Role of Calcium

Calcium is the most abundant mineral in the body and more than 99 percent of it is stored in bone tissue. Although only 1 percent of the calcium in the human body is found in the blood and soft tissues, it is here that it performs the most critical functions. Blood calcium levels are rigorously controlled so that if blood levels drop, the body will rapidly respond by stimulating bone resorption, thereby releasing stored calcium into the blood.  Thus, bone tissue sacrifices its stored calcium to maintain blood calcium levels. This is why bone health depends on dietary calcium intake and also why calcium blood levels do not always correspond to dietary intake.

Calcium plays a role in several different functions in the body like bone and tooth formation. The most well-known calcium function is to build and strengthen bones and teeth. Recall that when bone tissue first forms during the modeling or remodeling process, it is unhardened, protein-rich osteoid tissue. In the osteoblast-directed process of bone mineralization, calcium phosphates (salts) are deposited on the protein matrix. Calcium salts typically make up about 65 percent of bone tissue. When your diet is calcium deficient, the mineral content of bone decreases, causing it to become brittle and weak. Thus, increased calcium intake helps increase the mineralized content of bone tissue. Greater mineralized bone tissue corresponds to a greater BMD and greater bone strength. The remaining calcium plays a role in nerve impulse transmission by facilitating electrical impulse transmission from one nerve cell to another. Calcium in muscle cells is essential for muscle contraction because the flow of calcium ions is needed for the muscle proteins (actin and myosin) to interact. Calcium also plays an essential role in blood clotting by activating clotting factors to fix damaged tissue.

In addition to calcium’s four primary functions, calcium has several other minor functions critical for maintaining normal physiology. Calcium plays a role in acid-base balance, healthy blood pressure, and regulating hormones and enzymes. For example, without calcium, the hormone insulin could not be released from cells in the pancreas and glycogen could not be broken down in muscle cells to provide energy for muscle contraction.

Maintaining Calcium Levels

Because calcium performs such vital functions in the body, blood calcium levels are closely regulated by the hormones, parathyroid hormone (PTH), calcitriol, and calcitonin. When blood calcium levels are low, PTH is secreted to increase blood calcium levels via three different mechanisms. First, PTH stimulates the release of calcium stored in the bone. Second, PTH acts on kidney cells to increase calcium reabsorption and decrease its excretion in the urine. Third, PTH stimulates enzymes in the kidney that activate vitamin D to calcitriol, which acts on the intestinal cells and increases dietary calcium absorption. When blood calcium levels become too high, the hormone calcitonin is secreted by certain cells in the thyroid gland and PTH secretion stops.  At higher nonphysiological concentrations, calcitonin lowers blood calcium levels by increasing calcium excretion in the urine, preventing further absorption of calcium in the gut and directly inhibiting bone resorption.

Other Health Benefits of Calcium in the Body

Besides forming and maintaining strong bones and teeth, calcium has been shown to have other health benefits for the body, which include:

1. Decreased risk of cancer: There is enough scientific evidence to conclude that higher intakes of calcium decrease colon cancer risk and may suppress the growth of polyps that often precipitate cancer. Although higher calcium consumption protects against colon cancer, some studies have looked at the relationship between calcium and prostate cancer and found higher intakes may increase the risk for prostate cancer; however, the data is inconsistent and more studies are needed to confirm any negative association.
2. Reduced blood pressure: Multiple studies provide clear evidence that higher calcium consumption reduces blood pressure. A review of twenty-three observational studies concluded that for every 100 milligrams of calcium consumed daily, systolic blood pressure is reduced 0.34 millimeters of mercury (mmHg) and diastolic blood pressure is decreased by 0.15 mmHg.^[1]
3. Improved cardiovascular health: There is emerging evidence that a higher calcium intake prevents other risk factors for cardiovascular disease, such as high cholesterol and obesity, but the scientific evidence is weak or inconclusive.
4. Decreased risk of developing kidney stones: Another health benefit of a high-calcium diet is that it blocks kidney stone formation. Calcium inhibits the absorption of oxalate, a chemical in plants such as parsley and spinach, which is associated with an increased risk of developing kidney stones. Calcium’s protective effects on kidney stone formation occur only when you obtain calcium from dietary sources. Calcium supplements may actually increase the risk of kidney stones in susceptible people.

Dietary Reference Intake for Calcium

The recommended dietary allowances (RDA) for calcium are listed in Table 12.1 “Dietary reference intakes for calcium”. The RDA is elevated to 1,300 milligrams per day during adolescence because this is the life stage with accelerated bone growth. Studies have shown that a higher calcium intake during puberty increases the total amount of bone tissue that accumulates in a person. For adults above the age of fifty, the RDAs are also a bit higher for several reasons including that as we age, calcium absorption in the gut decreases, vitamin D3 activation is reduced, and maintaining adequate blood levels of calcium is important to prevent an acceleration of bone tissue loss (especially during menopause). Currently, the dietary intake of calcium for females above age nine is, on average, below the RDA for calcium.

Calcium Bioavailability

In the small intestine, calcium absorption primarily takes place in the duodenum (first section of the small intestine) when intakes are low, but calcium is also absorbed passively in the jejunum and ileum (second and third sections of the small intestine), especially when intakes are higher. The body doesn’t completely absorb all the calcium in food. Interestingly, the calcium in some vegetables such as kale, brussel sprouts, and bok choy is better absorbed by the body than dairy products. About 30 percent of calcium is absorbed from milk and other dairy products.

The greatest positive influence on calcium absorption comes from having an adequate intake of vitamin D. People deficient in vitamin D absorb less than 15 percent of calcium from the foods they eat. The hormone estrogen is another factor that enhances calcium bioavailability. Thus, as a woman ages and goes through menopause, during which estrogen levels fall, the amount of calcium absorbed decreases and the risk for bone disease increases. Some fibres, such as inulin, found in jicama, onions, and garlic, also promote calcium intestinal uptake.

Chemicals that bind to calcium decrease its bioavailability. These negative effectors of calcium absorption include the oxalates in certain plants, the tannins in tea, phytates in nuts, seeds, grains, and some fibres. Oxalates are found in high concentrations in spinach, parsley, cocoa, and beets. In general, calcium bioavailability is inversely correlated to the oxalate content in foods. High-fibre, low-fat diets also decrease the amount of calcium absorbed, an effect likely related to how fibre and fat influence the amount of time food stays in the gut. Anything that causes diarrhea, including sickness, medications, and certain symptoms related to old age, decreases the transit time of calcium in the gut and therefore decreases calcium absorption. As we get older, stomach acidity sometimes decreases, diarrhea occurs more often, kidney function is impaired, and vitamin D absorption and activation are compromised, all of which contribute to a decrease in calcium bioavailability. Finally, the consumption of high amounts of other minerals at the same time (e.g., iron, zinc magnesium, phosphorus) may impact our ability to absorb calcium; however, there is currently not enough evidence to suggest that this would result in a calcium deficiency.

Who May Require Extra Calcium?

Calcium inadequacy is most prevalent in adolescent girls and the elderly. Elderly individuals may experience age-related factors, such as decreased calcium absorption and limited mobility, which can affect their bone health. Additionally, it is crucial to consider the socioeconomic determinants of health, as individuals from lower-income communities may have limited access to calcium-rich foods and healthcare resources. Proper dietary intake of calcium is critical for proper bone health. Here we will take a closer look at particular groups of people who may require extra calcium intake:

1. Adolescent teens: A calcium-deficient diet is common in teenage girls as their dairy consumption often drops considerably during adolescence.
2. Amenorrheic women and the “female athlete triad”: Amenorrhea refers to the absence of a menstrual cycle. Women who fail to menstruate suffer from reduced estrogen levels, which can disrupt and negatively impact the calcium balance in their bodies. The “female athlete triad” is a combination of three conditions characterized by amenorrhea, disrupted eating patterns, and osteoporosis. Exercise-induced amenorrhea and anorexia nervosa-related amenorrhea can decrease bone mass.^[3][4] In female athletes, as well as active women in the military, low BMD, menstrual irregularities, and individual dietary habits together with a history of previous stress issues are related to an increased susceptibility to future stress fractures.^[5][6]
3. The elderly: As people age, calcium bioavailability is reduced, the kidneys lose their capacity to convert vitamin D to its most active form, the kidneys are no longer efficient in retaining calcium, the skin is less effective at synthesizing vitamin D, there are changes in overall dietary patterns, and older people tend to get less exposure to sunlight. As a result, the risk of calcium inadequacy among the elderly is quite high.^[7]
4. Postmenopausal women: Estrogen enhances calcium absorption. The decline in this hormone during and after menopause puts postmenopausal women, especially, at risk for calcium deficiency. Decreases in estrogen production are responsible for an increase in bone resorption and a decrease in calcium absorption. During the first years of menopause, annual decreases in bone mass range from 3–5 percent. After age sixty-five, decreases are typically less than 1 percent.^[8]
5. Lactose-intolerant people: Groups of people, such as those who are lactose intolerant, or who adhere to diets that avoid dairy products, may not have an adequate calcium intake.
6. Vegans: Vegans typically absorb reduced amounts of calcium because their diets favor plant-based foods that contain oxalates and phytates.^[9] In addition, because vegans avoid dairy products, their overall consumption of calcium-rich foods may be less.

If you are lactose intolerant, have a milk allergy, are a vegan, or simply do not like dairy products, remember that many plant-based foods have a good amount of calcium and there are also some low-lactose and lactose-free dairy products on the market.

Dietary Sources of Calcium

In the typical Canadian diet, calcium is obtained mostly from dairy products, primarily cheese. A slice of cheddar or Swiss cheese contains just over 200 milligrams of calcium. One cup of nonfat milk contains approximately 300 milligrams of calcium, which is about a third of the RDA for calcium for most adults. Foods fortified with calcium such as cereals, soy milk, and orange juice also provide one-third or greater of the calcium RDA.

Consuming Too Little Calcium

Many Canadians do not consume enough calcium^[11], and while there may be no short-term risks associated with this, it could result in osteoporosis in the long term. This can occur because to maintain blood calcium levels, calcium is taken from the bone, which will decrease bone mass and bone density in the long term. This can lead to a decrease in bone strength and an increased risk of fractures (broken bones).

A calcium deficiency, in conjunction with low Vitamin D intake, can also lead to rickets in children, which is when the bones become soft or weak.

Kidney disease, vitamin D deficiency, or diseases that affect the production of parathyroid hormone, can all result in hypocalcemia (low blood calcium), which can result in muscle spasms and convulsions.

Consuming Too Much Calcium

Generally, consuming too much calcium in the diet does not result in toxicity as most of it is excreted. However, it’s possible that excessive supplementation of calcium may result in health problems. For example, it may interfere with the absorption of other minerals such as magnesium, iron, and zinc, which could be problematic for older adults or those who have a low mineral intake.  An excess intake of calcium may also play a role in kidney stone formation. If hypercalcemia occurs due to metabolic disorders or disease, people can experience confusion, fatigue, loss of appetite, and constipation. Furthermore, hypercalcemia can result in calcium being deposited in tissues such as the kidneys and can result in kidney failure.

Calcium Supplements: Which One to Buy?

Many people choose to fulfil their daily calcium requirements by taking calcium supplements. Calcium supplements are sold primarily as calcium carbonate, calcium citrate, calcium lactate, and calcium phosphate, with elemental calcium contents of about 200 milligrams per pill. It is important to note that calcium carbonate requires an acidic environment in the stomach to be used effectively. Although this is not a problem for most people, it may be for those on medication to reduce stomach acid production or for the elderly, who may have a reduced ability to secrete acid in the stomach. For these people, calcium citrate may be a better choice. Otherwise, calcium carbonate is the cheapest. The body can absorb approximately 30 percent of the calcium from these forms.

Beware of Lead

There is public health concern about the lead content of some brands of calcium supplements, as supplements derived from natural sources such as oyster shell, bone meal, and dolomite (a type of rock containing calcium magnesium carbonate) are known to contain high amounts of lead. In one study conducted on twenty-two brands of calcium supplements, eight of the brands exceeded the acceptable limit for lead content. This was the case for supplements derived from oyster shells and refined calcium carbonate. The same study also found that brands claiming to be lead-free did, in fact, show very low lead levels. Because lead levels in supplements are not disclosed on labels, it is important to know that products are not derived from oyster shells and are generally low in lead content. In addition, it was also found that one brand did not disintegrate as is necessary for absorption, and one brand contained only 77 percent of the stated calcium content.^[12]

Diet, Supplements, and Chelated Supplements

In general, calcium supplements perform to a lesser degree than dietary sources of calcium in providing many of the health benefits linked to a higher calcium intake. This is partly attributed to the fact that dietary sources of calcium supply additional nutrients with health-promoting activities. It is reported that chelated forms of calcium supplements are easier to absorb as the chelation process protects the calcium from oxalates and phytates that may bind with the calcium in the intestines. However, these are more expensive supplements and only increase calcium absorption up to 10 percent. In people with low dietary intakes of calcium, calcium supplements have a negligible benefit on bone health in the absence of a vitamin D supplement. However, when calcium supplements are taken along with vitamin D, there are many benefits to bone health: peak bone mass is increased in early adulthood, BMD is maintained throughout adulthood, the risk of developing osteoporosis is reduced, and the incidence of fractures is decreased in those who already had osteoporosis. Calcium and vitamin D pills do not have to be taken at the same time for effectiveness. But remember that vitamin D has to be activated and in the bloodstream to promote calcium absorption. Thus, it is important to maintain an adequate intake of vitamin D.

The Calcium Debate

A recent study published in the British Medical Journal reported that people who take calcium supplements at doses equal to or greater than 500 milligrams per day in the absence of a vitamin D supplement had a 30 percent greater risk of having a heart attack.^[13]

Does this mean that calcium supplements are bad for you? If you look more closely at the study, you will find that 5.8 percent of people (143 people) who took calcium supplements had a heart attack, but so did 5.5 percent of the people (111 people) who took the placebo. While this is one study, several other large studies have not shown that calcium supplementation increases the risk for cardiovascular disease. While the debate over this continues in the realm of science, we should focus on the things we do know:

1. There is overwhelming evidence that diets sufficient in calcium prevent osteoporosis and cardiovascular disease.
2. People with risk factors for osteoporosis are advised to take calcium supplements if they are unable to get enough calcium in their diet. This includes calcium both from dietary sources and supplements.
3. Consuming more calcium than is recommended is not better for your health and can prove to be detrimental. Consuming too much calcium at any one time, be it from diet or supplements, impairs not only the absorption of calcium itself, but also the absorption of other essential minerals, such as iron and zinc. Since the GI tract can only handle about 500 milligrams of calcium at one time, it is recommended to have split doses of calcium supplements rather than taking a few all at once to get the RDA of calcium.