Reaching for the dietary supplements seems like a quick and easy way to make up the nutrients we may be missing in our busy lives - and there are plenty of adverts encouraging us to do just that.

But a report in the latest issue of Harvard Women's Health Watch questions whether nutritional supplements help or hurt.

It warns that supplements may not always deliver the promised health benefits and that some can even be dangerous if people exceed the recommended allowances.

Studies have suggested the health benefits of vitamin D in defending the body against the likes of cancer, diabetes, depression and common colds.

Omega-3 fatty acids have been linked to preventing strokes, and antioxidants like vitamins C and E, and beta carotene have been recommended against heart disease and cancer.

But the Harvard report says there is a "big caveat" and that is that many of the studies conducted on supplements have been observational, meaning that they were not tested against a placebo in a controlled setting.

The observational studies also do not fully control variables like diet and exercise habits.

Observational studies of vitamin E concluded that it could protect the heart. However, more rigorous testing showed that it increased the risk of bleeding strokes.

Similarly, folic acid and other B vitamins were believed to prevent heart disease and strokes, but further studies failed to confirm that and even linked it to an increased cancer risk when taken in high doses.

"Often the enthusiasm for these vitamins and supplements outpaces the evidence. And when the rigorous evidence is available from randomised controlled trials, often the results are at odds with the findings of the observational studies," explains Dr JoAnn Manson, chief of preventive medicine at Brigham and Women's Hospital, professor of medicine at Harvard Medical School, and principal investigator of a large randomised trial known as VITAL (Vitamin D and Omega-3 Trial).

"People who take supplements tend to be more health conscious, exercise more, eat healthier diets, and have a whole host of lifestyle factors that can be difficult to control for fully in the statistical models."

The report adds that healthy foods like fruit, veg and fish contain many nutrients not found in a pill.

Dr Manson's recommendation is a daily intake of a variety of nutrients that are sourced from real food rather than supplements.

She says: "Usually it is best to try to get these vitamins and minerals and nutrients from food as opposed to supplements."