Vitamin D: the New Zealand winter supplement

Why people take it

The body makes vitamin D from sunlight on the skin, and the sun is the main source. New Zealanders are much more likely to be low in late winter and early spring, most of all in the South Island, and few foods contain much vitamin D. It's needed for calcium absorption and bone health.

Why it comes up with GLP-1 medicines

Eating less means less of the little vitamin D food provides. Low vitamin D is also more common in people with obesity, partly because it's spread through a larger amount of body fat. Bone health matters during weight loss, since bone density can fall as weight drops.

What the evidence shows

Enough vitamin D prevents rickets in children and osteomalacia (soft bones) in adults. A blood test is how vitamin D is measured.

What it doesn't show

The VITAL trial, in more than 25,000 US adults, found vitamin D supplements did not lower the rate of cancer or major heart events overall. More isn't better: too much is harmful, as below.

Who should be careful

• →Too much vitamin D raises calcium in the blood, which can harm the kidneys and heart. The Australia–New Zealand upper limit for adults is 80 micrograms (3,200 IU) a day.
• →People with sarcoidosis or high calcium levels can be harmed by extra vitamin D, and people with kidney disease can be at risk of low levels. Talk to your doctor before taking it.
• →Vitamin D may interact with some medicines, including orlistat, statins, steroids and thiazide diuretics. Ask your pharmacist.

Food first, then a test

Oily fish, eggs and some fortified foods help a little. A blood test through your GP tells you whether you're actually low.