from 40 sites across the nation, our results can be generalized to a healthy population."

But because the study did not involve men, you can't say the results apply to them.

Neuhouser suggested that women concentrate on getting their nutrients from food rather than supplements:

"Whole foods are better than dietary supplements. Getting a wide variety of fruits, vegetables and whole grains is particularly important," she said.