By Dr Sarah Brewer
What does Vitamin D do?
Vitamin D is best known for its essential role in the absorption of calcium to maintain strong bones and teeth, but it has many other important actions, including supporting immunity. Vitamin D:
- Contributes to the normal function of the immune system in adults and children
- Helps with calcium absorption, contributes to the maintenance of bones and teeth
- Contributes to normal muscle function
- Is needed for growth and development of bones in children
Vitamin D deficiency in New Zealand
Although you can make vitamin D in your skin on exposure to sunlight, this is usually insufficient to meet your needs. In fact, you can only make vitamin D when the UV index is greater than three – as a rule of thumb, if your shadow is taller than you are, you're not producing enough vitamin D.
As a result, vitamin D deficiency becomes common during autumn and winter for people living in New Zealand, particularly in the South Island, where the UV index is usually less than three. Even when the sun is shining, many people also fail to make enough vitamin D because they, sensibly, avoid the sun, cover up or use high-SPF sunscreens.
Vitamin D foods
Because of this, it's important to get vitamin D from your diet. Vitamin D can be found in useful amounts in:
- Oily fish
- Liver products
- Food fortified with vitamin D
This can make it difficult for those following certain dietary regimes (particularly vegetarians and vegans) to get enough, or even those without particular dietary requirements during the colder months.
Vitamin D can be found in oily fish, liver, eggs and butter, so if you don't eat a lot of those foods, consider a supplement.
Vitamin D benefits and deficiency symptoms
Vitamin D supports many functions in the body, and deficiency can cause problems in anything from immunity to bones and muscles.
- Needed for the production of natural defences in the body
- Sufficient vitamin D levels help protect against colds, flu and pneumonia
Vitamin D helps to protect against a number of infections by increasing the production of proteins that are needed for the body's natural antimicrobial defences.1
Studies involving over 19,000 adults found that those with the lowest levels of vitamin D were 36 per cent more likely to develop a common cold than those with high levels, for example.2
The results from 25 clinical trials involving around 11,000 patients from 14 countries show that taking vitamin D supplements reduced the risk of acute respiratory infections such as cold, influenza and pneumonia by 12 per cent, with those who were initially lacking in vitamin D gaining the most benefit.
Because asthma is often triggered by a respiratory infection, taking vitamin D supplements can help to reduce the risk of a severe asthma attack that needs emergency treatment by as much as 61 per cent.
In pregnant women, low vitamin D levels are also associated with bacterial vaginosis (BV), which increases the risk of miscarriage and premature labour. As a result, pregnant women are advised to take a vitamin D supplement.3
Vitamin D deficiency and coronavirus
It has been suggested that one reason why a disproportionate number of people from BAME communities have experienced serious COVID-19 infections may relate to vitamin D status. This is because a higher level of melanin pigment in the skin reduces the ability to synthesise vitamin D on exposure to sunlight.
The National Institute for Healthcare and Excellence (NICE) in the UK recently carried out a 'rapid review' of five published studies to see if vitamin D might offer benefits against the current COVID-19 pandemic. Unfortunately, none of these studies were actual intervention trials of vitamin D supplements for preventing or treating COVID-19, which would have provided the strongest evidence.
Despite this, four of the studies did find an association with those having a lower vitamin D status having an increased risk of subsequently developing COVID-19. However, because four out of the five studies did not adjust for other confounding factors such as obesity and underlying health conditions, NICE concluded that, although there are health benefits associated with vitamin D, there is insufficient evidence to confirm that vitamin D might prevent or treat COVID-19.
- Needed for the absorption of calcium and phosphorous, essential for strengthening bones
- Deficiency can lead to a lack of bone mineral, particularly in post-menopausal women
Vitamin D is needed for the absorption of calcium and phosphorus, which are required in both adults and children to strengthen bones.
Lack of vitamin D is therefore a risk factor for loss of bone mineral, especially in post-menopausal women, and is therefore also a risk factor for bone fractures resulting from osteoporosis.
The results from 12 studies involving over 42,000 adults show that vitamin D supplements reduced the risk of hip fractures by nine per cent and other non-vertebral fractures by 14 per cent, with higher doses of over 10 mcg (400 IU) vitamin D per day reducing the risk of non-vertebral bone fractures by at least 20 per cent.4
- Has beneficial effects on muscle fibres
- Can reduce the risk of falling due to muscle weakness
Vitamin D also helps to maintain muscles, which in turn helps to reduce the risk of falling due to muscle weakness.
The results from 10 studies involving older adults show that taking a daily vitamin D supplement can reduce falls by 14 per cent compared with placebo.5 This is another reason why vitamin D helps to reduce the risk of bone fractures among men and women aged 60 and older.
Vitamin D3 versus vitamin D2
Vitamin D3 (cholecalciferol) is the form most important for human health, and is the best form to take as a supplement.
The plant form vitamin D2 (ergocalciferol) has a slightly different structure, which makes it less beneficial for maintaining our vitamin D levels.6
How much vitamin D do you need per day?
The nutrient reference value (NRV) for vitamin D is just 5 mcg (200 IU) per day for people aged 19 to 50, 10mcg a day for those aged 51 to 70, and 15 mcg per day for people over 70.
Your ability to make vitamin D in the skin falls as you grow older, and at least halves, or more than halves, between the ages of 20 to 80 years.
In one study, researchers found that people aged 62 to 80 years synthesised four times less natural vitamin D than those aged 20 to 30 years.7
It's therefore a good idea to take a supplement designed for your age. Healthspan's Medical Director Dr. Sarah Brewer recommends 25 mcg for adults aged up to 50 years, and 50 mcg per day thereafter.
Can you take too much vitamin D?
Excessive vitamin D intake can cause side effects due to disturbances in calcium metabolism, such as headache, loss of appetite, nausea, vomiting, diarrhoea or constipation, palpitations and fatigue.
An upper safe intake level of 80 mcg per day is therefore recommended.8
1Borella, E., Nesher, G., Israeli, E. and Shoenfeld, Y. (2014). Vitamin D: a new anti-infective agent?, Annals of the New York Academy of Sciences 1317(1), pp.76-83.
2Ginde, A.A., Mansbach, J.M. and Camargo, C.A. (2009). Association between serum 25-hydroxyvitamin D level and upper respiratory tract infection in the Third National Health and Nutrition Examination Survey, Archives of internal medicine 169(4), pp.384-390.
3Bodnar, L.M., Krohn, M.A. and Simhan, H.N. (2009). Maternal vitamin D deficiency is associated with bacterial vaginosis in the first trimester of pregnancy, The Journal of nutrition 139(6), pp.1157-1161.
4Bischoff-Ferrari, H.A., Willett, W.C., Wong, J.B., Stuck, A.E., Staehelin, H.B., Orav, E.J., Thoma, A., Kiel, D.P. and Henschkowski, J. (2009). Prevention of nonvertebral fractures with oral vitamin D and dose dependency: a meta-analysis of randomized controlled trials, Archives of internal medicine 169(6), pp.551-561.
5Kalyani, R.R., Stein, B., Valiyil, R., Manno, R., Maynard, J.W. and Crews, D.C. (2010). Vitamin D treatment for the prevention of falls in older adults: systematic review and meta-analysis, Journal of the American Geriatrics Society 58(7), pp.1299-1310.
6Houghton, L.A. and Vieth, R. (2006). The case against ergocalciferol (vitamin D2) as a vitamin supplement, The American journal of clinical nutrition 84(4), pp.694-697.
7Heaney, R.P. (2006). Barriers to optimizing vitamin D3 intake for the elderly, The Journal of nutrition 136(4), pp.1123-1125.
8Ministry of Health, Nutrient Reference Values
Dr Sarah Brewer is Healthspan's medical director. She is a GP, nutritionist and nutritional therapist and the author of more than 60 self-help books.