Vitamin D is essential for bone health

Essential is the right word for one half of this and the wrong word for the other, and the two halves are almost always sold as one. Without enough vitamin D the gut cannot absorb calcium properly and bone does not mineralise, which is rickets in children and osteomalacia in adults. Both are real deficiency diseases, both are preventable and treatable with vitamin D, and a global consensus of thirty-three experts across eleven scientific organisations recommends 400 IU daily for every infant in the first year.

What does not follow is that more vitamin D, once you have enough, does anything further for bone. The largest trial to test it found nothing. VITAL randomised 25,871 US adults to 2,000 IU a day or placebo for a median of 5.3 years and confirmed 1,991 fractures by medical record review. Total fractures came out at a hazard ratio of 0.98, nonvertebral 0.97, hip 1.01, none of them statistically significant. The result did not change with age, sex, race, body mass index or, notably, starting vitamin D level.

Pooling the whole literature says the same thing. The most recent pooled analysis, published in 2026, separates vitamin D taken alone from vitamin D taken with calcium. Across 36 trials of vitamin D alone and about 92,000 people, the relative risk was 1.00 for any fracture, 1.13 for hip fracture and 1.01 for falling, all rated high certainty, and the result did not vary with participants' vitamin D levels or with whether they were at high risk. An earlier pool of 81 trials found bone density differences at every measured site running from a 0.16 percent loss to a 0.76 percent gain, and its trial sequential analysis, which asks whether enough people have now been studied to rule an effect out rather than merely to have missed it, found the sample large enough to exclude a 5 percent reduction in total fracture. Hip fracture is the loose end: both pools put the estimate a little above 1, a 15 percent reduction is ruled out, and whether vitamin D might increase hip fractures is unresolved.

Two objections get raised at this point and both have been tested directly. The first is that these people were not deficient, so there was nothing to correct. In Ulaanbaatar, 8,348 schoolchildren aged 6 to 13 took 14,000 IU weekly or placebo for three years, and 95.5 percent of them started below 50 nmol/L. Blood levels rose. Fractures occurred in 6.4 percent of the vitamin D group and 6.1 percent of the placebo group. The second is that the doses were too low. In 311 Canadian adults given 400, 4,000 or 10,000 IU daily for three years, bone density at the radius fell 1.2, 2.4 and 3.5 percent across those groups, with bone strength unchanged. The comparator there was 400 IU rather than placebo, so it shows a dose gradient rather than a test against nothing.

The biology is not the weak link. Active vitamin D binds a receptor in the intestine and switches on the transport machinery for calcium, and deleting either that receptor or the enzyme that makes the hormone abolishes baseline calcium absorption in mice. NIH's Office of Dietary Supplements describes the same architecture and the same gap: insufficient vitamin D contributes to osteoporosis by reducing calcium absorption, while whether supplements reduce falls and fractures is not clear, and most trials gave calcium alongside vitamin D so the two are hard to separate. The 2026 pooled analysis addresses that last point directly, with 46 trials of vitamin D alone, and finds the same absence of effect. A nutrient can be indispensable at the low end and inert above it, and that is how vitamin D behaves for bone.