Reader question
“It’s the middle of winter, I’m stuck inside most of the day, and I keep reading I should be taking vitamin D. Is that true, and how much?” – Mark, aged 58.
Professor Grant Schofield’s response
Mark, this is the one supplement I’ve actually come round to. I’ve spent most of my career telling people that good food beats a cupboard full of pills. Vitamin D, along with creatine, is one of only two I now take myself.
It’s winter. A few foods give you some, mainly oily fish like salmon and sardines, plus egg yolks and liver. But food won’t get you there. Most of your vitamin D comes from the sun, or a supplement. We make it in the skin from UVB (midday) sunlight. In a New Zealand winter, at our latitude, there isn’t much of that, especially down south, so your levels fall over the cold months, right when you need them most.
How many of us are low?
A lot. In the last national survey, around a third of New Zealand adults had vitamin D below the adequate mark, and roughly 1 in 20 were severely deficient. Winter pushes those numbers higher. This survey was 17 years ago, it’s likely we are worse now.
Who’s most at risk?
If you have darker skin, you make less vitamin D from the same sun. That’s me.
Age. Your skin’s ability to make vitamin D drops about 13% per decade. A 70 year old makes roughly half what they did at 20 for the same sun.
Carrying extra body fat lowers your levels too, because vitamin D is fat soluble and gets locked away in fat tissue.
Does it actually do anything?
I think a big reason would be immune function.
The solid finding is a 2017 analysis pooling 25 trials that found vitamin D modestly cut the risk of coughs, colds and chest infections. The benefit was small. That’s because the positive effects were concentrated in people who were deficient to start with. It’s likely if you are at risk then you will benefit.
The big VITAL trial gave 2000 IU of Vitamin D3 a day to thousands of healthy, already-replete adults. It did not reduce cancer, heart disease or fractures. So it fixes a deficiency. It likely does nothing extra once you’re topped up. Even if you are topped up over summer, you can get low over winter.
My overall verdict?
If you work inside, are dark skinned, in NZ, and it’s winter, then you’ll probably benefit, especially in the South Island. There are lots of claimed benefits, but the main winter one, I think, is getting sick less and recovering faster when you do.
A cheap, easy to take and generally harmless supplement is worth it. No body likes colds and flu.
How to take it
Get a blood test if you can, although these aren’t routine in NZ unfortunately - they should be. It’s the only way to actually know your number. Below 50 nmol/L is where I’d act.
If you can’t test and you’ve got the risk factors (winter, darker skin, older, mostly indoors, heavier), a daily 1000 to 2000 IU of D3 is a sensible, safe dose for most adults. Take it with a meal that has some fat in it, as it absorbs better that way.
If you’re overweight, or you’ve got the metabolic warning signs (high blood pressure, high triglycerides, high blood sugar), you’ll need more to reach the same level. Vitamin D is fat soluble, so more body fat means more of it gets locked away. Studies show overweight people need about 1.5 times the dose, and obese people 2-3 times, to match someone lean. If that’s you, take the top of the range. One reason is that you have to activate the D3 into more usable forms in a two step process through the liver and kidney. If they aren’t healthy then you might have trouble doing that as efficiently as someone who is healthy. That’s why more might help.
One more, still early. In people with prediabetes, vitamin D slightly cut the number who went on to develop type 2 diabetes. A 2023 review of three trials put it at about 15% fewer, again strongest in those who started low. It’s not a weight loss pill, and its early research.
Pairing it with vitamin K2 and magnesium can help, since they work together. Don’t megadose. Very high doses over time can cause harm.
And get outside. A walk at lunchtime, even on a grey day, beats sitting under a light all winter. There’s a bigger reason too. In a 20-year study of nearly 30,000 Swedish women, those who avoided the sun lived shorter lives, a risk on the scale of smoking. It’s probably not all vitamin D, and it’s about regular sun, definitely not roasting yourself. Avoid burning, that is bad for you no matter what.
My last bit is advice I need as much as anyone. And if you can get a family winter holiday in the sun (Dr Fiji, Dr Rarotonga), you’ll know just how much a bit of winter sun helps everything.
This article was originally published by Stuff on 6 July 2026, as part of Grant’s HealthStuff column.
Disclaimer: This column is intended as general health information only and does not constitute medical advice. Please consult your own GP or healthcare provider for advice specific to your condition.
