What is the main function of calcium

Calcium: functions, bioavailability and supply

The human body contains around 1.0 to 1.5 kg of calcium. The hard tissue, i.e. bones and teeth, already make up about 99% of the total content. The chemical compound of calcium, which builds up the tooth and bone substance as a structural substance, is called hydroxylapatite - it has the formula Ca5 (PO4) 3OH. Apatite (in the natural occurrence the OH group is replaced by a fluorine or chlorine atom) is an extremely stable substance, which gives bones and teeth their high strength due to this property.

Bones contain 55% inorganic salts, apart from calcium phosphate also smaller amounts of calcium carbonate, magnesium and fluorine salts. 20% is water, 25% organic material. Glycoproteins and collagen are contained in dentin, the dentin. Both dentin and enamel consist of 35 to 36% calcium and 17% phosphate. The tartar consists of precipitates of calcium salts in saliva, which in turn can come from microorganisms and food residues.

The remaining 1% of the calcium in the body is outside the cells and on the surface of the bones. In the body fluids calcium is either dissolved as calcium ions or bound to organic substances (proteins and organic acids): 47% bound to proteins, 6% bound to low-molecular complexing agents such as citric acid and 47% as "free" calcium ions. The plasma calcium concentration is normally 2.5 mmol / l. Calcium is also partially bound to organic substances in plant-based foods - e.g. B. pectins, which belong to the group of higher molecular weight carbohydrates and nutritionally to the dietary fiber. The calcium salts of pectic acids are the main components of the middle lamellae of plant cells.

Absorption, metabolism and household

Calcium is not only a building material for the human skeleton, it also forms a reserve for calcium metabolism as a whole. Calcium plays an essential role in all forms of transmission of excitation and also in blood coagulation. Calcium is absorbed in the intestine (mainly in the duodenum under neutral to weakly alkaline conditions), with vitamin D contributing. Protein substances that can bind calcium during transport, citric acid as a complexing agent and lactose, which influences the intestinal flora, also promote the absorption of calcium.

Substances that form poorly soluble compounds with calcium, such as oxalic acid, phosphoric acid and also phytic acid, inhibit absorption. However, the phytin (calcium-magnesium salt) in grain is largely broken down during the dough preparation and baking process, so that this organic inositol hexaphosphoric acid does not play a negative role in our diet. Two processes play a role as the mechanism of absorption: the active transfer, in which a "calcium binding protein" is involved, and the diffusion of calcium ions or small calcium compounds through the intestinal mucosa.

The calcium metabolism and thus the concentration in the blood is regulated by hormones that work in the opposite direction. They ensure a constant blood calcium level (processes in the context of so-called homeostasis = balance of physiological body functions). In the parathyroid gland, the parathyroid hormone ensures that calcium is supplied to the blood by mobilizing calcium from the bones. In addition, the parathyroid hormone improves absorption from the intestine and also transports back calcium that is already in the kidneys.

This is also where vitamin D attacks, which supports the effect of the parathyroid hormone. The counterhormone, the antagonistic calcitonin, unfolds its effect in the thyroid gland. It lowers the blood calcium level by storing calcium in the bone substance.

If there are disruptions in the calcium balance, growth disorders (rickets) can occur in infants with a calcium deficiency; in adults, insufficient calcification of the bones can also be observed. If the deficiency persists, cramps also occur, i. H. Calcium affects the excitability of nerves and muscles. Excessive blood calcium values ​​can lead to pathological deposits (e.g. in the kidneys as kidney stones, but also in the brain).

With regard to possible calcium deficiency symptoms, osteoporosis (shrinkage of solid bone tissue combined with an increase in the marrow spaces) is of the greatest importance: In women after menopause, fractures occur more frequently, and femoral neck fractures in older men as well.

Calcium supply

Milk and dairy products, bread and baked goods, fruit and vegetables, but also fish, which are eaten with bones like Kiel sprats, and mineral waters (with high calcium levels of over 500 mg / l) best meet our calcium requirements. The relatively high calcium contents in rhubarb and spinach (see table) are bioavailable only to a small extent due to the high oxalic acid content (formation of poorly soluble calcium oxalate). The bioavailability of calcium is similar in fiber-enriched crispbread, where groups such as pectins, lignins and other fiber with functional groups can bind calcium ions.

Calcium tablets (for food supplements - usually with the note "supports bones and teeth") contain calcium salts (calcium carbonate) together with organic acids (often citric acid) and vitamin C (ascorbic acid) as well as sodium hydrogen carbonate and / or sodium carbonate (in effervescent tablets). If it contains phosphates, the bioavailability (see above) may be reduced. Additives with very different functions are often used as calcium salts, which in some cases also increases the calcium content in the food.

The following are permitted: calcium salts of sorbic and benzoic acid, sulphurous acid, propionic acid (as preservative), acetic acid, malic acid and tartaric acid (as acidulant), ascorbic acid (antioxidant), lactic acid (acid regulator, melting salt), citric acid (acidulant) , Stabilizer, antioxidant), phosphoric acid (melting salt, acidifier), ethylenediaminetetraacetic acid (EDTA; antioxidant and complexing agent), alginic acid (thickening, coating and gelling agent), hydrochloric acid (flavor enhancer, firming agent), sulfuric acid (acidity regulator, carrier , Release agent), silicic acid (calcium aluminum silicate as release agent), gluconic acid (acid regulator, stabilizer), glutamic, guanyl and inosinic acid as well as 5'-ribonucleosinic acid (flavor enhancer), cyclohexane sulfamic acid and saccharine (sweeteners).

In the diet regulation, calcium carbonate, calcium chloride, calcium salts of citric acid, calcium gluconate, calcium glycerophosphate, calcium salts of orthophosphoric acid, calcium hydroxide and calcium oxide are listed as additives of category 2 (minerals).

 

Author's address:
Prof. Dr. Georg Schwedt,
Clausthal University of Technology,
Institute for Inorganic and Analytical Chemistry,
Paul-Ernst-Str. 4,
38678 Clausthal-Zellerfeld,
Email: [email protected]

 

literature
G. Schwedt and I. Könecke: Contribution of element species analysis to the question of the bioavailability of calcium from different types of bread, Z Lebensm Unter Forsch 197: 389-394 (1993). G. Schwedt and F. Schütte: Element Species Analysis. Characterization of mineral preparations and anti-anemics, Deutsche Apotheker Zeitung 133 (No. 9): 697-701 (1993).