Supplement analysis backed by scientific evidence · PT

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Moderate evidenceVitamins & minerals

Calcium Citrate

Absorbs essentially the same as cheap calcium carbonate when taken with food — the well-run comparison found identical 24-hour serum calcium curves and concluded the cost analysis favours carbonate. Citrate earns its price only in specific situations: low stomach acid, acid-suppressing drugs, post-bariatric surgery, or dosing away from meals.

What the science says

  • No absorption advantage over carbonate with food Moderate evidence

    A randomised three-way crossover in 24 vitamin D-replete postmenopausal women compared marketed calcium carbonate, encapsulated carbonate and marketed calcium citrate: all three produced identical 24-hour serum calcium time courses and equivalent PTH suppression, with a non-significant edge for citrate on urinary calcium. The authors' explicit conclusion was that the cost-benefit favours the cheaper carbonate.

  • The genuine advantage is in low stomach acid or fasted dosing Moderate evidence

    In 11 fasting patients with achlorhydria, fractional calcium absorption was 0.45 from citrate versus 0.04 from carbonate — a tenfold gap. Crucially, giving the carbonate with a normal breakfast restored completely normal absorption, so the fix for most people is timing, not a different salt. Citrate remains the sensible choice on long-term proton pump inhibitors, after gastric bypass, and for anyone who cannot reliably take supplements with meals.

  • Calcium supplementation itself delivers less than the marketing implies Strong evidence

    A systematic review and meta-analysis found increasing calcium intake raises bone mineral density by only about 1-2%, an increase that does not accumulate over time and is unlikely to translate into meaningful fracture reduction. A companion systematic review found the evidence that calcium supplements prevent fractures is weak and inconsistent. Choosing a better calcium salt does not rescue a modest underlying effect.

Dosage & safety

Studied dose500-600 mg elemental calcium per dose (absorption efficiency falls above this), up to a total dietary plus supplemental intake of about 1,000-1,200 mg/day for adults. Calcium citrate is only about 21% elemental calcium versus 40% for carbonate, so pills are larger or more numerous for the same dose. Unlike carbonate, citrate does not need to be taken with food.
SafetyConstipation and bloating occur but less than with carbonate. Supplemental calcium raises kidney stone risk in some analyses, though citrate itself is used therapeutically to reduce stone formation and is generally the preferred form in stone formers. Do not exceed the tolerable upper intake of 2,000-2,500 mg/day of total calcium; hypercalcaemia and milk-alkali syndrome occur at excessive intakes, especially combined with high vitamin D. Calcium blocks absorption of levothyroxine, iron, tetracyclines, quinolones and bisphosphonates — separate by 2-4 hours. Contraindicated in hypercalcaemia and primary hyperparathyroidism without medical supervision.

Scientific references

Where to buy

21st Century, Calcium Citrate + D3 Maximum, 120 Tablets

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