What is vitamin D?

Vitamin D is a fat soluble vitamin which behaves like a steroid hormone in the body. We can make vitamin D3 from 7-dehydrohcolesterol in the dermis layer of our skin or we can eat foods containing vitamin D3, cholecalciferol, contained in animal products. These include cod liver oil, fish oil, dairy products, red meat and egg yolks. Another source is vitamin D2, also known as ergocalciferol, which is found in fungi and fortified plant food products. There has been some evidence to show that vitamin D3 may be a more effective than D2 because it may be more biologically available in the body (1).

Mushrooms exposed to UV light contain vitamin D. However, mushrooms that you find in the supermarket may have been grown in the dark and would not be a good source. It is possible to leave mushrooms that you have bought outside in the sun and they will produce more D2 but there is no way of knowing how effective that is. It is possible to buy mushrooms that have been grown under UV light and these should be labelled as enriched with vitamin D when you see them on the shelves. They are a key source of other nutrients and are especially good for gut health (27) so should be included as part of a healthy diet. Foods supplemented with vitamin D include almond milk, soy milk, orange juice and some cereals.

If you consume foods that contain healthy fats such as avocadoes, nuts, olive oil and seeds (not seed oils as they are highly processed) with vitamin D rich foods, this will help you to absorb more vitamin D. 

Why do we need Vitamin D?

Vitamin D is needed for a wide variety of purposes. It helps to maintain calcium, magnesium and phosphorous levels in the body by promoting their absorption from the gut. This is especially true for calcium when there are low levels in the diet (2,3). It helps to maintain the health of our bones and teeth. The active form of vitamin D is also formed in the bone building osteoclasts (4). The parathyroid works in conjunction with the kidneys to regulate vitamin D levels and ensure that adequate calcium levels are kept at an optimum level in the bloodstream.

The active form of vitamin D is also formed in immune cells. It has been found that patients with inflammatory bowel conditions such Crohn’s disease and ulcerative colitis, which are autoimmune diseases, have greater disease activity and poorer outcomes when they have low levels of circulating vitamin D (5). Some studies have shown that it may help to improve the symptoms of inflammatory bowel disease (6, 7).

A deficiency in vitamin D may lead to an increase in inflammation in the body which may result in insulin resistance and diabetes (8).

Vitamin D has been discussed recently in relation to the current COVID-19 outbreak where low serum levels of vitamin D have been linked with a higher rate of contracting respiratory tract infections (9). Of the people who have contracted COVID-19, a significant number regardless of their age, ethnicity, sex and race have lower levels of circulating vitamin D (10).

How do we get vitamin D from the sun?

Vitamin D is made by cholesterol in the skin reacting with optimum levels of ultra violet light. This inactive form is converted to calcidiol in the liver. It can be stored here or in our fat cells (11) to be used when it is needed. It is the parathyroid that stimulates production of the active form of vitamin D, calcitriol, in the kidneys when the body needs it.

The amount of time we are recommended to stay in the sun is not exact with anything from 5 to 30 minutes between 10am and 3pm at least twice a week (RNHRD.NHS.UK) with no sun screen. We are also advised to have our arms, legs or back exposed to ensure we can make sufficient amounts of the vitamin (RNHRD.NHS.UK). If you are out in the sun adjust any recommendations to suit your skin type and the severity of the sun and always remember to put sunscreen on afterwards if you are staying out.

Vitamin D from supplements

Vitamin D levels decline in the winter and spring (12) when the body has used up its supplies and then we are solely reliant on food sources or supplements. It is UVB light, in particular, that the body needs to make vitamin D and in the UK it is only at the optimum wavelength from late March to late September (13). There isn’t a great deal of consistency over the minimum we need to stay healthy. In the UK, if you want to supplement vitamin D, the recommended amount for healthy adults is 400IU or 10mcg (14) whereas in Europe and the US it is 600IU or 15mcg (15,16).  

There is also a lack of consensus on the levels of circulating vitamin D we need in our bodies for it to function well. This is because most of the studies have been done in vitro or on animals. At the moment, if you have a test and the results show that your serum levels are below 25nmols/l, this is seen as a severe deficiency that requires supplementation. If your measurements are between 25 and 50nmols/l, there is an insufficiency that requires you to eat vitamin D rich foods and possibly supplement. A measurement of 50nmols/L is deemed to be sufficient although older adults should aim for 75nmols/L (17).

Some medications can reduce the absorbency of vitamin D and Vitamin D may also react with some medications so please check with your healthcare provider before supplementing. Taking high doses of vitamin D is advised against. It is also unclear whether it is harmful to take higher doses of vitamin D without vitamin K. Vitamin K helps to put calcium into the bones and teeth. It can also prevent calcium building up in soft tissues such as blood vessels (18). It is therefore advisable to ensure that you consume foods that contain both vitamin D and vitamin K. Foods rich in vitamin K are dark green leafy vegetables such as kale, spinach and broccoli. You should find that you will get enough if you are eating a varied and balanced diet. Please be aware that you also have to careful when consuming foods rich in vitamin K if you suffer from any blood clotting disorder.

Why might we have lower levels of Vitamin D in our bodies

People suffering from liver (19) or kidney disease (20) or other health problems may have lower levels of the active form of vitamin D in the body.

We may differ genetically in how well be process vitamin D in the body and so there will be variations in levels that we can achieve from the sun and from food and supplements (21).

We may not be able to absorb vitamin D efficiently from our digestive tract (22).

People who are overweight or obese have also been found to have a deficiency in vitamin D (23).

Studies investigating the use of moderate levels of sun protection factor (SPF) in sunscreens have found no increased risk of using these sunscreens and having lower levels of circulating vitamin D (24). However, there are very few studies looking at the effects of using very high SPF sunscreens that are currently recommended and how these may affect vitamin D levels in the body.

Spending a lot of time indoors will decrease the amount of vitamin D your body is able to make from the sun. It is UVB light that is used to make vitamin D and even although you will be getting light through a window, the UVB light will be blocked (25).

Ageing can reduce the amount of vitamin D that our bodies are able to produce. This can be because kidney function has reduced or because the skin is producing less vitamin D (26).

If you are generally fit and healthy and have a balanced and varied diet, you shouldn’t need to worry about your vitamin D levels. You can also supplement the recommended amount in the winter months to ensure you don’t develop a deficiency.

Vitamin D tests are available privately but are not carried out as standard by the NHS. You have to have a health condition that may put you at risk of a deficiency or show a symptom of vitamin D deficiency.

This article is intended to provide information on how to maintain your health. It does not recommend treatments and the information provided has not been proven to be effective against Coronavirus. Please consult your GP or healthcare practitioner before taking supplements.

For the latest public health information from CDC: https://www.coronavirus.gov

For the latest research information from NIH: https://www.nih.gov/coronavirus


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