Vitamin D is known as the ‘sunshine vitamin’. This is because vitamin D is made in our body in response to UVB rays hitting our skin. Some vitamin D is found in foods, but only quite small amounts. Like most other vitamins, vitamin D has many important roles in the body – from supporting absorption of calcium for our bones, to maintaining a healthy immune system, to supporting muscle function.
Vitamin D is a fat-soluble vitamin, like vitamins A, E and K. Because of this, it is more easily stored in the body than water-soluble vitamins such as vitamin C. Our vitamin D stores should build up at sunny times of the year (or when we’re taking supplements) and then reduce slowly over winter or when we’re not exposed to sunlight (or not taking supplements).
Primary functions of Vitamin D
- Vitamin D helps us regulate the body’s calcium levels. It is needed for us to absorb adequate calcium, but it also ensures that enough calcium is reabsorbed in the kidneys too, rather than excreted in the urine. Having enough calcium is not only important for our bones and teeth, but for nerve impulses to travel around our body, for our muscles to work properly and for our blood to clot normally.
- Vitamin D is needed for strong bones and teeth. Primarily through ensuring optimal absorption of calcium and another important bone mineral – phosphorus. Vitamin D is also thought to promote mineralization of collagen in bone (i.e. binding of minerals such as calcium into the bone).
- Vitamin D is important for immune function. The cells of our immune system have receptors for vitamin D – this means that when vitamin D binds to the cells, it has an effect on their activity. Vitamin D is needed for the production of potent anti-bacterial proteins called ‘cathelicidin’ and ‘beta defensin’, which help to defend against infection. It is also said to have a role in regulating the immune response, i.e. preventing over-activation of the immune system.
- Vitamin D plays a role in our muscle function. It is thought to have a role in the production of muscle proteins. Muscle weakness is a common symptom of vitamin D deficiency, and supplementing vitamin D has been found to improve muscle strength in people who are deficient.
- Vitamin D is also thought to help protect the brain. There are receptors for vitamin D in various areas of the brain. It is thought to have an effect on nerve growth, and production of substances that regulate nerve growth and survival.
Are you getting enough vitamin D through sunlight?
Knowing whether we’re getting enough vitamin D through sunlight is quite complicated. The amount of vitamin D synthesized in our skin can depend on several factors: the time of the year and time of day, our skin pigmentation, our age, our body weight, how much of our skin we’re exposing, and whether we use sunscreen.
A light-skinned person who is sunbathing in full sun with no sunscreen in the middle of summer may make as much as 10,000 to 20,000 IU (international units) of vitamin D with just 10 to 12 minutes of sun exposure. For someone with very dark skin, it could take as long as two hours to make this amount in the same conditions. And applying a sunscreen of just SPF-10 can reduce vitamin D production in the skin by 90%!
In winter – November to March – anywhere further north than 35 degrees latitude (London is 51 degrees) doesn’t receive enough direct UVB rays for any vitamin D production in our skin, even if we go outside with bare arms and legs on a sunny day. This means that many people in the UK and around the world may become deficient in vitamin D over winter, even if we build up good stores in the summer.
So it’s important to get our sun exposure when we can, and spend at least some time in the sun without sun protection. Of course, we also have to be careful not to overdo it, due to the risk of skin cancers and sunburn. The US-based Vitamin D Council advises exposing our skin to sun without protection for half the time it takes for the skin to turn pink. After that, we should cover up with clothes, shade or sunscreen. See Vitamin D Council’s website for more information: https://www.vitamindcouncil.org/. 
Food sources of vitamin D
Foods generally provide quite small amounts of vitamin D – rarely enough to make up for lack of sunlight exposure.
The best natural food sources of vitamin D are:
- Fatty fish, such as salmon, mackerel and sardines. These may contain between around 200 and 500 IU (5 to 12.5µg) of vitamin D per 100g.
- Two eggs may contain between around 35 and 80 IU of vitamin D (just 1 or 2µg).
- Beef kidney and liver – around 16 to 32 iu per 100g (less than 1µg)
- Shiitake mushrooms – 20 iu per half cup (less than 1µg).
Other food sources tend to be ‘fortified’ foods, such as breakfast cereals and milk, where vitamin D has been added into the food and is not naturally occurring.
So we can see that these amounts are a lot lower than we can get in even just a few minutes of sunlight!
How much do we need in our diet?
This is a difficult one because it can depend how much vitamin D we are getting through sunlight (if at all).
The EU nutrient reference value for vitamin D for adults is just 400 IU (10µg). Nutrient reference values refer to the amount needed to ensure that the needs of nearly all the population (97.5%) are being met. However, some researchers believe that most people need more than that minimum value in order to prevent deficiency in absence of sun exposure. In supplement form, the Vitamin D Council recommends as much as 5000 IU per day for adults and 1000 IU per 25lb (11kg) body weight for children at times when we’re not getting any sun exposure.  See Dosages below for more information and our guidelines.
Deficiency signs and symptoms*
Symptoms of vitamin D deficiency may include:
- Muscle weakness and pain
- Bone weakening, osteoporosis, rickets or osteomalacia (‘adult rickets’)
- Immune system dysregulation (meaning either reduced immune defences or ‘over-activation’ of the immune system, e.g. in autoimmune conditions)
- Reduced cognitive function (reasoning, memory, attention, etc.)
- Low mood – especially linked to SAD (seasonal affective disorder)
*If you experience any of these symptoms, please consult your doctor or health practitioner.
Forms and bioavailability
The two types of vitamin D available in supplement form are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is the form made in our body in response to sunlight. Vitamin D2 is found in plants – it has a slightly different chemical structure to vitamin D3. Both forms can be used by our bodies, and both are actually inactive precursors and have to be converted through two further stages in the body to become active vitamin D.
While some studies suggest that vitamin D2 and D3 supplements have equal effects, some clinical trials have found that vitamin D3 is more effective at raising the body’s vitamin D stores compared to D2. For this reason, vitamin D3 is generally considered the better form to go for, and most modern vitamin D supplements are in the form of D3. Until recently, D3 supplements were all animal-sourced, but now vegan-friendly vitamin D3 supplements are available, sourced for example from lichen.
Adults: Vitamin D supplements for adults can provide dosages varying from 400 IU to 5,000 IU or even more. So how do you know how much to take? The best starting point is to get your vitamin D level tested. Your doctor can do this, or you can order a blood spot test for vitamin D at http://www.vitamindtest.org.uk/. When you receive the results, it’s a good idea to speak to your doctor or your health professional (e.g. nutritional therapist) to help you decide which dose to take (if any).
Here are our nutritionist’s guidelines for those who want to supplement vitamin D without seeking individual professional advice.
- 1000–2000 IU: for adults who already have an adequate level of vitamin D, but are not getting much sunlight and need to maintain this level, for example over winter. If you have not had a test done, you may be in this category at the end of the summer or in autumn if you have had regular sun exposure over the summer, or if you have recently had sun exposure on a sunny holiday.
- 3000–5000 IU: for adults who are deficient in vitamin D and need to increase their level.
For those who are taking more than 1000 IU per day, it’s also advisable to get a re-test every three months to check whether the dose needs to be adjusted up or down, or stopped completely.
Children: Children’s vitamin D supplements tend to provide between around 300 IU for infants to 400–600 IU for children of 3 and over. Practitioners may advise more than this where required.
What to look out for when buying a supplement
As well as deciding what dosage you need to take, there are a few other factors that may help you decide which supplement to go for:
- Vitamin D3 is generally considered a better form than D2 (most individual vitamin D supplements are D3, but it can be wise to check).
- Vitamin D is fat-soluble. This means it is generally better absorbed in an oil-based liquid or gel capsule rather than as a dry tablet or capsule. However, emulsified forms – which often come as liquids or sublingual sprays – can be even better for absorption, because the fat is broken down into tiny droplets to aid absorption.
- Do you need a vegan-friendly vitamin D? Most D3 supplements are made from lanolin (from sheep’s wool) or from fish liver oil, so you need to check that the label/product description says ‘vegan’.
If you are taking any medications or have any medical condition, please consult your healthcare practitioner before taking vitamin D.
Because vitamin D is stored in the body, it is possible to take too much if you are using high doses over a long period of time. As mentioned above, for those who are taking more than 1000 IU per day, it’s advisable to get a re-test every three months to check whether the dose needs to be adjusted up or down, or stopped completely.
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