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Vitamin D

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Vitamin D

In this Health Hub article, we explore sources of vitamin D, vitamin D supplementation, and who should be taking vitamin D.


Vitamins are used for the prevention and treatment of specific deficiency states or where the diet is known to be inadequate. Vitamin D, a fat-soluble vitamin, helps regulate the amount of calcium and phosphate in the body. These minerals are required to keep bones, teeth and muscles healthy. Vitamin D is particularly important for building and maintaining bones, because it is necessary for calcium absorption.

Facts & Figures

  • One in eight people aged over 50 suffer from low levels of vitamin D. During the winter this number increases to one in four over 50’s.
  • People of all ages living in Ireland have inadequate intakes of vitamin D and studies in Ireland have revealed that low vitamin D status and vitamin D deficiency are widespread in the population of Ireland.
  • Ultraviolet B (UVB) rays in sunlight are the main source of vitamin D for humans. Ireland’s northerly latitude means that between the months October to March the UVB rays required to make vitamin D are inadequate, therefore vitamin D cannot be produced.

Forms of Vitamin D

The term vitamin D is used for a range of compounds including; ergocalciferol (calciferol, vitamin D2), colecalciferol (vitamin D3), dihydrotachysterol, alfacalcidol (1-hydroxycholecalciferol) and calcitriol (1,25-dihydroxycholecalciferol). All of these analogues possess the property of preventing Vitamin D deficiency.

Vitamin D can be measured in International Units (IU) or in micrograms (mcg). These terms can be used interchangeably.

Where 200 IU = 5 micrograms of vitamin D3.

Sources of Vitamin D

The body can produce vitamin D from direct sunlight on the skin when outdoors. During the summer months most people should get all the vitamin D they need from sunlight. However, between the months October to March we don’t get enough vitamin D from sunlight.

Food Sources of Vitamin D include:

  • Oily fish – such as salmon, sardines, herring, mackerel and fresh tuna.
  • Red meat.
  • Liver.
  • Egg yolks.
  • Fortified foods – such as most fat spreads and some breakfast cereals and milk.

Dietary supplements are another source of vitamin D.

Vitamin D Definciency

Vitamin D deficiency can occur in people whose exposure to sunlight is limited and in those whose diet is deficient in vitamin D (such as vegans). People in Ireland of all ages have low levels of vitamin D and hence vitamin D deficiency has emerged as a public health problem in Ireland. It is not possible to safely get all the vitamin D we need from the sun and few foods contain vitamin D.

Severe and prolonged vitamin D deficiency can cause rickets in children and osteomalacia (softening of the bones) in adults. Less severe vitamin D deficiency, while not visible or symptomatic, may increase bone turnover and contribute to osteoporosis.

Vitamin D Supplementation

The Department of Health recommends that you take a daily supplement containing 10 mcg (400IU) of vitamin D throughout the year if you:

  • aren’t often outdoors – for example, if you’re frail or housebound
  • are in an institution like a care home
  • usually wear clothes that cover up most of your skin when outdoors

People with dark skin from African, African-Caribbean and south Asian backgrounds may also not get enough vitamin D from sunlight. They should consider taking a daily supplement containing 10 mcg of vitamin D throughout the year.

Recommended Daily Allowance of Vitamin D

Below is a table which describes the Recommended Daily Allowance (RDA) of vitamin D according to the Food Safety Authority of Ireland, 1999 and the Department of Health 1991.


Some people have medical conditions that mean they may not be able to safely take as much as the RDA. If in doubt, you should consult your doctor. Also, some people who have been identified with vitamin D deficiency, or other malabsorption conditions, may be prescribed much higher doses of vitamin D by their doctors to treat the deficiency.

Can You Take Too Much Vitamin D?

Yes. Taking too much vitamin D supplements over a long period of time can cause too much calcium build up in the body (hypercalcemia).

Taking multiple supplements which contain vitamin D can increase your chances over taking too much vitamin D. It is important to read the patient information leaflet of all medications and supplements to insure that you are not taking too much vitamin D. If you unsure, please discuss with your pharmacist or doctor.


  • Vitamin D toxicity is extremely rare.  Excessive sunlight exposure cannot cause vitamin D toxicity, as there is a limited ability by the skin to produce vitamin D.
  • The toxic threshold for vitamin D consumption is unknown but a daily dietary intake of up to 50mcg is considered safe. Extreme intakes of vitamin, in the range of >250mcg/d, has been found to cause adverse effects such as nausea, vomiting, poor appetite, constipation, weakness and weight loss.  
  • The main symptom of vitamin D toxicity is hypercalcemia, primarily resulting from intestinal calcium hyperabsorption and to a lesser degree from calcium release from the bone. Vitamin D toxicity can cause other serious adverse outcomes such as elevated serum calcium, which can result in severely high blood pressure and calcification of soft tissue such as the kidneys, heart, lungs and blood vessel walls.

Who Should Take Vitamin D?

Advice for infants and young children:  

  • All infants from 0 -12 months will need a daily vitamin D supplement to ensure sufficient intake of vitamin D (whether breast-fed, bottle-fed or on solid food).
  • Vitamin D3 (cholecalciferol) is the preferred form of Vitamin D supplement for infants. Parents should use an oral supplement in a form suitable for infants (liquid/drop form), containing only vitamin D3 and that provides 5 micrograms (5 µg) of vitamin D3.
  • Children aged 1-4 years old should be given a daily supplement containing 10 mcg of vitamin D.

Advice for adults and children over 5 years old:

  • During the autumn and winter, you need to get vitamin D from your diet because the sun isn’t strong enough for the body to make vitamin D.
  • Since it’s difficult for people to get enough vitamin D from food alone, everyone (including pregnant and breastfeeding women) should consider taking a daily supplement containing 10 mcg of vitamin D during the autumn and winter.
  • It is common for elderly people, especially postmenopausal women to be advised to take a combination product with Calcium and Vitamin D in order to prevent osteoporosis. It is usually sufficient to take a combination product such as those listed in the table below rather than a vitamin D supplement alone.

Products Available



Advice From The Pharmacist

  • Take vitamin D supplements alongside a meal if possible to increase absorption.
  • Vitamin D supplementation is recommended during winter months especially.
  • Take care with multiple supplements as often vitamin D is also found in omega 3 fish oils and in multivitamins.

Supports Available In Ireland


BNF 73

Mayo Clinic