Vitamin D at 1,600 or 3,200 IU a day did not cut fractures in older adults, most of whom were not deficient

In US national survey data, more than half of women 60 and older took a supplement containing vitamin D in 2015 and 2016, and nearly half of men that age did too. The vitamin’s best-known job is in the skeleton: without sufficient vitamin D, bones can become thin and brittle.

Writing in the journal Bone, Toni Rikkonen, Jyrki Virtanen and colleagues at the University of Eastern Finland report what extra vitamin D did for older bones. They randomized 2,495 older Finns for five years into three groups: two took vitamin D3 every day, one at twice the strength of the other, and the third took a placebo. Neither dose reduced the risk of fractures.

The volunteers were largely vitamin D sufficient, meaning most had enough in their blood before the trial started. The trial also turned up a puzzle: the weaker pill came with more fractures than the placebo, and the stronger one did not.

How the Finnish Vitamin D Trial tested two doses in older adults who mostly had enough

Among the large vitamin D3 supplementation trials, the Finnish one is unusual in comparing two doses, so it can ask whether more does more. The doses were 1,600 or 3,200 international units, or IU, a day. US recommended daily allowances range from 600 IU to 800 IU for vitamin D, so the people taking the vitamin got two or four times the top of that range.

Each pill held 1,600 IU, 3,200 IU or nothing, and the pills were otherwise identical, so neither the volunteers nor the researchers knew who took what. Chance decided who went where, which leaves the vitamin itself as the only systematic difference between the three groups. Rather than asking anyone whether they had broken a bone, the team pulled fracture diagnoses from two national care registers.

To take part, men had to be at least 60 and women at least 65, with no history of cardiovascular disease or cancer. About a third were already taking their own vitamin D supplements, which the rules allowed up to 800 IU a day. Few were short in the first place. US nutrition authorities set a blood level of vitamin D that counts as sufficient for most people, and in the subgroup of 550 participants tested before the trial, fewer than one in ten fell below it. The investigators have written that readings this high most likely reflect national food fortification policies Finland introduced in 2003 and 2010.

A fracture excess at 1,600 IU that the higher vitamin D dose did not repeat

During the roughly four years of treatment, there were 49, 73 and 52 fractures in the placebo, lower-dose and higher-dose groups. Counting on to a little over seven years, the tallies reached 104, 139 and 111. No single type of fracture showed a clear difference, and the pattern held regardless of age, sex, body mass index, a history of fracture or fall, an osteoporosis diagnosis or calcium supplement use.

So the 3,200 IU group tracked placebo closely, while the 1,600 IU group logged about half as many fractures again during supplementation. If the vitamin itself were weakening bone, those taking the most should have shown it most clearly. They did not. Nor do extra falls seem to explain it. A report from the same trial, published in January in the Journal of the American Geriatrics Society, found that just over half of the volunteers fell at least once whichever group they were in, a similar fall risk of 55% across the board.

The biology offers no obvious mechanism. Vitamin D promotes calcium absorption in the gut and keeps enough calcium and phosphate in the blood for bones to mineralize. That is why a shortage weakens them, but nothing in that job predicts harm at a middle dose that vanishes at a higher one. Chance is one candidate: two doses, several kinds of break and a list of subgroups give a single comparison plenty of room to look striking by luck. The excess could also be real for reasons nobody yet understands, and only other trials can settle which.

Fractures were a side question in a trial planned for 18,000 people

Other trials matter all the more because this one was not built to study bones. It was designed to test vitamin D against major chronic diseases, with cardiovascular disease and cancer as the main targets, and its planners wanted 18,000 participants. Interest turned out to be far lower than anticipated: of 30,000 invitation letters mailed in eastern Finland, 8.7% drew a yes, and the trial went ahead with 2,495.

In their main report, the investigators wrote that recruiting far fewer people than planned, along with fewer disease events than predicted, significantly limited the power of the trial, meaning its ability to detect a real difference. A response rate that low also raises the possibility of selection bias, they noted: the volunteers’ vitamin D status may not reflect older Finns in general. The trial was made up only of White, Finnish adults. The investigators have cautioned that its results may not be generalizable to more ethnically diverse populations, or to places where low vitamin D is more common.

What the trial offers, then, is a fair randomized test of extra vitamin D in well-supplied older Finns, with too few fractures to pin down a small effect.

The Finnish trial joins larger vitamin D studies that found no fracture benefit

Bigger studies have reached the same answer. In a US trial called VITAL, 25,871 generally healthy men 50 and older and women 55 and older took 2,000 IU a day or a placebo for about five years. Most participants were vitamin D sufficient there too, and the vitamin did not lower the risk of total fractures, hip fractures or fractures outside the spine.

A review published this year in the BMJ, the British Medical Journal, pooled 69 trials involving 153,902 people. It found little to no effect of vitamin D supplements on the risk of any fracture, and rated that conclusion as high certainty. Guidance is moving the same way: the US Preventive Services Task Force recommends against supplementation with vitamin D, with or without calcium, to prevent fractures in postmenopausal women and men 60 or older living at home, in a draft dated December 2024.

For older adults like the Finnish volunteers, the authors put it plainly: “The findings do not support use of high vitamin D doses for fracture prevention in such populations.”

Both verdicts have limits. The task force draft does not cover people with a history of osteoporotic fractures, a diagnosis of osteoporosis or vitamin D deficiency, or a condition that affects vitamin D absorption. And the panel still says that “ensuring adequate vitamin D and calcium intake is important for bone and overall health.” Older adults in care homes are another case: according to the National Institutes of Health, supplements of both nutrients together do help reduce fracture rates there.

For older adults who mostly had enough vitamin D, this trial found that five years of bigger doses did not keep bones from breaking.