Calcium

Is calcium safe?

For most adults, yes, within limits. Calcium is the mineral the body holds in the largest amount, and there is no question that people need it. The question is whether pills add risk that calcium in food does not. Here the evidence points to two possible downsides, kidney stones and a disputed rise in heart attacks, plus a long list of medicines that calcium blocks.

The US National Academies set the upper limit at 2,500 mg a day for adults up to 50 and 2,000 mg after that. EFSA uses 2,500 mg. Those limits count food and pills together, and that matters: someone who drinks milk, eats cheese and yoghurt and takes a 1,200 mg supplement can be near the limit without trying.

Calcium pills and heart attacks

This debate began in 2010. A meta-analysis in the BMJ pooled trials of calcium supplements of 500 mg a day or more and found that people given calcium had 31% more heart attacks than those on placebo (143 against 111 in the trials with patient-level data). A later reanalysis of the Women's Health Initiative, looking only at women who were not already taking their own calcium, estimated a 21% rise in heart attacks with 1,000 mg calcium plus vitamin D.

Other evidence points the other way. The CAIFOS trial in older women found no increase in vascular disease with 1,200 mg a day, and a meta-analysis of 18 trials covering 63,563 postmenopausal women found no rise in coronary disease or deaths. None of these trials was designed to measure heart outcomes as the main result, which is why the argument has not been settled.

One theory is that a pill raises blood calcium faster and higher than food does. The US National Osteoporosis Foundation and the American Society for Preventive Cardiology concluded that supplements are safe for the heart as long as total intake stays within the upper limit. We score organ toxicity at 70 because the signal is unproven but has not gone away.

Kidney stones

The clearest harm comes from the Women's Health Initiative itself. In that trial, 36,282 postmenopausal women took 1,000 mg of calcium carbonate with 400 IU of vitamin D or a placebo for about seven years. The calcium group had 17% more kidney stones, and the supplement did not significantly reduce hip fractures.

Food tells a different story. People who get more calcium from their diet tend to have fewer stones, probably because calcium binds oxalate in the gut. Taking calcium pills with meals may help for the same reason, though this has not been proven in trials.

Too much calcium in the blood

Calcium-alkali syndrome (once called milk-alkali syndrome) causes high blood calcium, kidney injury and sometimes confusion. It has been reported at 1.5 to 16.5 g a day, usually from calcium supplements plus calcium carbonate antacids such as Tums. Postmenopausal and pregnant women, people with bulimia and dialysis patients are the groups now most often affected. Count antacids as calcium.

Interactions with medicines

Calcium binds several drugs in the gut. Take these at least 2 hours before or 4 to 6 hours after calcium:

• Levothyroxine for thyroid disease
• Bisphosphonates such as alendronate for osteoporosis
• Quinolone and tetracycline antibiotics
• Dolutegravir, elvitegravir and raltegravir for HIV
• Iron supplements (iron is absorbed less when taken with calcium)

Some combinations raise blood calcium instead. Thiazide diuretics such as hydrochlorothiazide reduce calcium loss in urine, and lithium can raise calcium too. High doses of calcium may increase the risk of heart rhythm problems in people taking digoxin. Acid-reducing medicines such as omeprazole reduce absorption of calcium carbonate.

Product quality

Calcium from natural sources such as oyster shell, bone meal and dolomite can contain lead. A 1993 study found measurable lead in most of 70 supplements tested. Manufacturers have since cut lead sharply, and later testing found levels below tolerable limits. Calcium itself reduces lead absorption. A USP VerifiedThird-party testingTesting of a supplement by an independent lab to check that it contains what the label says and is free of set contaminants.More in the glossary mark is the simplest check.

How to take calcium sensibly

• Add up your food first. Three servings of dairy supply about 900 mg.
• If you need a supplement, take 500 mg or less at a time; absorption falls at higher doses.
• Take carbonate with food; citrate can be taken any time.
• Most calcium products include vitamin D, so count that too.
• If you have had kidney stones, have kidney disease or take a thiazide diuretic, talk to your doctor before starting.

How we scored it

Calcium scores 77/100. Its regulatory record is clean (88) and quality is good (80) now that lead levels have fallen. Organ toxicity (70) is the weakest factor because of the kidney stone result and the unresolved heart attack question, and interactions (72) are many, though most are managed by spacing doses. It scores below magnesium, which has a wider safety margin.