CalciumMineral

• Bone mineral densityPossibly helps

  Some systematic reviews find calcium, usually paired with vitamin D, modestly raises bone mineral density at some sites in older adults and postmenopausal women [1].
  
  Femoral neck BMD specifically often shows no effect at all, and how much the measured gains matter clinically is unclear [1]. ^[1]

• Fracture riskPossibly helps

  A meta-analysis of 8 RCTs (n=30,970) found calcium plus vitamin D cut total fractures 15% and hip fractures 30% [1].
  
  Other systematic reviews, covering tens of thousands more participants, found no significant fracture benefit [1].
  
  The US Preventive Services Task Force concluded, with moderate certainty, that daily doses under 1,000 mg calcium and 400 IU vitamin D do not reduce fractures in postmenopausal women [1].
  
  It called the evidence on higher doses inadequate. Unresolved, and we are not resolving it for you [1]. ^[1]

• Colorectal adenoma recurrencePossibly helps

  A systematic review and meta-analysis of 4 RCTs found 1,200-2,000 mg/day of elemental calcium over 36-60 months lowered recurrent-adenoma risk 11% (RR 0.89, 95% CI 0.82-0.96) [15].
  
  That works out to one recurrence avoided per 20 people supplemented (95% CI 12-61), resting on the Calcium Polyp Prevention Study (n=930) [15].
  
  A long-term follow-up (n=1,121, about 55 months after treatment ended) found no lasting effect - kept as an open conflict, not settled [1]. ^[15]

• Colorectal cancer riskPossibly helps

  Observational data are consistent: a dose-response meta-analysis across 15 cohorts and over 1.4 million participants found 8% lower colorectal cancer risk per 300 mg/day of extra calcium [1].
  
  Supplement RCTs have been mixed, which is the usual gap between what an intake correlates with and what a pill does [1]. ^[1]

• Prostate cancer riskPossibly helps

  Here the signal points the wrong way. Higher calcium intake, especially from dairy, has been linked to higher prostate cancer risk in observational studies [1].
  
  One French cohort found 2.4x the risk in the highest intake quartile against the lowest [1]. ^[1]

• Cardiovascular riskPossibly helps

  This is the sharpest unresolved conflict in the calcium literature, and no study or panel has closed it [9].
  
  A Cochrane review of 16 trials (n=3,048) found supplements modestly lowered blood pressure: systolic -1.43 mmHg, diastolic -0.98 mmHg [1].
  
  But a meta-analysis in 28,935 healthy postmenopausal women found calcium supplements raised CVD risk 15% and coronary heart disease risk 16% [7].
  
  A 2023 meta-analysis (n=55,438) found no significant excess for CHD or stroke specifically, while not ruling out a small absolute excess [8].
  
  A 2026 cohort of people with existing CVD found calcium-only supplementation associated with higher recurrence, more pronounced in men, with calcium plus vitamin D less consistent [9].
  
  Meanwhile the Nurses' Health Study (74,245 women, 24 years of follow-up) found no association at all [1].
  
  An expert panel from the National Osteoporosis Foundation and the American Society for Preventive Cardiology concluded calcium at or below the UL neither raises nor lowers CVD risk [1]. ^[9]

• Preeclampsia in pregnancyLikely helps

  A Cochrane review of 27 RCTs (n=18,064) found high-dose calcium, at 1,000 mg/day or more, lowered high blood pressure risk 35% [1].
  
  In women with low baseline intake, under 900 mg/day, preeclampsia risk fell 64% - though the review rated the evidence quality low [1].
  
  WHO and ACOG both recommend 1,500-2,000 mg/day for pregnant women whose dietary calcium is low, which ACOG puts at under 600 mg/day [1]. ^[1]

• Weight lossNot shown to help

  A meta-analysis of 41 RCTs found calcium supplements alone had no effect on body weight or body fat [1].
  
  Observational data link higher intake to 7% lower metabolic syndrome risk per 300 mg/day, especially in women, but the trial evidence there is very limited [1]. ^[1]

• Osteoporosis - fragile, low-density bone with higher fracture and fall risk. This is the chronic endpoint of long-term low intake, not an acute symptom.
• Rickets in children - growth cartilage fails to mineralize normally, causing irreversible skeletal changes.
• Osteomalacia in children and adults - defective bone mineralization, meaning softened bone.
• Vitamin D deficiency is a more common cause of rickets and osteomalacia than low calcium alone, and the two nutrients interact.
• Hypocalcemia means serum calcium under 8.5 mg/dL, or ionized calcium under 4.61 mg/dL.
• Its usual causes are not low dietary intake but vitamin D or magnesium deficiency, hypoparathyroidism, impaired bone resorption, critical illness, or drugs such as bisphosphonates, cisplatin, and proton pump inhibitors.
• Mild or chronic hypocalcemia is often symptom-free.
• When symptoms do appear, the most common is neuromuscular irritability - numbness around the mouth, tingling in hands and feet, muscle spasms or tetany.
• Severe cases can involve renal or brain calcification, neurologic symptoms including depression, cataracts, congestive heart failure, paresthesia, seizures, and rarely coma.^[1]

• Calcium carbonateCost and pill count. It is also the active ingredient in antacids like Tums and Rolaids, at 270-400 mg calcium per chew…

  Elemental content

  40% elemental calcium by weight - the most concentrated common form

  Absorption

  Insoluble, so it needs stomach acid to dissolve. Take it with a meal, and expect less absorption where gastric acid is low

  Best for

  Cost and pill count. It is also the active ingredient in antacids like Tums and Rolaids, at 270-400 mg calcium per chewable [1]

• Calcium citrateOlder adults, low stomach acid, and after gastric bypass.

  Elemental content

  21% elemental calcium by weight

  Absorption

  Absorbs 22-27% better than carbonate across 15 studies in 184 subjects - 27.2% better on an empty stomach, 21.6% with meals

  Best for

  Older adults, low stomach acid, and after gastric bypass. No food required, and less gas, bloating, and constipation than carbonate [1]

• Calcium lactate and calcium gluconateUncommon on the supplement shelf.

  Elemental content

  13% and 9% elemental calcium respectively

  Absorption

  Far less calcium per pill, so the tablet count climbs quickly

  Best for

  Uncommon on the supplement shelf. Mostly encountered in clinical or food-fortification settings [26]

• Coral calciumNothing. The FTC brought a false-advertising case against its marketers in 2003 over disease claims, and the Okinawa lo…

  Elemental content

  Chemically just calcium carbonate from fossilized coral reef deposits, plus trace minerals

  Absorption

  No clinical evidence shows any absorption advantage over ordinary calcium carbonate

  Best for

  Nothing. The FTC brought a false-advertising case against its marketers in 2003 over disease claims, and the Okinawa longevity story used to sell it was attributed to lifestyle [17]

• Digestive effects

  Gas, bloating, and constipation are more common with calcium carbonate than with calcium citrate [1].
  
  Older adults with reduced stomach acid get the worst of it [1]. ^[1]

• Drugs that lower calcium status

  Bisphosphonates, cisplatin, and proton pump inhibitors are recognized causes of hypocalcemia in their own right [1].
  
  That runs opposite to the interactions above - here the drug depletes calcium rather than calcium blocking the drug [1]. ^[1]

• Kidney stones, genuinely disputed

  Part of the upper limit rests on Women's Health Initiative data linking 1,000 mg/day of supplemental calcium over 7 years to more urinary-tract stones [1].
  
  Two later systematic reviews - one over 8,000 osteoporosis patients, another 51,419 adults over 50 - found no such association [1].
  
  Both kept, neither resolved [1]. ^[1]

• Timing changes the stone maths

  A controlled study in 32 subjects found calcium carbonate taken with meals avoided a significant rise in the calcium-oxalate stone-formation risk index, while bedtime dosing did not [14].
  
  The mechanism is that meal-timed calcium binds dietary oxalate in the gut, so less oxalate reaches the urine [13].
  
  The National Kidney Foundation advises that adequate dietary calcium with meals lowers oxalate-stone risk, and that a strict low-oxalate diet is not the right approach for most people [13]. ^[14]

• Dolutegravir (Tivicay, Dovato)

  Calcium chelates this HIV integrase inhibitor and can substantially reduce its blood levels [1].
  
  FDA labelling instructs taking dolutegravir 2 hours before or 6 hours after calcium [1]. ^[1]

• Levothyroxine (Synthroid)

  Calcium carbonate interferes with thyroid hormone absorption [1].
  
  The FDA-approved label says avoid levothyroxine within 4 hours of a calcium carbonate dose [1]. ^[1]

• Quinolone antibiotics

  Ciprofloxacin, gemifloxacin, and moxifloxacin absorb less when taken at the same time as calcium [1].
  
  Taking the antibiotic 2 hours before or after calcium avoids the problem [1]. ^[1]

• Lithium

  Long-term lithium use can itself cause hypercalcemia [1].
  
  Adding calcium supplements on top may increase that risk [1]. ^[1]

• Thiazide diuretics

  Hydrochlorothiazide and indapamide combined with large calcium amounts raise the risk of milk-alkali syndrome, a serious hypercalcemia [20].
  
  Mayo Clinic advises against large daily calcium amounts on these drugs, plus periodic blood pressure and calcium monitoring [20]. ^[20]

• Other blood pressure drugs, mostly fine

  Oral calcium does not meaningfully interfere with ACE inhibitors, ARBs, beta blockers, or renin inhibitors [20].
  
  Calcium channel blockers are the nuance: oral calcium leaves them alone, but intravenous calcium can blunt them, which is why IV calcium is used clinically in overdose [20]. ^[20]

• Iron, zinc, and magnesium

  Calcium competes with all three for intestinal absorption, and reduces heme and non-heme iron absorption specifically [1].
  
  Space them roughly 2 hours apart from a calcium dose [21]. ^[1]

Dairy dominates: about 72% of US dietary calcium comes from dairy and foods containing dairy ingredients [1].

Per-serving figures, compiled by NIH ODS from USDA data:

Plain low-fat yogurt, 8 oz - 415 mg (32% DV)

Calcium-fortified orange juice, 1 cup - 349 mg (27% DV)

Part-skim mozzarella, 1.5 oz - 333 mg (26% DV)

Canned sardines with bones, 3 oz - 325 mg (25% DV)

Nonfat milk, 1 cup - 299 mg (23% DV), matched exactly by calcium-fortified soymilk

Firm tofu made with calcium sulfate, half cup - 253 mg (19% DV)

Canned pink salmon with bones, 3 oz - 181 mg (14% DV)

Boiled spinach, half cup - 123 mg (9% DV)

Raw kale, 1 cup - 24 mg (2% DV) [1]

The milligram figure is only half the story. Absorption from dairy and fortified foods runs about 30%, but from spinach only about 5%, because oxalic acid binds calcium into an indigestible salt [1].

Collard greens, sweet potatoes, rhubarb, and beans are oxalate-rich in the same way [1].

Low-oxalate plants - broccoli, kale, cabbage - absorb about as well as milk, near 30%, even though they carry far less calcium per serving [1].

Calcium citrate malate, a well-absorbed form, is what juices are usually fortified with [1].

Caffeine and phosphorus trim net absorption slightly. Low vitamin D status cuts it far more, since vitamin D drives active intestinal calcium absorption [1].

The well-supported guidance here is about dose size, not clock time [1].

Absorption efficiency falls as the dose rises: about 36% of a 300 mg dose is absorbed, against about 28% of a 1,000 mg dose taken at once [1].

So NIH, MedlinePlus, and Mayo Clinic all recommend splitting anything above 500 mg/day into two or more doses across the day [21].

A clinical review says the same - single doses should not exceed 500 mg, whichever calcium salt is used [24].

Form decides the food question. Calcium carbonate needs a meal, since it needs stomach acid to dissolve, while calcium citrate works with or without food [22].

Keep calcium roughly 2 hours away from iron, zinc, multivitamins, levothyroxine, quinolone antibiotics, and dolutegravir [21].

On morning against evening there is no clinical-trial evidence establishing a winner. A supplement-brand claim that nighttime is better because bone turnover peaks overnight is not backed by any primary trial found here [21].

For people prone to stones, one small controlled study gives a reason to prefer meal-timed over bedtime dosing [14].