Vitamin D in winter: how to keep your levels up through the cold months

Australian winters are mild by global standards. We don't get the months of darkness Scandinavian countries do. Daytime temperatures in Sydney rarely drop below 8°C. The sun is technically out for nine hours a day even in mid-July.

And yet — vitamin D deficiency rates roughly double in Australia between May and September. In the southern states, more than one in three adults is deficient by the end of winter. The problem isn't temperature. It's UVB.

Why winter beats vitamin D

Your skin makes vitamin D when ultraviolet B radiation (specifically wavelengths between 290 and 315 nanometres) hits cholesterol molecules in the skin. Two things have to be true for this to work:

1. UVB has to reach your skin
2. The UV index has to be above about 3 for meaningful production

In summer, both happen easily. In Australian winter, both get harder.

The angle problem

UVB is absorbed by the atmosphere. The lower the sun sits in the sky, the more atmosphere UVB has to travel through, and the more gets filtered out before it reaches the ground. In Melbourne (latitude 37°S), the sun in June sits low enough that UVB is largely scattered before it hits you. In Hobart (latitude 43°S), it's worse — there are weeks in midwinter where no useful UVB reaches the ground at all.

This is the same physics that makes Scandinavian winters bad for vitamin D. We just flip the calendar.

The behavioural problem

Even on a clear winter day with workable UV, most Australians aren't outdoors at midday with skin exposed. We're at desks, in classrooms, in cars, in clothing that covers most of the skin. The hours when UVB is strongest — 10am to 2pm — are also the hours when almost everyone is indoors.

The UV index rule

The Cancer Council and Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) publish UV index data daily. The practical rule for vitamin D production:

• UV index above 3: meaningful vitamin D synthesis is possible. Sun protection is still recommended for skin cancer prevention, but incidental exposure to arms and face for a few minutes contributes meaningfully to your level.
• UV index 3 or below: negligible vitamin D production. No amount of standing outside will move the needle.

In Brisbane and tropical northern Australia, UV index stays above 3 most of winter. In Sydney, you get useful UV most clear days, but cloudy weeks contribute almost nothing. In Melbourne, Adelaide, Hobart, and most of southern Australia, the UV index sits at or below 3 for most of June and July. Your skin simply isn't making meaningful vitamin D for weeks at a time.

What this means in practice

Pre-winter testing

The smartest move is to know where you're starting from. A vitamin D test in March or April — before the worst of winter — tells you whether you have headroom to coast through, or whether you need to start supplementing now.

If your level is above 100 nmol/L in March, you can probably maintain through winter with minimal supplementation. If you're below 75 nmol/L, you're likely heading into deficient territory by August unless you do something.

Two groups where the winter risk is sharper

Most adults can ride out winter with a low-effort supplementation routine. Two groups should plan it more carefully:

• Athletes training through winter, particularly indoor sports and field sports played at dusk or dawn — the combination of low UV, sun-protective kit, and high turnover puts athletes at extremely high deficiency rates by August. See our Australian guide to vitamin D for athletes for the sport-by-sport breakdown.
• Women whose third trimester falls in winter — demand rises while UV synthesis collapses, and the baby's stores at birth track the mother's level. Our Australian guide to vitamin D in pregnancy covers the trimester-by-trimester targets.

Winter sun exposure: still worth it

On clear winter days when UV is above 3, brief midday exposure of arms, hands and face (without sunscreen on those areas) still contributes. The Cancer Council's specific recommendation for vitamin D in winter:

• Southern Australia (Tasmania, southern Victoria, southern South Australia): 2–3 hours of midday sun exposure per week across the body, in segments. Achievable but unrealistic for office workers.
• Mid-latitude Australia (Sydney, Melbourne, Adelaide, Perth): 30–60 minutes of midday sun exposure most days of the week.
• Northern Australia (Brisbane, Darwin, Cairns): Standard summer rules — minimal incidental exposure is enough.

Realistically, almost no working adult in southern Australia gets this much winter sun. Which leaves option two.

Winter supplementation

For most Australians, the math through winter looks like this: limited sun + limited dietary vitamin D = falling levels unless you supplement.

Standard winter supplementation for adults is 1,000–2,000 IU of vitamin D3 (cholecalciferol) daily, taken with a fat-containing meal. For people who started winter with a low level, with darker skin, or with a higher body weight, doses up to 4,000 IU daily may be needed.

Without a test, you're guessing whether your dose is doing the job. With a test, you can confirm.

The food contribution

Food in Australia contributes very little to vitamin D status. Unlike the US (where milk is mandatorily fortified) or Finland (where margarine and dairy are fortified at high levels), Australian fortification is limited. The realistic dietary contribution from a normal Australian diet is 100–300 IU per day — enough to delay deficiency, not prevent it.

The richest natural sources, if you want to maximise the food contribution:

• Wild salmon: ~400–600 IU per 100g
• Sardines (tinned): ~270 IU per 100g
• Mackerel: ~360 IU per 100g
• Egg yolks: ~40 IU each
• Mushrooms exposed to UV: variable, can be high if labeled "vitamin D enhanced"

The spring rebound

The good news: levels recover. UV index in most of Australia climbs back above 3 from September onwards. For people who supplement through winter and stop in spring, levels typically rebound naturally over 4–6 weeks of normal outdoor activity — even without sustained sun exposure, just incidental daily UV.

This is why the testing rhythm we recommend is twice a year: late winter (August) to confirm your supplementation strategy is working through the hard months, and late summer (February) to confirm you've topped up and have a buffer heading into autumn.

The simplest winter strategy

For an average Australian adult living in Sydney/Melbourne/Adelaide/Perth, the lowest-effort winter strategy is:

1. Test in March to know your baseline.
2. If you're above 75 nmol/L, start a maintenance dose of 1,000 IU daily in May.
3. If you're below 75 nmol/L, start 2,000 IU daily in May.
4. Get any clear-day UV you can over winter (lunch walks, weekends outside).
5. Retest in August to confirm the strategy worked.

That's it. The whole loop costs less than a single GP appointment and takes about ten minutes a year.

Ready to test? Our at-home Vitamin D Test uses the same gold-standard LC-MS/MS method run in hospital pathology labs. From $49.95/kit (2-pack) with free Australia-wide shipping. Results in 3–5 days, no GP referral required.