What are the primary sources of calcium found in supplements?

Primary Sources of Calcium in Supplements

The most commonly used calcium supplements are calcium carbonate and calcium citrate, with calcium carbonate being the preferred first-line option due to its high elemental calcium content (40%), low cost, and wide availability. 1

Main Calcium Supplement Formulations

Calcium Carbonate

• Provides the highest elemental calcium content at 40%, making it the most cost-effective option 1, 2
• Widely available over-the-counter and inexpensive 1
• Must be taken with meals because gastric acidity is required for optimal absorption 1, 2
• More likely to cause constipation and bloating compared to other formulations 1

Calcium Citrate

• Contains 21% elemental calcium, which is lower than calcium carbonate 1
• Can be taken with or without food since absorption is not dependent on gastric acidity 1, 2
• Preferred for patients with achlorhydria, GERD, or those taking acid-reducing medications (H2-blockers or proton pump inhibitors) 2, 3
• Better tolerated in patients with bothersome gastrointestinal symptoms 1
• Particularly suitable for elderly patients who may 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 not practical for oral supplementation because of their low calcium concentration 3

Critical Dosing Considerations

Doses should not exceed 500 mg of elemental calcium at one time to optimize absorption 2, 3

• If daily supplementation requires more than 500 mg of elemental calcium, divided doses throughout the day are recommended to improve absorption and minimize gastrointestinal side effects 1, 2
• Supplement labels list both total calcium salt content and elemental calcium content—dosing calculations must be based on elemental calcium 1

Practical Selection Algorithm

Choose calcium carbonate if:

• Cost is a primary concern 1, 2
• Patient has normal gastric acid production 1
• Patient can reliably take supplements with meals 1
• No significant gastrointestinal complaints exist 1

Choose calcium citrate if:

• Patient has achlorhydria or hypochlorhydria 2, 3
• Patient takes acid-reducing medications (H2-blockers, proton pump inhibitors) 2, 3
• Patient experiences constipation or bloating with calcium carbonate 1
• Patient cannot reliably take supplements with meals 1, 2
• Patient is elderly with potentially reduced gastric acid 2

Important Caveats

• Calcium supplements should complement, not replace, dietary calcium intake 2
• Dietary sources like broccoli and kale may have better absorption than supplements, though clinical outcome data are lacking 1
• Upper intake limits of 2000-2500 mg daily should not be exceeded due to increased risk of nephrolithiasis and potentially cardiovascular events 2
• Look for U.S. Pharmacopeia (USP)-verified supplements to ensure quality and manufacturing standards 3