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Ester-C

Ester-C is calcium ascorbate with trace vitamin C metabolites, sold on claims of superior absorption and 24-hour retention. Plasma data do not support either claim; its one demonstrated difference is a modest leukocyte-uptake edge in a single trial.

What the science says

  • Better absorbed than plain ascorbic acid Limited evidence

    A randomised, double-blind, placebo-controlled trial gave healthy adults 1000 mg/day for 8 weeks: plasma vitamin C did not differ significantly between Ester-C and plain ascorbic acid. Leukocyte vitamin C was higher with Ester-C at the 24-hour timepoint, which is the origin of the branding — a real finding, but a surrogate marker of uncertain clinical meaning from one study. A 2025 systematic review of alternative vitamin C forms in healthy adults found no consistent evidence of meaningful superiority for any of them, Ester-C included.

  • Stays in your system for 24 hours Limited evidence

    Vitamin C kinetics are set by SVCT1/SVCT2 transporters, tissue saturation and the renal threshold — not by the counterion attached to the ascorbate. Once absorbed, ascorbate from calcium ascorbate is indistinguishable from ascorbate from ascorbic acid. The claim rests on the single leukocyte timepoint above, not on any demonstrated change in half-life.

  • Gentler on the stomach Limited evidence

    Plausible and probably true in practice: calcium ascorbate is pH-neutral rather than acidic, so it is less likely to irritate at gram doses. But this has barely been tested in controlled trials, and it applies equally to cheap unbranded calcium or sodium ascorbate, which cost a fraction as much.

  • The metabolites do something Limited evidence

    Ester-C contains small amounts of vitamin C metabolites, chiefly calcium threonate. In vitro work suggests threonate may enhance ascorbate uptake into cells. There is no human trial showing this produces a health benefit, and the metabolite content is a minor fraction of the product.

Dosage & safety

Studied doseTypical products supply 500–1000 mg per serving; the published trial used 1000 mg/day for 8 weeks. The RDA is 75 mg (women) and 90 mg (men), and roughly 200 mg/day saturates plasma in most people. Note that 1000 mg of vitamin C as calcium ascorbate carries about 110 mg of elemental calcium, which counts toward your daily calcium total.
SafetyWell tolerated. Track the calcium if you also take a calcium supplement, have hypercalcaemia risk, sarcoidosis, or a history of calcium-based stones. Standard high-dose vitamin C cautions apply above ~2 g/day: osmotic diarrhoea, elevated urinary oxalate (relevant for stone formers and CKD), enhanced iron absorption (haemochromatosis, thalassaemia), and haemolysis risk in G6PD deficiency at very high doses. Vitamin C at high doses interferes with glucose meter readings and faecal occult blood testing. No drug interactions of clinical significance at ordinary doses. Not banned in sport.

Scientific references

Where to buy

Solgar, Ester-C® Plus, Vitamin C, 50 Capsules

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