What is vitamin D?
Vitamin D is one of four fat-soluble1 vitamins needed by the body. Following a diet low in fat might mean that vitamin D absorption from the foods we eat is not optimal.
Vitamin D is also often called the “sunshine vitamin” because it’s made in our skin upon exposure to the sun’s ultraviolet rays.
Interestingly, despite its name, vitamin D isn’t actually a vitamin at all. Most vitamins act as cofactors for enzymes. In contrast, vitamin D is rather a prohormone because in the body it is converted into a hormone called calcitriol with the help of the kidneys.1 The hormone calcitriol (also called the active form of vitamin D) helps to regulate the amounts of calcium and phosphorus in the body and supports the amount of calcium stored in bones, helping to maintain bone health. It does this by increasing production of calcium transport proteins in the intestines, helping to increase the uptake of calcium from the foods we eat.2
When taking a vitamin D supplement, consuming this with a meal that contains good fats like avocados or oily fish, will ensure even better absorption.
Can our bodies make vitamin D?
Yes, our bodies make vitamin D when the sun’s rays touch our skin. Although getting enough sun is important for maintaining vitamin D levels in the body, too much sun may be dangerous.
Applying sunscreen and wearing sun-protective clothing can help with avoiding excess sun exposure; however less vitamin D will be produced. Exposing your arms and legs or hands, arms and face to 5-10 minutes of sunlight two to three times a week will ensure your skin is protected but able to make the vitamin D your body needs.3
Conversely, in winter or on cloudy days, the sun’s ultraviolet rays your skin is exposed to aren’t strong enough to stimulate production of all the vitamin D our bodies need.
Consuming foods rich in vitamin D and supplementation may be helpful. In fact, the Scientific Advisory on Nutrition (SACN) recommends that between October and March in the UK, everyone over the age of 5 should take a daily vitamin D supplement containing 10 micrograms (or 400 IU) of vitamin D.
If your complexion is darker4, the natural melanin content in your skin may also lead to less production of vitamin D from the sun. Additionally, spending long hours in the office or working night shifts may make you more vulnerable to a vitamin D deficiency and in turn affect bone and immune system health.5,6
Can we get vitamin D from food?
Absolutely, but food options are rather limited, especially if you’re following a plant-based diet. Tofu and mushrooms are great vegan-friendly options. Beef liver, egg yolks, cheese and oily fish are non-vegan options.
Fortified foods may be a convenient way to ensure you’re getting more vitamin D in your diet too. Look out for foods like soya milk which may be fortified with vitamin D when you next buy some groceries.
Why do we need it?
Most of us know vitamin D is important for healthy bones and teeth. This is because it helps our bodies absorb calcium but it is also essential for the proper functioning of the immune system.
There are some studies to suggest having lower vitamin D levels may make one’s immune system less able to fight off germs effectively.7
What is the difference between vitamin D2 and vitamin D3?
Vitamin D is normally provided in two different forms: Vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is found primarily in mushrooms or plant foods. Vitamin D3 is the type synthesized from ultraviolet rays and found in egg yolks or oily fish.
Vitamin D3 tends to be the preferred form as it is more potent and bioavailable, so it gets to work faster in your body. Although vitamin D2 is the less potent and less bioavailable form of vitamin D, it has the advantage of being vegetarian. Newer innovations mean that now there are algae-based forms of vitamin D3 available through your favourite supplement or online shop for those following a plant-based diet.
How much vitamin D do we need?
The amount you need can depend on circulating vitamin D levels in your body. Ideally, vitamin D levels should be at least 25 nmol/L.8 You can ask you healthcare practitioner about testing your levels – it’s a simple blood test.
The daily recommended dose of vitamin D in winter months is 10 µg per day but up to 75 µg per day is considered the maximum level for vitamin D in a food supplement according to EFSA, CRNUK, HFMA and PGAB. Always follow your healthcare practitioner’s advice.
Who is at risk of a vitamin D deficiency?
- Older adults are at risk for suboptimal vitamin D status or deficiency mainly because ageing skin can lose its efficiency in producing vitamin D from the sun’s ultraviolet rays. Older adults are also less able to convert vitamin D into its active form.
- Individuals with darker skin are at risk due to natural content of melanin in their skin, which can absorb much more ultraviolet rays.
- Because vitamin D is fat soluble, those individuals who don’t tolerate fatty foods well or who follow a very-low fat lifestyle may have trouble absorbing sufficient vitamin D from food.
- Obese individuals tend to be at risk of vitamin D deficiency as their extra body fat may bind some vitamin D reducing its circulation.
Is taking a vitamin D supplement enough to support my immune system?
Although the immune system needs vitamin D to function normally, it’s also reliant on other nutrients and environmental factors. Apart from taking a vitamin D supplement, make sure to also eat a well-balanced and varied diet and incorporate exercise in your daily routine. These are essential to your immune system function too.
1. Nair, R. & Maseeh, A. Vitamin D: The sunshine vitamin. Journal of Pharmacology and Pharmacotherapeutics 3, 118–126 (2012).
2. Hinson, J., Raven, P. & Chew, S. HORMONAL REGULATION OF PLASMA CALCIUM AND CALCIUM METABOLISM. in The Endocrine System 147–159 (Elsevier, 2010). doi:10.1016/b978-0-7020-3372-8.00012-4
3. Holick, M. F. Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. The American journal of clinical nutrition 80, (2004).
4. Brown, L. L. et al. The vitamin D paradox in Black Americans: a systems-based approach to investigating clinical practice, research, and public health - expert panel meeting report. BMC Proc. 12, 6 (2018).
5. Coppeta, L., Papa, F. & Magrini, A. Are shiftwork and indoor work related to D3 Vitamin deficiency? A systematic review of current evidences. Journal of Environmental and Public Health 2018, (2018).
6. Sowah, D., Fan, X., Dennett, L., Hagtvedt, R. & Straube, S. Vitamin D levels and deficiency with different occupations: A systematic review. BMC Public Health 17, (2017).
7. Aranow C. Vitamin D and the Immune System Cynthia. J Investig Med. (2011). doi:10.231/JIM.0b013e31821b8755
8. Scientific Advisory Committee on Nutrition. Press Notice: SACN publishes new recommendations on vitamin D. Tso (2016).