Dietary Supplements

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Dietary supplements come in many different forms. As the name suggests they are intended to supplement a balanced diet.

This post will explain the role of various supplements, and many of the safety considerations to bear in mind.

The Role of Supplements

There are many situations where supplements can can play a really important nutritional role.

Vitamin D

Vitamin D is needed for regulating calcium and phosphorus in the body, which has a knock-on impact on the health of our bones, teeth and muscles.

In Ireland it is advised that babies are given 5 mcg vitamin D per day from birth until they are 1 year old.

People who live in Northern Europe may need to consider supplementing with vitamin D during winter. 10 mcg of vitamin D per day is the usual recommended dose.

More isn’t always better when it comes to a fat-soluble vitamin like vitamin D, as excess levels can build up in our body. It is recommended to supplement with no more than 25 mcg of vitamin D per day (on top of dietary intake), as excess vitamin D has been linked with: irregular heart beat, hardening of blood vessels (due to calcification) and kidney stones.

For more information on vitamin D check out this blog post.

Folic Acid

Women who are planning a pregnancy should take a folic acid supplement for at least 3 months prior to becoming pregnant until the 12th week of pregnancy in order to reduce the risk of neural tube defects (NTDs) like spina bifida developing in their baby.

Most women are advised to take 400 mcg of folic acid per day, however those who have a higher risk of having a child with an NTD may be advised to take a higher dose.

Correcting Deficiencies

Supplements can also be very important for correcting a nutritional deficiency, such as an iron or B12 deficiency.

Similarly, those who have a restrictive diet can benefit from certain supplements. For example, vegans often need to consider supplementing with vitamin B12, iodine and omega 3 (as well as the usual advice about vitamin D and folic acid outlined above).

Other Uses For Supplements

Other situations where specific supplements can be helpful include:

  • Sports supplements for athletes
  • Probiotics as part fo the treatment of IBS, or after a course of antibiotics (see this article I wrote for The Food Medic Educational Hub for more information about probiotics)
  • Part of the treatment of osteopenia or osteoporosis.
  • Improving immune response.
  • Certain supplements can play a role in the management of polycystic ovarian syndrome (PCOS), premenstrual syndrome (PMS) and migraines.
  • There is more evidence emerging related to likely benefits of older adults supplementing with folic acid and folate in terms of cognitive health.

For any of these situations, it is always best to get individual advice from a Registered Dietitian so that you are supplementing in a safe and useful way.

Supplements can be a useful addition to a balanced diet in certain circumstances. For anything beyond the government guidelines related to supplementing with vitamin D and folic acid, it is always best to seek individualised advice from a healthcare professional like a Dietitian.

Other Vitamins & Minerals

There is no strong evidence to suggest that taking a multivitamin improves overall health or reduces the risk of chronic disease. In fact, a large systematic review of the evidence from 2013, found no clear health benefits from taking vitamin and mineral supplements.

However, taking a multivitamin can sometimes be a safer option as compared with taking a ‘cocktail’ of various supplements which may interact with each other or contain the same nutrients.

There are situations where specific vitamin and mineral supplements can be useful, but there are safety considerations to be aware of.

Vitamin A

Vitamin A has many functions in our body, such as supporting our immune system and vision (particularly our night vision).

Pregnant women need to avoid taking supplements containing vitamin A as this can cause birth defects.

There is also some evidence that consuming more than 1.5mg of vitamin A per day over the course of years may weaken the bones and increase the risk of fractures later in life. It isn’t unusual for supplements to contain 1.5mg of vitamin A (particularly certain ‘skincare supplements’), so always check the labels, especially if you have a higher risk of osteoporosis or osteopenia.

A large study in 2004 called the CARET study was investigating the impact of supplementing smokers with vitamin A. However, this study was finished early because there was a significantly higher risk of death from lung cancer and heart disease in those who were supplementing with vitamin A (30mg of beta-carotene and 2.25mg of retinyl palmitate).

There is also a risk of liver damage, coma or even death from taking a very high dose of vitamin A. This was discovered when artic explorers developed dangerously high levels of vitamin A in their blood after eating polar bear liver (which contains a lot of vitamin A).

Vitamin E

Vitamin E acts as an anti-oxidant in the body, it also plays an important role in keeping our eyes, skin and immune system healthy.

High dose vitamin E supplements have been linked with a higher risk of hemorrhagic stroke (a bleed in the brain), and possibly a higher risk of prostate cancer. However, more research is needed to investigate this.

Vitamin E can also interfere with medical treatments including: chemotherapy, radiotherapy and blood thinning medication.    

The HSE in Ireland state that “taking 540mg or less a day of vitamin E supplements is unlikely to cause any harm”.

Vitamin K

Vitamin K is important for blood clotting and wound healing.

Vitamin K supplements can be useful in certain circumstances, such as improving bone health.

However, vitamin K supplements need to be avoided by anyone who takes anticoagulant medication (like warfarin).

Vitamin C

Vitamin C acts as an antioxidant in our body and is also important for: wound healing, healthy skin and blood vessels.

There is some evidence that regularly taking vitamin C supplements (about 1g divided into a few doses across the day) may help to reduce the duration and severity of colds, but not to prevent colds. This may be particularly useful in those who are under regular physical stress, such as athletes.

But taking higher doses of antioxidant supplements (including vitamin C) may impair muscle recovery in athletes, as oxidative stress is involved in muscle repair. So the best option is to obtain vitamins and minerals from a balanced diet where possible

However, taking more than 1g of vitamin C per day has been associated with nausea, diarrhoea and kidney stones.

B Vitamins

B vitamins play a number of important roles in our body, including in: cell metabolism and keeping our nervous system healthy.

As outlined above, there are circumstances where supplementing with folic acid and B vitamins is useful. B vitamins also tend to have a low risk of toxicity as they are water-soluble, so excess levels are usually peed out.

See here for more information on individual B vitamins.

Overall, vitamin D is the only vitamin supplement that most people need to consider taking. There are also certain groups of people who may benefit from supplementing with B-vitamins, and women planning a pregnancy need to take a folic acid supplement.


We need a variety of minerals in our diet for good health.

There are some cases where it is useful to supplement with minerals, for example:

  • Some vegans need to supplement with iodine
  • Calcium supplements may be used by those with osteopenia or osteoporosis
  • Iron supplements can be used to correct deficiency

However there are risks associated with taking high doses of mineral supplements. For example, some minerals may interact with certain medication. Similarly, taking calcium, iron, magnesium, and zinc at the same time in doses of 800mg or higher can cause these to compete for absorption, so the body struggles to absorb these.

Therefore, it is best to obtain our minerals from our diet where possible, and to get advice from a Dietitian if you are thinking about starting a mineral supplement.

Omega Oils

Omega 3 and omega 6 are essential fatty acids which we need to obtain from our diet so that our body can function properly.

Most people consume enough omega 6 in their diet, so it is rare that this would be needed in supplement form. Some fish oil supplements also contain omega-9 fatty acid, but this is unnecessary in supplement form as our body can make omega 9 by itself.

Omega 3 plays an important role in heart and eye health, and it also has an anti-inflammatory impact. Omega 3 also promotes healthy brain and eye development in babies.

It is best to get omega 3 from our diet by having oily fish at least once per week, as this is thought to have a much better impact on our heart health in particular.

But there are circumstances where omega-3 supplements can be useful, for example this is important for pregnant women who don’t consume oily fish. If a pregnant woman is using an omega-3 supplement it is important that this doesn’t contain any vitamin A and that it is omega-3 oil father than fish liver oil.

Check out this article for more information about omega-3.

Fat Burners

Most of these are unlikely to significantly impact our metabolism or to ‘burn fat’ as the claim to do.

However, some of these can be really dangerous.

For example, a banned supplement called dinitrophenol (DNP) has been linked with comas and deaths. Green tea supplements have also been linked with causing liver failure.

So I would strongly discourage anyone from using ‘fat burning’ or ‘metabolism boosting’ supplements.

Supplements vs. Whole Foods

Supplements can’t mimic the ‘food matrix’ of whole foods, which is the combination and interactions between all of the nutrients which are found in food. For example, fish oil supplements are not deemed to reduce the risk of heart disease in the same way that oily fish does.

However there are certain nutrients that we benefit from taking in supplement form, such as vitamin D and folic acid (as discussed above).

Furthermore, there isn’t any good evidence that so called ‘whole food supplements’ are any better than ordinary vitamin and mineral supplements. 

So overall it is best to get the nutrients we need from a balanced and varied diet where possible. If in doubt, it is always best to get individual advice from a Dietitian if you are unsure about which supplements to take.


Maeve has been consulting on The Food Medic Educational Hub for 12 months now and has been a huge asset to the team. Her ability to translate some very nuanced topics in nutrition into easy-to-follow, informative articles and infographics is really admirable.

Dr Hazel Wallace

Founder of The Food Medic

Maeve is incredibly talented at sharing scientific information in an easy to understand way. The content she shares with us is always really interesting, clear, and of very high quality. She’s one of our favourite writers to work with!

Aisling Moran

Senior UX Writer at Thriva Health

Maeve has written extensively for NHD magazine over the last few years, producing a wealth of dietetic and nutritional articles. Always evidence based and factual, Maeve creates material that is relevant and very readable. She provides high quality work with a professional and friendly approach. Maeve is a beacon of high quality knowledge and work within the nutrition writing community; and someone NHD magazine is proud to work with.

Emma Coates

Editor of Network Health Digest

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