Vitamin D is one of the most widely recommended supplements. It's also one of the most misunderstood, so it's worth separating what it does in the body from what the evidence says about taking extra.

What vitamin D actually is.

Vitamin D behaves more like a hormone than a typical vitamin. Your skin makes it when it's exposed to UVB sunlight, and smaller amounts come from food. Your liver and kidneys then convert it into its active form. In the UK, the sun is too low from roughly October to March for our skin to make it, so we rely on stores built up over summer, plus food and supplements.

What it does in the body

Bone health. This is its best-established role. Vitamin D helps your gut absorb calcium and helps stop your kidneys losing it. Without enough, bones can't mineralise properly. Severe deficiency causes rickets in children and osteomalacia (soft, painful bones) in adults. Milder, long-standing inadequacy can also push the body to draw calcium out of bone, which may contribute to bone loss over time.

Muscle function. Vitamin D receptors are found in muscle tissue, and deficiency is linked to muscle weakness and poorer balance. That's part of why it matters for falls risk in older adults, though as we'll see, extra vitamin D doesn't appear to improve strength or reduce falls in people who are already replete.

Immune function. Many immune cells have vitamin D receptors, and it helps regulate immune responses. Trials suggest a small reduction in respiratory infections, mostly in people who were deficient to start with. However a later analysis of VITAL published in the BMJ found that taking 2,000 IU/day of vitamin D3 for over five years reduced incident autoimmune disease by 22%, and when combined with marine omega-3 fatty acids further decreased autoimmune disease rates by 31%.

Other roles. Vitamin D is involved in cell growth and differentiation, and observational studies link low levels with a wide range of conditions, including heart disease, some cancers, depression and type 2 diabetes. That's where the story gets complicated, because observational links don't show that supplements fix the problem. Low vitamin D can also be a marker of poor health, less time outdoors, or higher body fat.

What the VITAL trial tested

VITAL is one of the largest randomised trials of vitamin D. It gave 25,871 generally healthy US adults (men 50+, women 55+) either 2,000 IU of vitamin D3 daily or a placebo for a median of 5.3 years.

Compared with placebo, vitamin D did not significantly reduce major cardiovascular events, total invasive cancer, fractures or falls. Participants weren't selected for deficiency, low bone density or osteoporosis, and their average starting level was around 77 nmol/L, which is comfortably adequate.

There was a possible signal for lower cancer mortality in secondary analyses, but that needs confirming before anyone should rely on it.
What VITAL does and doesn't prove

It suggests that taking extra vitamin D when your levels are already adequate is unlikely to prevent fractures, falls, heart disease or cancer.

It doesn't prove that vitamin D is unimportant, and it says very little about people who are deficient, housebound, have malabsorption, or have diagnosed osteoporosis. It also tested one dose in a US population, so it can't tell us about UK winters or other dosing strategies. And it wasn't a menopause trial.

Put simply, VITAL challenges the idea that "more is better", not the idea that we need enough.

What this means for menopause and bone health

Around menopause, falling oestrogen speeds up bone loss, which is why bone health becomes a priority for many women. Vitamin D is part of the foundation here, but it isn't a treatment for osteoporosis. Nothing in the evidence suggests it can reverse bone loss on its own.

If you have osteoporosis or take bone medication, adequate vitamin D matters because those treatments work best when levels are sufficient, and some can lower blood calcium. UK guidance from the Royal Osteoporosis Society says people with osteoporosis are often advised to take a supplement year-round, and may be prescribed 20 micrograms (800 IU) daily.

Bone health also depends on:
• Enough protein and calcium (1200mg of calcium per day if diagnosed with osteoporosis, otherwise it’s 700 mg)
• Magnesium, vitamin K2 and other minerals such as zinc, boron and silicon
• Resistance training and impact exercise, which load the bones
• Hormonal factors, including whether HRT is suitable for you
• Avoiding smoking and excess alcohol
• Overall nutritional status

Who's most likely to need a supplement

UK public health advice is that everyone should consider a daily 10 microgram (400 IU) supplement from around October to March. Year-round supplementation is advised for people who:

• spend little time outdoors, for example housebound people or care-home residents
• keep their skin covered when outside
• have darker skin, such as African, African-Caribbean or South Asian backgrounds
• have osteoporosis or take osteoporosis medicines

Food helps but rarely covers the gap. Oily fish, egg yolks, red meat, liver and fortified foods contain some vitamin D, but it's hard to reach the recommended amount through diet alone.

Do you need a test?

A level below 25 nmol/L is generally regarded as deficient. If you don’t know what your level is I recommend testing and if your GP won’t test I can help.

The takeaway

Vitamin D matters, and genuine deficiency should be corrected. But the evidence doesn't support taking high doses "just in case" when your levels are adequate. A better question than "which supplements should I take?" is "what does my body actually need?"