“Those that can heal can harm; those that can cure can kill.” Celia Rees

Many people believe that taking supplements represents a magic bullet for disease prevention and a long, healthy life. It’s been estimated that one in two adults take some forms of vitamin supplements daily, and one in six people on prescription medications are taking at least one supplement. 

We have evolved over thousands of years to eat real, not encapsulated, foods. The US Department of Agriculture states that “an adequate and balanced diet should provide necessary nutrients without the need for dietary supplements.” Vitamin manufacturers, however, argue that a standard diet doesn’t contain enough vitamins, and the more supplements we take, the better. Most of us assume that taking vitamins at least can’t do any harm. Scientists disagree. 

So, what are these dietary supplements and do we need them in the first place? 
Supplements are usually products that contain so-called dietary ingredients intended to supplement the diet. They are therefore considered foods, not medications. Most common supplements are vitamins, minerals, herbal products, amino acids and pretty much anything that can be bottled. The interesting thing is that since supplements are considered foods, not drugs, they are not regulated like prescription drugs. When we read about a new drug coming up, we hear more about the side effects than the potential benefits as all drugs, require rigorous clinical trials for safety and efficacy. Curiously, this doesn’t apply to supplements. They don’t need to be tested the same way as drugs.

Since many observational studies have shown a consistent association between greater fruit and vegetable consumption and a lower risk of heart disease, cancer and other chronic illnesses, companies started producing supplements that may capture the benefits and potentially improve health.

What does research say about supplementation?
In a Finish randomised controlled clinical trial published in the New England Journal of Medicine researchers were looking at how vitamin E and beta-carotene supplementation may influence the risk of lung cancer and heart disease. They enroled more than 29,000 men, all smokers, giving them a daily dose of vitamin E, beta-carotene, both or a placebo. The study found that men taking beta-carotene for five to eight years were more likely to die from lung cancer or heart disease than those in a placebo group.

A few years later the same journal published another randomised clinical trial on vitamin supplementation. In this study, researchers examined more than 18,000 people at high risk of lung cancer due to smoking or asbestos exposure. All participants received a combination of vitamin A and beta-carotene, or a placebo. The study ended when the researchers noticed that the risk of death from lung cancer in people taking the vitamins was 46% higher than in a placebo group. 

A large meta-analysis of 14 randomised clinical trials for the Cochrane Database found that supplementation with vitamins A, C, E, beta-carotene, and selenium, taken for intestinal cancer prevention, actually increased total mortality.

These findings were confirmed in another large review published in the Annals of Internal Medicine. Scientists studied 19 clinical trials of nearly 136,000 people and found again that vitamin E supplementation increased mortality. The results raised concerns and a few more studies examined the link between vitamin E supplementation and chronic diseases. One study on people with cardiovascular disease and diabetes found that vitamin E increased the risk of heart failure. Another study published soon after in The Journal of the American Medical Association linked vitamin E supplementation to a higher risk of prostate cancer. Finally, a Cochrane review stated that “beta-carotene and vitamin E seem to increase mortality, and so may higher doses of vitamin A.”

A few years ago researchers enroled more than 14,600 men age 50 years and older to see whether taking multivitamins may lower the risk of cardiovascular disease and cancer. All men were taking either multivitamins or placebo and were followed on average for more than eleven years. The researchers concluded that for the outcome of cardiovascular events taking multivitamins didn't make any difference. Regarding cancer, however, the results were different. It turned out that men taking multivitamins had an 8% reduced risk of all kind of cancer. 

Since the results were encouraging, the researchers started investigating how taking multivitamins may affect other types of cancer like breast cancer.

A study published a few years ago with 35,000 women was looking at whether the use of multivitamins may be associated with the risk of breast cancer. The outcomes were once again surprising: “these results suggest that multivitamins use is associated with an increased risk of breast cancer.” The researcher suggested that the culprit may be a folic acid. 

The results were confirmed again in another study that was looking at the link between the use of multivitamins and the risk of prostate cancer. Men taking multivitamins had double the risk of prostate cancer, probably due to zinc content. 

Observational studies haven’t clearly demonstrated an association between multivitamins use and a lower risk of either cardiovascular disease or cancer. Some studies suggest benefits, other no effect or even a risk. Large-scale, long-term randomised controlled trials designed to show whether multivitamins play any role in the prevention of major chronic illness are limited. 

It’s also very difficult to estimate the optimal dose and assess the quality of supplements. We don't know exactly what we get when asking for supplements. Often the form of the nutrient may be cheap or poorly absorbed by the body. The current regulations don’t require that the dose on the label needs to match the dose in the pill. 

We need to remember that all supplements are highly processed substances with lots of added colours, fillers and allergens. Many raw products, particularly herbs may not be tested for toxins like mercury as there are no regulations in this area either. 

Another problem is that most chronic diseases are inherently multifactorial. In other words, we know that there are many risk factors for cardiovascular disease, diabetes or cancer, and there is no way that supplements are going to be representing all of those factors. Additionally, when nutrients are isolated from whole foods they don’t always act the same way. 

The idea of a quick fix or "all in one pill" solution only distracts us from the importance of good quality nutrition and wise lifestyle choices. As a result, we often don’t realise that taking multivitamins could actually do more harm than good.

Can supplements help us stay well? Probably not, but we don’t know for sure. Despite the lack of benefits seen for most large clinical trials, their use persists. We still take them.