Vitamin D is often thought of as a supplement for the cooler months, but as natural medicine practitioners, we know its importance extends far beyond supporting immunity during winter cold and flu season. Vitamin D also plays critical roles in calcium absorption and bone health, immune modulation, cardiovascular function, insulin signalling and metabolic health, and mental health.¹ Ensuring patients maintain optimal levels all year round can prevent a host of subclinical and chronic conditions.

Seasonal, Geographical and Individual Considerations

The two main sources of vitamin D are sun exposure and oral intake, including vitamin D supplementation and dietary intake.² Previtamin D3 is formed in the skin when provitamin D3 (7-dehydrocholesterol) absorbs ultraviolet B (UVB) radiation from sunlight. The angle of the sun affects how much UVB we’re exposed to and therefore how much vitamin D we can synthesise endogenously.^3,4

The angle of the sun varies considerably with several factors: geographic location (different latitudes), season, time of day, cloud cover, altitude, air pollution, and reflection (off snow, water and sand).^4,5 Additionally, the amount of skin exposed to the sun, the length of time spent in the sun, wearing sunscreen, being behind glass, and age determine how much (or how little) vitamin D is produced.^3,4

Skin pigmentation is another factor that affects vitamin D synthesis. People with darker skin have more melanin pigment. The melanin pigment absorbs ultraviolet radiation (UVR), which protects underlying skin from damage caused by UVR, but also reduces the UVR available for vitamin D synthesis in the skin.⁶ This makes certain populations – particularly those with darker complexions living in southern Australian latitudes – more prone to suboptimal levels year round.

For comprehensive information on sun exposure recommendations, refer to the Australian Skin and Skin Cancer Research Centre (ASSC) Position Statement: Balancing the Harms and Benefits of Sun Exposure, available from https://www.assc.org.au/wp-content/uploads/2023/01/Sun-Exposure-Summit-PositionStatement_V1.9.pdf.

Health Implications of Low Vitamin D

Vitamin D deficiency or insufficiency is associated with a wide spectrum of conditions. Musculoskeletal issues such as osteoporosis are well-established associations. Immune dysregulation can manifest as increased susceptibility to recurrent respiratory infections, potential exacerbation of autoimmune conditions and even cancer. Evidence also links low vitamin D to type 2 diabetes mellitus (T2DM) and cardiovascular disease (CVD), as well as depression, schizophrenia, multiple sclerosis (MS), chronic obstructive pulmonary disease (COPD), and other chronic conditions. Children, adolescents, and pregnant and breastfeeding women have an increased physiological demand for vitamin D.⁷

Deficiency is generally defined as serum 25-hydroxyvitamin D [25(OH)D] levels <50 nmol/L, while levels between 50 to 75 nmol/L are considered insufficient for optimal health (suboptimal). Many experts advocate for maintaining levels between 75 to 125 nmol/L, though the definition of optimal levels remains debated.^3,7 In Australia, it is advised that serum 25(OH)D concentrations should be maintained at ≥50 nmol/L at the end of winter or early spring in the general population, and ≥75 nmol/L in people diagnosed with osteoporosis.⁸

Serum 25(OH)D Monitoring

When taking all of this into consideration, year-round monitoring vitamin D is crucial. Seasonal checks can guide targeted supplementation rather than reactive dosing during winter.

However, in Australian, routine serum 25(OH)D testing is not recommended unless the patient is at high risk of deficiency (i.e. limited skin exposure to UVB, dark skin, fat malabsorption, obesity, end-stage liver or kidney disease, or certain medications). Measuring vitamin D status consistently can be difficult and results from different immunoassays can be highly variable.⁸

Most natural medicine practitioners would agree that the scope of this high-risk patient group is not broad enough to encompass the scope of deficiency in Australia, nor respect the full range of benefits optimal vitamin D status has to offer.

Is Enough, Enough?

In 2011 to 2012, the Australian Bureau of Statistics (ABS) reported that, around 76% of Australian adults had sufficient levels (>50 nmol/L) of vitamin D. This is nowhere near optimal and serves to highlight that almost a quarter of Australians were in fact deficient.^9,10 Newer data reveals a slight decrease in deficiency (down to 20.6% Australian adults in 2022 to 2024),⁹ but that Australians – especially women and older adults – have increased their use of vitamin D supplements, overall rising from 3.5% in 2011 to 2012 to 9.3% in 2023.¹¹ This signals that the public is now more aware of the benefits of supplementing with vitamin D, and that there’s potentially a greater need for it.

Vitamin D3 Supplementation

Supplementation with cholecalciferol (vitamin D3) is preferred, since it is more effective at raising serum 25(OH)D levels and lowering parathyroid hormone (PTH) levels than ergocalciferol (vitamin D2).¹²

However, there is a lack of consensus about the recommended vitamin D supplementation regimen (doses, frequency/administration schedule, treatment duration, etc.), though daily oral cholecalciferol supplementation generally seems to be beneficial for improving vitamin D status.¹³

In Australia, vitamin D3 supplementation is recommended only for people who are vitamin D deficient (including asymptomatic people), and dose is categorised based on degree of deficiency:⁸

• 1000 to 2000 IU/day in mild deficiency (30 to 49 nmol/L)
• 3000 to 5000 IU/day for 6 to 12 weeks, or 50,000 IU once a month for 3 to 6 months, in moderate deficiency (12.5 to 29 nmol/L) and severe deficiency (<12.5 nmol/L)

Additionally, all patients are recommended to achieve the recommended dietary intake (RDI) for calcium.⁸

These Australian guidelines narrowly focus on treatment of deficiency, which differs from guidelines elsewhere in the world, where prevention of deficiency using cholecalciferol (and sunbathing) is recommended and individualised.⁷ The same overseas guidelines also recommend that serum 25(OH)D retesting be considered 8 to 12 weeks after, but no later, depending on dose of cholecalciferol therapy.⁷

Overall, dosing should be individualised and informed by baseline serum 25(OH)D levels, age,¹⁴ body weight (obesity),^14,15 race/ethnicity, genetics,^14,16 and comorbidities (extending to photosensitivity,¹⁷ dermatological disorders associated with low levels,² diseases and medications influencing efficient absorption and metabolism of vitamin D,¹⁴ etc.).

Clinical Takeaway

Vitamin D is not just a winter nutrient. Year-round monitoring allows for early identification of insufficiency. This is particularly relevant in patients with darker skin, limited sun exposure, chronic conditions, and those with recent history of vitamin D deficiency. Other patients can be monitored less frequently. Maintaining optimal vitamin D status through adequate dietary intake, safe sun exposure and strategic supplementation can support immune resilience, musculoskeletal integrity, cardiometabolic health, and overall wellbeing. Practitioners should regularly reassess serum 25(OH)D to ensure target levels are achieved safely. By proactively addressing vitamin D levels, practitioners can help patients achieve better long-term outcomes beyond cold and flu season.

References

1. Office of Dietary Supplements. Vitamin D [Internet]. Bethesda: National Institutes of Health; 2025 [cited 2026 Feb 13]. Available from: https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

2. Kechichian E, Ezzedine K. Vitamin D and the skin: an update for dermatologists. Am J Clin Dermatol. 2018 Apr;19(2):223-235. DOI: 10.1007/s40257-017-0323-8

3. Healthline. Vitamin D 101 — a detailed beginner's guide [Internet]. San Francisco: Healthline; 2019 [cited 2026 Feb 16]. Available from: https://www.healthline.com/nutrition/vitamin-d-101

4. Wacker M, Holick MF. Sunlight and vitamin D: a global perspective for health. Dermatoendocrinol. 2013 Jan;5(1):51-108. DOI: 10.4161/derm.24494

5. Cancer Council NSW. Factors that affect UV radiation levels [Internet]. Woolloomooloo: Cancer Council NSW; 2026 [cited 2026 Feb 13]. Available from: https://www.cancercouncil.com.au/cancer-prevention/sun-protection/understanding-uv-radiation/factors-that-affect-uv-radiation-levels/

6. Webb AR, Kazantzidis A, Kift RC, Farrar MD, Wilkinson J, Rhodes LE. colour counts: sunlight and skin type as drivers of vitamin D deficiency at UK latitudes. Nutrients. 2018 Apr;10(4):457. DOI: 10.3390/nu10040457

7. Płudowski P, Kos-Kudła B, Walczak M, Fal A, Zozulińska-Ziółkiewicz D, Sieroszewski P, et al. Guidelines for preventing and treating vitamin D deficiency: a 2023 update in Poland. Nutrients. 2023 Jan;15(3):695. DOI: 10.3390/nu15030695

8. NPS MedicineWise. Vitamin D supplementation in musculoskeletal health: what's new? [Internet]. Sydney: Australian Commission on Safety and Quality in Health Care; 2019 [cited 2026 Feb 17]. Available from: https://www.nps.org.au/news/vitamin-d-supplementation

9. Australian Bureau of Statistics. National Health Measures Survey: information on biomarkers of chronic disease and nutrition including selected health risk factors [Internet]. Canberra: Australian Government; 2025 [cited 2026 Feb 13]. Available from: https://www.abs.gov.au/statistics/health/health-conditions-and-risks/national-health-measures-survey/2022-24

10. Australian Bureau of Statistics. Vitamin D [Internet]. Canberra: Australian Government; 2013 [cited 2026 Feb 13]. Available from: https://www.abs.gov.au/articles/vitamin-d

11. Australian Bureau of Statistics. Dietary supplements: information on consumption of dietary supplements (including fibre and protein) in Australia [Internet]. Canberra: Australian Government; 2025 [cited 2026 Feb 13]. Available from: https://www.abs.gov.au/statistics/health/food-and-nutrition/dietary-supplements/latest-release

12. Balachandar R, Pullakhandam R, Kulkarni B, Sachdev HS. Relative efficacy of vitamin D2 and vitamin D3 in improving vitamin d status: systematic review and meta-analysis. Nutrients. 2021 Sep;13(10):3328. DOI: 10.3390/nu13103328

13. Giustina A, Bilezikian JP, Adler RA, Banfi G, Bikle DD, Binkley NC, et al. Consensus statement on vitamin D status assessment and supplementation: whys, whens, and hows. Endocr Rev. 2024 Sep;45(5):625-654. DOI: 10.1210/endrev/bnae009

14. Mazahery H, Von Hurst PR. Factors affecting 25-hydroxyvitamin D concentration in response to vitamin D supplementation. Nutrients. 2015 Jun;7(7):5111-5142. DOI: 10.3390/nu7075111

15. Ekwaru JP, Zwicker JD, Holick MF, Giovannucci E, Veugelers PJ. The importance of body weight for the dose response relationship of oral vitamin D supplementation and serum 25-hydroxyvitamin D in healthy volunteers. PLoS One. 2014 Nov;9(11):e111265. DOI: 10.1371/journal.pone.0111265

16. Powe CE, Evans MK, Wenger J, Zonderman AB, Berg AH, Nalls M, et al. Vitamin D-binding protein and vitamin D status of black Americans and white Americans. N Engl J Med. 2013 Nov;369(21):1991-2000. DOI: 10.1056/NEJMoa1306357

17. Rhodes LE, Webb AR, Berry JL, Felton SJ, Marjanovic EJ, Wilkinson JD, et al. Sunlight exposure behaviour and vitamin D status in photosensitive patients: longitudinal comparative study with healthy individuals at U.K. latitude. Br J Dermatol. 2014 Dec;171(6):1478-1486. DOI: 10.1111/bjd.13325