Grey skin in February. Tiredness that settles in as the days get shorter. And at the chemist, vitamin D sprays and tablets lined up by the till.
Your body makes vitamin D itself, in the skin, when it gets UVB rays from the sun. Food alone struggles to cover your needs, so the sun does most of the work. In a British winter, it hardly does any.
What winter sun no longer does
In 1988, in Boston, Michael Holick's team exposed human skin, and the molecule the skin turns into vitamin D, to sunlight. From November to February, even on clear days, nothing formed. In Edmonton, Canada, further north, this blank period ran from October to March. Further south, around 34 and 18 degrees of latitude, winter sun still did the job (Webb et al., 1988).
London sits at 51.5 degrees, almost level with Edmonton. Manchester and Edinburgh are further north still. The NHS says the same thing in plain terms, between October and early March we do not make enough vitamin D from sunlight. The stores you build in summer run down all winter.
A large European analysis re-measured blood samples from 55,844 people with a standardised method. Over the year, 13% had a level below 30 nmol/L. For samples taken between October and March, it was 17.7%. Using a cut-off of 50 nmol/L, the share rose to 40%. People with darker skin were far more affected (Cashman et al., 2016).
Some women start at a disadvantage. About 85% of women who see a doctor about the menopause are low in vitamin D, and the gut absorbs it less well with age. With PMOS, low levels are very common and go with a poorer response to insulin. With endometriosis, it matters too, because vitamin D helps regulate the immune system.
Why official advice skips the test
The NHS simply advises everyone to consider 10 micrograms, or 400 IU, a day in autumn and winter, without a test. In 2024, the US Endocrine Society also advised against routine testing in healthy adults (Demay et al., 2024).
Their reasoning holds up. The VITAL trial gave 2,000 IU a day or a placebo to nearly 26,000 Americans over 50, for five years on average. The supplement did not lower the number of cancers or of heart attacks and strokes (Manson et al., 2019). Most participants had decent levels at the start. Giving vitamin D to someone who already has enough does not do much.
That is exactly why I have levels tested. The women I work with who have PMOS, endometriosis or are going through the menopause are not the general population these guidelines are written for. A starting figure tells you whether you are deficient, and by how much. Without it, you either take a supplement blind or skip one you actually need. Ask your GP for the test.
Reading the result
The test measures 25-hydroxyvitamin D, the storage form. UK labs report it in nmol/L, while some others use ng/mL, and 1 ng/mL equals 2.5 nmol/L. Below 30 nmol/L counts as deficient. Below 50 nmol/L is generally seen as insufficient. These are the two cut-offs used in the European analysis above.
The season matters. A sample taken in February will not give the same figure as one taken in September, and you need to know that to compare two results.
If you are deficient, 2,000 to 4,000 IU a day usually corrects it safely. 4,000 IU is also the ceiling the NHS says not to go over unless your doctor advises otherwise. Taken above that over a long period, calcium builds up in the blood and can weaken bones and damage the kidneys and heart (NHS). Your doctor sets the amount and plans the follow-up test.
Getting out into morning light is still a good habit all winter, even for ten minutes, because it resets your body clock. For vitamin D between October and March, the blood test is what tells you where you stand.
This article is for information only and does not replace medical advice.