Vitamin D, when to test and how much to take

What vitamin D does and what the test measures

Vitamin D helps the body regulate calcium and phosphate, which keep bones and muscles healthy [1]. Most of it is made in the skin in sunlight, and it is found naturally in only a few foods, such as oily fish, red meat, liver and egg yolks [1].

The blood test measures 25-hydroxyvitamin D, the form that reflects your body’s stores. UK experts use 25 nmol/L as the level below which the risk to bone and muscle health rises [2].

Low levels are common in the UK, and they follow the seasons. In national surveys, around 30% to 40% of people were below 25 nmol/L in winter, compared with 2% to 13% in summer, and 16% of adults in London stayed below it even in summer [2].

Who should take a vitamin D supplement

UK advice is that everyone aged 4 and over needs about 10 micrograms of vitamin D a day, all year round [2]. Between October and early March the sun in the UK is not strong enough for the skin to make enough, so most people should consider a supplement in those months [3].

Some people are advised to take 10 micrograms every day of the year, because their skin makes little vitamin D even in summer [1].

• People who are rarely outdoors, or who live in a care home
• People who cover most of their skin when outside
• People with dark skin, for example of African, African Caribbean or South Asian family origin
• Anyone who is pregnant or breastfeeding [3]

When a blood test helps, and when it does not

NICE advises against routine vitamin D testing in people who feel well. A test makes sense if you have symptoms of deficiency, or a particularly high risk, such as very little exposure to sunlight [4].

If you simply follow the UK advice on a daily supplement, you usually do not need a test first. A test is most useful when a doctor needs to confirm a deficiency, set a treatment dose or check that treatment has worked.

How much vitamin D is safe

For adults, the safe upper limit for vitamin D from all sources is 100 micrograms (4,000 IU) a day, a level UK toxicity experts agreed with [2]. Too much over a long period can raise calcium in the blood, which can harm the kidneys and heart [2].

Labels give vitamin D in micrograms, international units or both, and 10 micrograms is the same as 400 IU [3]. Higher doses are sometimes used for a time to correct a proven deficiency, under a doctor’s care. Avoid taking several products that each contain vitamin D without adding up the total.

Benefits and limits of vitamin D supplements

UK advice on vitamin D is built on protecting bone and muscle health across the population [2]. For people aged 60 and over, the Great Britain claims register allows a label to say that vitamin D helps reduce the risk of falling linked to unsteadiness and muscle weakness, with the effect obtained at 20 micrograms a day from all sources [5].

Vitamin D has not been shown to protect against other diseases. UK experts found too little evidence to draw firm conclusions on cancer, heart disease, multiple sclerosis or type 1 diabetes [1], and a trial of 25,871 older adults found that 2,000 IU a day did not lower the rate of cancer or major heart events over a median of 5.3 years [6].

Results and what happens next

Results are given in nmol/L. A level below 25 nmol/L is the threshold UK experts use for low vitamin D status [2], and a doctor will explain what your number means and whether you need more than the standard daily dose.

• Given my skin and how much time I spend outdoors, do I need a test or simply a daily supplement?
• What dose should I take, and for how long?
• Do any of my medicines or conditions change the safe dose for me?
• When, if ever, should we test again?

How The Wellness can help

The Core Wellness blood test includes vitamin D alongside a full blood count, liver tests, lipids and HbA1c, and a doctor explains the results with you. If you would rather talk first, a GP appointment gives you time to decide together whether a test will change what you do.

Every doctor is GMC registered, and samples go to accredited laboratories. Once you and your doctor know what you need, our shop is there when you want it.