Vitamin D

Vitamin D

There is a lot of conflicting information about Vitamin D on the web, here is what science rather than hearsay tells us.


What is vitamin D and why do we need it?

Vitamin D is a family of nutrients that act as a hormone that’s needed for calcium and phosphorous metabolism. These micronutrients are key for muscloskeletal (bone, muscle and teeth) health. Insufficient amounts of vitamin D can cause rickets in children and osteomalacia in adulthood as well as impairing immune system function, causing fatigue and increasing the risk of depression (1).


Where do we get it?

Vitamin D primarily comes from sun exposure as it’s synthesized in the skin when it’s exposed to sunlight containing sufficient ultraviolet B (UVB), but can also be found in certain foods, as well as in the form of dietary supplements.

Dietary sources are essential for optimal health when sunlight exposure containing UVB radiation is limited (winter months) or exposure is restricted (limited time spent outdoors or little skin exposure).

Vitamin D in foods mainly comes in two chemical forms, vitamin D2 and vitamin D3. They both help to maintain a good vitamin D level but differ in a couple of ways.

Vitamin D3 mainly comes from animal sources and vitamin D2 comes mainly from plant sources. Vitamin D2 is cheaper, this is what most vitamin-fortified foods contain.

Vitamins D2 and D3 get absorbed similarly by the human body but get metabolised in the liver differently. Because of this the same amount of vitamin D3 yields more calciferol (the main form of vitamin D actively circulating in the blood), than vitamin D2. This is why research shows vitamin D3 supplementation is better at maintaining and raising blood levels of vitamin D (2).

A small terminology note, without a subscript, vitamin D refers to vitamin D2, D3 or both.


There are certain groups that are under increased risk of vitamin D deficiency:

  • Infants
  • Pregnant and breastfeeding women
  • Adults aged 65+
  • People with limited sunlight exposure
  • People wearing clothing that covers most of the skin while outdoors


What are the recommendations?

Scientific Advisory Committee on Nutrition / SACN published a new guideline for Vitamin D in 2016. The current recommended daily intake* is 10 micrograms/400 IU for everyone in the UK aged 4 and above. This recommendation includes pregnant and breastfeeding women.

For infants aged 0-1 the safe intake is 8.85-10 micrograms /340-400 IU per day and for children between the ages of 1-4 years the safe intake is 10 micrograms / 400 IU per day.

 *Daily intake: RNI – Reference Nutrient Intake


These RNIs and safe intakes are developed to make sure that the majority of the UK population has enough vitamin D levels to protect bone, muscle and teeth health.

However, you should bear in mind that preventing disease due to deficiency and optimal health are not the same, so when it comes to RNIs, we are talking about bare essentials.


Something else to consider is that these recommendations do account for the amount of sunlight the person is exposed to, which varies not just by season, but also due to time of day, altitude, cloud cover, sunscreen and even clothing.

At latitudes above 37ᵒN, UVB radiation is not sufficient all year round for vitamin D synthesis. Because of this, sunlight induced vitamin D synthesis is only effective between April and September in the UK. 


What does 10 micrograms / 400 IU of vitamin D per day look like?

It’s possible to get all the vitamin D you require from food, here is what that would look like.

Sources of vitamin D are, oily fish like salmon, sardines, mackerel, trout, herring; egg yolks, red meat, wild mushrooms, fortified foods such as some breakfast cereals, some yogurts, margarine and dairy alternatives.


Examples of foods that give the daily minimum intake of 10 micrograms / 400 IU of vitamin D per day:

  • 4-5 egg yolks (depending on the size of the egg) (3)
  • 100-120 grams (roughly a fillet) of oily fish like salmon, mackerel, herring (3)
  • 4-6 glasses of fortified dairy alternative milk (4)
  • 60 grams of fortified breakfast cereal (5)

However, as you might imagine, it can be difficult to keep up a sufficient intake of these foods every day for months at a time.


So, do I need to supplement?

Probably, especially over the autumn and winter months.

  • All adults and children over the age of 1 year of age living in the UK, are advised for taking a daily supplement containing 10 micrograms / 400 IU of vitamin D over the autumn and winter months.
  • People under increased risk of vitamin D deficiency are advised for taking a supplement containing 10 micrograms of vitamin D all year around.
  • Infants under the age of one year who are breastfeeding or mixed feeding, should be given a daily supplement of 8.5-10 micrograms, unless they have more than 500 mls of fortified formula milk.


For supplementation, it is better to supplement with vitamin D3 rather than vitamin D2 for the previously mentioned reasons.

Once again it is useful to keep in mind that these guidelines are set to prevent diseases caused by vitamin D deficiencies, not for the optimal health of an individual.

If you currently have a vitamin D deficiency, a supplement containing a higher dose of vitamin D can be useful. The safe upper intake for vitamin d is 100 micrograms / 4000 IU per day (6). Vitamin D can be toxic at high levels, so if you are not sure about whether you have vitamin D deficiency or not, and you are thinking about supplementing for more that 10 micrograms per day, it would be a good idea to talk to your GP and get your blood levels tested before starting supplementing with a higher dose.



  1. SACN. Vitamin D and Health Report. 2016.
  2. Tripkovic L, Lambert H, Hart K, Smith C, Bucca G, Penson S et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. The American Journal of Clinical Nutrition. 2012;95(6):1357-1364.
  3. Composition of foods integrated dataset (CoFID) [Internet]. GOV.UK. 2015 [cited 15 August 2018]. Available from:
  4. Almond Drink | Original Chilled | Alpro [Internet]. 2018 [cited 15 August 2018]. Available from:
  5. [Internet]. 2018 [cited 15 August 2018]. Available from:
  6. Ross A, Taylor C, Yaktine A, Cook H. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press; 2011.
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