Vitamins C and E might reduce effectiveness of some types of cancer chemotherapy.


A supplement, as the name implies, is any product that aims to “supplement” the diet in nutrients that could be potentially missing. There are hundreds out there ranging from the more familiar vitamins and minerals, to herbals and botanicals, amino acids, enzymes, and many other products. Dietary supplements come in a variety of forms: traditional tablets, capsules, and powders, as well as drinks and energy bars. For this article we will not be discussing oral nutrition support supplements such as Ensure, Fortisip etc.

Here are the key points if someone asks you about supplements:

  1. The first thing to flag is that most people can get everything they need from a healthy balanced diet and do not need supplements. There are certain groups that may need a little extra support in meeting the recommended amounts for key micronutrients such as teenage girls and iron. A brief summary is shown in the table below:

  1. Supplements have increased in popularity recently due to the rise of the so-called experts on social media and the increase of the interest of the public in their own health. Clinical advice should never be replaced by the growing power of unqualified social media influencers and the public should be told this. For example, most dentists will say that charcoal does not whiten teeth and its abrasive nature can actually harm teeth.
  2. Aside from vitamins and minerals, robust evidence is lacking for a lot of dietary supplements. For example, there is no conclusive research that supports kelp as a weight loss aid. As “food” they do not require the same level of scrutiny as prescription only medications (POMs). Whilst many people think a “natural” product will not harm them, cases reported in the news, including the potential hepatotoxic effects of green tea capsules, should serve as a cautionary tale.
  3. A lack of regulation means that there is no guarantee on the quality of the product. Iodine is mainly found in fish and dairy food; vegans may be concerned about a lack of it in their diet and be drawn towards kelp supplements, however the amount of iodine in kelp and therefore kelp supplements is highly variable. Additionally, as with medications, internet products may not meet the UK standards and can be an inert substance or have toxic levels of the active or other ingredients. Buying from a reputable source such as a pharmacist or chemist should prevent this.
  4. Only a certain amount of each nutrient is needed for our bodies to function, and higher amounts are not necessarily better. At high doses, some substances may have adverse effects, and may become harmful. Supplements can therefore only be legally sold with an appropriate daily dose recommendation, and a warning statement not to exceed that dose. We cannot store water soluble vitamins so there is no point exceeding the dose. For fat soluble vitamins (A, D, E, K) the maximum dose is there to prevent toxic effects. Make sure people are not doubling up by taking different supplements at the same time.
  5. If someone is receiving any medical treatment, advise them to see their doctor or healthcare professional. Supplements may have interactions with some medication and some are unsafe if you suffer from certain medical conditions. Vitamin K can reduce the ability of blood thinners to prevent blood from clotting. St. John’s Wort can speed the breakdown of many drugs (including antidepressants and birth control pills) and thereby reduce these drugs’ effectiveness. Vitamin A can also reduce bone strength and should be used in caution with those at risk of osteopenia and osteoporosis.

Antioxidant supplements, like vitamins C and E, might reduce the effectiveness of some types of cancer chemotherapy. Fish liver oil should not be taken by pregnant women as it contains vitamin A; large amounts can be teratogenic. Vitamin E supplements should be avoided by people with cardiovascular disease as it can increase the risk of further heart attacks. Effervescent (fizzy) vitamin supplements contain approximately a gram of salt per tablet and may not be appropriate for those advised to limit salt intake.

In summary, pharmacists should be cautious with recommending dietary supplements to the general public. If someone is concerned about their health or diet then they should go and see a doctor or dietitian who could potentially prescribe a medicine or diet to meet any deficit.

The author is a registered NHS dietitian and prescriber.

This article appears in the July issue of Pharmacy Business.

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