Why Auckland Winters Crash Your Vitamin D (And What to Do About It)

Reviewed by Dr. Michael Blandy (NZCC, Sports Chiropractor, 20 years) and Dr. Jun Chung (AWC Founder). Last reviewed May 2026.

Why we wrote this

If you live in Auckland or south of it, your body genuinely cannot make adequate Vitamin D from sun exposure between April and September. This is not marketing fear. It is a result of UV physics, latitude, and skin biology. Many of our AWC patients have low D3 on bloods in winter, and most do not realise it is happening.

The 1-line summary

Auckland sits at roughly 37°S. Between April and September, the sun's angle is too low to deliver UVB strong enough for Vitamin D synthesis in skin. Even sitting outside in winter sun for hours produces little to no Vitamin D. Supplementation through winter (800–2,000 IU per day) is the practical fix, especially after 40.

The UV physics in plain terms

Vitamin D is made when UVB rays (specifically 290–315 nm) hit the cholesterol in your skin. UVB is shorter wavelength than UVA — it gets absorbed by the atmosphere much more easily. In winter, the sun comes in at a lower angle, so it has to travel through more atmosphere to reach you. By the time UVB reaches Auckland skin in July, almost all of it has been filtered out.

UV index below 3 = essentially no Vitamin D synthesis. Auckland in June averages a UV index of 1–2 at solar noon. Below 35°S you can keep making Vitamin D year-round; above 35°S you cannot. Most of New Zealand sits above that line.

How common is winter Vitamin D deficiency in NZ?

• NZ Adult Nutrition Survey 2008/09: roughly 32% of adults below the 50 nmol/L threshold (insufficient) by late winter. Higher in Māori, Pacific peoples, and South Asian populations.
• Bolland 2007: 49% of Auckland adults had Vitamin D below 50 nmol/L in late winter.
• Common symptoms of low D3: persistent fatigue, low mood ("winter blues"), more frequent colds, muscle aches, bone tenderness, slower workout recovery.

Why this matters for Kiwis over 40

Three reasons it gets worse with age:

1. Skin synthesis drops. A 70-year-old produces roughly 25% of the Vitamin D a 20-year-old produces from the same UV exposure.
2. Bone density acceleration. Vitamin D enables calcium absorption. Low D3 + reduced oestrogen (women) or testosterone (men) compound bone loss.
3. Hormonal pathways. Low Vitamin D correlates with lower testosterone in men (Pilz 2011, PMID 21154195) and altered insulin sensitivity in postmenopausal women.

What is enough?

Goal	Daily Vitamin D3 dose	Notes
Avoiding insufficiency	800–1,000 IU/day	Most healthy adults reach 50 nmol/L on this
Optimising	1,500–2,000 IU/day	Often needed to reach 75–100 nmol/L target
Correction of deficiency	4,000–5,000 IU/day, short-term	Under GP supervision; bloods at 12 weeks
Upper safety limit	4,000 IU/day long-term (Endocrine Society)	Higher doses need monitoring

Practical NZ winter protocol

1. April: Start daily D3 800–1,000 IU with breakfast.
2. June–August (peak winter): 1,000–2,000 IU/day if you are over 40 or work indoors.
3. September–October: Continue until UV index regularly hits 4–5 (typically late October Auckland).
4. Test at 12 weeks: ask your GP for a 25-hydroxyvitamin D blood test (best winter marker). Target 75–100 nmol/L.
5. Take with fat: D3 is fat-soluble. With breakfast eggs, butter, or olive oil works.

Where WIIP fits in

WIIP Joint Comfort+ includes 800 IU Vitamin D3 per 2-capsule serve, alongside Green Lipped Mussel and L-Carnitine. For most healthy adults this covers maintenance through winter. If you have a known deficiency, your GP may recommend a higher dose separately (NZ pharmacies sell stand-alone D3 affordably).

What about sun beds, foods, oils?

• Sun beds: cancer risk vs marginal D3 gain — not recommended.
• Foods: oily fish (salmon, mackerel, sardines) provide some D3 but not enough to correct winter deficit at typical intakes.
• Cod liver oil: high Vitamin A. Use cautiously.
• Mushrooms exposed to UV: D2, less effective than D3 in NZ adults.

Risk of overdose?

Toxicity from D3 supplementation is rare and almost always involves doses above 10,000 IU/day for many months. The Endocrine Society safety limit for unsupervised long-term use is 4,000 IU/day. Stay under this without specific medical guidance.

Related reading

• Anti-inflammatory supplements NZ
• Supplements for women over 40 NZ
• Supplements for men over 40 NZ
• Supplements for runners NZ
• Joint Comfort+ (includes 800 IU D3)

References

1. Pilz S, et al. Vitamin D and testosterone in men. Horm Metab Res. 2011. PMID 21154195.
2. Bolland MJ, et al. Vitamin D insufficiency in healthy adults in Auckland. NZ Med J. 2007. PMID 17853914.
3. Charoenngam N, Holick MF. Immunologic effects of vitamin D. Nutrients. 2020. PMC7409331.
4. Holick MF. Vitamin D deficiency. NEJM. 2007. PMID 17634462.

Editorial standards: WIIP content is reviewed against the NZ Therapeutic Advertising Code 2026. Dietary supplements are not medicines and are not intended to diagnose, treat, cure, or prevent disease.

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