Getting enough Vitamin D in winter months?

Last night I watched my mum count out her medications into her monthly pill dispenser. It’s a bit of an undertaking!

You may not think of vitamins as medication, but it is important to spare them some thought. Vitamin D is a good one to think about at this time of year because our main natural source of this vitamin is affected by the seasons.

Our bodies use sunlight - UVB rays - to make Vitamin D via our skin, so when we have more reasons to stay inside (wet or cold weather for example) and when there are fewer hours of sufficient sunlight (in the winter) we have fewer opportunities to make it. UVB rays do not penetrate through glass so sitting in the sun by the window is not an effective substitute for going outside!

The research showing 3.5 serves of dairy on the risk of falls and fractures in the elderly was done on subjects whose Vitamin D levels were adequate. The magic is in the triad of protein, calcium and Vitamin D.

People at higher risk of lower Vit D status are:

  • people with darker skin – melanin offers some protection against DNA photodamage

  • people with older skin – the skin has a declining ability to synthesise Vitamin D

  • people who spend a lot of time inside – those in hospital, who have low mobility, who are bed/chair bound or frail, people with hip fractures

  • people who avoid any sun exposure – perhaps due to a history or concern with skin cancer

  • people who eat very little or no meat or eggs – they already have reduced dietary sources of Vitamin D

  • people routinely using anti-inflammatory medications (e.g. ibuprofen) or laxatives

  • people on long-term proton pump inhibitors - a decrease in stomach acidity affects the ability to absorb the proteins which carry Vitamin D  

  • people with declining kidney function – resulting in a decline in the ability to activate Vitamin D

Sources of Vitamin D

Vitamin D is a fat-soluble vitamin, so you won’t find it in lettuce but in fattier parts of animal products, such as the fatty skin of fish like tuna and salmon, and in egg yolks. Mushrooms are sometimes listed as containing Vitamin D but this is only if they have been exposed to light that contains the UVB rays needed to activate Vitamin D synthesis. So, humans have something in common with mushrooms! Button mushrooms grown in the dark without exposure to UVB rays before packaging will have negligible Vitamin D.

In NZ we don’t usually fortify our food with Vitamin D as is done in the U.S. so it is smart to do a lifestyle check.
Are you able to be outside several times a week with 10-15 mins in the sun?
Does your diet contain meat, eggs, fatty fish?  

The most reliable way to get Vitamin D is to be outside regularly with exposed skin. In the winter the best time is around noon when the UVB levels are highest. Exposing face and arms will suffice, for as little as 10 minutes, depending on the UVB levels.  There is skin damage to consider, but according to NIWA (NZ’s National Institute of Water and Atmospheric Research) damage occurs with exposure for over an hour when UV levels are above 3, which rarely happens in winter. The NIWA website provides a daily UV Index for sites throughout NZ (www.niwa.co.nz) if you want to check UV index in your area.


Testing your Vitamin D levels and taking a supplement

Testing your Vitamin D levels is expensive and not routinely necessary. The Auckland-based ViDA study completed in 2015 showed that people who already have good levels of Vitamin D (50 nanomoles/L blood) gained no real benefit from supplementation, but those whose levels are less that this will benefit from supplementation.

It doesn’t take much either – ViDA researcher Professor Scragg recommends 25 to 50 mcg daily, “particularly in winter when benefit from supplements will be greatest”. If your Vitamin D levels are already good, this level won’t hurt you, but if you’re under, it will be of benefit. The ViDA study showed that the intervention dose of 80mcg/day was safe for subjects who already had adequate Vitamin D levels.

Vitamin D supplements can be bought over the counter. Vitamin D3 (colecalciferol) is favoured for easy absorption reasons. If you think you might consider Vitamin D supplementation, run it past your GP first.  It’s always important that they know when you start taking a Vitamin or nutrition supplement.

And if you can – when it isn’t raining! - get outside for 10 minutes a day with face, hands and arms exposed to the sunlight.

In winter, when regular UVB exposure is not a given, taking a supplement is worth considering, especially for elders. 

To learn more about Vitamin D: 


To learn more about the ViDA research and results: https://www.auckland.ac.nz/en/news/2021/06/01/robert-scragg-drills-into-data-vitamin-D.html

To find NIWA’s daily UV Index: