Calcium

Description

Calcium is the most abundant mineral in the human body. More than 99% of the calcium is present in bones and teeth, the remaining 1% is found in other tissues and in the blood.

In bones, calcium complexes with phosphorus and carbonate to form a characteristic crystalline structure (inorganic matrix) around a framework of protein (organic matrix), primarily collagen. Many other minerals are necessary for strong bones (magnesium, zinc, copper, sodium, fluoride, etc.), but calcium is the major mineral component in bones.

In addition to its major function in bones, calcium serves many other important and vital functions in the body. Calcium is necessary for muscular contraction, the clotting of blood and the activation of many enzymes. Calcium is also required for nerve transmission and regulation of the heart beat.

Absorption
Calcium absorption is dependent on several important factors. Calcium is only absorbed when present in the intestines in its ionized water-soluble form. Stomach acid is necessary to ionize calcium, ingested inorganic calcium salts (such as calcium carbonate, the most widely used calcium supplement), or calcium complexed to large protein molecules like those occuring in milk. If an individual secretes insufficient amounts of stomach acid, these forms of calcium will be largely unavailable for absorption in the intestine.

Components of vegetables and grains may also decrease calcium bioavailability by binding the calcium. Oxalic acid and phytic acid are examples of these types of compounds. Oxalic acid is found in high concentrations in rhubarb, spinach, chard and beet greens. Rich sources of phytic acid include the outer husks of grains, such as oatmeal, and unleavened bread. In addition, high intake of dietary fiber may also decrease ionized calcium availability. Once the ionized calcium reaches the intestine, it requires vitamin D for absorption. Since vitamin D can be produced in our bodies by the action of sunlight on 7-dehydrocholesterol in the skin, many experts consider it more of a hormone than a vitamin.

The sunlight changes the 7-dehydrocholesterol into vitamin D3 (cholecalciferol). It is then transported to the liver and converted by an enzyme into 25-hydroxycholcalciferol (25-OHD3) which is five times more potent than cholecalciferol (D3). The 25-hydroxycholecalciferol is then converted by an enzyme in the kidneys to 1, 25-dihydroxycholecalciferol (1,25-(OH)2D3), which is ten times more potent than cholecalciferol and the most potent form of vitamin D3.

Disorders of the liver or kidneys result in impaired conversion of cholecalciferol to more potent vitamin D compounds, and a calcium deficiency subsequently develops.

Recently, much attention has been given to the role of calcium in regulating blood pressure. Several studies have indicated that populations having a higher calcium content in their drinking water or a higher dietary intake were at a lower risk of developing hypertension. Clinical studies have demonstrated that calcium supplementation often lowers blood pressure, supporting the population studies.

Once absorbed, the calcium is transported to bone and other tissues. About 60% of the calcium circulating in the blood is ionized and physiologically active, about 35% is bound to blood proteins and the remainder is complexed with citrate, bicarbonate and phosphate.

The concentration of calcium in the blood is strictly maintained within very narrow limits. If serum levels of calcium start to decrease, there is a resultant increase in the secretion of parathyroid hormone by the parathyroid glands and a decrease in the secretion of calcitonin by the thyroid and parathyroids. As serum calcium levels increase, there is a corresponding decrease in the secretion of parathyroid hormone and an increase in the secretion of calcitonin.

Parathyroid hormone increases serum calcium levels primarily by increasing the activity of the cells that break down bone (osteoclasts), Parathyroid hormone also decreases the excretion of calcium by the kidneys and increases the absorption of calcium in the intestines. In the kidneys, parathyroid hormone increases the conversion of 25-OHD3 to 1,25-(OH)2D3.

One of the theories relating calcium loss to estrogen deficiency as occurs in postmenopausal osteoporosis is as follows: an estrogen deficiency makes the cells that break down bone (osteoclasts) more sensitive to parathyroid hormone, resulting in increased bone break down thereby raising serum calcium levels. This leads to a decreased parathyroid hormone level, which results in diminished levels of active vitamin D and as well as increased calcium excretion. Evidence in osteoporosis patients seems to support this theory. Calcitonin lowers serum calcium levels by increasing the activity of the cells that build bone (osteoblasts). Low calcitonin levels are found in postmenopausal osteoporosis and may be responsible for this type of bone loss.

Recently calcitonin (isolated from salmon) has demonstrated remarkable effects in clinical studies and holds much promise in treating severe osteoporosis. Since calcitonin secretion can be increased by an elevation in serum calcium levels, this may be one way calcium supplementation exerts its protective effect against osteoporosis.

Method of Action

Because calcium is involved in the control of so many processes, it has been described as "second messenger" that mediates cellular metabolism in a manner analogous to the cyclic nucleotides.

Calcium's role as a secondary messenger is mediated by an intracellular calcium-binding protein, called calmodulin. Calmodulin, thus far found to be present in every nucleated cell type examined, binds calcium ions in response to a stimulus. When bound to calmodulin, the shape and activity of the calcium molecule is altered, activating specific enzymes involved in cyclic nucleotide metabolism, protein phosphorylation, secretory function, muscle contraction, micotubule assembly, glycogen metabolism or calcium flux.

Many new pharmaceutical preparations, such as the so-called calcium channel blockers, are being developed that interact with calmodulin and affect intracellular calcium metabolism. Many herbs and herbal components also have been shown to interact with calmodulin and affect intracellular calcium metabolism.

Calcium, Phosphorus and Protein Interrelationships
Calcium status is determined by the interrelationship of calcium intake, phosphorus intake and protein intake. Increasing protein intake results in increased calcium loss in the urine; when the dietary calcium to phosphorus ratio is lower than 2:1, significant bone loss results. From this evidence it appears protein intake should not exceed the RDA for an individual and the dietary calcium:phosphorus should be above 2:1.

Properties & Uses

Calcium has been used in the management of uremic bone disease, a disorder in which chronic renal failure produces abnormally high serum phosphate levels and, reciprocally, low serum calcium levels.

It is thought that unexplained calcium deficiencies may be related to excessive magnesium depletion. Calcium supplementation helps to prevent the excessive resorption of calcium from the bones. Experimentation has shown adequate calcium intake can prevent and, on occasion, reverse resorption of bone in human periodontal disease.

Calcium and vitamin D supplements are effective in the treatment of osteomalacia; vitamin D promotes increased calcium absorption within the duodenum. Although not a proven cure, increased calcium and vitamin D intake is beneficial for the elderly with osteoporosis, as it counteracts the high rate of calcium efflux characteristic of this disease. In conjunction with vitamin D, calcium is effective in alleviating symptoms of vitamin D-dependent rickets.

Consequence of Deficiency

Calcium deficiency may result from many factors, including: insufficient dietary intake, vitamin D deficiency, lactose intolerance, lack of gastric juices and chronic diarrhea. In children, calcium deficiency is termed rickets, while in the adult it is termed osteomalacia.

Rickets is usually not due to a deficiency in dietary calcium, but rather to a deficiency in vitamin D (discussed below in absorption and utilization). This deficiency causes a decreased absorption of calcium. In rickets, the failure to deposit sufficient amounts of calcium in bones results in growth retardation, bowed legs and skeletal abnormalities.

Osteomalacia, or adult rickets, is also associated most often with a vitamin D deficiency, rather than low dietary calcium. The vitamin D deficiency may be due to a lack of sunlight or decreased dietary intake of this mineral, but in many cases it is the result of malabsorption of fat-soluble vitamins, kidney and liver disease, drugs or some other secondary factor. Osteomalacia is frequently confused with osteoporosis. They differ in that in osteomalacia only the mineral portion (inorganic phase) of the bone is affected, while in osteoporosis both the mineral portion and the protein network (organic phase) is reduced.

A calcium deficiency may cause increased irritability of nerve fibers and nerve centers, resulting in muscle spasms such as leg cramps. This condition is known as tetany.

Toxicity Factors

A thorough review of the literature on the calcium needs of humans was conducted by Irwin and Kienholz in l973. They concluded it is difficult to find clear-cut symptoms of toxic, an extremely high intake of calcium in conjunction with a high intake of vitamin D can induce hypercalcemia. This condition can result in excessive calcification of bone and soft tissue (e.g., the kidney), or in the formation of kidney stones.

The German Commission E notes the increased effectiveness and side effects of simultaneously administered calcium with Lily of the Valley and squill.

References:

Blumenthal, M (Ed.): The Complete German Commission E Monographs: Therapeutic Guide to Herbal Medicines. American Botanical Council. Austin, TX. 1998.

Recommended Dietary Allowance

age	RDA (mg)	RNI (mg)
infants/children
0-6 months	400	525
6-12 months	600	350
1-10 years	800	450
males
11-24 years	1200	1000
25+ years	800	700
females
11-24 years	1200	800
25+n years	800	700
pregnant	1200	-
lactation	1200	1250

Determining the recommended dietary allowance (RDA) of calcium is based on data obtained from calcium-balance studies. These studies measure the intake and excretion of calcium over periods of time. To determine the minimum calcium requirement, the calcium intake is reduced until excretion of calcium exceeds calcium intake (negative calcium balance).

From these studies, it became evident, if given time, calcium balance could be maintained over a very wide range of calcium intakes. It is very possible to maintain calcium balance even at very low calcium intakes, i.e. 200 to 400 mg. per day.

The 1980 revision of the RDA by the National Research Group recommends 800 mg. of calcium per day in view of the high levels of protein and phosphorus consumed in the "typical American diet." The RDA is based on an average calcium loss per day of 320 mg. Since only 20 to 30% of dietary calcium is absorbed, 800 mg. would be required to maintain balance.

Need for calcium is increased during pregnancy and lactation; therefore, the RDA for calcium is increased to 1,200 mg. per day during these times. The RDA for children from 1 to 10 years is 800 mg. and for children from 11 to 18 it is 1,200 mg. daily. For best absorption, calcium should be supplemented at mealtimes.

Complications of excessive calcium intake are usually quite rare, as excess dietary calcium is usually not absorbed. However, in certain disease states, calcium supplementation may be quite detrimental. Examples are hyperparathyroidism, vitamin D intoxication, sarcoidosis and cancer.

For over thirty years, Recommended Daily Amounts has existed in the United Kingdom. It has been used to measure the adequacy of an individual's diet. However, in 1991 the Committee on Medical Aspects of Food Policy (COMA) gave forth a whole new set of figures upon the request of the Department of Health's Chief Medical Officer. Reference Nutrient Intake (RNI) is one of these sets collectively known as "Dietary Reference Values." RNI is an amount of a nutrient that is enough for almost every individuals, even someone who has high needs for the nutrient. This level of intake is, therefore, considerably higher than what most people would need. If individuals are consuming the RNI of a nutrient they are most unlikely to be deficient in that nutrient.

Food Sources

Artichoke	Beans (dried)
Beet greens	Broccoli
Cheese	Collard greens
Dandelion greens	Ice cream
Kale	Milk
Mustard greens	Okra
Orange sherbet	Parsnip
Rhubarb	Rutabaga
Salmon	Sardines
Spinach	Swiss chard
Tangerine	Turnip greens
Watercress

Abstracts

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