Calcium

Calcium is the fifth most abundant element in the human body (3); 99% is present in the skeleton and teeth. It is complexed with phosphorus in the molecule hydroxyapatite (Ca10(PO4)6(OH)2), which provides strength for bones to support movement, and a reservoir with which to regulate blood calcium levels (3, 4)through liberation from bone. Otherwise, the only true source of calcium for humans is from the diet; it cannot be made in the body.

Calcium homeostasis is tightly regulated via three major transport systems: intestinal absorption, renal reabsorption and bone turnover (3, 4). Calcium homeostasis relies on an intricate mechanism involving parathyroid hormone (PTH), calcitonin, 1,25-dihydroxyvitamin D (1,25(OH)2D), ionised calcium and the calcium sensing receptor (5). Extracellular calcium concentrations are retained at around 2.5 mmol/L, irrespective of bone calcium status and dietary intake. A diet deficient in calcium does not necessarily cause low blood calcium concentrations but instead results in the liberation of calcium from bone to maintain blood calcium homeostasis (6).

Calcium requirements are determined by the relationship between intestinal absorption, excretion (in urine, faeces, sweat, saliva, skin, hair and nails) and accretion for bone growth and development. In adults, a calcium balance is achieved when the rate of calcium absorption is equal to excretion, thereby preserving the skeleton from calcium depletion. In children and adolescents calcium is required to cover skeletal growth requirements until peak bone mass is achieved (7).

Dairy products contribute over 50% of dietary calcium in some diets, and small fish such as sardines, provide a rich source especially if consumed whole (including the bones). Mineral-rich drinking waters may provide substantial contributions to calcium intakes in some areas (2). However, dietary phytic or oxalic acid content may negatively influence calcium absorption (8). Similarly, some plant foods, particularly green leafy vegetables and legumes also contribute to dietary calcium intakes, but their actual contribution to dietary calcium is impacted by impaired intestinal absorption if dietary oxalates (found in spinach, sweet potato, rhubarb, beans) or phytates (found in unleavened bread, raw beans, seeds, nuts, grains, soy isolates) are concurrently high (9). Calcium absorption from rice, a staple crop for many global populations, is particularly affected (8).

Globally, there is a general lack of data on population calcium intakes. A systematic review of dietary assessment (recall or food frequency questionnaires) data on global calcium intakes published in 2017 (10)found that 75% of the available data were not nationally representative, and 123 countries had no data (fitting review inclusion criteria) on population or population sub-group calcium intakes. However, this review does illustrate the variation in calcium intakes worldwide, in that some regions have higher intakes compared to others; largely due to the consumption of cow’s milk and milk products.

Calcium deficiency can be caused by:

- Low calcium intakes, due to unavailability or avoidance of certain food groups, e.g. cow’s milk and other dairy produce made from cow’s milk; Impaired intestinal calcium absorption due to interactions between the calcium content of foods and dietary phytates or oxalates (see above);
- Low calcium absorption, due to increasing age , physiological disorders such as hypochlorhydria (11), or vitamin D receptor disorder in the gut (12); Increased calcium excretion due to elevated protein or sodium intakes (12).

Severe hypocalcaemia (serum calcium <1.9 mmol/L) is a rare phenomenon that may cause cardiac arrhythmias leading to emergency hospital admission (13). It is not typically linked to dietary intakes (unless in the severely malnourished); instead the most likely causes are hypoparathyroidism, chronic kidney disease and vitamin D deficiency (13, 14), or malabsorption e.g. in coeliac disease and/or in particular, where malabsorption of vitamin D is concomitant (14).

In healthy populations, a lower calcium absorption will usually be met with a lower urinary calcium excretion, thereby maintaining calcium homeostasis. In the case of mild calcium deficiency, study results reporting on the health benefits of calcium supplementation that are often reported in the media, were found by a thorough review by the Institute of Medicine to be mixed and inconclusive (7).

Very low calcium intakes are also associated with non-infantile rickets, either alone or together with low, but not necessarily deficient, vitamin D status (15).

Though calcium is an essential element in the diet, the use of supplements to maintain optimal intakes, and thereby prevent osteoporotic fractures, is controversial. Most studies show little evidence of a relationship between calcium intake, bone density or fracture prevention (16). Instead, calcium supplements in older people likely reduce bone loss through effects on bone remodelling rather than restitution of an underlying calcium deficiency (16).

Long-term calcium supplementation use may be associated with cardiovascular disease (17) and may increase the risk of non-oxalic kidney stones (16, 18).

An excessive dietary intake of calcium is uncommon, although may rarely occur through excessive use of calcium containing supplements. "Milk-alkali" syndrome has been reported in response to high doses of calcium carbonate, with symptoms including renal failure, metabolic acidosis and precipitation of calcium salts in renal tissue (19). However, it has also been suggested that this phenomenon may equally be associated with the carbonate salt as the calcium intake itself (19). Under normal circumstances of calcium supplement use, calcium absorption rises slowly, negating the risk of hypercalcuria.

The principal causes of hypercalcaemia are hyperparathyroidism, malignancies especially with bone metastases, reduced renal excretion due to certain medications e.g. lithium or thiazide diuretics, and increased intestinal absorption of calcium secondary to increased vitamin D activity (20).