South Island winters make low vitamin D more likely, but sunlight production does not simply switch off from May to August. Your exposure and individual circumstances matter. 1

Why South Islanders run low through winter

UVB reaching the skin starts vitamin D production. Winter's lower sun reduces the available UVB, while cold weather means more covered skin and fewer opportunities for exposure. 1

NZ guidance recognises some synthesis even in southern winters. That does not mean a lunchtime walk will meet everyone's needs, particularly when little skin is exposed. 1

Older skin produces vitamin D less efficiently. Darker pigmentation, limited outdoor time and conditions affecting fat absorption can also increase the risk of low levels. 2

Keep sun protection in the picture. SunSmart encourages outdoor activity while protecting skin. If your exposure is limited, discuss supplements rather than deliberately extending unprotected time outdoors. 3

What the NZ data actually shows

The 2008/09 NZ Adult Nutrition Survey found 4.9% of adults had serum vitamin D below 25 nmol/L. A further 27.1% were between 25 and 50 nmol/L: about one-third below 50 overall. 4

Deficiency was more common among Māori and Pacific adults. The survey had too few Asian participants providing blood samples for reliable estimates, so it cannot establish a South Asian prevalence figure. 4

Deficiency peaked from August to October. During those months, adults in the southern region were about three times as likely to be deficient as northern adults, after adjustment for age, sex and ethnicity. 4

These are historical population findings, not a current estimate or a diagnosis for every South Island resident. They also do not show that everyone's summer stores disappear by August. 4

What vitamin D actually does

The liver converts vitamin D to 25(OH)D. Further conversion, mainly in the kidneys, produces the active hormone calcitriol. Its roles include calcium absorption, bone mineralisation, muscle function and immune signalling. 2

Biology is not the same as a supplement benefit. Trials have not established vitamin D as a general treatment for low mood or a reliable way to improve glucose control in everyone. 2

In VITAL, 25,871 US adults received 2,000 IU daily or placebo for a median 5.3 years. Vitamin D did not significantly reduce total, hip or nonvertebral fractures. 5

Participants were not selected for deficiency, low bone mass or osteoporosis. The result argues against routine supplementation for fracture prevention in similar adults, not against treating established deficiency. 5

Some groups differ. The 2024 Endocrine Society guideline supports supplementation in adults aged 75 and older and selected other groups, including high-risk prediabetes alongside lifestyle changes. 6

Blood levels and when testing helps

The usual status test is serum 25(OH)D. NZ consensus guidance identifies levels below about 25 nmol/L as deficient and uses 50 nmol/L as a practical adequacy benchmark, interpreted with clinical circumstances. 1

A result between 50 and 75 nmol/L is not automatically inadequate. The 2024 prevention guideline does not establish a universal blood target for better health, including a 75 to 125 nmol/L longevity range. 6

Healthy adults generally do not need routine screening or repeat testing to adjust supplements. This recommendation does not cover every clinical situation or replace investigation of suspected disease. 6

Testing may help when there is suspected severe deficiency, unexplained low calcium, atypical osteoporosis or unexplained proximal limb pain in an older person. A clinician may recommend supplementation without testing. 1

If testing is indicated, late winter or early spring can capture a seasonal low. The survey's August-to-October pattern is not an individual timetable or a guarantee about the rest of your year. 4

Dosing context for NZ adults

Separate prevention from treatment. Neither 1,000 nor 2,000 IU daily guarantees a particular blood result: response varies with starting level and individual factors. 2

For adults aged 50 and older who need supplementation or treatment, the Endocrine Society favours daily lower doses over intermittent higher doses. This is a conditional recommendation, not proof that every monthly regimen is harmful. 6

The NZ Vit.D3 data sheet includes 50,000 IU monthly for some treatment situations. Follow your prescription, and ask your prescriber before changing its dose or frequency. 7

The US adult upper intake limit is 4,000 IU daily from all sources. It is not a suggested starting dose or a guarantee of safety for every person. 2

Taking vitamin D with a meal containing some fat can improve absorption, although absorption still occurs without it. Eggs, fish or a meal with olive oil are practical options. 2

What about K2 and magnesium?

Vitamin K supports proteins involved in bone and calcium regulation. That provides a rationale for studying K2, but does not establish that adding MK-7 to D3 prevents fractures or arterial calcification. 8

K2 is not a proven safeguard against excessive vitamin D. If you take warfarin, do not add a vitamin K supplement without advice from the clinician managing your anticoagulation. 8

Magnesium participates in vitamin D metabolism. A 180-person randomised trial found that supplementation changed vitamin D metabolites differently depending on starting levels. 9

It measured biomarkers, not fewer fractures or longer life. The finding does not establish that everyone taking vitamin D also needs a magnesium supplement. 9

If you are comparing magnesium products, read which magnesium supplement is best before adding another supplement.

What food can contribute

Food is useful, even when it is not enough on its own. Oily fish such as salmon and sardines contribute vitamin D; egg yolks and liver also contain it. 3

Some dairy products, plant milks and spreads are fortified. Check the label rather than assuming all milk contains the same amount. Fortification and your usual portions both matter. 3

Practical options include sardines on toast, salmon with kūmara, or a vitamin D-fortified milk with breakfast. These are food choices, not a treatment prescription or a measured daily ceiling. 3

Diet alone can make adequate intake difficult. If deficiency is established, food should support rather than replace the treatment agreed with your clinician. 1

What to do this week

  • Review your outdoor routine and food sources. Keep being active outside while following sun-protection advice. 3
  • If exposure is very limited or you have relevant health concerns, ask whether you need supplementation, testing or neither. 1
  • Bring all supplement labels to your GP or pharmacist so they can check the combined vitamin D and calcium amounts. 7
  • Seek advice before supplementing if you have kidney disease, kidney stones, high calcium or sarcoidosis, or take medicines such as thiazide diuretics or digoxin. 7
  • If treatment is prescribed, agree how it will be reviewed. Do not automatically increase the dose or book repeat tests to chase a longevity target. 6