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Calcium is Canada’s go-to strategy for preventing bone loss and osteoporosis. It’s in dairy products, canned fish, beans, broccoli and kale. It’s pumped into soy beverages, mixed into multivitamins and added to orange juice, breakfast cereals and energy bars. Then there are the calcium supplements that many of us take, often on doctors’ orders, just to be safe. The more, the better, right?

Not quite. The safety of calcium supplements has become the subject of heated debate over the past 10 years, as researchers have produced evidence of side effects ranging from kidney stones to cardiovascular disease. Earlier this month, a study linking calcium supplements to precancerous colon growths added another potential health risk to the list. And while at least 40 per cent of Americans take calcium supplements, clinical trials have cast doubt on the effectiveness of these chalky tablets in preventing bone fractures.

So should Canadians ditch their calcium pills? No one disputes the importance of calcium for healthy bones. But the pros and cons of supplementary calcium may depend on your health status, the amount of supplementation – and which scientists you choose to believe.

Dr. Mark Bolland, associate professor of medicine at the University of Auckland, was among the first to sound the alarm about calcium supplements, in a 2010 BMJ report linking them to an increased risk of heart attack.

Various studies have confirmed his findings, but others, including a 2016 review in the Annals of Internal Medicine, have shown no increased cardiovascular risk. According to Bolland, these reassuring reports have either chosen to measure different cardiovascular outcomes, or “did not include all the data from all the studies.” One explanation for this, he wrote in an email, involves “tight links between industry, academics and special societies in this area.”

Dr. Erin Michos, a cardiologist at Johns Hopkins Medicine, agrees. Michos and colleagues scanned the coronary arteries of 5,448 adults from different ethnic backgrounds to confirm they had no calcium deposits, in a study published in 2016 in Journal of the American Heart Association. Ten years later, those who took calcium supplements had a 22-per-cent increased risk of developing calcification in their heart arteries, compared to non-supplement users. “We didn’t see this with dietary calcium,” she said, adding that supplements are “a billion-dollar industry.”

Earlier this month, other researchers linked calcium supplements to serrated polyps, a less common type of colon growth that can become cancerous. In a study published in the journal Gut, patients with a history of these polyps who took calcium supplements, with or without vitamin D, had a twofold increased risk of developing more of these growths within six to 10 years, compared to those who took no calcium supplements, or vitamin D alone.

The adverse effects of calcium supplements remain controversial, but so are the potential benefits.

In a 2006 study in the New England Journal of Medicine, women aged 50 to 79 who took a 1,000-mg calcium supplement with vitamin D had about the same hip fracture risk after seven years as those who took a placebo. Supplement users in this large clinical trial, involving 36,282 American women, showed just a 1-per-cent increase in hip-bone density – and more kidney stones.

On the flip side, a 2016 review in Osteoporosis International found that calcium supplementation plus vitamin D lowered the risk of total fractures by 15 per cent and hip fractures by 30 per cent. This study came from the U.S. National Osteoporosis Foundation, one of the organizations Bolland has described as “compromised” by the influence of companies that market supplements and nutrition-related laboratory tests.

Calcium has made headlines since 1984, when the U.S. National Institutes of Health declared osteoporosis “a major public health problem.” Newspapers warned about osteoporosis, and the supplement industry stepped in to solve the nation’s “calcium deficiency.”

The theory that porous bones signalled a need for more calcium made sense, since calcium is the major mineral in bone, said Aileen Burford-Mason, a Toronto-based immunologist, cell biologist and author of Eat Well, Age Better. But, she added, “that story was wrong.” Calcium is just part of the picture, since it is poorly absorbed without vitamin D, and requires vitamin K2 to help bind it to bone. Magnesium, too, plays a crucial role in bone health. In fact, “there is no essential nutrient that isn’t involved in keeping bones strong.”

In 2010, Health Canada lowered its recommended dietary allowance of calcium to 1,000 mg a day for adults under age 51, 1,200 mg for men over 51 and 1,200 for all adults over 70. The agency recommends vitamin D supplementation, and cautions that long-term intakes of calcium above 2,500 mg a day for adults under 51, and 2,000 mg for those over 51, “increase the risk of adverse health effects, such as kidney stones.”

Nevertheless, many physicians may be unaware of the current guidelines, said Dr. Angela Cheung, former chair of Osteoporosis Canada’s scientific advisory council and professor of medicine at the University of Toronto. Back in 2001, when guidelines recommended a daily calcium intake of 1,000 to 1,500 mg for older adults, “a lot of physicians interpreted that as three [tablets] of 500 mg [per day],” she said, without considering the calcium patients were getting from food.

Since Canadian adults tend to be low on calcium, Cheung encourages people to increase their intake from dietary sources, if possible. Adults can start by tracking their current dietary intake using Osteoporosis Canada’s calcium calculator.

Calcium supplements can be useful for patients with specific health conditions, she added, but “for most people, I don’t think they need a supplement.”