Vitamin D & Winter

It is always better to get your nutrition from food, unless you live with a disease that causes issues around absorption.

However vitamin D, the unique vitamin created in your skin from sunlight, that also acts as a hormone, is less available in foods. Most people get enough with safe exposure to the sun, although this is affected by the seasons and other factors.

In some cases vitamin D supplementation is a good idea. Vitamin D deficiency can be serious and lead to:

  • Increased risk of bone fractures and reduced bone density in adults
  • Risk of rickets causing bowed legs and knocked knees in children

Why bones? Vitamin D helps your body absorb calcium, which makes and keeps bones strong. Your life long bone density is reached in teenage years, as such childhood and teenage nutrition is important to prevent bone density issues later in life.

  • There is also suggestions that low vitamin D status can have a knock on effect on depression, cardiovascular diseases, autoimmune diseases, multiple sclerosis, type 1 diabetes and schizophrenia.

So for whom and when should people supplement, and how much by?

People more at risk of deficiency

People who may be more at risk inlcude:

  • The elderly in particular who may spend less time outside
  • Anyone spending more time inside (due to a pandemic for example)
  • People at high risk of skin cancer who may avoid sun exposure
  • Regularly wearing clothing that covers all or most of the skin
  • Living at more extreme latitudes such as the South Island in New Zealand (May-August), or Nordic countries during Northern winter months
  • Kidney, liver disease and the medication may increase risk of vitamin D deficiency
  • Having darker skin, as it takes longer to top up your vitamin D levels, as well as experiencing any of the above risk factors

How to tell you are deficient?

The only gold standard way to test for deficiency is to go to your GP, and ask for a blood test of your 25-hydroxy D (25-OHD). In Australia and New Zealand vitamin D tests measure the amount of 25-OHD in the blood, which is a form of vitamin D. The measurement is in nano mols per Litre of blood. A nano mol is a measurement that is very small, and is common in measuring particles, atoms and molecules in blood and other solutions.

New Zealand & Australian Measurements
Hypercalcaemia (toxicity)220 nmol/L
High levels125.0 nmol/L
Recommneded amount
and over
50.0 nmol/L
Below recommended25.0-49.9 nmol/L
Moderate deficiency< 25.0 nmol/L
Severe deficiency<12.5 nmol/L
Retreived from &

If people are at risk then they can take a multi faceted approach to dealing with vitamin D deficiency.

Dealing with Deficiency

Sun Exposure

This is the quickest way to top up your vitamin D. However there are some caveats. First of all chemical or physical sunscreen blocks your bodies ability to absorb UVB, and create vitamin D. So exposure needs to be at safe times to reduce skin cancer risk.

Making sure sun exposure is safe and limited, means no sun beds. If living in extreme northern or southern latitudes you may need at least 40 minutes of full body sun at noon in winter. However this may not be easy, or the sun may be too weak even then. If you have darker skin it will take even longer.

If living in countries with more sun, be careful aim for exposure early or late in the day for less time – max 20 minutes. In winter you may get enough sun just from exposed hands/arms and face.


There are food sources of vitamin D. Natural sources include:

  • oily fish, cod liver oil
  • milk and milk products
  • eggs
  • mushrooms that have been left out in the sun (gills up)
  • liver

Fortified food products include:

  • margarine and fat spreads
  • some reduced-fat dairy products
  • plant based dairy substitutes
  • liquid meal replacements

Check ingredients lists, The amounts in food can be quite low, so a supplement may be needed if the sun and food are not adequate.


A supplement in the form of D3 may be most appropriate in situations such as:

  • Living in an extreme latitude, especially with dark skin in winter months.
  • Elderly people who get out less and may also have reduced appetites in winter.
  • Children with auto-immune diseases.
  • Pregnancy and Lactation in winter.

Supplements are available in many forms. The standard amount is 25 micro grams (ugrams). Or they may be 1000 International Units (IU). An international unit is just another type of measurement, 1000 IU is 25 ugrams. One a day, over winter should replenish vitamin D levels to recommended levels.

What else can you do?

There are a few other things you can do to help maintain a normal vitamin D range, and healthy bones.

Adequate calcium

Having enough dietary calcium will help maintain higher concentrations of 25-OHD in the body, as well as supporting bone health. Low calcium may be an issue for people who have absorption problems. Which means reduced vitamin D levels may be a result of poor calcium absorption.

As such eating calcium containing and fortified foods is integral to not only bone health but also vitamin D status.

Increase physical activity

It is not clear whether increased physical activity helping with vitamin D status is due to more sun exposure, or better mobilisation of fat stores (also where vitamin D is stored). Studies have shown indoor and outdoor physical activity increased blood levels of vitamin D (Fernandes & Dos Reis Barreto Junior, 2017). Overall, higher levels of physical activity are usually a predictor of good vitamin D status and physical activity is better for you in the long run.


People cannot get toxicity from vitamin D produced from the sun.

Hypercalcaemia from supplements is from excess vitamin D causing a build up of calcium in the blood, leading to nausea, vomiting, weakness and frequent urination. It is rare to get toxicity from supplementation. In rare cases, infants and older adults may experience toxicity. Typically though the rare cases are from excessive doses that are not appropriate for age and body size.

Taking the typical supplement amount is unlikely to lead to toxicity.


Vitamin D is essential for good bone health and may be linked to a other mental, metabolic and auto-immune diseases. If you live in New Zealand’s south island or Nordic countries in winter, you may want to look at supplementation. Look for 25 ugrams or 1000IU, take one a day. You do not need any more than that.

Above all else, eat a balanced diet full vegetables, fruit, whole foods, and proteins that suit your dietary choices.


Fernandes, M, R. & Dos Reis Barreto Junior, W. (2017, June). Association between physical activity and vitamin D: A narrative literature review, Revista da Associacao Medica Brasileria, 63(6); 550-556. Retrieved from

Vitamin D, The Ministry of Health (2018, July 5), retrieved from

Nowson, C.A. etal., (2012, June 18). Vitamin D and the health in adults in Australia and New Zealand: A position statement, The Medical Journal of Australia, 196(11). Retrieved from

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