Sources of Calcium in Supplements
The most commonly used calcium supplement sources are calcium carbonate (providing 40% elemental calcium), calcium citrate (providing 21% elemental calcium), and to a lesser extent calcium lactate and calcium gluconate. 1
Primary Calcium Supplement Formulations
Calcium Carbonate
- Provides the highest elemental calcium content at 40%, making it the most cost-effective and widely available option 1, 2
- Must be taken with meals because gastric acidity is required for sufficient absorption 1, 2
- More likely to cause constipation and bloating compared to other formulations 1
- Not recommended for patients with achlorhydria (absent stomach acid) 3, 4
Calcium Citrate
- Contains 21% elemental calcium, which is lower than calcium carbonate but still clinically useful 1, 2
- Can be taken with or without meals since absorption is not dependent on gastric acidity 1, 2
- Preferred for patients with gastrointestinal symptoms, GERD, or those taking acid-reducing medications (histamine-2 blockers or proton-pump inhibitors) 2, 3
- Better tolerated in elderly patients who often have reduced gastric acid production 2
Less Common Formulations
- Calcium lactate and calcium gluconate are used to a lesser extent due to their lower elemental calcium content 1, 2
- These preparations are less concentrated and not practical for routine oral supplementation 3
- Hydroxyapatite has limited research supporting its use and is not recommended 3
Critical Dosing Considerations
If daily supplementation exceeds 500 mg of elemental calcium, divided doses are recommended to improve absorption and minimize gastrointestinal side effects 1, 2, 3
- The maximum dose of elemental calcium that should be taken at one time is 500 mg to optimize absorption 2, 3
- Supplement labels include both total milligrams of calcium salt and milligrams of elemental calcium—dosing should be based on elemental calcium content 1
Common Pitfalls to Avoid
- Do not confuse total calcium salt content with elemental calcium content—the dose required to meet daily requirements is based on elemental calcium only 1
- Avoid prescribing calcium carbonate to patients with achlorhydria or those on chronic acid-suppressing medications, as absorption will be significantly impaired 2, 3, 4
- Do not exceed the upper intake level of 2000-2500 mg daily due to increased risk of nephrolithiasis and potentially cardiovascular events 2
- Calcium supplements should ideally complement dietary calcium intake (primarily from dairy products) rather than replace it completely 2