Major Minerals Include Calcium Phosphorus and Electrolytes

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The major minerals make up most of the mineral in our body by weight. Because of this our dietary needs are higher than minor minerals. In addition to foods, supplements make a significant contribution to many people's intake. For instance, calcium-based supplements are among the most popular with consumers who take them primarily for bone support.

Calcium (Ca) 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 do not understand how calcium functions. Calcium is found in foods and the body as an atom with a +2 charge (Ca++ or Ca2+). Calcium atoms, therefore, are most stable after they have given up two electrons (see Chapter 1). Because of this heavy positive charge, calcium strongly interacts with substances bearing a negative charge. This allows it to form mineral complexes found in bone and teeth as well as interact with proteins to make things happen in certain cells.

Calcium is a large, charged atom and is the most abundant mineral in our body.

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 milligrams of calcium (Table 10.2). 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,

Table 10.2 Calcium Content of Select Foods

Food

Calcium

Food

Calcium

(mg)

(mg)

Milk and Milk Products

Vegetables

Yogurt low-fat (1 cup)

448

Collard greens (% cup)

110

Milk, skim (1 cup)

301

Spinach (% cup)

90

Cheese, Swiss (1 ounce)

272

Broccoli (% cup)

70

Ice cream (1 cup)

180

Legumes and products

Ice milk (1 cup)

180

Tofu (% cup)

155

Custard (% cup)

150

Dried beans (% cup)

50

Cottage cheese (% cup)

70

Lima beans (% cup)

40

mustard greens, and turnip greens. Other vegetables such as spinach, rhubarb, chard, and beet greens contain respectable amounts of calcium. 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 of the forms of calcium supplements, calcium carbonate supplies the most calcium (by weight); this form also possesses a slightly greater efficiency of absorption for most people when taken with food. Calcium citrate malate (CCM) is found in some supplements and research suggests that it is better absorbed than the forms above. However, using CCM results in a more expensive supplement so manufacturers tend to either not use it or use it in combination with other calcium forms.

Calcium supplements should be taken with a meal unless the meal contains fiber-rich foods which often contain phytate and oxalates. Calcium carbonate is the form found in many antacids. Meanwhile, calcium citrate is itself an acid and therefore may be better suited for people lacking the ability to produce adequate stomach acid. Recently, supplements containing hydroxyapatite have also begun to appear on the shelves.

Several forms of calcium exist in supplements including calcium carbonate, citrate and citrate malate.

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 (Table 10.3). It is estimated that as little as 5 percent of the calcium is absorbed from spinach because of the presence of inhibiting substances in the digestive tract.

In addition, factors such as normal stomach acidity and the presence of certain amino acids in the small intestine seem to increase the efficiency of calcium absorption. Because of this calcium supplements should be taken with a meal and the one with the least amount of vegetables. Furthermore, a diet having a higher phosphorus-to-calcium ratio may reduce calcium absorption and the ratio of phosphorus to calcium in the diet should not exceed 2:1.

What Are the Recommendations for Calcium Intake?

The recommended intake for calcium is the highest among the nonenergy providing essential nutrients with the only exceptions being phosphorus and water, the latter of which does not have a RDA. The Adequate Intake (AI) for adults (including pregnant and lactating women) is 1,000 milligrams of calcium daily until the age of 51 then the AI increases to 1,200 milligrams. Pregnant or lactating females 18 years old or younger the AI for calcium is 1,300 milligrams daily. 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.

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

Table 10.3 Influence of Various Factors on Calcium Absorption

Calcium Absorption

Calcium Absorption Decreased

Increased

Vitamin D and PTH

Vitamin D and PTH

Lactose during same meal

Phytate, fiber, and oxalates during same meal

Need (growth, pregnancy,

Need

lactation)

PTH = parathyroid hormone.

PTH = parathyroid hormone.

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 similar to flagstone: 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 (Figure 10.1). 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,

Hydroxyapatite containing calcium and phosphate

Hydroxyapatite containing calcium and phosphate

Figure 10.1 Sheets of hydroxyapatite (calcium and phosphate crystals) coating collagen fibers in bone.

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. Scientists sometimes call this 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.

Besides providing hardness to bone, calcium is involved in blood clotting and muscle and hormone action.

How Is the Level of Calcium in the Blood Regulated?

One thing is for certain: calcium is very busy in the body. Again, 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 we 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 (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 as a result of calcium deficiency during childhood, this disorder seems similar to 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. The differences will be explored in Chapter 12.

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. This is because the level of calcium in the blood is more influenced by the hormones mentioned previously in the short run (over a period of days and weeks). 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 hormones that regulate blood calcium levels.

Is Calcium Toxic in Large Amounts?

Today, it is fairly common for people to take in more calcium than years gone by because of supplementation practices and the large number of calcium-fortified foods. Based on this it is possible for people to exceed the AI. 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 Upper Limit (UL) has been set at 2,500 milligrams for children and adults, a level that is usually only achieved with the assistance of supplementation. Beyond this intake level the risk of undesirable effects increases, and can include loss of appetite, nausea, vomiting, constipation, abdominal pain, dry mouth, thirst, and frequent urination. In addition, 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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