Chapter 3

Considering Calcium

In This Chapter

Breaking down the physiology of calcium

Selecting the best sources of calcium

Following the U.S. government’s recommended dietary allowances

Understanding the importance of bone

Giving credit to other bone minerals

Calcium is tightly linked to many of the roles that vitamin D plays in the body. In bone health (and other physiologic systems), calcium is a key player, and more needs to be said about it.

Calcium plays numerous roles in the body, from building bones to permitting movement of nerve impulses. In this chapter, I explain what calcium does, how it interacts with vitamin D, and how much of it you need. I also include information about two other minerals whose use by the body are affected by vitamin D — phosphorous and magnesium.

Understanding the Physiology of Calcium

Calcium is a mineral that must be constantly eaten to build bone and maintain the blood level of calcium.

In the body of a 70-kilogram (150-pound) person, about 2 to 3 of those kilograms (or about 6 pounds) is calcium. Ninety-nine percent of the calcium in your body is found in your skeleton. Together with phosphorus (another mineral), calcium helps stabilize the bones, and calcium from bones is used to maintain the calcium in the blood and the tissues.

In the following sections, I list some of the many ways your body uses calcium, and explain how the body regulates its levels of calcium.

Considering the functions of calcium

When people hear about calcium, they think “bone.” Bone is formed by special cells in the body. These cells make a scaffold with proteins and then calcium and phosphorus form a crystal on top of this scaffold. That’s how the scaffold gets strong — like putting concrete on top of an iron structure to make a strong building.

A baby begins to accumulate calcium in bone during the third trimester of pregnancy. Accumulation of calcium in bone continues until its peak in early adulthood. Then the amount of bone, as well as the calcium level in bone, begins its gradual decline at the rate of 1 percent per year. The decline occurs because the continual remodeling of bone switches from an excess of bone formation during growth to an excess of bone breakdown in adulthood.

Even though only 1 percent of the calcium in the body is found outside of bone, this form of calcium is critical for many functions in the body. Therefore, its level is maintained in a narrow range in the blood and tissues (more on this in the following section). Consider some of the key non-bone functions of calcium:

It’s essential for blood clotting.

It stabilizes blood pressure.

It contributes to normal brain function.

It’s critical for communicating essential information among cells.

Normally the amount of calcium inside a cell is very low relative to the amount that’s in your blood. Cells let calcium inside in response to a large number of chemicals, such as hormones. This chemical stimulus of calcium rushing into a cell makes them perform all sorts of critical functions. For example, it

Helps insulin open cells to glucose

Is needed for the release of chemicals that transmit a signal from a nerve cell to a target cell (for example, when a nerve tells a muscle to move)

Facilitates the actual process of contraction of the muscle cell

Assists the movement of sperm into an egg to fertilize the egg

Learning how calcium is controlled

The concentration of calcium in the blood is normally 9 to 10.5 milligrams per deciliter (mg/dl) [2.25-2.63 mmol/L]. Half of that is free in the blood; the other half is bound to proteins or complexed to bicarbonate and citrate. If the amount of protein in the blood declines, the total calcium falls whereas the free calcium remains normal. The body has developed elaborate ways to make sure blood calcium levels don’t dip or raise much.

Changes in the blood levels of parathyroid hormone and calcitriol (the active form of vitamin D) control the level of free calcium. Parathyroid hormone levels increase when free calcium levels fall. In response the kidneys make more calcitriol. Together, parathyroid hormone and active vitamin D work to increase the amount of calcium in the blood. Both hormones mobilize calcium from bone. Both hormones make sure less calcium is lost in the urine. Calcitriol makes the intestine more efficient at absorbing calcium from your diet. All of these things increase free calcium in the blood; as the free calcium increases, it lowers the serum level of parathyroid hormone and calcitriol. This is a classic example of a biological feedback loop where all the events are tied together in a circle.

When a person normally eats a diet rich in calcium every day, the secretion of calcitriol decreases so that less calcium is absorbed. However, if dietary calcium is low for even one week, more active vitamin D is secreted by the kidney and this makes the intestine more efficient at absorbing calcium from the diet. As a result, despite large changes in dietary intake of calcium, the total daily amount of calcium absorbed each day is relatively constant at 200 to 400 mg.

In a healthy adult, the daily intake of calcium is balanced by a daily loss of about the same amount of calcium in the urine. Calcitriol can affect the amount of calcium in the urine, but this loss is mainly under the influence of parathyroid hormone. Both hormones help the kidneys effectively remove calcium from the urine and return it to the blood, especially when the free calcium falls. Calcium from your body is also lost from the intestine, from sweat, and from other secretions.

High levels of protein and salt in the diet can prompt the kidneys to get rid of too much calcium.

If dietary calcium intake is too low, calcium is released from bone under the influence of parathyroid hormone and calcitriol. The body maintains the level of calcium in the blood, even if it means tearing down the skeleton to do it.

Chapter 4 describes rickets in children or osteomalacia in adults as diseases of vitamin D deficiency. They can also be caused by an inadequate intake of calcium.

Getting the Calcium You Need

Because your body doesn’t make minerals such as calcium, you need to eat foods rich in calcium for optimal health. In the following sections, I tell you how much calcium is recommended, list calcium-rich foods that can help you meet the guidelines, and outline the dangers of having too much or not enough calcium in your diet.

Following the U.S. government’s guidelines on calcium

As with other nutrients that you consume, expert panels of scientists determine the acceptable level of calcium intake for optimal health. The expert panel comes up with recommendations depending on your age and gender, as well as whether you’re pregnant or nursing. You can read the full report from the expert panel at http://www.iom.edu/Reports/2010/Dietary-Reference-Intakes-for-Calcium-and-Vitamin-D.aspx.

Table 3-1 shows the calcium recommendations for children; girls and women; and boys and men.

Men get osteoporosis and suffer hip fractures just like women. By age 80, 25 percent of men will have had a hip fracture. Eat your calcium and vitamin D-rich foods, men!

You can easily fulfill these recommendations by taking one calcium tablet twice daily as an adult and eating a portion of any of the foods listed among the best sources of calcium (see the following section).

Selecting the best sources of calcium

Many of the best sources of calcium are also good sources of vitamin D, especially dairy foods. Table 3-2 lists the best sources of calcium to include in your diet. (See Chapter 12 for more about good food sources of vitamin D.)

Other good sources are any cheeses, tofu processed with calcium, enriched farina, almonds, dried beans, and soybeans.

Research shows that most women consume less calcium than is recommended. This is probably because women view calcium-rich dairy products as fattening. Regardless, women who don’t like calcium-rich foods are advised to take supplements. One hint about taking calcium supplements — you don’t absorb calcium efficiently if you take a supplement with the entire requirement at a time, so rather than take a large amount at one time, the recommendation is to take 500 mg of calcium twice daily.

Take your calcium supplement with a meal; it’s easier to remember that way. More importantly, the acid released in response to food breaks the calcium tablets down into a form that makes them more readily absorbed. Also, eating food with your supplement causes fewer side effects, like bloating and gas. Take combined calcium and vitamin D to make sure you benefit from both.

Vegetarians may have difficulty meeting the daily requirements for vitamin D and calcium if they avoid dairy foods. Consider these suggestions for vegetarians:

If you drink fortified nondairy milk, shake the milk: the calcium added to these products settles to the bottom.

The body readily absorbs calcium in kale, broccoli, collard greens, and soy milk.

The body poorly absorbs calcium in spinach, Swiss chard, and beet greens. (Chemicals in these foods tie up the calcium and prevent it from being absorbed.)

Dealing with too much and too little calcium

For the most part, your body adapts well to changing levels of dietary calcium. In the following section, I explain what happens in the relatively rare event when your serum calcium levels become too high or too low.

When calcium intake is high, perhaps only more than 2,000 mg a day, this may gradually cause calcification of arteries and increase the risk of heart attack and stroke. Therefore “more is better” isn’t always true. Most people need a calcium intake in the range of 1,000 to 1,500 mg per day and it’s probably wise not to exceed that unless it’s known that you don’t absorb calcium well.

When calcium intake is very high — for example more than 4,000 mg in a day — even though the hormones that increase calcium absorption turn off, large amounts of calcium can pass into the body from the intestine and a state of hypercalcemia (too much calcium in the blood) occurs. This can cause kidney stones and, in extreme cases, kidney failure. This condition used to be more common when people took large amounts of antacids for stomach ulcers.

More acute symptoms may accompany the condition of hypercalcemia. If the elevation of calcium is mild, such as 11 or 12 mg/dl, no symptoms may arise. If the calcium level is greater than 12, symptoms may be severe. Among them are these symptoms:

Abdominal pain

Confusion

Excessive thirst and urination

Fatigue and lethargy

Loss of appetite

Muscle weakness

Nausea and vomiting

Treatment of symptomatic hypercalcemia is done under a doctor’s supervision. The patient is given fluids to improve hydration, and they are given a variety of drugs that lower serum calcium levels rapidly. The doctor measures the blood calcium every few hours to determine the success of therapy.

The symptoms of hypocalcemia (low blood calcium) are quite different. They may occur when total calcium is below 9 mg/dl. The most important are these symptoms:

Anxiety and depression

Dementia and mental retardation

Low blood pressure

Muscle stiffness and spasms

Papilledema (swelling of the optic disc caused by increased pressure in the brain)

Seizures

Treatment of hypocalcemia is also done under a doctor’s supervision. If the condition is mild, oral calcium and vitamin D is taken. If it’s severe, intravenous calcium is administered. Blood calcium levels are measured every few hours to follow the progress of the treatment if intravenous calcium is given.

Realizing the Purpose of Other Minerals for Bone

The body requires several other bone minerals to make hard bone and to assist in other actions of calcium.

Focusing on phosphorus

Similar to calcium, the bulk of the phosphorus found in the body is in bone (80 to 90 percent of all the body’s phosphorus). The rest is in blood and tissue fluids. In its free form in the blood, phosphorous is in a chemical form called phosphate. The normal amount of phosphate in the blood is 2.7 to 4.5 mg/dl (0.86 to 1.44 mmol/L).

The major function of phosphorus is to combine with calcium to mineralize the skeleton; however, like calcium, phosphorus has critical roles in all cells. For example it’s a part of our genetic material — DNA. Phosphorous is also critical to how the body uses the energy from carbohydrates, proteins, and fats. It’s the main component of ATP, the form of energy that all cells use. Even organisms that don’t have bones, such as bacteria or worms, need phosphorus for these essential roles.

Phosphate is available in many foods, and the body absorbs it well even without vitamin D (although vitamin D can raise the absorption). The best sources are dairy, meat, and fish. Calcium and phosphorus exist together, so your good source of calcium is also your good source of phosphorus. Phosphorus deficiency is rare in the United States.

We usually absorb about 500 to 1,000 mg of phosphate daily. We lose phosphate through the kidneys. Table 3-3 shows recommended values for phosphorus intake.

Although phosphorus deficiency is rare in the United States, phosphorus can be low in the body as a result of malnutrition or conditions that affect phosphorus absorption (or both, such as alcoholism). The chief signs and symptoms of low phosphorus are the following:

Bone pain

Confusion

Muscle weakness

Rickets and short stature in children

Seizures

If an adult’s phosphate level is lower than 2.7, it can be treated with phosphate supplementation under the care of a doctor.

Giving credit to magnesium

Magnesium is present in small amounts in the body and is important to body function. Magnesium is required for the secretion of parathyroid hormone, which, in turn, controls calcium removal from bone. It’s also required for more than 300 enzymes to perform their functions, including enzymes that make DNA and RNA, the building blocks of the genes, and it helps ATP, the chemical that carries energy in the body, work.

Sixty percent of the body’s 24 grams of magnesium is present in the bones, where it helps form the structure of the bone. Thirty-nine percent is found inside cells, and only 1 percent is found in blood and tissue fluid. The usual level of magnesium in the blood is 1.8 to 2.4 milliequivalents per liter, but blood is a poor measure of the body’s magnesium.

Consider some of the major functions of magnesium (in addition to its requirement for enzyme reactions):

Muscle and nerve function

Normal heart rhythm

Normal immune system

Strong bones

The U.S. government has established daily dietary reference intakes for magnesium, as shown in Table 3-4.

Table 3-4 Recommended Dietary Allowances (RDA) and Tolerable Upper Intake Levels (UL) for Magnesium

Age	RDA: Milligrams Per Day	UL: Milligrams Per Day**
0 to 6 months	30*	ND***
7 to 12 months	75*	ND
1 to 3 years	80	65
4 to 8 years	130	110
9 to 13 years	240	350
14 to 18 years Males Females	410 360	350
19 to 30 years Males Females	400 310	350 350
Older than 31 Males Females	420 320	350 350
Pregnant 14 to 18 years 19 to 30 years 31 to 50 years	400 350 360	350 350 350
Breastfeeding, to 18 years	Same as for age group	350

* An adequate intake (AI) value was set instead of an RDA.** The UL for magnesium refers only to the amount of magnesium you would get from a supplement, does not include the amount that you get from food or water.*** ND is short for not determined yet.

The richest sources of magnesium are cereals, coffee, cocoa, fish, bananas, apricots, avocados, nuts, spices, tea, and vegetables. It seems that the intake of magnesium has decreased in the last decades.

The best dietary supplement for magnesium is probably magnesium citrate.

Although magnesium, like calcium, contributes to the strength of the skeleton, there is no proof that taking magnesium improves the bones of someone with osteoporosis. On the other hand, magnesium irritates the bowels and can cause diarrhea (don’t forget magnesium is a laxative when taken as “Milk of Magnesia”!). And so combination calcium and magnesium supplements can be useful because they counteract the constipating effect of calcium carbonate with the liberating actions of magnesium.

As with phosphorus, nutritional magnesium deficiency is rare. However, low serum magnesium levels occur in certain diseases and from taking certain medications:

Aging, which is associated with decreased intake, reduced absorption, and increased loss by the kidneys

Alcoholism

Anticancer medications, like cisplatin

Certain antibiotics, like gentamicin and amphotericin

Diabetes mellitus that is uncontrolled

Diseases that cause poor absorption, like irritable bowel syndrome, ulcerative colitis, and Crohn’s disease

Diuretics, medicines that promote water loss, like furosemide, bumetanide, ethacrynic acid, and hydrochlorothiazide

Symptoms of magnesium deficiency include the following:

Agitation and anxiety

Confusion

Low blood pressure

Nausea and vomiting

Restless leg syndrome

Seizures

Sleep disorders, like insomnia