Calcium
Calcium is the mineral of bone, and the skeleton doubles as the body's calcium reservoir, drawn down whenever diet falls short. For decades calcium pills were the default prescription for bone health. A 2010 meta-analysis linked them to more heart attacks, and the default became food first, which is where the evidence still points.
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What It Does
Parathyroid hormone and calcitriol, the active form of vitamin D, hold serum calcium in a narrow range. When intake is low, calcitriol drives active transport across the intestinal wall; at higher intakes, calcium moves passively between cells. Adults absorb roughly a quarter to a third of the calcium they eat, and the fraction falls as the dose rises, which is why single doses above 500 mg waste much of the extra. When intake stays low, parathyroid hormone keeps serum calcium normal by drawing on bone, so a normal blood calcium says nothing about whether intake is adequate.
The fracture benefit of supplements in the general population is small. In the Women's Health Initiative, 36,282 postmenopausal women took 1,000 mg of calcium and 400 IU of vitamin D or placebo for seven years. Hip bone density rose about 1%, hip fractures did not fall significantly overall, and kidney stones increased by 17% (Jackson 2006).
The cardiovascular question is the calcium paradox. A 2010 meta-analysis of trials of calcium without vitamin D found about 30% more heart attacks in those taking supplements (Bolland 2010), and a reanalysis that added the Women's Health Initiative data found a similar signal for calcium with vitamin D (Bolland 2011). One proposed mechanism is the spike in serum calcium that follows a supplement dose and does not follow a meal. In 2016, the National Osteoporosis Foundation and the American Society for Preventive Cardiology reviewed the same body of evidence and concluded that calcium with or without vitamin D, within the UL, does not raise cardiovascular risk in healthy adults (Kopecky 2016). The data conflict. What both sides agree on is that calcium from food has not been linked to harm.
Vitamin K2 activates matrix Gla protein, which inhibits calcium deposition in arteries, and it is often sold alongside calcium and D3 for that reason. The mechanism is sound; trials have not shown that adding K2 prevents cardiovascular events.
Forms and Bioavailability
Absorption ratings are broad tiers. Head-to-head human trials rarely support a finer ranking among well-absorbed forms.
| Form | Elemental % | Absorption | Typical dose | Cost | Labels |
|---|---|---|---|---|---|
| Calcium Carbonatecalcium from oyster shell, calcified seaweed | 40% | ModerateNeeds stomach acid to dissolve. Taken with a meal it is absorbed about as well as citrate; fasting or on acid suppression, absorption falls. | 500-600 mg elemental per dose, with food | Low | 1,075 |
| Calcium Citratecalcium citrate malate (related) | 21.1% | ModerateDissolves without stomach acid, so absorption holds up when fasting, on PPIs, and after gastric bypass. | 200-500 mg elemental per dose | Moderate | 1,061 |
| Tricalcium Phosphatecalcium phosphate | 38.8% | ModerateAbsorbed about as well as carbonate when taken with food; supplies phosphorus as well. | 500-600 mg elemental per dose | Low | 209 |
| Microcrystalline Hydroxyapatite (MCHC)hydroxyapatite, bone meal concentrate | Varies | VariableGround bovine bone with calcium, phosphorus, and bone matrix proteins. Absorption data are limited. | Per product | Moderate | 149 |
| Calcium Gluconate | 9.3% | ModerateSoluble, but only about 9% calcium, so a useful oral dose takes many tablets. | Rarely used orally | Moderate | 149 |
Labels column: number of labels named as calcium products that list this form. A product can list more than one form, and some labels do not state a form at all.
Carbonate is 40% calcium and needs stomach acid to dissolve. Taken with a meal, it is absorbed about as well as citrate and costs less per milligram of calcium (Heaney 2001). Taken fasting by someone with little stomach acid, absorption drops sharply, while a meal restores it (Recker 1985). That makes citrate, at 21% calcium, the better choice with PPIs or after gastric bypass. Oyster shell, coral, and algae calcium are mostly carbonate with marketing. Hydroxyapatite (MCHC) is ground bone with limited absorption data, and gluconate carries so little calcium per tablet that it is mainly used intravenously.
Typical Dosing
| Group | RDA |
|---|---|
| Men 19-70 | 1,000 mg |
| Men 71+ | 1,200 mg |
| Women 19-50 | 1,000 mg |
| Women 51+ | 1,200 mg |
| Upper limit (UL) | 2,500 mg per day from all sources for ages 19-50; 2,000 mg for 51 and older |
The fraction absorbed falls as the dose rises, so take 500 mg or less at a time. Count calcium from food first and supplement only the gap.
Food sources: yogurt, milk, cheese, fortified soy milk, canned sardines and salmon with bones, tofu made with calcium sulfate, kale, fortified orange juice.
What to Look For
Count food first. A cup of milk or yogurt, an ounce and a half of hard cheese, or a cup of calcium-fortified soy milk each provide roughly 300 mg. Add up a typical day, subtract from your RDA, and supplement only the gap. Many people who eat dairy need no supplement at all.
If you need one, read the elemental calcium on the panel and keep each dose at 500 mg or less. Take carbonate with food; take citrate any time. Keep calcium at least two hours from iron, zinc, and tetracycline or quinolone antibiotics, and four hours from levothyroxine. Bisphosphonates must be taken alone on an empty stomach. Make sure vitamin D status is adequate; the Vitamin D Repletion calculator can estimate the dose needed to reach a target level.
Red flags: 1,200 mg supplements taken on top of a dairy-rich diet, coral calcium disease claims, and bone meal products without heavy metal testing. Green flags: elemental calcium per tablet stated, doses of 500 mg or less, and USP Verified or similar certification.
Calcium Carbonate
The most calcium per tablet at the lowest cost. Works well with meals in people with normal stomach acid.
What to look for: 500-600 mg elemental calcium per serving, taken with food. Expect some constipation.
Find on iHerb Affiliate linkCalcium Citrate
The form that does not depend on stomach acid. Bigger tablets, gentler on the gut.
What to look for: Elemental calcium per serving stated; 200-500 mg per dose. The choice with PPIs or after bariatric surgery.
Find on iHerb Affiliate linkProducts on Titrate
37,382 of the 214,745 labels in the NIH Dietary Supplement Label Database list calcium as an active ingredient (19,618 on market). 3,213 are sold as calcium products by name.
Top on-market labels, ranked by label completeness and simplicity:
- Calcium Citrate Malate
- Calcium Citrate Malate 400 Milligrams
- Calcium Malate Chelate
- Calcium Malate Chelate
- Calcium D-Glucarate 500 mg
Calcium does not have its own search category yet. Each product above links to its full label in the NIH database.
Related Tools
References
NIH Office of Dietary Supplements. Calcium: Fact Sheet for Health Professionals.
Recker RR. Calcium absorption and achlorhydria. N Engl J Med. 1985;313(2):70-73.
Heaney RP, Dowell MS, Bierman J, Hale CA, Bendich A. Absorbability and cost effectiveness in calcium supplementation. J Am Coll Nutr. 2001;20(3):239-246.
Jackson RD, LaCroix AZ, Gass M, et al. Calcium plus vitamin D supplementation and the risk of fractures. N Engl J Med. 2006;354(7):669-683.
Bolland MJ, Avenell A, Baron JA, et al. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis. BMJ. 2010;341:c3691.
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;342:d2040.
Kopecky SL, Bauer DC, Gulati M, et al. Lack of evidence linking calcium with or without vitamin D supplementation to cardiovascular disease in generally healthy adults: a clinical guideline from the National Osteoporosis Foundation and the American Society for Preventive Cardiology. Ann Intern Med. 2016;165(12):867-868.
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