Calcium metabolism

Calcium is the 5th most abundant element on earth and sea, and the most abundant element in the human body. The body stores 99% of calcium in bones and teeth.  Calcium can also be found in blood, muscles and cells.  

Calcium serves as a structural element in bones.  It is essential for nerve conduction and muscle contraction.  In the heart, it is necessary for conduction of electrical impulses and hence, cardiac rhythm and rate.  Calcium is also necessary for the clotting of blood. 

The average human body contains about 1 kg of calcium.  Calcium levels are tightly controlled in various body compartments.  The major regulators of calcium are parathyroid hormone, vitamin D, and calcitonin.  

The parathyroid hormone which is released from the parathyroid glands, is responsible for increased absorption of calcium.  The parathyroid hormone responds to low levels of calcium in the blood.  It increases the gastrointestinal absorption of calcium.  It releases calcium from the bones.  And it decreases the excretion of calcium by the kidneys.  

Vitamin D is necessary for the absorption of calcium.  It is obtained from diet and can be produced by sunlight exposure. 

Calcium is deposited on bones and when needed, it is released from the bones.  Short term deficiency of calcium does not affect serum calcium levels.  Long term deficiency can cause bone demineralization.  

Requirements

Normal daily requirement of calcium depends on age.            

            Infants 0-6 months- 400 mg/day

            6-12 months- 600mg/day

            1-10 yrs- 800 mg/day

            11-24- 1200 mg/day

            Adults and elderly- 1500 mg/day 

Sources of calcium

-         milk, milk products, cheese, and yogurt

-         sardines

-         salmon

-         tofu, turnip, kale, Chinese cabbage, spinach 

Supplements 

Calcium carbonate and calcium citrate are the two main forms of calcium supplementation.  Both forms are well absorbed.  Calcium carbonate is 40% elemental calcium, and calcium citrate is 21%.  

Amount of calcium absorbed from the stomach depends on the acidity of the stomach.  Calcium carbonate is absorbed more with food.  Calcium citrate is absorbed equally with or without food.  If the amount of calcium supplemented is increase, the percentage of absorption is decreased, so more than 500mg of calcium supplementation is not advised at one time, and therefore should be taken more than once a day. Vitamin D improves the absorption of calcium.  Caffeine and alcohol decrease the absorption.