Calcium: Benefits, Best Forms, Dosing, and Side Effects

Table of Contents

• Current view (reviewed 22 October 2026)
• Key safety points
• Brad's videos on this topic
• Overview
• Forms and Bioavailability
• Evidence for Health Benefits
• Recommended Dosing
• Safety and Side Effects
• Drug Interactions
• Dietary Sources
• References

Current view (reviewed 22 October 2026)

Brad's view is that calcium the mineral matters, but the evidence says to get it from food rather than a pill: dairy, leafy greens and fish (Brad, 7 Aug 2026). He points to a 2017 analysis that pooled more than 51,000 people and found calcium, with or without vitamin D, was not linked to fewer hip, spine or other fractures (Brad, 7 Aug 2026). His guidance cites a meta-analysis of placebo-controlled trials of calcium supplements given without vitamin D that found more heart attacks in the calcium groups (hazard ratio 1.31, 95% CI 1.02 to 1.67) (Brad's current guidance, 14 Oct 2026). He notes that the US Preventive Services Task Force recommends against daily supplements of 1,000 mg or less of calcium with 400 IU or less of vitamin D to prevent fractures in postmenopausal women living in the community (Brad, 7 Aug 2026). If your doctor has put you on calcium supplements for a genuine deficiency or because you don't eat dairy, he says that is not a reason to stop, but a conversation to have with your doctor (Brad, 7 Aug 2026). People taking calcium supplements often report stomach problems and constipation, and some find a higher risk of kidney stones (Brad, 14 Oct 2024).

Brad's videos on this topic

• 7 Aug 2026, NASA Found How To Stop Osteoporosis: calcium supplements did not prevent fractures in pooled trials and were linked to heart attacks, so get calcium from food; heavy resistance exercise builds bone.
• 3 Jul 2025, Do This Daily to STOP Bone Loss After 40: calcium supplements have mixed evidence, with the lower-bias trials showing no fracture benefit; exercise comes first.
• 15 May 2025, This Incredibly Popular Supplement DAMAGES Your Blood Vessels: calcium supplements are linked to more heart attacks and artery calcification while dietary calcium is not; vegan women may need them.
• 14 Oct 2024, How to Include More Calcium-Rich Foods in Your Diet: diet first, and going above the recommended intake may raise heart disease risk. Superseded on supplements to fill dietary gaps: the August 2026 post says to get calcium from food rather than a pill.
• 11 Aug 2024, Which Popular Supplements Should You Actually Try to Avoid?: calcium supplements do not seem to prevent fractures and may add to kidney stones; vegans and people who avoid dairy may need them.

1. Overview

Calcium is the most abundant mineral in the body. More than 99% of it is stored in bones and teeth as calcium hydroxyapatite. The small ionized pool circulating in blood and other body fluids helps blood vessels contract and dilate, and supports muscle function, blood clotting, nerve transmission and hormone secretion ^[1].

At birth the body holds about 26 to 30 g of calcium, rising to about 1,200 g in women and 1,400 g in men by adulthood ^[1]. Levels then stay steady in men but start to fall in women at menopause, as lower estrogen speeds bone remodeling ^[1]. On average, women lose about 1% of their bone mineral density (BMD) a year after menopause ^[1]. About 30% of postmenopausal women in the United States and Europe have osteoporosis, and at least 40% of them have at least one fragility fracture ^[1].

Many people eat less calcium than recommended. In 2007 to 2010 US survey data, 49% of children and teenagers and 39% of everyone aged 4 and older took in less than the Estimated Average Requirement from foods and supplements ^[1]. Average intakes from foods and drinks are 1,083 mg for men 20 and older and 842 mg for women ^[1]. Inadequate intakes are more common among non-Hispanic Black and non-Hispanic Asian people (47% to 48%) than among Hispanic (30%) and non-Hispanic White people (24%) ^[1]. Among adults 50 and older, the risk of inadequate intake is 11.6% higher in households earning less than $20,000 a year ^[1].

Calcium is absorbed by active transport, which needs vitamin D and handles most absorption when intakes are low, and by passive diffusion, which takes a growing share as intakes rise ^[1]. The more calcium you take in, the smaller the share absorbed: about 45% at intakes of 200 mg a day but only 15% when intakes are higher than 2,000 mg a day ^[1]. Net absorption is as high as 60% in infants and young children, falls to about 25% in adults and keeps declining with age ^[1].

Blood calcium is held in a narrow range, typically 8.8 to 10.4 mg/dL (2.2 to 2.6 mmol/L), so a blood test does not show nutritional status ^[1]. A dual-energy X-ray absorptiometry (DEXA) scan of bone mineral density reflects calcium status over a lifetime ^[1].

Hypocalcemia (serum calcium less than 8.5 mg/dL) usually results from vitamin D or magnesium deficiency, low parathyroid hormone, critical illness, or medicines such as bisphosphonates, cisplatin or proton pump inhibitors ^[1]. The most common symptoms are numbness around the mouth, tingling in the hands and feet, and muscle spasms; severe cases can cause seizures and, rarely, coma ^[1].

2. Forms and Bioavailability

A supplement's form sets how much elemental calcium each gram provides, and it can affect side effects ^[1].

Comparison Table

Elemental calcium figures are from NIH ODS ^[1]. Absorption figures are from one study of eight healthy fasting adults who took a 500 mg calcium dose of each salt; the differences were not statistically significant ^[15].

Key Principles for Form Selection

For most people: The form may matter little. In eight healthy fasting adults, absorption from carbonate, acetate, lactate, gluconate and citrate salts, and from whole milk, was similar ^[15]. In 37 healthy adults, calcium carbonate taken with a light breakfast was absorbed no worse than calcium citrate ^[18].

For people taking proton pump inhibitors (PPIs): Calcium citrate is less dependent on stomach acid than calcium carbonate ^[1]. Evidence that PPIs reduce absorption is mixed: in postmenopausal women, 30 days of omeprazole 40 mg a day did not reduce calcium absorption from their usual meals ^[20].

For post-bariatric surgery patients: In a crossover trial in people after gastric bypass or sleeve gastrectomy, calcium citrate gave slightly higher relative bioavailability than calcium carbonate (AUC 76.1 mg/dL·h vs 74.7 mg/dL·h) and lowered parathyroid hormone more ^[16].

Plant-based calcium (algae-derived): One study of AlgaeCal, an algae-derived bone supplement, was open-label and had no placebo group: adults followed a bone-health plan with one of two versions of the supplement for six months, and both groups gained more bone density than expected, but only one version raised it significantly from baseline ^[24].

How to Read Supplement Labels

The weight of a calcium compound is not all calcium: calcium carbonate is 40% calcium by weight, whereas calcium citrate is 21% calcium ^[1]. The Supplement Facts panel lists the elemental calcium, so there is no need to work it out from the compound weight ^[1].

Calcium and Magnesium: Timing Considerations

In a metabolic study of five men, raising magnesium intake to 826 mg a day as magnesium oxide did not change calcium absorption at calcium intakes of 241 or 812 mg a day ^[27].

3. Evidence for Health Benefits

Bone Health and Fractures

Bone is constantly remodeled. Falling estrogen during menopause, and for about 5 years after, makes bone resorption outpace formation, so bone mass drops quickly ^[1]. FDA has approved a health claim that supplements containing calcium and vitamin D reduce the risk of osteoporosis, but not all research supports it ^[1].

Bone mineral density in postmenopausal women: Across 15 trials in 78,206 postmenopausal women, calcium with vitamin D, or dairy fortified with both, increased total BMD and BMD at the lumbar spine, arms and femoral neck; in subgroup analyses, calcium had no effect on femoral neck BMD ^[1]. In a 2-year trial in 500 healthy postmenopausal women, skimmed milk providing 900 mg calcium and 600 IU vitamin D a day increased femoral neck BMD ^[1]. Earlier reviews linked calcium with vitamin D to higher BMD in older men, though whether these gains matter clinically is unclear ^[1].

In young girls: In a trial in identical twin girls aged 9 to 13 years, 800 mg of calcium with 400 IU of vitamin D3 a day for 6 months increased trabecular density and bone strength at the lower leg and forearm compared with placebo ^[32].

Women's Health Initiative (WHI): In this trial, 36,282 postmenopausal women aged 50 to 79 years took 1,000 mg of elemental calcium as calcium carbonate with 400 IU of vitamin D3 a day, or placebo, for an average of 7.0 years ^[113]. Hip bone density was 1.06 percent higher with supplements, but hip fracture was not significantly reduced (hazard ratio 0.88, 95% CI 0.72 to 1.08), and kidney stones increased (hazard ratio 1.17, 95% CI 1.02 to 1.34) ^[113].

Vegan women: In a cohort of 34,542 adults, vegan women had a higher risk of hip fracture than non-vegetarians (hazard ratio 1.55, 95% CI 1.06 to 2.26) ^[35]. Vegan women using both calcium and vitamin D supplements had no extra risk, and diet pattern was not linked to hip fracture in men ^[35].

Pre/peri-menopausal women: In 1,490 women who were 42 to 52 years old at baseline, followed for 10 to 12 years, calcium supplement users lost less BMD but had no fewer fractures than non-users ^[1].

Elderly care facility residents: In a 2-year cluster randomised trial in 60 Australian aged care facilities (7,195 residents, mean age 86.0), extra milk, yoghurt and cheese raised calcium intake and brought protein to 1.1 g/kg body weight ^[37]. All fractures fell by 33% (hazard ratio 0.67), hip fractures by 46% (hazard ratio 0.54) and falls by 11% (hazard ratio 0.89) ^[37].

Meta-analyses on fracture reduction: A meta-analysis of eight trials in 30,970 people found that calcium with vitamin D reduced total fractures by 15% and hip fractures by 30% ^[38]. Other analyses disagree. A 2017 meta-analysis of 33 trials in community-dwelling adults older than 50 years found no link between calcium, vitamin D or both and hip, vertebral or total fractures ^[116]. A 2015 review found that calcium supplements reduced total fracture across 20 trials (relative risk 0.89, 95% CI 0.81 to 0.96), but had no effect on fracture at any site in the four trials at lowest risk of bias (n=44,505) ^[114]. In a review of 11 trials in 51,419 adults 50 and older, vitamin D with calcium had no effect on total or hip fracture ^[1]. The USPSTF found adequate evidence that daily supplementation with 400 IU or less of vitamin D and 1000 mg or less of calcium has no benefit for preventing fractures in community-dwelling postmenopausal women, and inadequate evidence on higher doses ^[40].

Cancer

Overall cancer incidence: In a 4-year trial in 2,303 healthy postmenopausal women, 1,500 mg/d of calcium with 2,000 IU/d of vitamin D3 did not significantly lower cancer incidence (hazard ratio 0.70, 95% CI 0.47 to 1.02) ^[42]. In the WHI, calcium with vitamin D did not change invasive cancer incidence during the trial (hazard ratio 0.98) ^[43]. A meta-analysis of 10 trials in 10,496 people taking 500 mg or more of calcium without vitamin D for a mean of 3.9 years found no change in total cancer risk ^[1]. A 4-year trial in 1,179 women in Nebraska reported 60% lower cancer incidence with 1,400 to 1,500 mg calcium plus 1,100 IU vitamin D3 and 47% lower with calcium alone, but scientists have questioned these findings because the trial was designed to study bone, not cancer ^[1].

Colorectal cancer: In a dose-response meta-analysis of 15 prospective cohort studies, each 300 mg/day increase in total calcium intake was linked to an approximately 8% lower risk of colorectal cancer (summary RR 0.92, 95% CI 0.89 to 0.95) ^[46]. A meta-analysis of four trials found that 1,200 to 2,000 mg elemental calcium a day for 36 to 60 months modestly reduced recurrent adenomas (RR 0.89, 95% CI 0.82 to 0.96) but not advanced adenomas ^[47]. In the WHI, colorectal cancer incidence did not differ between groups after an average of 11 years (hazard ratio 0.95, 95% CI 0.80 to 1.13) ^[48]. In a trial of people with a previous adenoma given 1,200 mg/day of elemental calcium, 1,000 IU/day of vitamin D3, both or neither for 3 or 5 years, there was no effect on sessile serrated polyps during treatment, but in the later observational phase the risk was higher with calcium alone (adjusted RR 2.65, 95% CI 1.43 to 4.91) and with calcium plus vitamin D (3.81, 95% CI 1.25 to 11.64) ^[49].

Prostate cancer: In 2,776 French men followed for a median of 7.7 years, those in the highest quartile of calcium intake had a higher risk of prostate cancer than those in the lowest (RR 2.43, 95% CI 1.05 to 5.62) ^[50]. A meta-analysis of cohort studies linked dietary calcium to slightly higher prostate cancer risk (summary RR 1.05 per 400 mg/d, 95% CI 1.02 to 1.09), but not nondairy or supplemental calcium; supplemental calcium was linked to fatal prostate cancer ^[51]. In a trial of 672 men, 1,200 mg of calcium a day as calcium carbonate for 4 years left prostate cancer rates no higher over a mean of 10.3 years (rate ratio 0.83, 95% CI 0.52 to 1.32) ^[52].

Breast cancer: Mixed evidence. A meta-analysis of 11 prospective cohort studies in 872,895 women found an 8% lower breast cancer risk with the highest calcium intakes ^[1]. In the WHI, invasive breast cancer was similar in the supplement and placebo groups (hazard ratio 0.96, 95% CI 0.85 to 1.09) ^[54].

Ovarian cancer: A meta-analysis of 15 studies in 493,415 women found that ovarian cancer risk was 20% lower with the highest dietary calcium intakes, but the difference was not statistically significant when supplemental calcium was included ^[1].

Cancer mortality: After a median of 22.3 years of WHI follow-up, cancer mortality was lower in the supplement group than on placebo (hazard ratio 0.93, 95% CI 0.87 to 0.99) ^[34]. Two large observational analyses found no association between calcium intake and cancer mortality ^[1].

Cardiovascular Disease

The debate centres on calcium from supplements rather than food. Several large observational studies link lower calcium intakes with higher risk of hypertension, stroke and atherosclerosis, while some cohort studies and trials suggest supplements might raise cardiovascular risk and others find no link ^[1].

Heart attack risk: In a meta-analysis of placebo-controlled trials of calcium supplements given without vitamin D, calcium was linked to more heart attacks: hazard ratio 1.31 (95% CI 1.02 to 1.67) in the five trials with patient-level data, and a pooled relative risk of 1.27 (95% CI 1.01 to 1.59) in the trial-level analysis ^[58]. A reanalysis that added WHI women not taking their own calcium supplements found that calcium or calcium and vitamin D raised heart attack risk (relative risk 1.24, 95% CI 1.07 to 1.45) ^[59]. In the Auckland trial in 1,471 healthy postmenopausal women, adjudicated heart attacks were more common with calcium (relative risk 2.12, 95% CI 1.01 to 4.47) ^[115]. In a Korean cohort of people with osteoporosis, calcium with vitamin D was not linked to more cardiovascular disease, but calcium without vitamin D was (hazard ratio 1.54, 95% CI 1.17 to 2.04), especially non-fatal heart attack (hazard ratio 1.89, 95% CI 1.23 to 2.91) ^[60]. In a German cohort, calcium supplement users had more heart attacks than non-users of any supplements (HR=1.86; 95% CI 1.17 to 2.96), more so with calcium-only supplements (HR=2.39; 95% CI 1.12 to 5.12) ^[61].

Stroke risk: In a nested case-control study of adults aged 40 to 89, calcium with vitamin D was not linked to ischemic stroke, but calcium alone at 1,000 mg/day or more was (odds ratio 2.09, 95% CI 1.25 to 3.49) ^[62].

CVD mortality: In US adults aged 50 to 71 years followed for a mean of 12 years, supplemental calcium above 1,000 mg a day was linked to higher cardiovascular death in men (relative risk 1.20, 95% CI 1.05 to 1.36) but not in women, and dietary calcium was not linked to it ^[63]. In Swedish women followed for a median of 19 years, dietary intakes above 1400 mg/day were linked to higher death rates from cardiovascular disease (hazard ratio 1.49, 95% CI 1.09 to 2.02) and from all causes (hazard ratio 1.40, 95% CI 1.17 to 1.67); among calcium tablet users (500 mg per tablet) with such intakes, the hazard ratio for death from all causes was 2.57 (95% CI 1.19 to 5.55) ^[64]. After a median of 22.3 years of WHI follow-up, cardiovascular mortality was higher among women given calcium and vitamin D (hazard ratio 1.06, 95% CI 1.01 to 1.12) ^[34].

Coronary artery calcification: In 1,567 adults without coronary calcium at baseline followed for 10 years, high total calcium intake was linked to lower risk of new calcification, but calcium supplement use was linked to higher risk (relative risk 1.22, 95% CI 1.07 to 1.39) ^[65]. In a pooled analysis of 9 trials in patients with known coronary disease (447 taking calcium supplements, 4,700 not), supplement use was linked to a faster yearly rise in coronary calcium on intravascular ultrasound (odds ratio 1.15, 95% CI 1.05 to 1.26) ^[66]. Calcium supplements raise blood calcium for up to 8 hours after each dose, and researchers suggest this may relate to the cardiovascular risk ^[17].

Cholesterol effects in postmenopausal women: In a placebo-controlled trial in Chinese women with dyslipidemia, 800 mg calcium a day raised total cholesterol and carotid intima-media thickness in postmenopausal women aged 50 to 60, but not in premenopausal women ^[68].

Blood pressure: A Cochrane Review of 16 trials in 3,048 adults found that calcium supplements (typically 1,000 to 2,000 mg/day) reduced systolic blood pressure by 1.43 mmHg and diastolic blood pressure by 0.98 mmHg, with the largest effects in adults younger than 35 years and at doses higher than 1,500 mg/day ^[1]. The DASH diet, with added low-fat dairy, lowered blood pressure by 5.5/3.0 mmHg, though the independent contribution of calcium cannot be determined ^[1].

Cholesterol: A meta-analysis of 23 trials in 4,071 people found calcium supplements with or without vitamin D were associated with LDL cholesterol 4.6 mg/dL lower and HDL cholesterol 1.9 mg/dL higher ^[1].

Position statements: The National Osteoporosis Foundation and the American Society for Preventive Cardiology found moderate-quality evidence that calcium with or without vitamin D, from food or supplements, has no relationship to cardiovascular disease in generally healthy adults, and that intakes within the upper limit (2000 to 2500 mg/d) should be considered safe from a cardiovascular standpoint ^[72].

Age-Related Macular Degeneration (AMD)

Evidence on calcium and AMD is mixed and comes from observational studies only. In the Blue Mountains Eye Study of adults aged 49 years or above, followed for 15 years, falling dairy and calcium intake was linked to more late AMD ^[74]. In the Age-Related Eye Disease Study, the top tertile of supplementary calcium was linked to less neovascular ("wet") AMD (hazard ratio 0.70, 95% CI 0.50 to 0.97) ^[75]. In US survey data on 3,191 adults aged 40 and older, those reporting more than 800 mg/d of supplemental calcium had higher odds of AMD (odds ratio 1.85, 95% CI 1.25 to 2.75) ^[76].

Mortality

In 38,772 older women in the Iowa Women's Health Study, calcium supplement use was linked to lower total mortality (hazard ratio 0.91, 95% CI 0.88 to 0.94) ^[77]. In a Canadian cohort of 9,033 adults followed for 10 years, calcium supplement use was linked to lower mortality in women (hazard ratio 0.78, 95% CI 0.66 to 0.92), with significant reductions at doses up to 1000 mg/d, and no clear association in men ^[78].

Preeclampsia

Calcium supplements during pregnancy might reduce preeclampsia risk, but the benefit might apply only to women with low calcium intakes, and much of the evidence comes from studies with methodological weaknesses ^[1]. A Cochrane review of 27 trials (18,064 women) found that at least 1 g daily of calcium cut high blood pressure in pregnancy (risk ratio 0.65, 95% CI 0.53 to 0.81) ^[79]. In women with low dietary calcium (less than 900 mg/day), high doses reduced preeclampsia risk by 64%, though the quality of this evidence was low ^[1]. An earlier meta-analysis of 10 trials in 24,787 women found that 1,500 to 2,000 mg/day reduced preeclampsia risk by 38%, but the effect was no longer significant in trials with 4,000 or more women ^[1]. The World Health Organization advises 1,500 to 2,000 mg/day for pregnant women with low dietary calcium intakes, and similar advice comes from other groups, including the International Society for the Study of Hypertension in Pregnancy ^[1].

Weight Management

Early studies suggested calcium might aid weight loss, but later evidence is mixed. A meta-analysis of 41 trials in 4,802 adults found that more calcium from dairy did not change body weight or body fat except alongside an energy-restricted diet, and calcium supplements had no effect ^[1]. In the WHI, women whose intakes were less than 1,200 mg calcium at baseline and who took calcium and vitamin D were 11% less likely to gain 1 kg or more ^[1].

Metabolic Syndrome

In US survey data on 9,148 adults, women in the highest quintile of calcium intake (1,172 mg/day or more) were 27% less likely to have metabolic syndrome than the lowest quintile ^[1]. A meta-analysis of 10 observational studies in 63,017 people found risk fell 7% with every extra 300 mg/day of dietary calcium ^[1]. Clinical trial evidence is very limited ^[1].

Dementia

In 700 Swedish women aged 70 to 92 years, calcium supplement users had higher odds of dementia at 5 years (odds ratio 2.10, 95% CI 1.01 to 4.37), mainly women who had had a stroke or had white matter lesions ^[86]. Trials have not confirmed this. In a 5-year trial of 1,200 mg/day calcium carbonate in 1,460 older Australian women followed over 14.5 years in total, calcium did not increase dementia events (hazard ratio 0.90, 95% CI 0.71 to 1.15) ^[87]. In the WHI Memory Study, calcium with 400 IU of vitamin D3 did not change incident dementia during a mean follow-up of 7.8 years (hazard ratio 1.11, 95% CI 0.71 to 1.74) ^[88].

Dental Health

In a 3-year trial in 145 adults aged 65 and older, fewer people taking calcium and vitamin D supplements lost one or more teeth than those taking placebo (13% vs 27%; odds ratio 0.4, 95% CI 0.2 to 0.9) ^[118].

PMS Symptom Reduction

In a trial in 466 women with moderate to severe premenstrual symptoms, 1200 mg of elemental calcium per day as calcium carbonate for three cycles reduced total symptom scores by 48% from baseline, compared with 30% on placebo ^[117].

4. Recommended Dosing

Recommended Dietary Allowances (RDAs)

From the Institute of Medicine (now National Academy of Medicine) ^[1]:

*Adequate Intake (AI). RDAs are not set for this age group due to insufficient data.

Note: The RDA during pregnancy and lactation is the same as for other women of the same age ^[1].

Tolerable Upper Intake Level (UL)

The ULs are based on WHI evidence linking supplemental calcium (1,000 mg/day for 7 years) with more kidney stones, though two later systematic reviews found no such link ^[1].

Practical Dosing Guidance

General principle: Food first, supplements to fill gaps. Nutritional needs should be met mainly through the diet, with supplements useful where food alone falls short ^[1].

Dose per serving: Absorption from supplements is highest with doses of 500 mg or less; the body absorbs about 36% of a 300 mg calcium dose and 28% of a 1,000 mg dose ^[1].

Timing: Calcium carbonate is best taken with a meal, while calcium citrate can be taken without food; absorption of either is greater with food ^[1].

With vitamin D: Vitamin D is required for calcium to be absorbed by active transport ^[1]. Many fracture trials tested calcium with vitamin D, with mixed results (see Bone Health and Fractures).

Dosing for specific situations:

• Women older than 50 years: The RDA rises to 1,200 mg, from all sources, to lessen bone loss after menopause ^[1]
• Building bone in girls aged 9 to 13 years: 800 mg calcium with 400 IU vitamin D3 a day in one trial ^[32]
• Preeclampsia prevention (low-calcium pregnant women): 1,500 to 2,000 mg/day ^[1]
• PMS symptom reduction: 1200 mg of elemental calcium per day as calcium carbonate in one trial ^[117]

Product mention: Dr Brad Stanfield's MicroVitamin does not contain calcium but includes Vitamin K2 MK-7 90 mcg (which activates osteocalcin to direct calcium to bone), Vitamin D3 1,000 IU (supports calcium absorption), and Boron 1 mg (supports bone metabolism).

Antacids as Calcium Sources

Some over-the-counter antacids, such as Tums and Rolaids, contain calcium carbonate. Depending on strength, each chewable or soft chew supplies roughly 270 to 400 mg ^[1].

5. Safety and Side Effects

Common Side Effects

Some people taking calcium supplements get gas, bloating, constipation or a combination of these ^[1]:

• Constipation, gas and bloating: Calcium carbonate appears to cause more of these than calcium citrate, especially in older adults with lower levels of stomach acid ^[1].

Switching to another form, taking smaller doses more often during the day, or taking the supplement with meals can ease symptoms ^[1].

Kidney Stone Risk

Calcium supplements may raise kidney stone risk, though the evidence is mixed:

• WHI trial: 500 mg of calcium (as calcium carbonate) plus 200 IU vitamin D3 twice daily increased self-reported urinary tract stones (hazard ratio 1.17, 95% CI 1.02 to 1.34; 449 vs 381 women) ^[90].
• USPSTF review: Vitamin D with calcium increased kidney stones (pooled absolute risk difference 0.33%, 95% CI 0.06% to 0.60%), but calcium alone did not ^[39].
• Urine calcium monitoring: In 163 older women given calcium citrate to reach a total intake of about 1,200 mg/day with vitamin D, hypercalciuria occurred in 30.6% and hypercalcemia in 8.8% over a year, and hypercalciuria was as common in the placebo group ^[92].
• Dietary calcium may protect. In 120 men with repeated calcium oxalate stones and hypercalciuria, a diet with a normal amount of calcium (30 mmol per day) and less animal protein and salt led to fewer relapses at 5 years than a low-calcium diet (12 of 60 vs 23 of 60 men; relative risk 0.49, 95% CI 0.24 to 0.98) ^[93]. Urinary oxalate rose on the low-calcium diet ^[93].

Hypercalcemia

Hypercalcemia (serum calcium more than 10.5 mg/dL) is rare in healthy people and usually results from cancer, primary hyperparathyroidism or other conditions ^[1]. It can cause poor muscle tone, kidney problems, constipation, nausea, weight loss, fatigue, frequent urination and heart rhythm problems ^[1]. Too much calcium taken with high doses of vitamin D can cause milk-alkali syndrome, and thiazide diuretics and lithium can also raise blood calcium ^[124]. Anyone with symptoms of high calcium should have their blood calcium checked, and people taking calcium and vitamin D supplements should ask their provider about the correct dose ^[124].

Cardiovascular Risks

See Section 3 (Cardiovascular Disease) for detailed evidence. Some trials and cohort studies link calcium supplements, particularly without vitamin D, to more heart attacks, ischemic stroke, coronary calcification and cardiovascular death ^{[58, 60, 62, 63, 65]}. An expert panel found no relationship in generally healthy adults ^[72]. In a post hoc analysis of people with coronary disease, supplement users had a faster yearly rise in coronary calcium ^[66]. In a retrospective study of 2,657 people aged 60 or older with mild to moderate aortic stenosis, calcium supplements with or without vitamin D were associated with higher all-cause mortality (hazard ratio 1.31, 95% CI 1.07 to 1.62), cardiovascular mortality (hazard ratio 2.0, 95% CI 1.31 to 3.07) and aortic valve replacement (hazard ratio 1.48, 95% CI 1.24 to 1.78) ^[127]. If you have coronary artery disease or aortic stenosis, ask your doctor before taking calcium supplements.

Prostate Cancer Risk

Higher calcium intakes might raise prostate cancer risk, possibly only when the calcium comes from dairy foods, though not all studies confirm this ^[1]. In one trial, calcium supplements did not increase prostate cancer ^[52].

Early Menopause

The Nurses' Health Study II reported no higher risk of early menopause with calcium: dietary calcium in the highest quintile (median 1246 mg/d) was linked to a borderline lower risk (hazard ratio 0.87, 95% CI 0.76 to 1.00), while high supplement use was not associated with lower risk ^[96].

Carotenoid Interaction

In a crossover study in healthy adults, a meal with 19 mg lycopene (from tomato paste) plus 500 mg calcium carbonate gave 83% lower lycopene bioavailability than the meal alone ^[97].

Interference with DXA Scans

Undissolved calcium tablets in the gut during a DXA scan add only small errors to lumbar spine BMD, not enough to change the diagnosis, but they can cause misclassification when only two or three vertebrae can be read ^[98].

Strontium Concerns

Some bone health supplements contain strontium. A case report described DRESS (drug reaction with eosinophilia and systemic symptoms), a severe drug reaction, linked to a strontium citrate supplement ^[100].

6. Drug Interactions

In a survey of 62 older adults, calcium and multivitamin-mineral supplements were the most common supplements with potential medication interactions ^[101]. If you take prescription medicines, check with your doctor or pharmacist before starting.

Calcium Reduces Drug Absorption

These medications should be separated from calcium supplements:

Drugs That Affect Calcium Levels

Other Interactions

Iron: Calcium reduces iron absorption. In 126 adults, adding 300 to 600 mg of calcium, given as calcium chloride, to wheat rolls reduced iron absorption by 50% to 60% ^[125].

Aluminum-containing antacids: In eight healthy men, calcium citrate taken with aluminum hydroxide raised urinary aluminum excretion 5.3 to 11.1 times; the authors say the two must not be prescribed together in kidney failure ^[109].

Micromineral interactions: Giving 600 mg of calcium as calcium carbonate alongside a test meal cut zinc absorption from it by 50%, and extra zinc in the supplement offset the effect ^[126].

Digoxin: Case reports suggest that high blood calcium may predispose to digoxin toxicity ^[123].

7. Dietary Sources

Milk, yogurt, and cheese are the richest natural sources, providing approximately 72% of calcium intakes in the United States ^[1]. Nondairy sources include canned fish with bones, certain vegetables, and fortified foods.

Top Food Sources

Source: USDA FoodData Central via NIH ODS ^[1].

Absorption from Food Sources

Absorption varies by food because some plant compounds bind calcium:

• Dairy products and fortified foods: About 30% absorbed ^[1]
• Low-oxalate vegetables (broccoli, kale, cabbage): Absorption similar to milk, though the amount per serving is much lower ^[1]
• High-oxalate foods: Spinach, collard greens, sweet potatoes, rhubarb and beans are high in oxalic acid ^[1]
• Spinach: In 13 healthy adults, calcium absorption averaged 5.1% from spinach versus 27.6% from milk, because oxalate binds the calcium ^[111]

Practical Notes

• Oxalate and phytates bind calcium into indigestible salts, but when people eat many different foods these interactions probably have little or no nutritional effect ^[1].