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What is calcium?

Without question calcium is one of the most recognizable and popular minerals. Perhaps this is well deserved, as calcium is about 40 percent of total body mineral weight and about 1.5 percent of total body weight. Furthermore, calcium tends to be portrayed as a hero for protecting the human body from osteoporosis. However, most people really don’t understand how calcium functions. Calcium is found in foods and the body as an element (atom) with a +2 charge, which is why you sometimes see Ca (calcium) with ++ or 2+ superscript. Because of this heavy positive charge, calcium strongly interacts with substances bearing a negative charge like phosphate, carbonate, citrate and malate. This allows it to form mineral complexes found in bone and teeth, interact with proteins to make things happen in certain cells. It also helps explain the different forms of calcium used in food fortification and dietary supplements.   


What foods provide calcium to our diet?

Without question, dairy products are the greatest contributors of calcium to the diet. Perhaps more than 55 percent of the calcium in the American diet comes from dairy products. For instance, a cup of milk or yogurt or 1.5 oz of cheddar cheese supplies about 300 mg of calcium. Other good or reasonable calcium sources include sardines, oysters, clams, tofu, red and pinto beans, almonds, calcium-fortified foods, and dark green leafy vegetables such as broccoli, kale, collards, mustard greens, and turnip greens. Other vegetables such as spinach, rhubarb, chard, and beet greens contain respectable amounts of calcium. Some food manufacturers (e.g., orange juice manufacturers) use. Calcium has also become a popular nutrient for fortification in foods such as bread.







What forms of calcium are common in nutrition supplements?

Most calcium supplements contain calcium carbonate or calcium citrate. Other forms of common calcium supplements include calcium gluconate, calcium acetate, and calcium lactate. Of all the forms of calcium supplements, calcium carbonate supplies the most calcium (by weight) and is a good economical choice and best taken with food. Calcium carbonate is the form found in many antacids too. Calcium supplements should be taken with a meal unless the meal contains fiber-rich foods which often contain phytate and oxalates. For the best absorption efficiency, limit supplementation of calcium to 500 milligrams at one time. For people that produce lower amounts of stomach acid or treated with drugs to reduce stomach acid, calcium citrate is often recommended.

What dietary factors can influence calcium absorption?

Plants can contain substances called oxalates and phytate which can bind to calcium in the digestive tract and decrease its absorption. It is estimated that as little as 5 percent of the calcium is absorbed from spinach due to the presence of inhibiting substances in the digestive tract. In addition, factors such as normal stomach acidity and the presence of certain amino acids from protein may increase the efficiency of calcium absorption in the small intestine. Because of the different factors potentially influencing absorption, calcium supplements should be taken with a protein-based meal and the one with the least amount of vegetables or fiber supplements. Meanwhile, a diet with a higher phosphorus-to-calcium ratio may reduce calcium absorption at that same meal. Health professionals often recommend a phosphorus to calcium ratio in the diet not exceeding 2:1.



What are the recommendations for calcium intake?

The recommended intake for calcium is the highest among the non-energy providing essential nutrients with the only exceptions being phosphorus and water, the latter of which does not have an RDA. The RDA for men 19 to 70 years of age is 1000 milligrams daily which is the same for women, ages 19 to 50, and then increases to 1200 milligrams daily from 51 to 70. After the age of 70, the RDA for both men and women is 1200 milligrams daily. For children and teens, ages from 9 to 18, the RDA is 1300 milligrams daily the accommodate greater need during growth of bone and other tissue. During pregnancy and lactation, the RDA for calcium matches those of females from 14 to 50 years of age. 

Recommendations for calcium takes into consideration daily losses of calcium from the body by way of urine, skin, and feces along with an absorption rate of about 20 to 40 percent for adults and up to 75 percent for children and during pregnancy. Food labels in North America will include a Daily Value of 1300 milligrams for Calcium. Both the milligram amount and the percentage if Daily Value per serving are included to assist in informed food choices and diet planning. 


Where is calcium found in the body?

About 99 percent of the calcium in the body can be found in the bones and teeth. Only a small portion of the body’s calcium (1 percent) is found outside bone and teeth and is distributed in tissue throughout the body such as muscle, glands, and nerves. This calcium is found in the blood as well as distributed in other tissues throughout the body including muscles, nerves, and glands. However, despite the relatively smaller quantity, it is this portion of calcium that is more important to human existence on a millisecond-to-millisecond, second-to-second, ­minute-to-minute basis. That’s because this calcium will play a role in the beating of the heart, muscle action, blood clotting and nerve and hormone activity.


What is calcium’s role in bone and teeth?

.Without question the most recognizable function of calcium is to make the bones and teeth hard. The two major calcium-containing complexes in these tissues are calcium phosphate [Ca3(PO4)2] and hydroxyapatite [Ca10(PO4)6OH2] with the latter being the most abundant. Hydroxyapatite crystals have a structure somewhat like construction drywall: they are basically long and flat. This design allows hydroxyapatite to lie on top of collagen fibers in bones and teeth, thereby complementing the strength of collagen with hardness and rigidity. Calcium phosphate is a little different from hydroxyapatite in that it is broken down more readily than hydroxyapatite, which allows it to serve as a resource of both calcium and phosphate to help maintain blood levels of these minerals. Furthermore, calcium phosphate can be used to make hydro­xy­apatite in bones and teeth.







What role does calcium play in the heart and skeletal muscle?

Calcium is involved in the function of excitable tissue (muscle and nerves). Before the heart can “beat,” special cells in a region of the heart called the sinoatrial node (SA node) must spontaneously initiate an electrical impulse. This impulse then stimulates the rest of the heart to contract. Calcium is fundamentally involved in initiating that impulse in the SA node. Calcium is also involved in the contraction of heart muscle, as well as contraction of skeletal muscle. In doing so calcium is the factor that initiates the physical action of heart beats and muscle movement.


What role does calcium play in nerves and hormone action?

Neurotransmitters and hormones are the means by which cells in the body can communicate with each other. However, in order for these substances to provide this service efficiently, they must be released from glands and nerve cells at appropriate times. Calcium is involved in the release of several of these substances. Furthermore, calcium is essential for certain hormones to have an impact upon certain cells. This means that when some hormones interact with their receptors, the result is an increase in the calcium concentration in that cell. As the level of calcium increases in these cells it will then interact with specific proteins and evoke the desired effect in that cell. Calcium sometimes can act as a middleman or intermediate factor as hormones cause things to happen. This is often referred to as a “second messenger” role, whereby the first messenger was the hormone itself.

How is calcium involved in blood clotting?

Calcium is also involved in proper blood clotting. When a hemorrhage occurs, clotting factors in the blood become activated and ultimately a clot is formed at the site of the hemorrhage. A clot is somewhat analogous to a bicycle tire patch that is placed specifically to seal off a hole. The clotting process consists of many steps, some which require calcium to proceed. Calcium binds to the clotting factors and allows them to become more active. Therefore, with a less than optimal amount of calcium in the blood, it might take longer to stop a hemorrhage.

How is the level of calcium in the blood regulated?

One thing is for certain: calcium is very busy in the body. On an instant-to-instant basis, the calcium found in the blood and other tissues is more vital than the calcium complexes in bones and teeth. As alluded to, bones serve as a reservoir for calcium to safeguard against falling blood calcium levels. Blood calcium levels are very tightly regulated; two hormones and one vitamin are directly involved in blood calcium status. Parathyroid hormone (PTH), calci­tonin, and vitamin D all function with blood calcium levels in mind.


PTH is released into circulation from the parathyroid gland when blood calcium levels begin to decline. PTH increases the activation of vitamin D in the kidneys and, along with vitamin D, PTH decreases the loss of calcium in urine. Vitamin D and PTH also increase the release of calcium from bone into the blood as well as increase the efficiency of calcium absorption from the small intestine. The net result is an increase in the level of calcium in the blood, thus returning it to normal, which is 8.8 to 10.8 milligrams/100 milliliters of blood. On the contrary, the level of the hormone calcitonin in the blood increases when calcium levels increase above the normal range. Calcitonin is made by the thyroid gland and generally works opposite to PTH and vitamin D. Calcitonin decreases bone release of calcium and with the help of urinary loss of calcium promotes a reduction in blood calcium, thus returning it to the more optimal range.

How does a calcium deficiency impact bone health?

A deficiency of calcium results in bone abnormalities. If the deficiency occurs during growing years, poor bone mineralization will occur. Bones become soft and pliable due to a lack of mineralization. As bowed legs are often seen because of calcium deficiency during childhood, this disorder seems like rickets, which results from a vitamin D deficiency. If a calcium deficiency develops later in life, the result is a loss of mineral that renders bone less dense and more susceptible to fracture. This process is referred to as osteomalacia, which is often confused with osteoporosis.


Can blood calcium levels be used to assess body calcium status?

It is important to keep in mind that poor calcium intake may not be reflected by reductions in blood calcium for a while. This is because the level of calcium in the blood is influenced in shorter periods of time by PTH and calcitonin as well as vitamin D status, than by calcium intake. However, if calcium intake remains poor for longer periods of time, such as months, blood calcium levels can indeed begin to decrease. Therefore, an assessment of blood calcium levels is somewhat incomplete without an assessment of the other factors that regulate blood calcium levels.


Is calcium toxic in large amounts?

Today, it is more common for people to take in more calcium than years gone by because of supplementation practices and large number calcium fortified foods. Based on this it is possible for people to well exceed the RDA. Although the efficiency of calcium absorption decreases as more is ingested and body calcium status is optimal, this can still lead to increased entry of calcium into the body. The Tolerable Upper Limit (UL) has been set at 2500 milligrams for children and adults, a level that is usually only achieved with the assistance of supplementation.


Beyond the UL level of intake, the risk of undesirable effects increases which can include loss of appetite, nausea, vomiting, constipation, abdominal pain, dry mouth, thirst, and frequent urination. Also, since most forms of kidney stones are calcium oxalate, higher levels of calcium in the urine can increase the risk of kidney stones in people prone to them. Very high intakes of calcium from supplements and usually in combination with calcium containing antacids, over time, can lead to increased calcium content in tissues such as muscle (including our heart), blood vessels, and lungs. This will affect the activity of the tissues by making them more rigid.



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