Calcium - Introduction

Calcium, the most abundant mineral in the body, is found in some foods, added to others, available as a dietary supplement, and present in some medicines (such as antacids). Calcium is required for muscle contraction, blood vessel expansion and contraction, secretion of hormones and enzymes, and transmitting impulses throughout the nervous system [1]. The body strives to maintain constant concentrations of calcium in blood, muscle, and intercellular fluids, though less than <1% of total body calcium is needed to support these functions.

The remaining 99% of the body's calcium supply is stored in the bones and teeth where it supports their structure [2]. Bone itself undergoes continuous remodeling, with constant resorption and deposition of calcium into new bone [1]. The balance between bone resorption and deposition changes with age. Bone formation exceeds resorption in growing children, whereas in early and middle adulthood both processes are relatively equal. In aging adults, particularly among postmenopausal women, bone breakdown exceeds formation, resulting in bone loss that increases the risk of osteoporosis over time [1].

Recommended Intakes

Intake recommendations for calcium and other nutrients are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board (FNB) at the Institute of Medicine of the National Academies (formerly National Academy of Sciences) [1]. DRI is the general term for a set of reference values used for planning and assessing the nutrient intakes of healthy people. These values, which vary by age and gender [1], include:

• Recommended Dietary Allowance (RDA): average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%-98%) healthy individuals.
• Adequate Intake (AI): established when evidence is insufficient to develop an RDA and is set at a level assumed to ensure nutritional adequacy.
• Tolerable Upper Intake Level (UL): maximum daily intake unlikely to cause adverse health effects [1].

The FNB established AIs for the amounts of calcium required to maintain adequate rates of calcium retention and bone health in healthy people. They are listed in Table 1 in milligrams (mg) per day.

Table 1: Adequate Intakes (AIs) for Calcium [1]

mg = milligrams

Dietary supplements

The two main forms of calcium in supplements are carbonate and citrate. Calcium carbonate is more commonly available and is both inexpensive and convenient. Both the carbonate and citrate forms are similarly well absorbed, but individuals with reduced levels of stomach acid can absorb calcium citrate more easily. Other calcium forms in supplements or fortified foods include gluconate, lactate, and phosphate. Calcium citrate malate is a well-absorbed form of calcium found in some fortified juices [8]. The body absorbs calcium carbonate most efficiently when the supplement is consumed with food, whereas the body can absorb calcium citrate equally effectively when the supplement is taken with or without food [9].

Calcium supplements contain varying amounts of elemental calcium. For example, calcium carbonate is 40% calcium by weight, whereas calcium citrate is 21% calcium.

The percentage of calcium absorbed depends on the total amount of elemental calcium consumed at one time; as the amount increases, the percentage absorption decreases. Absorption is highest in doses ≤500 mg. So, for example, one who takes 1,000 mg/day of calcium from supplements might split the dose and take 500 mg at two separate times during the day.

Some individuals who take calcium supplements might experience gas, bloating, constipation, or a combination of these symptoms. Such symptoms can often be resolved by spreading out the calcium dose throughout the day, taking the supplement with meals, or changing the brand of supplement used.

Medicines

Because of its ability to neutralize stomach acid, calcium carbonate is found in some over-the-counter antacid products, such as Tums® and Rolaids®. Depending on its strength, each chewable pill or softchew provides 200 to 400 mg of calcium. As noted above, calcium carbonate is an acceptable form of supplemental calcium, especially for individuals who have normal levels of stomach acid.

Calcium and Health

Many claims are made about calcium's potential benefits in health promotion and disease prevention and treatment. This section focuses on several areas in which calcium is or might be involved: bone health and osteoporosis; blood pressure regulation and hypertension; cancers of the colon, rectum, and prostate; kidney stones; and weight management.

Bone health and osteoporosis

Bones increase in size and mass during childhood and adolescence, reaching peak bone mass around age 30. The greater the peak bone mass, the longer one can delay serious bone loss with increasing age. Everyone should therefore consume adequate amounts of calcium and vitamin D throughout childhood, adolescence, and early adulthood.

Osteoporosis, a disorder characterized by porous and fragile bones, is a serious public health problem for more than 10 million U.S. adults, 80% of whom are women. (Another 34 million have osteopenia, or low bone mass, which precedes osteoporosis.) Osteoporosis is associated with fractures of the hip, vertebrae, wrist, pelvis, ribs, and other bones [57]. An estimated 1.5 million fractures occur each year in the United States due to osteoporosis [58].

When calcium intake is low or ingested calcium is poorly absorbed, bone breakdown occurs as the body uses its stored calcium to maintain normal biological functions. Bone loss also occurs as part of the normal aging process, particularly in postmenopausal women due to decreased amounts of estrogen. Many factors increase the risk of developing osteoporosis, including being female, thin, inactive, or of advanced age; smoking cigarettes; drinking excessive amounts of alcohol; and having a family history of osteoporosis [59].

Various bone mineral density (BMD) tests are available. The T-score from these tests compares an individual's BMD to an optimal BMD (that of a healthy 30-year old adult). A T-score between +1 and -1 indicates normal bone density, -1.0 to -2.5 indicates low bone mass (osteopenia), and lower than -2.5 osteoporosis [60].

Although osteoporosis affects individuals of all races, ethnicities, and both genders, women are at highest risk because their skeletons are smaller than those of men and because of the accelerated bone loss that accompanies menopause. Adequate intakes of calcium and vitamin D as well as regular exercise (both weight-bearing such as walking, running, and activities where one's feet leave and hit the ground and work against gravity, as well as resistance exercises such as calisthenics and that involve weights) are critical to the development and maintenance of healthy bones throughout the life cycle.

In 1993, the U.S. Food and Drug Administration authorized a health claim related to calcium and osteoporosis for foods and supplements [61]. In January 2010, this health claim is expanded to include vitamin D. Model health claims include the following: "Adequate calcium throughout life, as part of a well-balanced diet, may reduce the risk of osteoporosis" and "Adequate calcium and vitamin D as part of a healthful diet, along with physical activity, may reduce the risk of osteoporosis in later life" [61].

Blood pressure and hypertension
 

Several clinical trials have demonstrated a relationship between increased calcium intakes and both lower blood pressure and risk of hypertension [62-64], although the reductions are inconsistent. The authors of a systematic review of the effects of calcium supplements for hypertension found any link to be weak at best, largely due to the poor quality of most studies and differences in methodologies [65]. Other observational and experimental studies suggest that individuals who eat a vegetarian diet high in minerals (such as calcium, magnesium, and potassium) and fiber and low in fat tend to have lower blood pressure [56,66-69].

The Dietary Approaches to Stop Hypertension (DASH) study was conducted to test the effects of three different eating patterns on blood pressure: a control "typical" American diet; one high in fruits and vegetables; and a third diet high in fruits, vegetables, and low-fat dairy products. The diet containing dairy products resulted in the greatest decrease in blood pressure [70], although the contribution of calcium to this effect was not evaluated. Additional information and sample DASH menu plans are available on the National Heart, Lung, and Blood Institute Web site (http://www.nhlbi.nih.gov/health/public/heart/hbp/dash/index.htm).

 

Cancer of the colon and rectum
 

Observational and experimental studies on the potential role of calcium in preventing colorectal cancer provide mixed results. Several studies have found that higher intakes of calcium from foods (low-fat dairy sources) and/or supplements are associated with a decreased risk of colon cancer [71-74].

Supplementation with calcium carbonate has led to reductions in the risk of adenoma (a nonmalignant tumor) in the colon, a precursor to cancer [75,76], even as long as 5 years after the person stopped taking the supplement [77]. In two large prospective epidemiological trials, men and women who consumed 700-800 mg per day of calcium had a 40%-50% lower risk of developing left-side colon cancer [78].

But other observational studies have found the associations to be inconclusive [74,79,80]. In the Women's Health Initiative, a clinical trial involving 36,282 postmenopausal women, daily supplementation with 1,000 mg of calcium and 400 IU of vitamin D₃ for 7 years produced no significant differences in the risk of invasive colorectal cancer compared to placebo [81]. Given the long latency period for colon cancer development, long-term studies are needed to understand whether calcium intakes affect colorectal cancer risk.

Cancer of the prostate
 

Several epidemiological studies have found an association between calcium intakes of 600 mg or more per day, intakes of more than 2.5 servings of dairy foods, or both and an increased risk of developing prostate cancer [82-88]. However, others have found only a weak relationship, no relationship, or a negative association between calcium intake and prostate cancer risk [89-92]. The authors of a meta-analysis of prospective studies concluded that high intakes of dairy products and calcium might slightly increase prostate cancer risk [93]. Additional research is needed to determine whether a man's risk of prostate cancer is affected by the amount of dairy products or calcium consumed.

Kidney stones
 

Kidney stones in the urinary tract are most commonly composed of calcium oxalate. In the Women's Health Initiative clinical trial, postmenopausal women who consumed 1,000 mg of supplemental calcium and 400 IU of vitamin D per day for 7 years had a higher risk of kidney stones than subjects taking a placebo [94]. Other studies, however, have found that high dietary calcium intakes decrease this risk [95-97]. For most individuals, other risk factors for kidney stones, such as high intakes of oxalates from food and low intakes of fluid, appear to play a bigger role than calcium [98].

Weight management
 

Several studies have linked higher calcium intakes to lower body weight or less weight gain over time [99-102]. Two explanations have been proposed. First, high calcium intakes might reduce calcium concentrations in fat cells by decreasing the production of two hormones (parathyroid hormone and an active form of vitamin D) that increase fat breakdown in these cells and discourage fat accumulation [101]. Secondly, calcium from food or supplements might bind to small amounts of dietary fat in the digestive tract and prevent its absorption [101,103,104]. Dairy products, in particular, might contain additional components that have even greater effects on body weight than their calcium content alone would suggest [105-110].

Despite these findings, the results from clinical trials have been largely negative. For example, a meta-analysis of 13 randomized controlled trials concluded that neither calcium supplementation nor increased dairy product consumption has a statistically significant effect on weight reduction [111]. A more recent clinical trial found dietary supplementation with 1,500 mg/day of calcium (from calcium carbonate) for 2 years to have no clinically significant effects on weight in overweight and obese adults as compared with placebo [112]. Any apparent effects of calcium and dairy products on weight regulation and body composition are complex, inconsistent, and not well understood [101,108-110,113-115].

Source :  http://ods.od.nih.gov/factsheets/calcium.asp

·         Consume appropriate amounts of calcium to stay healthy and reduce your risk for developing osteoporosis and calcium kidney stones.

What increases calcium absorption?

The calcium you consume from your diet or as a supplement is absorbed by the body in the small intestine. Not all the calcium you eat will be absorbed, some will pass through your body and be excreted as waste. How much calcium is absorbed by the body depends on the type of calcium you consume, how well the calcium dissolves in the intestines, and the amount of calcium in your body.

Acidic conditions in the intestine

Calcium carbonate requires an acidic environment in order to be dissolved in the intestine and absorbed into the blood. Stomach acid production increases in the presence of food, creating an acidic environment. Therefore, calcium carbonate supplements should be taken with a meal. Calcium citrate does not require the presence of extra stomach acid to dissolve and be absorbed and can be taken on an empty or full stomach.

Vitamin D

Calcium absorption is dependent on an adequate level of the active form of vitamin D. Often vitamin D is supplemented, along with calcium. Vitamin D has been shown to produce adverse side effects at above 50 micrograms or 2,000 International Units a day. Vitamin D supplements are usually not necessary because vitamin D is available from vitamin D fortified milk, foods such as fish and egg yolks, and exposure to sunlight by the skin. In general, you only need 15 minutes of sunlight exposure to maintain an adequate vitamin D level. However, the amount of sunlight that your skin absorbs is dependant on the weather, latitude, time of year, the amount of skin exposed, and sunscreen use.

Estrogen

Estrogen is a hormone that plays an important role in helping increase calcium absorption. After menopause, estrogen levels drop and so may calcium absorption. Hormone replacement therapy has been shown to increase the production of vitamin D thus increasing calcium absorption.

Low calcium intakes

Your body absorbs calcium less efficiently as your intake increases, therefore it is best to take your calcium in smaller doses throughout the day to aid absorption. You should not take more than 500 milligrams of calcium at one time and allow 4 to 6 hours between doses.

Low blood calcium

When the calcium level in your blood is low, parathyroid hormone is released and increases the production of vitamin D. The vitamin D helps increase calcium absorption, returns the amount of calcium in your blood to normal levels, and makes calcium available to be deposited in the bones.

Lactose

Lactose, the major carbohydrate in milk, aids calcium absorption.

Source: ag.arizona.edu/pubs/health/az1042.pdf