Vitamin D – are you getting enough?

Now that British Summer Time is over, unless you are lucky enough to be on holiday in a sunnier climate, it’s time to consider you much vitamin D you should be taking. In 2016, Public Health England issued guidance(1) stating that all adults in the UK need to obtain at least 10mcg (400IU) vitamin D every day to keep bones, teeth and muscles healthy. PHE continued to advise that insufficient vitamin D cannot be obtained from sunlight during the autumn and winter, so people either need to be consuming sufficient vitamin D in their diet or else taking a dietary supplement during the autumn and winter. People who obtain little or no sun exposure, those with darker skins and children from birth to aged 4 would benefit from supplementing all year round.

winter sun

In this post, I’ll explain where we can get vitamin D from, how to find out how much we need and what we need it for  – it’s not just for our bones!

What is vitamin D?

Strictly speaking, vitamin D is not even always a vitamin! Confused already? Vitamins are organic compounds which are required by the body for normal health and to avoid disease but which cannot be made by the body itself. During summer months, when sufficient light energy falls on our skin, we can convert this to vitamin D. In northern latitudes during the autumn, winter and early spring there is less energy in the sunlight and we need vitamin D from dietary sources – in this context it becomes a vitamin.


Whether we get vitamin D from sunlight or from a supplement, the body has to convert it into its active form. Light falls on the skin and irradiates the cholesterol contained in the layer of fat immediately under the skin to produce a compound called cholecalciferol. (This is the same compound in supplemental vitamin D3.) It then has to undergo two further chemical reactions, one in the liver and one in the kidneys, before turning into the active form(2). This means that keeping our liver and kidneys in good health is important for ensuring that the vitamin D we get (from the sun, food or supplements) can work properly! The active form is really a hormone, in that it fits into specific receptors on the surface of target cells, like a lock into a key.

It’s also worth emphasising that sunlight needs cholesterol to form vitamin D. That’s right, we need a certain amount of cholesterol. Vitamin D is also fat soluble, so if the diet is too low in fat it cannot be absorbed properly.

What foods contain vitamin D?

Small amounts of vitamin D are found in oily fish, eggs, liver and butter. (Milk isn’t fortified with vitamin D in the UK.) However, animal products will only contain vitamin D if the animals themselves have been exposed to sunlight – butter from cows which spend most of their time indoors will contain much less vitamin D than butter from outdoor-reared herds.

vitamin D in oily fish

Certain mushrooms can synthesise a vitamin D2 (a plant source) if they are exposed to sunlight, but this is not as bioavailable as vitamin D3 from animal products. Nevertheless, some studies have shown that eating mushrooms exposed to sunlight can raise serum vitamin D levels(3).

However, the amounts of vitamin D that can be obtained from dietary sources may no longer be sufficient for optimum health, given that many of us spend less time outdoors in the sunshine than our ancestors did. And when we do go out in the sun, we reduce vitamin D synthesis by always covering ourselves in suncream. This is why supplements are recommended.

What does vitamin D do?

Vitamin D is best known for its role in regulating calcium. It increases absorption of calcium in the intestines, and, as calcium is important for mineralising bones, vitamin D is therefore extremely important for bone health. Overt vitamin D deficiency leads to rickets in children (osteomalacia in adults).

However, vitamin D is vitally important for other reasons as well. Its other roles include(4):

Regulation of immunity. As well as boosting innate immunity, vitamin D plays an interesting role in regulating autoimmunity (when the immune system mistakenly attacks the body). Sufficient levels of vitamin D are needed to make sure that the immune system switches off a potential autoimmune attack. This has been established for multiple sclerosis, but is likely to be true for other autoimmune conditions.

Regulation of hundreds of genes. Our genes are fixed, but whether or not they are expressed is influenced by the environment in the body; vitamin D influences gene expression.

Brain development. Low levels of vitamin D in early life may affect brain structure and function, and are also linked to conditions such as depression and Alzheimer’s disease.

Cardiovascular health. Vitamin D protects heart function, lowers blood pressure and reduces the risk of thrombosis.

Anti-cancer activity. Vitamin D inhibits cell proliferation and stimulates differentiation (ie making cells more like they should be for the tissue type they are in – cancer cells are undifferentiated). It also stops cancer cells from forming new blood vessels.

How much vitamin D do we need?

There is no easy answer to this question. NICE guidelines state that vitamin D levels are “adequate” if serum levels of 25-hydroxyvitamin D are above 50nmol/L. However, only bone health is taken into account when arriving at this figure. There is growing evidence that higher levels of vitamin D are beneficial for optimum health, and for people facing particular health challenges. Some examples include:

  • In subjects with multiple sclerosis, supplementing with enough vitamin D to raise serum levels from 50nmol/L to 115nmol/L significantly reduced the amount of lesions on the subjects’ brains(5)
  • Supplementing with vitamin D after a breast cancer diagnosis appears to improve survival rates. Serum levels as high as 200nmol/L may offer benefit(6)
  • Higher levels of vitamin D help prevent falls in the elderly. Minimum levels of 75nmol/L have been suggested(7).

These are by no means all the conditions in which higher levels of vitamin D than 50nmol/L may be beneficial, but it gives an idea.

Because vitamin D is fat soluble rather than water soluble, any excess is not easily eliminated in the urine. Very high levels of vitamin D in the blood can lead to a build-up of calcium in the blood, which can be dangerous.

It’s therefore important to test your vitamin D levels before beginning to supplement. Ideally you would then test twice a year on an ongoing basis: once in late winter when levels are at their lowest and again at the end of summer when levels are at their highest. Working with your doctor, a nutritional therapist or a naturopath, you can then determine how much to supplement to reach your target level. How much it takes to reach a target will be individual and can depend on factors such as how well nutrients are absorbed in the digestive tract.

The government RNI of 400IU/day may not correct a vitamin D insufficiency. Note that the European Food Safety Authority has set a Tolerable Upper Intake Level of 4000IU/day for adults. This is a safe upper limit which applies to all the general population. The figure of 4000IU was obtained after applying a safety factor to the results of studies in which no adverse effect was found when supplementing with 10,000IU/day(1). In other words, supplemental levels much higher than the recommended 400IU/day are likely to be safe, especially if you are deficient to start with.

What form of supplement is best?

Vitamin D is available in tablet form, softgels, drops and even sprays. Does is make a difference which you take? My own experience is that I supplemented at quite a high dose with softgels for a couple of years without it having much impact on my vitamin D levels. It was only when I switched to sublingual drops (which absorb under the tongue, bypassing the digestive system) that I managed to get my levels to where I wanted them to be. I would expect that oral sprays would have the same result.

Remember that vitamin D increases calcium in the blood? It’s important that that calcium gets directed to where it is needed (the bones, teeth etc) and doesn’t get deposited in arteries where it can cause stiffening. Vitamin K can help with this. Vitamin K can be obtained from green leafy vegetables or made by some of our gut bacteria. But if you’re not eating enough green leafies or you have digestive health challenges, it would be worth considering a supplement combining vitamin D with vitamin K.

How do I get my serum vitamin D levels tested?

If you have not had your vitamin D tested, you should be able to get it checked via your GP. However, NICE does not recommend continued monitoring once you are supplementing, so you may want to test privately. Better You offers a home test for £28, complete with a complementary oral spray at a strength recommended for you based on your results. Given the wide-ranging health benefits of vitamin D and the importance of having optimum levels, it may be worth your while!


  1. Scientific Advisory Committee on Nutrition (SACN) (2016). ‘Vitamin D and Health’, Public Health England. Available at (Accessed 14 November 2017).
  2. Bender, D.A. (2009). ‘The Vitamins’, in Gibney, M.J., Lanham-New, S.A., Cassidy, A. and Vorster, H.H. (eds.) Introduction to Human Nutrition. 2nd edn. Chichester: Wiley-Blackwell. pp141-146.
  3. Urbain, P., Singler, F., Ihorst, G. et al. (2011). ‘Bioavailability of vitamin D₂ from UV-B-irradiated button mushrooms in healthy adults deficient in serum 25-hydroxyvitamin D: a randomized controlled trial’, European Journal of Clinical Nutrition, 65(8), pp965-971. [Online]. Available at (Accessed 14 November 2017).
  4. Vitamin D Council (no date). What is vitamin D? Vitamin D Council [Online]. Available at (Accessed 14 November 2017).
  5. Soilu-hänninen, M., Aivo, J., Lindström, B.M. et al. (2012). ‘A randomised, double blind, placebo controlled trial with vitamin D3 as an add on treatment to interferon β-1b in patients with multiple sclerosis’, Journal of Neurology, Neurosurgy & Psychiatry, 83(5), pp565-571. [Online]. Available at (Accessed 14 November 2017).
  6. Lim, S.T., Jeon, Y.W. and Suh, Y.J. (2015). ‘Association between alterations in the serum 25-hydroxyvitamin d status during follow-up and breast cancer patient prognosis’, Asian Pacific Journal of Cancer Prevention, 16(6), pp2507-2513. [Online]. Available at (Accessed 14 November 2017).
  7. Bischoff-Ferrari, H.A., Dawson-Hughes, B., Willett, W.C. et al. (2004). ‘Effect of Vitamin D on falls: a meta-analysis’, JAMA, 291(16), pp1999-2006. [Online]. Available at (Accessed 14 November 2017).