Vitamin D and bone density: the honest numbers

Vitamin D is the most over-claimed supplement in the cabinet and one of the more useful ones, which is an awkward combination. The honest version is narrower than the marketing and more useful than the backlash: vitamin D is necessary, not sufficient, and the numbers are worth knowing because they are unusually specific.

Here is the number that frames everything below: bone loss around menopause is fast, and it has been measured precisely enough to quote. Sowers' 15-year cohort of 629 women found spine bone density falling about 1.7% a year in the two years before the final period and 3.3% a year in the two years after, leaving average spine density in the last hormonal stage roughly 6.4% below the first. That is the fastest rate of loss most people will ever experience, and it happens in a window you can see coming.

What D actually does

Vitamin D's core job in bone is logistical. It drives calcium absorption in the gut and helps maintain the calcium and phosphate concentrations that mineralisation requires. Run short and your body compensates by raising parathyroid hormone, which pulls calcium out of bone to keep blood levels steady. Severe long-term shortfall is a medical matter, and it is not what most people are dealing with.

What D does not do is build bone on its own. This is where the loudest claims come apart. Reid, Bolland and Grey's 2014 meta-analysis in The Lancet pooled 23 randomised trials of vitamin D on bone mineral density in adults and found no clinically meaningful benefit at most sites — a small effect at the femoral neck, nothing worth celebrating elsewhere. LeBoff's VITAL fracture analysis in the New England Journal of Medicine in 2022 tested 2,000 IU daily in more than 25,000 adults who were mostly not deficient and found no reduction in fractures.

Read those two carefully, though. Both tested supplementation in largely replete populations. They are evidence that topping up someone who is already sufficient does very little — not evidence that vitamin D status is irrelevant. Those are different claims, and the internet routinely conflates them.

How much is enough

The 2011 Institute of Medicine reference intakes remain the sober baseline: 600 IU daily for adults up to 70, 800 IU from 71, with a tolerable upper intake of 4,000 IU daily and a target blood level of 20 ng/mL (50 nmol/L) described as sufficient for bone health in almost the whole population. The Endocrine Society's 2011 clinical practice guideline sets a higher bar for people it classes as at risk, preferring 30 ng/mL.

That gap between 20 and 30 ng/mL is where most of the internet argument lives. It is a smaller disagreement than it looks: both documents agree that going far above the range does nothing extra, and the evidence on that is unusually blunt. Burt's Calgary trial in JAMA in 2019 randomised healthy adults to 400, 4,000 or 10,000 IU daily for three years and measured bone with high-resolution imaging. The high-dose groups did not gain bone — the 10,000 IU group came out with slightly lower radial bone density than the 400 IU group.

So the practical range for most adults is roughly 1,000–2,000 IU daily through the year, more if a test says you are low, and no reason at all to chase five figures.

Frame is deliberately boring for this reason: vitamin D3 as cholecalciferol, 2,000 IU, one softgel with a meal, made in the USA. It sits at the top of the sensible range rather than the top of the marketable one. Take it with food that contains some fat — D is fat-soluble and absorption is better with a meal than on an empty stomach.

The practical bit: 1,000–2,000 IU daily with a meal, all year. Test if you want the number. Ignore anything selling 50,000 IU as a lifestyle.

Testing without guesswork

The test is serum 25-hydroxyvitamin D, usually written 25(OH)D. Rough reading of results: below 12 ng/mL (30 nmol/L) is a level to take to a clinician; 12–20 is insufficient; 20–30 is the contested band; 30–50 is comfortable; above 50 buys nothing further.

Timing matters more than people expect. A test in late summer flatters you and a test in February does the opposite, so the same person can move 10–15 ng/mL across a year without changing anything. If you only ever test once, test in late winter — that is your floor, and the floor is the number worth managing.

You do not need to retest often. Once to find your baseline, then again about three months after changing dose, and after that only if something changes. Levels move slowly and repeat testing mostly measures the season.

D3 + strength training

Here is the part the supplement conversation keeps missing. Bone is a load-responsive tissue. It adapts to mechanical strain, not to nutrients — nutrients are the raw material, load is the instruction.

Watson's LIFTMOR trial, published in the Journal of Bone and Mineral Research in 2018, is the clearest demonstration. Postmenopausal women with low bone mass did eight months of supervised high-intensity resistance and impact training, twice a week, thirty minutes a session. They improved bone mineral density at the spine and hip, plus height, function and strength — in a group that conventional advice often steers away from heavy lifting entirely. Adverse events were minimal under supervision.

The stack, in order of effect size: heavy-ish resistance training two or three times a week, enough protein to build with, adequate calcium from food, adequate vitamin D year-round. The supplement is the last line of that list, not the first — and it is genuinely part of it.

Winter strategy

Above roughly 37° latitude — most of the United States, all of the UK and northern Europe — the sun sits too low from about October to March for skin synthesis to contribute much, whatever the sky looks like. Cloud cover, glass, sunscreen, darker skin and age all reduce production further; a 70-year-old makes markedly less vitamin D than a 20-year-old from identical exposure.

A workable plan, in four lines: take D3 daily from autumn through spring rather than trying to remember it seasonally, because a daily habit survives where a seasonal one does not. Eat oily fish twice a week if you like it — salmon, sardines, mackerel are the few genuinely useful food sources. Keep the strength sessions running through the dark months, since that is the stimulus that actually directs bone. And take the winter blood test rather than the summer one.

None of this is exciting. It is, however, the version supported by the trials rather than by the label.

Sources

1. Institute of Medicine. “Dietary Reference Intakes for Calcium and Vitamin D.” National Academies Press, 2011.
2. Holick MF, et al. “Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline.” Journal of Clinical Endocrinology & Metabolism, 2011.
3. Reid IR, Bolland MJ, Grey A. “Effects of vitamin D supplements on bone mineral density: a systematic review and meta-analysis.” The Lancet, 2014.
4. Burt LA, et al. “Effect of high-dose vitamin D supplementation on volumetric bone density and bone strength: a randomized clinical trial (Calgary Vitamin D Study).” JAMA, 2019.
5. LeBoff MS, et al. “Supplemental vitamin D and incident fractures in midlife and older adults (VITAL).” New England Journal of Medicine, 2022.
6. Watson SL, et al. “High-intensity resistance and impact training improves bone mineral density and physical function in postmenopausal women with osteopenia and osteoporosis: the LIFTMOR randomized controlled trial.” Journal of Bone and Mineral Research, 2018.
7. Sowers MR, et al. “Amount of bone loss in relation to time around the final menstrual period and follicle-stimulating hormone staging of the transmenopause.” Journal of Clinical Endocrinology & Metabolism, 2010.