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Dietary supplements are everywhere, from the local supermarket shelves to pharmacy aisles and Instagram ads, with promises of boosted immunity, energy or longevity. With easy access and glossy marketing, it’s no surprise that many people assume they’re safe and beneficial for everyone. But the truth is more complex. Used appropriately, supplements can be helpful but when misused, they may do more harm than good.
Nutrition professionals agree supplements have a crucial role in specific clinical contexts. For example, folic acid is recommended before and during early pregnancy to help prevent neural tube defects in babies; iron supplements are an effective treatment for diagnosed iron-deficiency anaemia; and vitamin B12 supplements are essential for individuals with gastrointestinal conditions that impair absorption. But taking supplements “just in case” is a very different matter—often unnecessary, and sometimes unsafe.
For starters, supplement users tend to eat a more nutritious diet than the average person, meaning they already have a higher intake of dietary nutrient than their peers. This means they’re often the least likely person to need extra nutrients. In other words, many are topping up a tank that’s already full, which means they are a waste of time and money.
Plus, more pills doesn’t always mean better health when it comes to supplements. Take vitamin A as a prime example; it is essential in small amounts but toxic in high doses. Excess intake has been linked to reduced bone density, liver damage and birth defects during pregnancy (a clear risk for women of reproductive age, given that many pregnancies are unplanned).
Similarly, vitamin E supplements were once considered a healthy option and a potential means of reducing cancer risk, until the SELECT trial revealed in 2011 that the high-dose vitamin E supplements given to their healthy male participants had increased the men’s prostate cancer risk by 17%.
Similarly, the CARET study, conducted in the 1990s, sounded a warning about vitamin supplements that was as loud as a foghorn. They found that a high-dose beta-carotene and vitamin A supplement actually raised lung cancer rates by 28% and overall mortality by 17% in smokers, prompting the early termination of the clinical trial.
Even widely used minerals such as selenium and iron can become problematic in excess. The SELECT trial also found that selenium offered no cancer protection and might raise the risk of type 2 diabetes. Randomly taking iron supplements can be dangerous for certain people with undiagnosed hereditary haemochromatosis, a common genetic condition in people of European descent where the body absorbs and stores too much iron.
The problem is that supplement users often have no idea what constitutes too much. For example, a 2018 study from Iceland found that nearly 40% of older adults were exceeding the tolerable upper intake level for vitamin D due to widespread use of supplements. This increases the risk of hypercalcaemia, kidney stones and renal damage.
Many people also risk potentially dangerous interactions between their prescribed medications and dietary supplements. One US study found that more than 30% of older adults took both nutritional supplements and prescription medicines, and that some combinations of these carried a significant risk of adverse drug-supplement interactions, including increased bleeding or altered drug effectiveness.
Plus, adverse events from supplements are not just theoretical. An extensive US analysis published in 2023 reviewed thousands of supplement-related emergency department visits. The researchers found that supplements caused or contributed to over 79,000 adverse outcomes in the US between 2004 and 2021, such as cardiovascular symptoms, allergic reactions and liver damage – especially when taken in high doses or combined with medications.
The safer and more effective strategy is to eat a varied, balanced diet. Whole foods provide a complex mix of vitamins, minerals, fibre and phytochemicals in combinations created by nature. These natural combinations cleverly work together in ways we’re still trying to understand.
A supplement can never fully replicate the nutritional value of these foods. This doesn’t mean all supplements are harmful. In certain situations they are useful. But just because a supplement is natural or available over the counter doesn’t make it safe or necessary. Supplements are not magic pills; rather, they are best used with care and professional guidance.
As well as Jennifer Bowden’s columns in the NZ Listener, listener.co.nz subscribers can access her fortnightly myth-buster column which explores food and nutrition myths.
