A comprehensive review of clinical trials involving a wide range of popular dietary supplements has found that with the exception of vitamin D and omega-3 fatty acids, there is little evidence to support their use in Western countries by the general population.
Dietary supplements are big business, with around half of the Australian population using at least one type per year; most commonly a multivitamin and mineral pill. Many people take supplements as a form of dietary insurance in case they are not meeting their nutrient needs from foods alone. Others take them as a form of health insurance – to protect against certain diseases. Some just take them out of habit.
Supplements do have a role to play in some situations. People with a diagnosed deficiency, those with malabsorption conditions, women planning pregnancy, and people with very poor diets all can benefit from specific nutrient supplementation.
But what about the benefit of supplements across the broad population? The mantra of most health professionals, which is even etched on the label of many supplements, is that supplements may only be of assistance if dietary sources are lacking. To see if this is supported by solid evidence, a systematic review  of 63 clinical trials involving some form of supplementation in non-pregnant adults was carried out by a team of medical researchers.
The clinical trials were broad in their nature with many nutrients studied including vitamins A, B6, B12, C, D and E; folic acid; calcium; selenium, omega-3 fatty acids; ginkgo biloba; glucosamine; saw palmetto; and milk thistle. All of the studies were randomised controlled trials and evaluated a range of health outcomes including risk of heart disease, cancer, diabetes, bone fractures, cognitive function and all-cause mortality.
What the review uncovered
From the 63 studies, no benefit was seen in 45 studies with 10 of these showing a trend towards harm and just two showing a trend towards a benefit. Of the remaining studies, four showed harm with a greater risk of cancer with beta-carotene and a higher risk of fractures with a once-yearly dose of vitamin D.
One study showed a lower risk of cancer with selenium supplementation, but this was not validated in much larger trials. Twelve studies showed a benefit and these mostly involved vitamin D supplementation in elderly people or omega-3 fatty acids in people with cardiovascular disease.
So what to make of it all? Even before this review, the evidence linking omega-3s and a range of health benefits has been very well documented and its use is supported by many peak health bodies such as the Heart Foundation. Vitamin D deficiency is now recognised as a major health issue across many populations so supplementation to correct known deficiencies unsurprisingly elicits positive health outcomes in most cases. As for the case for ‘the rest’ of the popular dietary supplements, there is little positive evidence for their widespread use across and there is reasonable evidence that in some cases their use may be harmful.
What is all means
In some clinical conditions, dietary supplements do have a role to play, but for most people they appear to offer little benefit. A varied diet will contain far more beneficial micronutrients than could possibly fit into a pill, and with food variety comes insurance for getting some of all the nutrients needed, but with little risk of overdosing.
Confused about the mixed soup of nutrition messages being stirred through the media? Tim maintains an active nutrition blog at www.thinkingnutrition.com.au  where you'll find the latest nutrition research and controversies discussed in straight forward language, distilling out what you need to know for your better health.