The NZ Vitamin D Problem: Why Most Kiwis Are Deficient

TL;DR: New Zealand has one of the highest rates of vitamin D insufficiency in the developed world. Our latitude (34–47°S), sun-safety messaging, and modern work/lifestyle patterns mean most Kiwis don’t produce enough vitamin D from May to August. Supplementing with vitamin D3 + K2 is one of the most impactful foundational health decisions you can make.

New Zealand sits between 34°S and 47°S latitude. During winter months (May–August), UVB rays are insufficient for vitamin D synthesis in much of the country — particularly the South Island. A landmark NZ study found that 73% of New Zealand children aged 5–14 had vitamin D levels below optimal (75 nmol/L) during winter.

Citation: Rockell, J.E. et al. (2006). Season and ethnicity are determinants of serum 25-hydroxyvitamin D concentrations in New Zealand children aged 5-14 y. The Journal of Nutrition, 135(11), 2602-2608. https://doi.org/10.1093/jn/135.11.2602

Immune Function

A major meta-analysis of 25 randomised controlled trials (over 11,000 participants) found that vitamin D supplementation reduced the risk of acute respiratory infections by 12% overall, and by 70% in those with severe deficiency.

Citation: Martineau, A.R. et al. (2017). Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ, 356, i6583. https://doi.org/10.1136/bmj.i6583

Bone Health & Healthy Ageing

Vitamin D is essential for calcium absorption. A 2019 meta-analysis confirmed that vitamin D supplementation (especially with calcium) significantly reduced fracture risk in older adults. When paired with K2, calcium is directed into bone rather than arteries.

Citation: Yao, P. et al. (2019). Vitamin D and Calcium for the Prevention of Fracture: A Systematic Review and Meta-analysis. JAMA Network Open, 2(12), e1917789. https://doi.org/10.1001/jamanetworkopen.2019.17789

Mood & Mental Health

Research increasingly links low vitamin D to depression. A 2023 umbrella meta-analysis of 41 studies found that vitamin D supplementation significantly reduced depressive symptoms, particularly in those with clinical depression.

Citation: Musazadeh, V. et al. (2023). Vitamin D protects against depression: Evidence from an umbrella meta-analysis on interventional and observational meta-analyses. Pharmacological Research, 187, 106605. https://doi.org/10.1016/j.phrs.2022.106605

Cardiovascular Health

A 2023 analysis from the D-Health trial (over 21,000 Australian participants — similar latitude to NZ) found vitamin D supplementation reduced major cardiovascular events by 9%. Small difference with further studies required.

Citation: Thompson, B. et al. (2023). Effect of Vitamin D Supplementation on Major Cardiovascular Events: D-Health Randomised Controlled Trial. BMJ, 381, e075230. https://doi.org/10.1136/bmj-2023-075230

Why D3 + K2?

Vitamin K2 (MK-7 form) directs calcium to bones and teeth rather than arteries. Taking D3 without K2 may increase calcium absorption but not optimise where that calcium goes. The synergy between D3 and K2 is supported by narrative reviews on bone and cardiovascular health.

Citation: van Ballegooijen, A.J. et al. (2017). The Synergistic Interplay between Vitamins D and K for Bone and Cardiovascular Health: A Narrative Review. International Journal of Endocrinology, 2017, 7454376. https://doi.org/10.1155/2017/7454376

This is not medical advice. BioHackNZ provides general information based on published research. Always consult your healthcare provider before starting any supplement regimen. Individual results vary.

Last updated: 12 May 2026 | BioHackNZ Research Team

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