Do you frequently take a multi-vitamin supplement as an insurance policy on your health? Many do; estimates suggest about one in three people regularly take them.

But what is a multi-vitamin supplement and are they needed to be healthy? Here's the evidence.

What is a multi-vitamin?

From a scientific perspective, the term 'multi-vitamin' is vague. They are supposed to contain most essential vitamins and minerals at levels close to recommended daily requirements, but some define them with just three different vitamins and minerals present.

There is no standard regulatory definition of what nutrients or what level a multivitamin must contain. This makes it difficult to generalise on their benefits as they also tend to be formulated differently for children, adults, men, women, pregnant women and older adults.

Are there health benefits?

Science shows people who take multivitamins already tend to have higher micronutrient intakes from their diet than nonusers. And these same people are more likely to have a higher education level, higher incomes, lower body mass index and higher physical activity levels – all factors that are linked to better health.

There is indeed some validity to the claim that many users of multivitamins sit within the 'worried well' group and are already quite healthy to start with.

So do multivitamins give an additional boost to health outside of what diet and lifestyle can offer? Most of studies so far can't prove definitively if taking multivitamins does, or does not, have a health benefit. The problem is that people who take these supplements are more likely to have healthier diets and lifestyles.

But if you look at the observational research, some studies suggest a benefit, some show adverse health problems and others no benefit. Very much a mixed bag indeed.

Evidence is mixed on their benefits

One of the largest observational studies looking at multivitamins and health involved over 160,000 postmenopausal women aged 50 to 79 years. They were all part of the Women's Health Initiative study that explored health markers and risk of cancer, heart disease and osteoporosis.

Over 40 percent of women were taking a multivitamin supplement, yet over the 8 years of the study, there was no link between taking these supplements with the risk of developing cancer, cardiovascular disease or dying earlier.

For heart disease, the most recent systematic review and meta-analysis of 18 observational studies involving over 2 million participants found no association between multivitamin supplementation and cardiovascular disease outcomes including mortality.

Randomised-controlled trials are better than observational studies for investigating any direct effects of multivitamins on health. But way back in 2006, a comprehensive review looking at only randomised-controlled clinical trials found that taking a multivitamin did not reduce the risk of any chronic disease compared to people taking a placebo pill.

More recently and published in 2012, The Physicians Health Study II looked at the benefit of multivitamins using a randomised-controlled trial design. Involving over 14,000 male physicians in the United States aged 50 years and older, there was no benefit seen from taking a broad-spectrum multivitamin in reducing the risk of having a heart attack, stroke or dying from a cardiovascular-related event.

Men taking the multivitamin also saw no benefit in their risk of dying earlier compared to men taking a placebo. A further analysis of data from this same group of men did find however that taking a multivitamin was linked to an 8 percent lower risk of developing cancer. But these same men saw no lowering in their risk from dying from cancer.

Moving to the most recent research and assessment of the evidence in the field, a 2015 critical review of observational studies and randomised-controlled trials explored the efficacy of multivitamins in reducing the risk of chronic disease.

What did it find? Most scientific studies investigating the use of supplements in chronic disease risk reduction reported no significant effect. The review, however, did note prior research that suggested some benefit of multivitamins in reducing the risk of developing cancer at least in men, yet no strong evidence shows that women may also benefit.

On that topic of cancer, a comprehensive evaluation of the research field by the World Cancer Research Fund and the American Institute for Cancer Research recommended against the use of dietary supplements for cancer prevention because of the unpredictability of potential benefits and risks, as well as the possibility of unexpected adverse events.

There is some evidence that multivitamins that include high doses of antioxidants may help to reduce the risk of cataracts and age-related macular degeneration, but we are still waiting on higher-quality studies to confirm this. Use of high-dose antioxidants is not without risk, so may offset any eye-health benefit.

What do experts say?

We asked five experts whether multivitamins make you healthier. The expert view was mixed with most giving a qualified 'yes', but the context of their use is key.

Dr Cornelie Nienaber-Rousseau, a nutrition expert from North-West University, commented that in the updated review from US Preventive Services Task Force they concluded that the evidence is insufficient to recommend for or against the routine use of supplements of vitamins A, C or E; multivitamins with folic acid; or antioxidant combinations for the prevention of cancer or cardiovascular disease.

In fact, some supplements can be harmful, she wrote. Beta-carotene supplements taken alone or with other vitamins increases the risk of lung cancer in smokers. In fact, a 2010 meta-analysis of randomised controlled trials that used antioxidant supplements to help prevent cancer found no clinical evidence to support a benefit of antioxidant supplements containing vitamin A, vitamin C, vitamin E, beta-carotene or selenium.

Dr Gideon Meyerowitz-Katz, an epidemiologist from the University of Wollongong in Australia, has also evaluated the evidence for multivitamins and wrote that there does not appear to be strong evidence that multivitamins improve health for people who are otherwise healthy and have a balanced diet.

Positive relationships between supplements and health that do exist are likely confounded by healthier people being the group more likely to take them.

Multivitamins can help with deficiencies

It is not all negative news for multivitamins. Taking a multivitamin supplement helps people obtain the recommended intakes of vitamins and minerals when they cannot meet these needs from food alone. And there are many situations where this is the case such as:

• People more likely to have a poor food intake such as those on restricted diets or the elderly
• Women planning pregnancy where taking folic acid and other nutrients such as iodine, iron and vitamin D before and during pregnancy is well supported by scientific evidence
• People following a vegan diet where vitamin B12 deficiency can be an issue
• People who are chronic abusers of alcohol
• People who have undergone bariatric weight-loss surgery
• People with malabsorption problems such as celiac disease, Crohn's disease, cystic fibrosis or pancreatitis

Dr Gideon Meyerowitz-Katz explained that there is usually a good rationale for taking multivitamins when someone is deficient. For example, supplements containing iron can help women who are at risk of iron deficiency during their reproductive years because of additional iron losses from menstruation.

From Dr Nienaber-Rousseau's analysis, she cited people who follow a vegan diet as an ideal example for multivitamin candidates – since they often miss out on nutrients.

Food is more than vitamins and minerals

All this discussion on multivitamin supplements can obscure the forest for the trees. Humans eat food, and food is a complex source of vitamins, minerals and phytochemicals (plant chemicals), which all work together.

Supplements tend to work in isolation and only contain a fraction of the nutrients that can be found in a diverse diet. Food also contains vitamins in different forms. For example, vitamin E naturally occurs in 8 different forms, but supplements usually contain just one of these forms. Each of these forms of vitamin E has different levels of bioavailability and even activity.

That is a lot of potential health benefits to be gained from eating whole plant-based food and likely why most studies do not show that multivitamins offer many health benefits. Food is much more than just essential vitamins and minerals.

What to look for in a supplement

If you do decide to take a multivitamin supplement, what should you look for?

Firstly, it should deliver close to the recommended daily intake for most of the vitamins and minerals. A level of 75 percent is a good baseline to work from. But a true multivitamin cannot hope to achieve this for all nutrients and still pack it all into a one-a-day pill. Calcium is a good example, as the amount we need is so large – about 1 gram per day.

Choose a multivitamin tailored to your age, gender and other characteristics such as if you are pregnant. Multivitamins for women usually contain additional iron, whereas those for seniors typically provide more calcium and vitamins D and B12.

Taking a basic multivitamin that provides nutrients at close to the recommended amounts is unlikely to pose a safety risk for healthy people. However, be aware that eating a diet high in fortified foods may mean it is possible to consume some nutrients at levels exceeding the upper level of intake.

If you feel that you could be lacking in certain nutrients, it may be better to look at changing your diet rather than reaching for supplements. If you need help, see your doctor or a dietitian.