In fact, excess calcium may be a hidden killer … especially if much of it comes from supplements.
Being an epidemiological study, the new analysis cannot prove a cause-effect relationship between calcium and death.
However, while the evidence is mixed, a number of studies find risks at the low (600mg or less) and high ends (1400mg or more) of daily calcium intake.
And the proven adverse effects of excess calcium on artery health could explain a large part of any apparent increase in death risk due to excess calcium.
Based on their data analysis, the Swedish scientists calculate that – compared with women taking between 600mg and 999mg of calcium per day – women who consume more than 1400mg per day were more than twice as likely to die.
As the research team from Uppsala University wrote, “The increase [in death risk] was moderate with a high dietary calcium intake without supplement use, but the combination of a high dietary calcium intake and calcium tablet use resulted in a more pronounced increase in mortality.” (Michaëlsson K et al. 2013)
Unsurprisingly, modest intake of calcium appeared benign: “For most women with lower intakes we observed only modest differences in risk.” (Michaëlsson K et al. 2013)
How much calcium can women take safely?
The U.S. Institute of Medicine (IOM) – which sets the RDAs for essential nutrients – recommends that women over 50 get 1,200mg of calcium daily.
But the IOM also sets Tolerable Upper Intake Levels (UL) for nutrients, which are the points at which no adverse effects are known.
For calcium, the UL is 2,000 mg daily from all sources combined, including foods and supplements. (For reference, cup of low-fat yogurt has 415mg, a cup of 2% milk has 293mg, a 3 oz can of salmon has 1981mg and 8 oz of firm tofu has 253mg.)
The new paper is the latest to detect possible risks from calcium supplements, such as arterial stiffness and increased risk of heart conditions and stroke.
Remarkably, the authors of one recent analysis argued that “the indiscriminate use of calcium supplements ‘should be abandoned”, because the risks of heart attack outweigh any benefit from osteoporosis prevention (Shah SM et al. 2010; Bolland MJ et al. 2008; 2010; 2011; Reid IR et al. 2011).
Their analysis showed that for every thousand people who take calcium supplements for five years, six suffer a heart attack and three avoid a fracture.
The new Swedish study is the first to pursue these concerns by looking for any associations between calcium intake and deaths from all causes, including cardiovascular disease.
Swedish data analysis reveals possible calcium risks
The Uppsala team analyzed data from the Swedish Mammography study, which followed 61,443 Swedish women born between 1914 and 1948 for an average of 19 years (Michaëlsson K et al. 2013).
The study authors’ estimated each woman’s dietary calcium intake based on the answers to diet questionnaires, and compared that data in the Swedish death registry, which records every citizen’s death date and cause.
As the Swedish team wrote, “The highest rates of death from all causes, cardiovascular disease, and ischemic heart disease but not stroke were observed among those with a dietary calcium intake higher than 1400mg/day.” (Michaëlsson K et al. 2013)
(Ischemic heart disease is characterized by reduced blood supply to the heart muscles, usually due to narrowing of the coronary arteries. Symptoms include reduced exercise capacity and chest pain during exertion.)
Compared with dietary intakes between 600 and 1000mg per day, intakes above 1400mg per day raised a woman’s death risk by 40 percent, raised the risk of death from any cardiovascular cause by almost 50 percent, and doubled the risk for death from ischemic heart disease.
Conversely, low calcium intake appeared risky, too, since mortality rates were higher among women with an intake below 600mg per day.
The Swedish team suggested that that diets very low or very high in calcium can strain the capacity of the body’s calcium-control mechanisms, allowing unhealthful changes in blood levels.
And evidence that calcium supplements help prevent osteoporosis or fractures is weak … compared with use of vitamin D alone (Warensjö E et al. 2011).
As the authors of a recent meta-analysis wrote, their work “… supports the use of vitamin D3 of 800 IU daily to reduce the incidence of osteoporotic non-vertebral, hip, and non-vertebral-non-hip fractures in elderly women. (Bergman GJ et al. 2010)
- Bergman GJ, Fan T, McFetridge JT, Sen SS. Efficacy of vitamin D3 supplementation in preventing fractures in elderly women: a meta-analysis. Curr Med Res Opin. 2010 May;26(5):1193-201. doi: 10.1185/03007991003659814.
- Bolland MJ, Avenell A, Baron JA, Grey A, MacLennan GS, Gamble GD, Reid IR. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis. BMJ. 2010 Jul 29;341:c3691. doi: 10.1136/bmj.c3691. Review.
- Bolland MJ, Barber PA, Doughty RN, Mason B, Horne A, Ames R, Gamble GD, Grey A, Reid IR. Vascular events in healthy older women receiving calcium supplementation: randomised controlled trial. BMJ. 2008 Feb 2;336(7638):262-6. doi: 10.1136/bmj.39440.525752.BE. Epub 2008 Jan 15.
- Bolland MJ, Grey A, Avenell A, Gamble GD, Reid IR. Calcium supplements with or without vitamin D and risk of cardiovascular events: reanalysis of the Women's Health Initiative limited access dataset and meta-analysis. BMJ. 2011 Apr 19;342:d2040. doi: 10.1136/bmj.d2040. Review.
- Michaëlsson K, Melhus H, Warensjö Lemming E, Wolk A, Byberg L. Long term calcium intake and rates of all cause and cardiovascular mortality: community based prospective longitudinal cohort study. BMJ. 2013 Feb 12;346:f228. doi: 10.1136/bmj.f228. Osteoporos Int. 2011 Jun;22(6):1649-58. doi: 10.1007/s00198-011-1599-9. Epub 2011 Mar 16.
- Reid IR, Bolland MJ, Avenell A, Grey A. Cardiovascular effects of calcium supplementation. Shah SM, Carey IM, Harris T, DeWilde S, Cook DG. Calcium supplementation, cardiovascular disease and mortality in older women. Pharmacoepidemiol Drug Saf. 2010 Jan;19(1):59-64. doi: 10.1002/pds.1859.
- Scragg R. Do we need to take calcium with vitamin D supplements to prevent falls, fractures, and death? Curr Opin Clin Nutr Metab Care. 2012 Nov;15(6):614-24. doi: 10.1097/MCO.0b013e328359ef21
- Warensjö E, Byberg L, Melhus H, Gedeborg R, Mallmin H, Wolk A, Michaëlsson K. Dietary calcium intake and risk of fracture and osteoporosis: prospective longitudinal cohort study. BMJ. 2011 May 24;342:d1473. doi: 10.1136/bmj.d1473.