How do I know whether I have enough vitamin D, and should I be supplementing?
A blood test (25-OHD) tells you whether you have a deficiency; if your value is below 20 ng/mL, supplementing is worthwhile, especially for your bones. Vitamin D has no proven effect on your heart.
A blood test gives you the answer: the value measured is called 25-hydroxyvitamin D (abbreviated 25-OHD). If it falls below 20 ng/mL (or 50 nmol/L), doctors classify this as a deficiency. If you fall between 20 and 30 ng/mL, your level is insufficient but not yet a full deficiency. Above 30 ng/mL is considered adequate. If you have a family history of bone problems, spend little time outdoors, or have darker skin, such a test is certainly worth having.
Pay attention to symptoms as well: typical signs of a deficiency include symmetrical lower back pain, weakness in the upper-leg or gluteal muscles, muscle pain, and a throbbing pain in your bones that occurs when you press on your sternum or shinbone. These complaints do not go away on their own by supplementing; a measurement is needed first.
If you want to know whether you should supplement: the strongest evidence applies to preventing fractures and falls in adults. A daily minimum of 700 to 800 IU of vitamin D demonstrably reduces that risk. For children and infants, vitamin D is essential for healthy bone development; 400 IU per day through diet or supplement is the standard recommendation. When a deficiency has been established, a doctor may prescribe a higher temporary dose, followed by a daily maintenance dose of 800 to 1000 IU of vitamin D3.
You do not need to take vitamin D for cardiovascular protection. Low levels are associated with more cardiovascular problems in observational research, but large randomised trials show no protective effect whatsoever from supplementation on heart attacks or strokes. Supplementing serves no purpose in this regard.
One caveat about the blood test: genetic variations in a protein that transports vitamin D in your blood can mean that the same measured value represents more or less active vitamin D in one person than in another. The standard measurement is therefore not equally reliable for everyone. If you are in doubt and have symptoms that do not match your test result, discuss this with your doctor.
All claims are based on one primary review study (PMID 19835345) and additional studies (PMID 32183681, 25883412, 18676559). The cardiovascular null finding comes from Mendelian randomisation studies and RCTs (PMID 32183681), which makes the evidence stronger in that area. The genetic variability claim (PMID 25883412) is observational and preliminary.
Does vitamin D prevent me from breaking bones?
Vitamin D alone does not demonstrably reduce the risk of bone fractures. Combined with calcium it does help, especially in older adults and people with a deficiency.
Should you take vitamin D together with vitamin K2?
For people with osteoporosis there is reasonable evidence that vitamin K2 and D together do more for bone density than either one alone; for healthy people without bone problems, the necessity of combining them has not been demonstrated. When in doubt, consult your doctor, especially if you are taking high doses of vitamin D.
How do I know whether my bones are strong enough (osteoporosis)?
With a DXA scan and, if needed, a FRAX risk calculation, you can have your GP check whether your bones are strong enough -- especially if you have risk factors or have already broken a bone from a minor fall.
What does sunlight do for your bones, apart from vitamin D?
Outside of vitamin D, there is preliminary evidence that red and near-infrared light can directly stimulate bone cells, but this has been demonstrated mainly with controlled light therapy, not with ordinary sunlight. Bear in mind that unprotected exposure to sunlight always carries the risk of skin damage.
How much vitamin D should you take per day?
For healthy adults under 75, extra vitamin D is probably not necessary, but for children, people aged 75 and older, pregnant women, and those at high risk of diabetes, supplementation is worthwhile. If you are unsure whether you belong to a risk group, it is worth discussing this with your general practitioner.
Can you get too much vitamin D?
Too much vitamin D is genuinely harmful, but that risk only arises at blood levels far above the recommended values. Stick to the dosage on the packaging, and unsure about your level? Have it measured by your GP.