Every year, about 4000 people in New Zealand fracture their hip. Judging from how my mother seemed last year when it happened to her, it is worth avoiding. The crumpled top of her femur, which was pronounced to consist of thin bone, was promptly replaced, and she received brochures advising her to drink and eat plenty of calcium – the big sources of which include dairy, sardines and vegetables such as squash and bok choy.
This advice is standard, but it does nothing to reduce the risk of future fractures, says Ian Reid, a professor of medicine at the University of Auckland. “I used to be a good boy who believed in calcium, but the evidence turned me into a calcium sceptic,” he says.
Reid is internationally renowned for his research into calcium metabolism and osteoporosis, and he sees patients in an osteoporosis clinic. He remembers when bone was understood to be like chalk that absorbs calcium and when the best available evidence on calcium requirements came from calcium balance studies. “If you gave people extra calcium and found that what was going in wasn’t coming out in urine or faeces, the conclusion was that it must be going into bone.”
Recommendations for high calcium intake – 1000mg a day or more – are often derived from those studies, he says. But bone density scans tell a different story. “Even people shown to be in positive calcium balance [when more goes in than comes out] were losing bone density.”
He co-authored a 2019 study of nearly 700 older women to monitor their dietary calcium and bone density. The women’s daily calcium intakes ranged from just under 300mg (the amount in one cup of cows’ milk or fortified plant milk) to more than 2000mg.
“The change in their total body bone mineral was completely unrelated to their calcium intake,” he says. That result wasn’t changed by adjusting for bone-thinning culprits (smoking, heavy drinking, low height and weight) or physical activity, which builds bones.
What about supplements? “When you [assign] half the people [in a trial] to a dollop of extra calcium every day for years, their bone density increases,” says Reid. But there’s a catch: their fracture risk stays the same.
“The increase in bone density is only about 0.5%, which is not enough to affect fracture risk.” That doesn’t justify calcium supplements, especially as the extra calcium can accumulate and increase the risk of heart attacks and kidney stones.
This accords with discoveries of bone biology and calcium metabolism in recent decades, he says. “The foundation of bone is type 1 collagen with a bit of calcium and phosphorus interwoven between the collagen fibres. You need enough calcium and phosphorus, but the amount of bone you get depends on how busy the cells are that make bone and how busy the cells are that remove bone. It’s a fundamental misunderstanding of bone biology to place emphasis on calcium.”
The body is clever with dietary calcium. It absorbs more and excretes less when needed, such as during pubertal growth spurts and pregnancy. “On a typical western diet, you’d absorb 20%, whereas at a lower calcium level you’d absorb 60%,” says Reid. “Our bodies have quite powerful mechanisms to stop calcium precipitating into soft tissues like hearts and arteries, and one is getting rid of it – peeing it out.”
There’s some controversy over Reid’s conclusions. The official recommended daily intake for older adults in New Zealand and Australia is 1300mg. That hasn’t been updated since 2006, but a Ministry of Health spokesperson says it could be reviewed if certain criteria are met. The recommended limit is the same as the United States guideline, but more than the UK’s recommendation of 700mg and freshly revised Nordic advice to consume 950mg. A 2023 Nordic review concluded that “convincing evidence that the intake of calcium above 1000mg a day in healthy adults prevents fractures is lacking.”
