Calcium is the most abundant mineral in the body, 99% of calcium in the body is in the teeth and bones. The other 1% is found in the blood and extracellular fluid and it is vital to maintain these calcium levels t a particular concentration range for normal physiological functions.

In order for calcium to be absorbed from the gut and utilised correctly, adequate Vitamins D and K, Magnesium and Phosphorus are necessary (See Metabolic Vitamin D, K and Magnesium).

Calcium cannot be manufactured by the body and must be obtained from food. Food sources of Calcium are milk and cheese products, some nuts and dark green leafy vegetables.

Calcium is used for;

Maintenance of normal teeth and bones and the normal growth and development of bone in children. The mineral part of bone is known as hydroxyapatite that contains large amounts of calcium and phosphate. Bone cells consist of osteoblasts (bone building cells) and osteoclasts (bone dissolving cells). During normal growth, especially growth in children, bone formation is greater than bone resorption and adequate calcium is imperative. Bones are not as strong as teeth, which consist mostly of a calcium-based tissue called dentine. They are however porous allowing minerals both in and out of them. Demineralisation can occur when bacteria in plaque produce acids when sugars and carbohydrates are eaten. This acid dissolves the hydroxyapatite (tooth enamel) causing demineralisation. When the sugar has gone mineral ions, including calcium ions dissolved in the saliva can rush in through the porous teeth and remineralise them.

Normal blood clotting.There are 7 vitamin K dependent clotting factors and calcium ions are required for activation of all these. If there is inadequate calcium then clots may not form with bleeding, leading to haemorrhage.

Normal muscle contraction. The cell membranes of muscle cells contain calcium channels that are voltage dependent. When muscles receive nerve impulses instructing them to contract, calcium channels in the cell membranes open allowing calcium ions into the muscle cells. These calcium ions bind to activator proteins in the cell that release more calcium ions from vesicles in the cell. The calcium binding to the activator protein sets off a series of steps that result in muscle contraction.

Normal energy metabolism. The endoplasmic reticulum in the cell is the major reservoir of calcium. Stored calcium is released to adjacent mitochondria through an ion channel receptor. This calcium release continues at a low level all the time. Mitochondria depend on the continuous calcium transfer to make enough ATP (energy) through oxidative phosphorylation, to support normal cell metabolism. In the absence of this transfer, the mitochondria fail to make enough ATP, which triggers an cell destruction, where the cell “eats itself”

Ref: Essential Regulation of Cell Bioenergetics by Constitutive InsP3 Receptor Ca2 Transfer to Mitochondria. Cell 2010 July Volume 142, Issue 2, 270-283, 23.

University of Pennsylvania School of Medicine.

Normal nerve transmission. An entire nerve impulse passes through a neuron in 7 milliseconds. A resting nerve is polarized which means that the outside of the nerve membrane contains excess sodium ions, and the inside contains excess potassium ions. Although the inside of the cell contains positively charged potassium ions, there is an overall negative charge inside the cell because the there are negatively charged proteins and nucleic acids inside. Cell membranes allow sodium and potassium ions to cross backwards and forwards, however there are sodium and potassium pumps that pump the sodium back outside and the potassium back inside. When a nerve impulse reaches a resting neurone, gated channels on the membrane open allowing sodium ions to rush into the cell. Calcium ions are like a door to these gated channels, being in charge of them. Calcium must move to release the ions and return before the impulse will stop. So it is essential to have adequate calcium ions to allow this nerve transmission to occur.

Cell division and specialisation.There is emerging evidence that calcium ion gradients are involved in different stages of cell development

Calcium gradients are associated with every aspect of cell physiology including cellular proliferation

(Calcium ion signalling, Genes and the Cell cycle by Khaled Machaca. Dept. of physiology and biophysics, Weill Cornell Medical College in Qatar).

Normal blood calcium levels.If there is inadequate dietary intake of calcium, proteins in the parathyroid glands, sense the lack of calcium and instruct the parathyroid glands to secrete parathyroid hormone. This then stimulates the conversion of vitamin D to its active form, calcitrol in the kidneys and calcitrol increases the absorption of calcium from the small intestine. Parathyroid hormone and calcitrol will actually release calcium from the bone from the skeleton and decrease urinary calcium loss in order to control and keep a constant blood calcium level. Basically calcium is moved out of the bones into the blood to keep the exact amount of calcium in cells and the blood.

Normal function of digestive enzymes. Cells of the pancreas produce powerful digestive enzymes. Calcium ion signals are sent by acetylcholine or cholecystokinin (CCK) to cells in the pancreas that releases the enzyme trypsin. Trypsin is a protease enzyme that breaks down protein role of calcium in cell division by P K Hepler, Dept of Biology, University of Massachusetts USA).

(Calcium ion signalling, genes and the cell cycle by Khaled Machaca. Dept. of physiology and biophysics, Weill Cornell Medical College in Qatar).

Bioavailability of calcium supplements

As with any mineral compound, the amount of calcium available to the body depends on a number of factors. In young people the bioavailability of calcium is believed to be 8-12%. However after the age of 35-40 the bioavailability of different forms of calcium decreases to 3-5%. As with any mineral compound, the amount of calcium available to the body depends on a number of factors.

  • How much calcium is in the compound
  • How soluble that compound is. The body can only use elemental calcium so a product must dissolve before it can be absorbed. Most water soluble forms of calcium dissolve easily in the gut
  • Absorption. This means the ability of the elemental calcium to be taken into the body in a useful form. Most absorption occurs in the small intestine into the blood but to be useful to the body it needs to get into the cells in an ionised form
  • Calcium absorption is improved by adequate levels of vitamin D and K, potassium, magnesium, zinc, manganese, copper and boron. All these assist in the assimilation of calcium.
  • Adequate levels of stomach acid. The acid environment makes calcium salts more soluble
  • High levels of fat, phytates, and oxalates inhibit calcium absorption.
  • High protein diets, smoking, refined sugar intake, caffeine and alcohol intake all decrease calcium absorption

    How should Calcium supplements be taken?

    Generally calcium supplements are best taken with food, so there is enough stomach acid available to break down the calcium compound. However Calcium citrate can be taken with or without food and ionic calcium taken at any time. Calcium compounds should be taken in doses of no more than 500mg at a time for best absorption and to prevent stomach irritation.

    Alison Claire