Calcium strengthens your bones

Calcium raises bone density by a small amount, and in the trials that counted broken bones, calcium supplements on their own made little or no difference to fractures. Across 59 randomised trials in adults over 50, extra calcium from food or supplements raised bone mineral density by roughly 0.7 to 1.8 percent, most of it within the first year, with no further growth after that. The newest and largest pooled analysis, published in 2026 and covering 69 trials, found no clear reduction in fractures from calcium supplements alone: across 11 trials and 9,067 people the risk of any fracture was 9 percent lower, with a range that includes no effect at all.

Fracture reductions appear in two places, and in both calcium came with something else. Calcium taken with vitamin D lowered the risk of any fracture by 9 percent and hip fracture by 16 percent across about 51,000 people in the 2026 analysis, about 1 fewer fracture for every 100 people treated; the authors judged that below the size that matters clinically and found it largely driven by one trial in a very high risk population. Separately, a two-year trial in 7,195 residents of Australian care homes added milk, yoghurt and cheese to their menus and recorded 33 percent fewer fractures and 46 percent fewer hip fractures; the extra food raised protein as well as calcium, so the two cannot be separated. The largest single trial also recorded a trade-off: kidney stones were 17 percent more common in women taking calcium with vitamin D than in women taking placebo. The US and Canadian reference report used that result to set the upper intake level for adults over 50 at 2,000 mg a day of total calcium, and states that the risk appears to come from supplements added to diet rather than from calcium in food. These trials tested calcium itself as the treatment. For people taking osteoporosis drugs, the FDA label for alendronate and NICE guidance treat an adequate calcium intake as part of that treatment.

The bone density effect held across every subgroup the pooled trials examined. It was about the same whether the extra calcium came from food or from pills, whether or not vitamin D was added, whether the dose was above or below 1,000 mg a day, and whether people started with intakes above or below 800 mg a day. What it does not do is build: the gain at two and a half years was about the size of the gain at one year. The reviewing authors judged increases of this size unlikely to produce a clinically meaningful drop in fractures.

The Women's Health Initiative randomised 36,282 postmenopausal women aged 50 to 79 to 1,000 mg of calcium, as calcium carbonate, with 400 IU of vitamin D daily, or placebo, for an average of seven years. Hip bone density ended 1.06 percent higher in the supplement group. Hip fracture came out at a hazard ratio of 0.88, clinical spine fracture 0.90 and total fractures 0.96, none statistically significant. Among women who kept taking their pills the hip fracture hazard ratio was 0.71, but that analysis sets aside the women who stopped, so it no longer compares randomised groups.

Pooled analyses reach different answers depending on which trials they include. One review of 26 trials found calcium supplements lowered total fracture risk by 11 percent and vertebral fracture by 14 percent, with no change in hip or forearm fracture. Restricted to the four trials at lowest risk of bias, 44,505 people, there was no effect at any site, and the review found signs that published results lean toward calcium. The one trial with clear reductions enrolled frail elderly women in residential care with low calcium intake and low vitamin D. In the 2026 analysis vitamin D alone made no difference, and calcium alone showed a hip fracture relative risk of 1.63, which the authors describe as very uncertain and imprecise. For calcium from food, the 2015 review found only two small randomised trials, and most cohort studies of dietary calcium, milk and dairy found no link with fracture; the care home trial of dairy foods, published in 2021, is far larger, with 7,195 residents.

In clinical practice calcium sits beside osteoporosis drugs rather than in place of them. The FDA label for alendronate directs supplemental calcium when dietary intake is inadequate and requires low blood calcium to be corrected before treatment starts. The same label states that calcium supplements interfere with alendronate absorption and directs a wait of at least half an hour after the dose before any other oral medication. NICE guidance on osteoporosis drugs assumes treated women have an adequate calcium intake and says supplementation should be considered where that is uncertain. NIH's Office of Dietary Supplements describes the fracture evidence as mixed and notes that an FDA-approved health claim for calcium and vitamin D reducing osteoporosis risk is not supported by all of the research.

The biology is well mapped. About 98 percent of the body's calcium is stored in bone, which is broken down and rebuilt continuously. What the trials test is narrower: whether adding calcium on top of a typical diet changes bone in a way that shows up as fewer fractures.