What is it?

Description

Calcium is an essential mineral that is commonly used to promote healthy bones and teeth. It is often found in dairy products, though various nuts, seeds, leafy greens, and cruciferous vegetables also contain calcium. Daily requirements for calcium vary by age and whether or not the individual is pregnant or lactating. Most guidelines indicate that individuals older than 19 years of age require between 1,000 to 1,300 mg per day. (37) In infants (0 to 12 months), the National Institute of Health recommends 200 to 260 mg as an adequate intake, whereas the recommended dietary allowance (RDA) rises to 700 mg and 1,000 mg in children 1 to 3 and 4 to 8 years of age, respectively. Older children and adolescents (ages 9 to 18) are recommended to consume 1,300 mg per day. (108

In healthy individuals, approximately 99% of calcium is stored in bone, while circulating blood levels are maintained between 8.8 to 10.4 mg/dL (2.2–2.6 mmol/L). (142) The homeostatic-monitoring of calcium levels acts to regulate brain, cardiovascular, hormonal, muscular, nerve, and skeletal functions, (96)(163) and ensures that the intake of calcium from the diet or from supplements produces little variability in circulating levels. (135) However, calcium supplementation may be indicated in populations at risk for hypocalcemia due to low or non-existent intake from the diet or for individuals taking certain medications. Considering average intakes in Western countries are under the RDA at approximately 700 to 800 mg per day, (147) a proportion of the population may safely benefit from supplementation if increased dietary intake is unrealistic. In general, long-term supplementation of up to 500 mg per day of calcium may be most effectively and safely used with a meal, in combination with vitamin D, and when the dietary intake of calcium does not exceed 800 mg per day. (26) Higher divided doses may be required if intake is still below the recommended levels.

Please note that while calcium is often combined with other ingredients such as vitamin D or magnesium, this review will only detail evidence for the use of calcium supplementation on its own. It should be noted that a significant amount of evidence (e.g., calcium plus vitamin D in the treatment of osteoporosis) is omitted from this ingredient review.

Main uses

Blood pressure regulation
Bone health
Hypocalcemia and hyperphosphatemia regulation
Prenatal health

Formulations

Formulation
Characteristics
Calcium carbonate
Contains 40% elemental calcium
Most commonly used form, and regularly used in research
Least expensive, but requires stomach acid to be properly absorbed; should be taken with a meal to reduce the likelihood of adverse GI events (68)(144)(163)
It is unclear whether calcium carbonate provides better, (85) lesser, (86) or equivalent (20) effects on the suppression of bone resorption markers than calcium citrate when taken with a meal, but it does appear to be less effective when taken on an empty stomach (at bedtime) than calcium citrate-malate. (44)
Calcium citrate
Contains 21% elemental calcium
Commonly used form and regularly used in research
More expensive than calcium carbonate, but does not need to be taken with a meal; should be used in people using H2 blockers or proton pump inhibitors, or in individuals with achlorhydria (144)(163)
Provides better absorption on an empty stomach and with meals than calcium carbonate, gluconolactate-carbonate, and tricalcium phosphate (60)(64)(70) (71) (116) (124) (133) (155)
Some studies show equivalent bioavailability to calcium carbonate when taken with food (67)(68) or in a fasted state. (46)(139)
Calcium lysinate
Contains 30% elemental calcium
Provides greater bioavailability than calcium carbonate or calcium citrate-malate (138)
Calcium chloride
Contains 27% elemental calcium
Used intravenously to treat hypocalcemia (163) as it may provide greater bioavailability than intravenous calcium gluconate (21)
Calcium fumarate
Contains 26% elemental calcium
Provides equal bioavailability to calcium gluconate (110)
Calcium acetate
Contains 25% elemental calcium
Provides similar bioavailability to calcium carbonate, calcium citrate, calcium gluconate, and calcium lactate in a fasted state (139)
May reduce serum phosphorus to a greater extent than calcium carbonate in Px with chronic kidney disease (162)
Calcium hydroxyapatite (microcrystalline hydroxyapatite complex, MCHC; ossein-hydroxyapatite complex, OHC)
Contains 21% elemental calcium; derived from animal bone and is therefore not appropriate for vegetarians or vegans
A complex containing calcium, phosphorus, and bone metabolism proteins including osteocalcin, type I collagen, insulin growth factor type I and II, & transforming growth factor-ꞵ (TGFꞵ).
A meta-analysis shows that calcium hydroxyapatite users had 1.02% higher BMD (mainly via an attenuation of bone loss) than calcium carbonate. (30)
Comparative studies published since the release of the above meta-analysis have continued to support this relationship, with a similar if not slightly better (~5% difference) adverse effect profile; (29)(36) calcium hydroxyapatite may provide a greater analgesic effect in osteopenia than calcium carbonate. (31)
Calcium hydroxyapatite increased serum calcium to a lesser extent than calcium carbonate or citrate, but produced greater phosphate and calcium-phosphate concentrations. However, it also resulted in equivalent reductions in bone resorption markers and bone turnover. (20) Calcium citrate, carbonate, pidolate, and gluconolactate reduced PTH to a greater extent than calcium hydroxyapatite. (46)
Calcium hydroxyapatite provided similar benefits to bone turnover (statistically equivalent) as tricalcium phosphate, but may provide a small clinical advantage for BMD over tricalcium phosphate. (1)
Calcium pidolate
Contains 14% elemental calcium
Provides equivalent bioavailability to calcium carbonate, citrate, lactate-gluconate, and lactate gluconate-carbonate, but less effectively suppressed PTH than citrate or carbonate in a fasted state (46)(56)
Calcium lactate
Contains 13% elemental calcium
Provides similar bioavailability to calcium carbonate, calcium citrate, calcium gluconate, calcium lactate, and calcium acetate in a fasted state (139)
Calcium lactate gluconate
Contains 12% elemental calcium
Provides similar bioavailability to calcium carbonate, citrate, and pidolate, but lower PTH suppression than carbonate and citrate in a fasted state (46)
Calcium citrate-malate
Contains 11% elemental calcium
Provides better bioavailability than calcium carbonate and tricalcium phosphate-lactate (69)(103)
Calcium gluconate
Contains 9% elemental calcium
Provides similar bioavailability to calcium carbonate, calcium citrate, calcium fumarate, calcium lactate, and calcium acetate in a fasted state (110)(139)(164)
Other forms without comparative evidence
Calcium phosphate: contains 38% elemental calcium
Calcium aspartate: contains 23% elemental calcium
Calcium malate: contains 23% elemental calcium
Calcium glycinate: contains 21% elemental calcium
Calcium butyrate: contains 19% elemental calcium
Calcium-D-glucarate: contains 16% elemental calcium
Calcium caprylate: contains 12% elemental calcium
Calcium undecylenate: contains 10% elemental calcium
Calcium ascorbate: contains 9% elemental calcium
Calcium folinate: contains 8% elemental calcium
Calcium pantothenate: contains 8% elemental calcium

Dosing & administration

Adverse effects

Calcium supplementation may induce gastrointestinal-related effects. Supplementation with calcium increased the relative risk of constipation, cramping, bloating, diarrhea, pain, or other symptoms by 43 to 47% compared with placebo. (94)(117)

It should be noted that controversy related to whether or not calcium supplementation increases the risk of cardiovascular events, fractures, or kidney stones exists. (12)(34)(150) Some analyses indicate that supplementation increases the risk of cardiovascular events, (15)(16)(95)(165)(167) while others do not. (7)(35)(93)(161) Similarly, an increased risk for kidney stones has been observed in some analyses, (81) but not in others. (28)(83

Overall, supplementation with calcium may need to be more closely monitored to avoid unnecessary use in excess of recommended dietary intakes to reduce the risk of negative outcomes or adverse events. (150) In general, total daily intake of calcium (from diet and supplements) should not exceed the tolerable upper intakes levels of 2,000 to 2,500 mg per day to reduce the risk of adverse cardiovascular outcomes, (35)(88) as well as to avoid reducing the uptake of other nutrients such as iron, zinc, or phosphorus. (99)(144

Calcium is also one of the most documented nutrients with interactions with other drugs. (156) Detailing these interactions is outside the scope of this review, but healthcare practitioners should evaluate the risks and benefits of adding calcium supplements to a treatment plan containing other pharmaceuticals. 

Pharmacokinetics

Absorption

  • The body can absorb up to 500 mg of calcium at a time, with very little further absorption with higher doses. (66)
  • The average absorption of calcium intake is ~25%; (79) however, absorption can be highly variable (e.g., varying between 15 to 58% in healthy women). (18)
  • Several factors can influence calcium absorption; estrogen deficiencies and fibre intake (reducing intestinal transit time) may reduce absorption, while increased absorption may occur with an adequate vitamin D status or when supplementation is taken with food (increasing transit time). (18)
  • While adding vitamin D to improve calcium absorption is common practice, there was no better or very little improvement in calcium absorption with the addition of vitamin D in some trials. (58)(74)(105)
  • The majority (95%) of calcium is absorbed in the duodenum and proximal jejunum via saturable active transport (limiting large single amounts from being absorbed). (18)
  • Approximately 5% is absorbed via passive diffusion, and absorption occurs throughout the entire intestine. (18)
  • Intake of calcium from dairy products, mineral waters, or supplements produce equivalent bioavailability. (14)(61)(74)(105)

Distribution

  • Approximately 99% is stored in skeletal tissue and fractional amounts circulate in the blood or are found in soft tissues. (96)(142)
  • The average adult stores between one to two kilograms of calcium. (18)

Metabolism

  • In a state where serum-free calcium levels are low, reduced calcium-sensing receptor activity in the parathyroid gland stimulates secretion of parathyroid hormone (PTH).
  • PTH stimulates PTH receptors in the kidney to reabsorb calcium.
  • PTH stimulates 25-hydroxyvitamin D 1α hydroxylase to convert vitamin D to calcitriol, which increases intestinal calcium levels.
  • PTH also stimulates resorption of calcium from bone.
  • These actions ultimately increase calcium levels in the blood and provide negative feedback to reduce further parathyroid gland activity.
  • In a state where serum-free calcium levels are high, increased calcium-sensing receptor activity inhibits the release of PTH (reducing the above effects) and stimulates the thyroid gland to secrete calcitonin.
  • Calcitonin reduces intestinal absorption and renal reabsorption of calcium and increases calcium bone deposition, thereby reducing circulating calcium levels. (96)(142)

Excretion

  • On average, 22% of calcium is lost in urine and 75% in feces, though some can be lost via the skin, hair, and sweat. (79)
  • Between 100 to 250 mg of calcium can be lost in the urine, and 100 to 200 mg is excreted in the feces every day. (18)
  • Estrogen deficiencies and caffeine can also increase calcium excretion. (18)
References
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