Vitamin D is not just a vitamin. It is a fat-soluble pro-hormone. It is obtained through the action of sunlight on skin and scarcely sourced from dietary sources. The action of specific ultraviolet sunlight on skin converts 7-dehydrocholesterol to pre-vitamin D3, which is then metabolised to vitamin D3.


Vitamin D deficiency associated with the development of CVD, cancer, IBD & AI disorders – Holick, (2007)
Most notably, deficiencies lead to Rickets in growing children.

Dietary sources of vitamin D are limited and impractical. To obtain 2500IU of Vitamin D through the diet you would have to consume

• 6.25 litres of whole milk
• 143 large eggs
The main natural source is from the action of sunlight on skin. However, from October to the beginning of April in the UK/Ireland there is no sunlight of the appropriate wavelength for skin synthesis of vitamin D. UK National surveys suggest that around a fifth of adults and 8 to 24% of children have low Vitamin D status – less than 25 nmol/litre in the blood.

Hall et al., (2009) estimated that white skin individuals with high sun exposure need 1300 IU/d vitamin D intake in the winter and African Ancestry individuals with low sun exposure need 2100-3100 IU/d year-round to achieve near-optimal Vitamin D3 status (≥75nmol/L)
Basically, everyone should have access to Vitamin D3 if necessary and it may be necessary in any of these cases:

  • Infants and children aged under 4 pregnant and breastfeeding women, in particular, teenagers
  • Young women
  • People over 65
  • People who have low or no exposure to the sun, for example, those who cover their skin for cultural reasons.
  • People who are housebound or confined indoors for long periods.
  • People with darker skin, for example, people of African, African-Caribbean or South Asian family origin.


Another population that may benefit from Vitamin D3 status testing and supplementation are those suffering from PCOS (Polycystic Ovarian Syndrome) (Wehr et al. 2011Selimoglu et al. 2010)

The current reference nutrient intakes (µg/day) for vitamin D are:
•10 micrograms of vitamin D per day, throughout the year, for everyone in the general population aged 4 years and older
•10 micrograms of vitamin D per day for pregnant and lactating women and population groups at increased risk of vitamin D deficiency.
(10 micrograms = 400iu)
What does reference intake mean?

The Reference Nutrient Intake (RNI) is the amount of a nutrient that is enough to ensure that the needs of nearly all the population (97.5%) are being met.

Does that mean you are ensuring optimal intakes?

No. Most of the benefits listed above are found in studies where upwards of ~2500iu per day are taken.
Do you have to take it daily?

No. As an adult, you can take large doses of Vitamin D3 to meet your needs for a week or more at a time. This is because Vitamin D3 is fat soluble and stored for long periods in the body.
During the winter months, if we do not supplement we rely on body stores from sunlight exposure in the summer and dietary sources to maintain vitamin D levels.
The tolerable upper daily limit given by the Endocrine Society is 10,000 IU but even prolonged daily intakes of 10,000 IU of D3 are considered to be safe and most cases of vitamin D intoxication have been attributed to prolonged and unintended daily intakes of >40,000 IU. (Prietl et al., 2013)
Sun Exposure
UVA penetrates glass (although more weakly than direct exposure) and over long periods of exposure will cause skin damage. However, the vitamin D-inducing UVB does not penetrate glass.
People who choose to expose their skin to strong sunlight to increase their vitamin D status should be aware that prolonged exposure (for example, leading to burning or tanning) is unlikely to provide additional benefit.
You need to weigh up the costs versus benefits of sun exposure. The risk of sunburn is greatest between 11am and 3pm especially in the summer months (June being the most intense) it takes a briefer exposure on the skin to synthesise Vitamin D but longer for those with the darkest of skin. Before 11am and after 3pm: it takes longer to synthesise sufficient vitamin D but the risk of sunburn is also less.

  • It is not possible to get enough vitamin D by sitting next to a closed sunny window.
  • It is not possible to get enough vitamin D from sunlight between October and March in the UK.
  • It is not possible to get enough vitamin D by wearing sunscreen.

Know your Skin Type for Optimal Vitamin D absorption & Health

6 different skin types have been identified which will make it easier for you to decide on a healthy sun exposure routine.

  • Type I: Often burns, rarely tans. Tends to have freckles, red or fair hair, blue or green eyes.
  • Type II: Usually burns, sometimes tans. Tends to have light hair, blue or brown eyes.
  • Type III: Sometimes burns, usually tans. Tends to have brown hair and eyes.
  • Type IV: Rarely burns, often tans. Tends to have dark brown eyes and hair.
  • Type V: Naturally brown skin. Often has dark brown eyes and hair.
  • Type VI: Naturally black-brown skin. Usually has black-brown eyes and hair.


Short (less than the time it takes for skin to redden or burn), frequent periods of sunlight exposure are best for vitamin D synthesis. In addition, this type of exposure is less likely to result in skin cancer.




Get out in the sun, improve your natural circadian rhythm, light and dark cycles, and with that your sleep quality and mood. Even if it’s somewhat overcast, the benefits are there to be reaped.

Be a conscientious Vitamin D skin synthesizer, supplementation though is advisable for most of us.

Information for this article was largely gathered from The NICE Guidelines (2014) and as always special thanks to Mac-Nutrition Uni.

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