Do you need more Vitamin D this winter?

With the shorter days and decrease in our sun exposure, maintaining optimal Vitamin D levels becomes important in winter - both for general health and athletic performance. 

Often referred to as the “sunshine vitamin,” Vitamin D plays a critical role in immune function, muscle health, bone strength, and recovery - all areas that can suffer during the colder months when natural synthesis from sunlight drops.

In this expert piece, performance dietitian Cushla Holdaway (pictured), who holds a Master's in Sports and Exercise Nutrition, explains the essential functions of Vitamin D, why your levels might fall in winter, and how it impacts everything from immunity to sport-specific performance.

Cushla writes:
Vitamin D is one of the fat-soluble vitamins (A, D, E, and K) and is an important part of health, performance, growth, immunity, calcium absorption, and protein synthesis. It is a modulator for ~2,000 genes! 

Due to its role in antimicrobial peptides in respiratory cells, it is thought to influence susceptibility to influenza and the common cold.

Blood concentrations of vitamin D (serum 25-hydroxyvitamin D) are the best indicator and ideally levels >75nmol/L should be maintained (reference range in NZ blood test labs 50 – 150nmol/L). 

Higher vitamin D status appears to be important for muscle strength, reducing fatigue and recovery, but there have been mixed results in human studies compared with animals which highlights the complexity of extrapolating findings to humans.

Table 1. Vitamin D blood concentration reference ranges (5)

(definitive cut offs for Vitamin D are less scientifically established compared with other clinical markers)

Signs and symptoms of vitamin D deficiency

·  Elevated parathyroid concentration.

·  Muscle weakness or discomfort.

·  Weakened bones, increased risk of bone stress injuries, reduced bone density.

·  Possibly low mood, depressive symptoms, and seasonal affective disorder.

·  Reduced immunity, increased susceptibility to respiratory illnesses.

·  Serious deficiencies can lead to osteomalacia (softening of bones), known as rickets in children.

Although a ‘vitamin’, it is unique in the fact that our needs can be entirely met from the sun. Ultraviolet (UVB) radiation in sunlight on our skin initiates conversion of active Vitamin D in the body. It is important to note glass blocks most UVB rays, so sitting in the sun indoors doesn’t help!

We can also consume small amounts in our diet through foods such as oily fish (100g canned salmon contains 320IU), fortified dairy products, and egg yolks (160IU in two eggs). Note, 40 IU is equivalent to 1ug (related units).

What influences our vitamin D status?

·  Genetics.

·  Body fat percentage.

·  Regular sunscreen use.

·  People living in institutional organisations.

·  Babies partially/exclusively breasted by a Mum who is deficient.

·  Increasing age as vitamin D synthesis reduces by ~75% at age 70y.

·  Time of day (solar zenith angle), atmospheric pollution, cloud cover, ozone layer.

·  Skin pigmentation (including many people from South Asia, Africa & the Middle East).

·  Medical conditions that affect vitamin D or calcium metabolism, e.g. liver failure, renal failure, IBD, coeliac disease.

·  Wintertime latitude >35° north or south (approx. south of Auckland) and/or those who spend very little time outdoors around midday between May and August in NZ.

·  People with minimal sun exposure due to religious, cultural, personal or medical reasons (e.g. full coverage clothing, history of skin cancer, taking photosensitive medicines like Accutane).

So, living in New Zealand, should we be supplementing?

Short answer, yes in winter time and especially if you meet several of the risk factors above.

In my experience, when a blood test is taken on a healthy and active individual, most sit in the sub-optimal or deficient categories by mid to late winter unless they have been supplementing.

My advice is to consider requesting a pre- and end of winter vitamin D test via your relevant health professional to guide appropriate supplementation. This will also allow you to safely monitor dose and effect which is key. Alternatively, you can request your own at any NZ blood test lab (~$30 to $70) or via I-screen at a higher cost of $142.

How much vitamin D do we need?

If your levels are optimal, the Endocrine Society recommends 1,500 – 2,000IU/day to maintain sufficient levels when adequate sun exposure is not attainable.

Ideally choose oral cholecalciferol (vitamin D3) and ensure it is batch tested i.e. Informed Sport or HASTA.

As vitamin D synthesis via the skin is self-limiting, it is unlikely high sun exposure will cause toxicity. However, high level supplementation (>4,000IU daily) pose more of a risk, especially when taken longer term (>6 months).

Depending on sun exposure during summer, you may consider supplementing all year around. Generally, safe sun exposure of 5-min (very fair skinned) to 30-min (darker skin) to arms, legs, and back around noon several times per week is enough to maintain sufficient status in warmer months. However, balance is key particularly in NZ where we have very high skin cancer rates!

Lastly, although vitamin D is very important, it is not the magic bullet and adequacy of this should be considered amongst a well-rounded diet and lifestyle.

Cushla is an Accredited Sports Dietitian based in Mid-Canterbury, specialising in nutrition for endurance athletes from beginners to elites. Cushla is also the dietitian for the New Zealand Defence Force (Burnham), runs online education courses, and hosts a podcast ‘Nourish Your Potential.’

With a BSc in Human Nutrition and a Master of Dietetics (dist), she offers personalised consultations across New Zealand, both in-person and online. 

Passionate about running and evidence-based nutrition, she helps athletes optimise performance and health through practical, realistic, and individualised advice.

Catch-up reading in the series so far: 

Part 1 - click here

Part 2 - click here

Part 3 - click here

References

1.     Health New Zealand | Te Whatu Ora. (2024). Companion statement on vitamin D and sun exposure in pregnancy and infancy in Aotearoa New Zealand. Retrieved from https://www.tewhatuora.govt.nz/publications/companion-statement-on-vitamin-d-and-sun-exposure-in-pregnancy-and-infancy-in-new-zealand

2.     Ministry of Health and Cancer Society of New Zealand. (2012). Consensus statement on vitamin D and sun exposure in New Zealand. Retrieved from https://www.health.govt.nz/publications/consensus-statement-on-vitamin-d-and-sun-exposure-in-new-zealand

3.     The Medicines Optimisation Team. (2017). Primary care guidelines for the management of vitamin D deficiency in adults. Retrieved from https://swlimo.southwestlondon.icb.nhs.uk/wp-content/uploads/2021/06/Vitamin-D-Guidelines-for-Adults.pdf

4.     NZ Food Composition Database. Retrieved from https://www.foodcomposition.co.nz/

5.     GSSI. The Importance of Vitamin D for Athletes. Retrieved from https://www.gssiweb.org/en/sports-science-exchange/article/sse-148-the-importance-of-vitamin-d-for-athletes

6.     Giustina A, Bilezikian JP, Adler RA, et al. Consensus Statement on Vitamin D Status Assessment and Supplementation: Whys, Whens, and Hows. Endocr Rev. 2024 Sep 12;45(5):625-654. doi: 10.1210/endrev/bnae009. PMID: 38676447; PMCID: PMC11405507. Retrieved from https://pmc.ncbi.nlm.nih.gov/articles/PMC11405507/

7.     Holick, M.F., Binkley, N.C., Bischoff-Ferrari, H.A., Gordon, C.M., Hanley, D.A., Heaney, R.P., Murad, M.H., & Weaver, C.M. (2011). Evaluation, treatment, and prevention of vitamin D deficiency: An Endocrine Society clinical practice guideline. J Clin Endocrinol Metab, 96(7), 1911-1930

8.     Kuwabara A, Matsumoto M, Hatamoto Y, Fujita S. Vitamin D and muscle health: insights from recent studies. Curr Opin Clin Nutr Metab Care 2024;27(6):499-506. Retrieved from https://pmc.ncbi.nlm.nih.gov/articles/PMC11486990/pdf/cocnm-27-499.pdf

9.     Examine. Retrieved from https://examine.com/?srsltid=AfmBOooAF_HKvHfC9A4QWhB63OXXkDuHfjAT7b7I4r3Phz7RzcxotPS_