Calcium is the most abundant cation in the human body, and a very important mineral for the maintenance of neuromuscular function. It constitutes 2% of the total body weight and 26% of the dry weight of bones and teeth, as phosphates and carbonates. About 0.5 g serum calcium is exchanged with bone calcium every day.
ABSORPTION
The absorption of calcium depends on the factors listed below.
1. Body needs. In a normal person only 30% of calcium is absorbed, but in a deficient person the proportion absorbed is higher.
2. Concentration. Other factors being equal, the higher the concentration of calcium in the food, the greater the absorption.
3. Intestinal motility. Intestinal hurry diminishes the absorption of calcium. With chronic diarrhoea and prolonged use of purgatives, calcium absorption is diminished considerably.
4. Fat. When fat absorption is deficient, as in steatorrhoea, calcium .combines with fatty acids in the bowel lumen to form
insoluble calcium soaps. Decreased fat absorption also diminishes absorption of vitamin D.
5. Ingested food. Absorption of calcium from food is similar whether it is taken as whole milk, chocolate milk, yoghurt or
cheese. The absorption is also similar whether calcium is taken in milk or as calcium acetate, lactate, gluconate, citrate, or carbonate.1 The amino acid L-Iysine helps in positive calcium balance, not only because it enhances calcium absorption, but also because it improves conservation of absorbed calcium. Lactose (milk sugar) and amino acids, such as arginine and tryptophan, enhance calcium absorption. When co-administered with glucose polymer, the absorption of calcium chloride increased 1.5 to 5-fold even in vitamin D-deficient patients.
Cereals contain phytic acid (inositol hexaphosphate), which combines with calcium to form insoluble calcium phytate which is not absorbed. Similarly, foods containing oxalate, such as spinach, combine with calcium to form insoluble calcium oxalate. An excess of magnesium apparently diminishes absorption of calcium. Intestinal lactase deficiency leads to decreased milk (and consequently calcium) intake, and possible also decreases calcium absorption.
6. Vitamin D. The most important factor in promoting calcium absorption is vitamin D, which stimulates the formation of a calcium-binding protein that enhances calcium absorption. When vitamin D is not consumed, as with a strict vegetarian (vegan)
diet (which excludes even milk as it is considered to be of animal origin), or if the skin is not exposed adequately to sunlight, severe vitamin D deficiency results in poor absorption of calcium.
Vitamin D3 (calcitriol: 1,25 dihydroxy vitamin 03) 400-800 IV may be of value in ensuring calcium absorption. With high doses of vitamin 0, the absorption of calcium from foods can be almost complete, and it is possible to maintain a positive calcium balance even on a low intake.
Glucocorticoids and alcohol decrease intestinal calcium absorption.
7. Calcium-phosphorus ratio. The ratio of calcium to phosphorus in food has an important influence on absorption. Most foods
containing calcium also have a proper proportion of phosphorus. Milk, eggs, and some leafy vegetables like lettuce provide these elements in the required proportions.
BLOOD LEVELS
Calcium is present in plasma but not in red blood cells. The serum level is about] 0 mg per 100 ml (2.4 mmol/l), about half
of which is ionized and the remainder protein-bound. Ionized calcium is diffusible. Its normal serum level is maintained by the parathyroid glands. Alkalosis reduces the level of ionizable serum calcium. Diminution of ionic calcium leads to increased muscle irritability and tetany.
The rest of the serum calcium is bound to proteins, mainly albumin, so that when the plasma albumin falls the level of serum
calcium is low; but since this decrease is only in the non-diffusible form, tetany does not develop.
REGULATING FACTORS
Parathyroid hormone (PTH) mobilizes calcium and phosphorus from the bones to maintain the blood level.
Hyperparathyroidism causes reabsorption of minerals together. with bone matrix, giving an X-ray appearance of cysts in the
bones (osteitis fibrosa cystica), most apparent in the skull. It may also manifest as kidney stones without detectable bone changes. PTH also stimulates the mitochondria of the kidney to increase conversion of calcidiol to calcitriol, which enhances
intestinal calcium absorption.
Calcitonin is a peptide hormone produced by the C cells of the thyroid gland. Having a very short half-life, it is secreted
continuously. The secretion increases with rising serum calcium. It helps to maintain blood level by preventing calcium release from the bones and increasing urinary excretion.
Therapeutically, calcitonin is used when serum calcium is high, as in Paget’s disease (where it decreases bone reabsorption),
and in hyperparathyroidism and breast carcinoma (where it increases urinary calcium excretion). Estrogen raises the level of calcitonin and prevents bone loss. Postmenopausal bone loss is ascribed to decreased estrogen; therefore, calcitonin rather
than estrogen therapy is suggested for the prevention of postmenopausal bone loss, as well as for the treatment of
osteoporosis.
STORAGE
Most of the body calcium (99%) is stored in the bones and teeth. The skeleton is the dynamic reservoir where calcium is continuously deposited and reabsorbed. This dynamic activity is high in the young and declines with age. About one per cent
of total body calcium is in the circulation and soft tissue.
One episode of mild to moderate malnutrition, occurring during the first year of life, is associated many years later with
increased caries in both the deciduous and permanent teeth.
At puberty, under the influence of sex hormones, girls accumulate about 200 mg, and boys 400 mg, calcium a day. Positive calcium balance occurs only if the calcium intake is over 1000 mg a day.
EXCRETION
About half the daily calcium intake is excreted in the feces, representing unabsorbed calcium and calcium from endogenous
sources. If a high dose of vitamin D is taken, the calcium content of the stools decreases.
About half the total daily calcium intake is excreted through the urine. Urinary excretion of calcium is the net result of
glomerular filtration and tubular reabsorption. The urinary excretion is under the control of the parathyroid glands, and
represents mostly endogenous calcium. The normal range of urinary calcium in adults on a normal diet is 100-300 mg (2.5-7.5 mmol) per day for men and 100-250 mg (2.5-6.5 mmol) per day for women. The kidneys conserve calcium during deficiency.
With normal kidney function, about eight per cent of filtered calcium is excreted. With the administration of the diuretics
frusemide or ethacranic acid, resorption of calcium by the kidneys is inhibited and the urinary loss is increased to 12%.
Thiazides, on the other hand, conserve calcium by increasing kidney tubular reabsorption, and the urinary loss is only three
per cent. Therefore, in older patients with high blood pressure-particularly in women-bone loss and osteoporosis occur with frusemide. Thiazide is recommended for prevention as well as treatment of osteoporosis. By decreasing calcium excretion,
thiazide is also useful in preventing recurrence of kidney stones.
High-protein diet decreases reabsorption of calcium by the kidneys and increases urinary calcium excretion.
FUNCTIONS
Calcium and phosphorus are necessary for bone formation.
Bones consist of a protein matrix (framework) in which calcium phosphate is deposited. Proteins, vitamins A and C, and sex
hormones are necessary for the formation of the matrix. The calcium in the bones is always being turned over with calcium in
the blood and other tissues, at a rate of 0.5 g (12.5 mmol) per day.
Ionic calcium affects neuromuscular excitability of both the voluntary and involuntary muscles. When the ionic serum calcium
concentration. is diminished, neuromuscular excitability is considerably increased, as in tetany.
Calcium is also necessary for clotting of blood and maintenance of capillary permeability.
SOURCE
Milk and milk products are the best sources of calcium. Calcium in milk is better absorbed than in other foods, because lysine and lactose (milk sugar) enhance calcium absorption. Fatty preparations like butter, cream and ghee are poor in calcium but contain an adequate amount of vitamin D which aids in calcium absorption.
The daily calcium requirements of a man are easily supplied by a cup of buffalo milk. Molasses and vegetables, like peas, beans, pulses, potatoes, cauliflowers, and dried figs, also supply calcium. Pumpkin leaves have a calcium content of 240300 mg (6-7.5 mmol) per 100 g. Chewing betel leaves with lime ( calcium hydroxide) augments dietary calcium. Hard water contains calcium, which can be better absorbed in tropical countries with vitamin D derived from exposure to sunlight. Those who chew fish bones are also assured of adequate calcium intake. A solution of bones soaked in vinegar for several weeks, a practice of the Indo-Chinese, is a good source of calcium.
The tropical diet of rice, wheat or millet is deficient in calcium, but body exposure to sunlight produces vitamin D, which
improves absorption and tends to compensate for the low intake. In the West, the compulsory addition of calcium (fortification) to wheat flour ensures adequate calcium intake.
Calcium carbonate is the cheapest commercial source of calcium.
REQUIREMENTS
Humans have a remarkable capacity to adapt to low calcium intake; it is compensated for by diminished urinary excretion and
increased intestinal absorption.
The daily requirement for calcium ingestion varies according to factors favorable or otherwise to its absorption. In the
tropics, with exposure to sunlight, a positive calcium balance is maintained even on a calcium intake lower than is generally
recommended. The requirement increases with physical activity, and is greater in athletes.
The current calcium intake of American girls during pubertal growth period is not adequate, and increased intake has been
recommended.
Pregnancy increases the demand for, as well as the absorption of, calcium. An infant at birth has more than 20 g (500 mmol)calcium, the major portion of which is accumulated in the last three months when the bones of the fetus are formed. Even if extra calcium is not taken by the mother, the fetus still derives calcium from the mother’s bones. When there is marked deficiency of calcium, the mother may suffer from osteomalacia. About 1.2 g (30 mmol) calcium per day is adequate for the
mother during the last trimester.
Lactation involves loss of calcium; human milk contains 20-30 mg calcium per 100 ml (5-7.5 mmol/l). If 1000 ml of milk is
secreted, 300 mg (7.5 mmol) extra calcium has to be absorbed by the mother. Even’ if the mother’s calcium intake is not
adequate, the concentration of calcium in her milk is maintained at the necessary level by mobilization from her bones. To allow for losses and daily -needs, a lactating mother requires 1.2 g (30 mmol) calcium daily.
Infants who are breast-fed require 50 mg (1.25 mmol) calcium per kg of body weight daily. Artificially-fed infants require two to three times this quantity of calcium, as absorption from artificial sources is not as efficient as from mother’s milk.
DEFICIENCY (HYPOCALCEMIA)
Low serum calcium levels are seen fn the situations listed below.
1. Hypoparathyroidism and pseudohypoparathyroidism.
2. Vitamin D deficiency (i) due to poor diet; (ii) in regions where women do not expose themselves to sunlight; or (iii) due
to diminished absorption in steatorrhea.
3. Low serum proteins (hypoproteinemia), in which the non-diffusible fraction bound to albumin is reduced.
4. Kidney disease, in which there is retention of phosphate or failure to acidify the urine, leading to systemic acidosis and
loss of calcium.
5. Acute pancreatitis, where the pancreatic enzyme lipase is released into the peritoneum and blood, splitting fat into
glycerol and fatty acid, the latter combining with calcium to form soaps.
6. Ingestion of drugs and toxins like sodium, fluoride and viomycin.
7. Following hypothermia for cardiac surgery.
8. Following gastric surgery, probably due to deficient vitamin D absorption. These patients respond to daily oral doses of 32.5 micrograms, or weekly injections of 25 micrograms of vitamin D. Some cases of hypocalcemia after gastric surgery may be due to hypoproteinemia.
9. High-protein diet, which increases urinary calcium excretion.
10. Rheumatoid arthritis, where considerable calcium is lost early in the disease.