CALCIUM-PHOSPHORUS SERUM BALANCE
A final level of calcium balance is that which calcium maintains with phosphorus in the blood serum. Amounts of these two minerals in the blood are normally maintained in a definite relationship because of their relative solubility. This relationship is called the calcium-phosphorus serum balance. This serum balance is the solubility product of calcium X phosphorus, expressed in milligrams per deciliter of each mineral. Normal serum levels are 10 mg/dl of calcium in children and adults, and 4 mg/dl of phosphorus in adults and 5mg/dl in children. Thus the normal serum calcium-phosphorus solubility products are 10x4=40 for adults and 10x5-50 for children. Any situation that causes an increase in the serum phosphorus level would cause a resulting decrease in the serum calcium level to hold the calcium-phosphorus solubility product constant. In such a case, the decreased serum calcium may bring on signs of tetany from lack of neuromuscular controls. To help maintain the normal calcium-phosphorus serum balance, the ideal dietary calcium-phosphorus balance is 1 to 1.5 for children and women during pregnancy and lactation. Other adults require a 1 to 1 dietary balance, meaning equal amounts of dietary calcium and phosphorus, for ideal absorption and utilization.
Control agents for calcium balance
Two main control agents work together to maintain these vital levels of calcium balance in the body: parathyroid hormone and vitamin D hormone (cal-citriol). The cooperative action of these two factors is a good example of the synergistic behavior of metabolic controls. Consider the interdependent relationship of these agents:
1. Parathyroid hormone. The parathyroid glands, lying adjacent to the thyroid glands, are particularly sensitive to changes in the circulating blood level of free ionized calcium. When this level drops, the parathyroid gland releases its hormone, which then acts in three ways to restore the normal blood calcium level: (1) it stimulates intestinal mucosa to absorb more calcium, (2) it withdraws more calcium rapidly from the bone compartment, and (5) it causes kidneys to excrete more phosphate. These combined activities then restore calcium and phosphorus to their correct balance in the blood.
2. Vitamin D hormone. In general, along with parathyroid hormone, vitamin D hormone controls absorption of calcium. It also, with the help of parathyroid hormone, affects the deposit of calcium and phosphorus in bone tissue. Thus these two agents balance each other, with vitamin D hormone acting more to control calcium absorption and bone deposit and parathyroid acting more to control calcium withdrawal from bone and kidney excretion of its partner serum phosphorus.
A third hormonal agent, calcitonin, is also involved in calcium balance. Produced by special C cells in the thyroid gland, it prevents abnormal rises in serum calcium by modulating release of bone calcium. Thus its action counterbalances that of parathyroid hormone to help regulate serum calcium at normal levels in balance with bone calcium.
PHYSIOLOGIC FUNCTIONS OF CALCIUM
The physiologic function of 99% of the body calcium is to build and maintain skeletal tissue. This is done by special types of cells that are in constant balance between depositing and withdrawing bone calcium.
Special tooth-forming organs in the gums deposit calcium to form teeth. The mineral exchange continues as is in bone. This exchange in dental tissue occurs mainly in the dentin and cementum. Very little deposit осcurs in the enamel once the tooth is formed.
General metabolic functions
The remaining 1% of the body's calcium performs a number of vital physiologic functions.
1. Blood clotting. In the blood clotting process, serum calcium ions are required for cross-linking of fibrin, giving stability to the fibrin threads.
2. Nerve transmission. Normal transmission of nerve impulses along axons requires calcium. A current of calcium ions triggers the flow of signals from one nerve cell to another and on to the waiting target muscles.
3. Muscle contraction and relaxation. lonized serum calcium helps initiate contraction of muscle fibers and control following the contraction. This catalyzing action of calcium ions on the muscle protein filaments allows the sliding contraction between them to occur. This action of calcium is particularly vital in the constant contraction-relaxation cycle of the heart muscle.
4. Cell membrane permeability. lonized calcium controls the passage of fluids and solutes through cell membranes by affecting membrane permeability. It influences the integrity of the intercellular cement substance.
5. Enzyme activation. Calcium ions are important activators of specific cell enzymes, especially ones that release energy for muscle contraction. They play a similar role with other enzymes, including lipase, which digests tat, and with ѕоme members of the protein-splitting enzyme system.
A number of clinical problems may develop from imbalances that interfere with the various physiologic and metabolic functions of calcium.
A decrease in ionized serum calcium causes tetany, a state marked by severe, intermittent spastic contractions of the muscle and by muscular pain.
A deficiency of vitamin D hormone causes rickets. When there is inadequate exposure to sunlight or deficient dietary intake of vitamin D, proper bone formation cannot take place.
The usual form of osteoporosis, which is characterized by bone mineral loss, occurs mainly in older persons, especially in postmenopausal women. In women affected, the most rapid rate of loss occurs in the first 5 years after menopause. There is a negative calcium balance of about 40 to 120 mg/clay, indicating loss from both the outer bone layer—cortex — and the small develорing needlelike projections of bone—trabecula — into the central marrow forming the calcium anchoring network of bone matrix. Afterward the rate of bone loss is about 1% a year. In some patients it is not corrected by increased dietary calcium alone but is often improved by exercise and sometimes coupled cautiously with vitamin D supplements under medical/nutrition supervision. Outpatient clinic programs usually center on individual assessment and nutrition care plans, with focus on sufficient protein, energy calories, and vitamin D, with a regular individual exercise program. Current medical approaches to stimulate new bone growth involve hormonal therapy with both vitamin D hormone and estrogen. An idiopathic osteoporosis occurring in young adults does not respond to calcium therapy.
Resorptive hypercalciuria and renal calculi
These two conditions can tilt the usually fine-tuned calcium deposition-mobilization balance maintained by the controlling hormones. When this normal balance is disturbed, resorption of calcium from bone and subsequent increased urinary calcium excretion occurs. A main factor leading to such a condition is prolonged immobilization. Such an imbalance may occur, for example, from a full body cast after orthopedic surgery or a spinal cord injury, or with a body brace following a back injury. In such cases, normal muscle tension on bones that is necessary for calcium balance is lessened, and the risk of renal stones from increased urinary calcium is increased.
Recognizing that peak bone mass is attained by about age 25, the RDA standard has been raised for young adults. An intake of 1200 mg/day for both sexes from ages 11 to 24 has been recommended to cover the important accelerated growth years of adolescence and early adulthood. A conservative estimate of a 40% absorption ratе is assumed. The previous allowance for older adults is continued at 800 mg/day. An allowance of 1200 mg/day is made for pregnancy and lactation, irrespective of age. The standard for formula-fed infants is 400 mg /day for the first б months and 600 mg/day for the remainder of the first year. For children ages 1 to 10, the standard is 800 mg/day.
FOOD SOURCES OF CALCIUM
Dairy products provide the bulk of dietary calcium. One quart of milk contains about 1 g of calcium. Cheese is also a major source. Other sources, including eggs, green leafy vegetables, broccoli, legumes, nuts, and whole grains, contribute smaller amounts. Table 8.2 provides some comparative food sources of calcium.