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Calcium regulation in the human body. [6]The plasma ionized calcium concentration is regulated within narrow limits (1.3–1.5 mmol/L). This is achieved by both the parafollicular cells of the thyroid gland, and the parathyroid glands constantly sensing (i.e. measuring) the concentration of calcium ions in the blood flowing through them.
The US Institute of Medicine (IOM) established Recommended Dietary Allowances (RDAs) for calcium in 1997 and updated those values in 2011. [6] See table. The European Food Safety Authority (EFSA) uses the term Population Reference Intake (PRIs) instead of RDAs and sets slightly different numbers: ages 4–10 800 mg, ages 11–17 1150 mg, ages 18–24 1000 mg, and >25 years 950 mg. [10]
Calcium is a common constituent of multivitamin dietary supplements, [8] but the composition of calcium complexes in supplements may affect its bioavailability which varies by solubility of the salt involved: calcium citrate, malate, and lactate are highly bioavailable, while the oxalate is less.
Calcium citrate malate is a water-soluble calcium supplement. It is the calcium salt of citric acid and malic acid with variable composition. Calcium citrate malate's bioavailability stems from its water-solubility and its method of dissolution. When dissolved, it releases calcium ions and a calcium citrate complex.
Calcium citrate is recommended for patients with achlorhydria and those on medications that decrease stomach acidity. While calcium carbonate is the most common and least expensive form of calcium and contains 40% of elemental calcium, calcium citrate supplements contain only 21% calcium, requiring more tablets for equivalent dosage.
Therefore, bioavailability for dietary supplements can be defined as the proportion of the administered substance capable of being absorbed and available for use or storage. [11] In both pharmacology and nutrition sciences, bioavailability is measured by calculating the area under curve (AUC) of the drug concentration time profile.
In many individuals, bioavailability of calcium citrate is found to be equal to that of the cheaper calcium carbonate (CaCO 3). [3] However, alterations to the digestive tract may change how calcium is digested and absorbed. Unlike calcium carbonate, which is basic and neutralizes stomach acid, calcium citrate has no effect on stomach acid.
Calcium formate is shown to be more readily absorbed form of calcium than calcium carbonate and calcium citrate. [7] No optic nerve damage has been observed with calcium formate supplementation – along with formaldehyde , formate is a major metabolic product of methanol , which can cause blindness upon ingestion.
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