Vitamin D for Osteoporosis

Vitamin D is not a treatment for osteoporosis on its own. A meta-analysis of 81 randomised trials found no effect on total fractures (RR 1.00), hip fractures or falls, and no clinically relevant change in bone density (Bolland 2018). The 25,871-person VITAL trial likewise found no effect of 2,000 IU a day on fractures (LeBoff 2022).

Combined with calcium, the picture is a little better: daily vitamin D plus calcium cut hip fractures by 16% (RR 0.84) in a 2019 meta-analysis (Yao 2019), and the Cochrane review found the same figure (Avenell 2014). That works out to about 9 fewer hip fractures per 1,000 older adults a year in care homes, but only about 1 per 1,000 in people living at home. The Bone Health and Osteoporosis Foundation still advises getting enough vitamin D and calcium alongside osteoporosis medicines (BHOF).

Vitamin D lets the gut absorb calcium. NIH's Office of Dietary Supplements notes that osteoporosis is most often linked to low calcium intake, and that insufficient vitamin D contributes by reducing calcium absorption (NIH ODS). Low vitamin D and low calcium intake raise parathyroid hormone in older people (Chapuy 1992), and when the body lacks calcium it takes it from bone (BHOF).

That chain is what the early trials targeted. In a subsample of 142 women from a trial of 3,270 women averaging 84 years, vitamin D3 with calcium lowered parathyroid hormone by 44% and raised blood 25-hydroxyvitamin D (25(OH)D, the marker of vitamin D status) by 162% over 18 months (Chapuy 1992). Vitamin D is also needed for normal muscle fibre growth, which matters for balance and falls (NIH ODS).

The biology explains why deficiency harms bone, not why extra vitamin D would help someone who already has enough. In VITAL's bone sub-study, 2,000 IU a day for two years did not change bone density in 771 adults not selected for low vitamin D; only a subgroup with low free vitamin D gained a little spine density (0.75% vs 0%) (LeBoff 2020).

More can even be worse. In healthy 55- to 70-year-olds, 4,000 or 10,000 IU a day for three years lowered forearm bone density more than 400 IU did (Burt 2019).

The trial with the clearest fracture benefit gave 800 IU of vitamin D3 plus 1.2 g of calcium daily (Chapuy 1992). The Women's Health Initiative used 400 IU with 1,000 mg of calcium (Jackson 2006), VITAL used 2,000 IU a day (LeBoff 2022), and D-Health used 60,000 IU once a month, with no overall effect on fractures (Waterhouse 2023). In a pooled analysis of 11 trials, only the group with the highest actual intake (median 800 IU a day, range 792-2,000) had fewer hip fractures (HR 0.70), though that comparison was not randomised (Bischoff-Ferrari 2012).

For reference, the US recommended intake is 600 IU a day at ages 51-70 and 800 IU over 70, with an adult upper limit of 4,000 IU (NIH ODS). The Bone Health and Osteoporosis Foundation suggests 800-1,000 IU a day for healthy adults 50 and older, and asks people with osteoporosis or low bone mass to review their vitamin D level with their clinician (BHOF). For adults over 50 who take vitamin D, the Endocrine Society suggests daily doses rather than intermittent high doses (Demay 2024).

A 2018 meta-analysis of 81 randomised trials (53,537 adults) found vitamin D had no effect on total fracture (RR 1.00, 95% CI 0.93-1.07), hip fracture (RR 1.11) or falls (RR 0.97), including in trials using more than 800 IU a day (Bolland 2018). The Cochrane review rated as high quality the evidence that vitamin D alone is unlikely to prevent hip fracture (RR 1.12, 95% CI 0.98-1.29) or any new fracture (Avenell 2014).

The largest recent trials agree. VITAL gave 2,000 IU a day to 25,871 adults for a median 5.3 years: total fractures HR 0.98 and hip fractures HR 1.01, and the effect did not differ by baseline vitamin D level (LeBoff 2022). D-Health gave 60,000 IU a month to adults aged 60-84 for up to 5 years: total fractures HR 0.94 (95% CI 0.84-1.06) (Waterhouse 2023). None of these trials selected people with osteoporosis.

Adding calcium changes the result. In the Cochrane review, vitamin D plus calcium lowered hip fracture risk (RR 0.84, 95% CI 0.74-0.96; 9 trials, 49,853 people) and any fracture (RR 0.95) (Avenell 2014). A 2019 meta-analysis of 6 large trials using 400-800 IU of vitamin D with 1,000-1,200 mg of calcium found 6% fewer fractures overall and 16% fewer hip fractures (Yao 2019).

The benefit leans on trials in very old, institutionalised people. In 3,270 women averaging 84 years in nursing homes, 800 IU of vitamin D3 with 1.2 g of calcium for 18 months gave 43% fewer hip fractures among those who completed the study (P=0.043) and 32% fewer nonvertebral fractures (Chapuy 1992). In 36,282 healthy postmenopausal women aged 50-79, 400 IU with 1,000 mg of calcium for 7 years left hip density 1.06% higher than placebo (measured at three centres) but did not significantly reduce hip fracture (HR 0.88, 95% CI 0.72-1.08) (Jackson 2006). An umbrella review concluded that the combined benefit is possibly driven by institutionalised people, with no reduction in reviews of community-dwelling adults (Chakhtoura 2022).

Across 23 trials in 4,082 adults, vitamin D raised bone density only at the femoral neck, by 0.8%, with no effect at the total hip or spine (Reid 2014). The 2018 meta-analysis put the differences at -0.16% to 0.76% over 1-5 years, which is not clinically meaningful (Bolland 2018). In VITAL, 2,000 IU a day for 2 years did not change spine, hip or whole-body density (LeBoff 2020).

In 2,256 women aged 70 and older at high fracture risk, a single 500,000 IU dose each autumn or winter for 3-5 years increased fractures (RR 1.26, P=.047) and falls (RR 1.15), with the excess in falls greatest in the 3 months after each dose, a post hoc finding (Sanders 2010). In the Calgary trial, 311 healthy adults taking 4,000 or 10,000 IU a day for 3 years lost more forearm bone density (2.4% and 3.5%) than those taking 400 IU (1.2%), although bone strength did not differ (Burt 2019).

No large trial has tested whether vitamin D improves the effect of bisphosphonates. One cross-sectional study of 120 women with osteoporosis on alendronate or risedronate found that those whose bone density fell or who fractured had lower vitamin D levels (22.4 vs 26.6 ng/mL), and women below 30 ng/mL had about 4-fold higher odds of an inadequate response (OR 4.42, 95% CI 1.22-15.97) (Peris 2012). It shows an association, not cause, but it supports the standard advice to correct deficiency before and during treatment.

At usual doses vitamin D is well tolerated. In the Cochrane review, mortality was not affected, but hypercalcaemia (high blood calcium, usually mild) was more common with vitamin D or its analogues, with or without calcium (RR 2.28; highest with calcitriol), and vitamin D with calcium slightly increased gastrointestinal symptoms (RR 1.04) and renal disease (RR 1.16) (Avenell 2014). Calcium plus vitamin D raised kidney stones by 17% in the Women's Health Initiative (Jackson 2006), and the US Preventive Services Task Force found adequate evidence that the combination increases kidney stones (USPSTF 2018).

The adult upper limit is 4,000 IU a day. Vitamin D toxicity causes marked hypercalcaemia, with nausea, vomiting, muscle weakness, excessive thirst and kidney stones, and in extreme cases kidney failure (NIH ODS). People with sarcoidosis are a special case: they can develop high urine or blood calcium after even small amounts of dietary vitamin D (Endotext).

Most vitamin D interactions involve calcium balance or absorption. The ones that matter in osteoporosis are thiazide diuretics, which can combine with vitamin D to raise blood calcium, and steroid tablets, which reduce calcium absorption and impair vitamin D metabolism (NIH ODS).