Calcium
mineral
ApprovedGeneral information about nutrients. No diagnosis, no medical advice. If you take medication, are pregnant or have a medical condition, talk to your doctor before taking supplements.
Overview
Calcium is needed for the maintenance of normal bones. Calcium contributes to normal muscle function. Calcium contributes to normal neurotransmission.
Interactions
Vitamin D3 is essential for calcium absorption in the small intestine. Without D3, only 10-15% of dietary calcium is absorbed; with D3, this rises to 30-40%.
K2 activates osteocalcin, which binds calcium into the bone matrix.
Calcium inhibits both heme and non-heme iron absorption by competing for DMT1 transporters in the intestinal epithelium. Studies show 300mg Ca reduces iron absorption by 50-60%. Take at different meals.
Calcium and zinc share intestinal absorption pathways (DMT1 transporter). High-dose calcium supplementation (>600mg) significantly reduces zinc absorption. Separate intake by 2+ hours.
Calcium and magnesium compete for absorption via TRPM6/7 channels. High calcium-to-magnesium ratios (>2:1) reduce Mg absorption. Take separately or maintain balanced ratios.
Calcium and manganese compete for DMT1 transporters in the intestinal epithelium. High calcium intake significantly reduces manganese absorption, so they should be taken separately.
High-dose calcium supplementation interferes with copper absorption by competing for shared divalent cation transporters in the intestine. Separate intake timing to avoid copper depletion.
Research status
Excess supplementation may increase the risk of cardiovascular calcification, so food sources are preferred.
Top 5 food sources
Expert opinions
NIH
The NIH UL of 2,500 mg/day (2,000 mg over age 50) reflects the risk of hypercalcemia, kidney stones, and milk-alkali syndrome at chronic excess. Meta-analyses link supplemental calcium above 1,000 mg/day to a modestly increased myocardial-infarction risk. The RDA is 1,000 mg/day (1,200 mg for women 50+ and men 70+), best met through dairy, leafy greens, and fortified foods. Supplementation is reserved for insufficient dietary intake, always paired with adequate vitamin D3.
Recommended dose: 1000–1200 mg
Source: NIH
Current Research
High-dose calcium supplements (>1,000 mg) correlate with accelerated coronary calcification in several cohort studies. The proposed mechanism: bolus dosing transiently raises serum calcium beyond what diet produces. Recent trials (Bolland 2020, Tankeu) reinforce preferring food over pills. When supplementation is necessary, split doses below 500 mg and always co-dose vitamin D3 and K2-MK7.
Source: Current Research
Dr. Strunz
Strunz warns against isolated calcium supplementation without K2 and D3. He supplements 500 to 1,000 mg elemental calcium only when dietary intake is insufficient, always with MK-7 plus magnesium to balance the Ca:Mg ratio. Preferred forms are citrate or hydroxyapatite (bone matrix) over carbonate, citing superior absorption and gentler stomach tolerance. Blood calcium and PTH are monitored rather than relying on total dose, since homeostasis controls actual uptake.
Recommended dose: 500–1000 mg (with K2+D3)
Source: Dr. Strunz
Expert profiles summarise publicly available statements. The named people do not cooperate with Stackwell and have not authorised these summaries. Not medical advice.
Shift toward dietary sources. Supplement carefully, always with K2 and D3.