Vitamin D is not really a vitamin. Your body synthesises it in skin exposed to UVB light, converts it in the liver and kidneys, and it then acts on receptors found in almost every tissue, including bone, muscle, immune cells and the testes.
That last point matters here: the cells producing testosterone carry vitamin D receptors, and correcting a genuine deficiency modestly raises testosterone in deficient men. Not dramatically, and not in men who are already sufficient.
Between October and March, UK sunlight is too weak for the skin to make any vitamin D at all. The angle of the sun means UVB does not reach the ground in sufficient quantity.
This is why UK guidance recommends everyone consider a supplement through autumn and winter, and why deficiency is the norm rather than the exception at this latitude.
Nine factors raise risk further, and this check scores them: skin tone, time outdoors, sunscreen use, covering clothing, body weight, age, digestive conditions, certain medications, and diet.
Two deserve particular note. Darker skin requires substantially longer sun exposure to make the same amount, because melanin absorbs UVB. And higher body weight lowers circulating levels, because vitamin D is fat-soluble and distributes into fat tissue.
The blood test measures 25-hydroxyvitamin D. UK thresholds: below 25 nmol/L is deficiency, 25 to 50 nmol/L is insufficiency, and above 50 nmol/L is considered adequate.
Some sources argue for higher targets. The evidence for benefits above 75 nmol/L is weaker than commonly claimed.
Testing is worth it if you have risk factors or symptoms. Otherwise, a standard supplement through winter is cheaper than the test.
UK guidance suggests 10 micrograms, or 400 IU, daily through autumn and winter for the general population.
People with several risk factors often need more, and 1,000 to 2,000 IU daily is commonly used and well within safe limits. Doses above 4,000 IU daily should be supervised, because vitamin D is fat-soluble and can accumulate.
Take it with a meal containing fat, which substantially improves absorption. Vitamin D3 is more effective at raising blood levels than D2.
Often nothing, which is why it goes unnoticed. When symptoms occur they are vague: fatigue, muscle aches, low mood, frequent infections, and bone pain in severe cases.
Because those overlap with almost everything else, vitamin D is worth checking rather than assuming, particularly if you have several risk factors.
Related reading: Sunlight and testosterone · Protecting your bones · The supplements guide · What causes low testosterone
Educational only, not medical advice.