An increasing number of individuals are relying upon calcium supplements as a means of obtaining sufficient calcium intake (1). In recent years the National Institutes of Health (NIH) have increased their current recommendations on adequate daily calcium intake for certain subgroups in the population. For example young adults aged 11-24 years are now recommended to ingest 1,200 - 1,500 mg. of calcium per day. Men and women older than 25 should ingest 1,000 mg. per day, postmenopausal women not taking estrogen replacement therapy require 1,500 mg. per day as do men over 65 and women over 65 (even if she is taking estrogen replacement therapy). Today, one in four women already have significant bone demineralization by age 50, partly as a result of poor calcium intake throughout their lives. Osteoporosis affects more than 25 million people in the United States and is the major underlying cause of bone fractures in postmenopausal women and the elderly. Previous surveys have revealed that the U.S. population experiences more than 1.5 million fractures annually at a cost in excess of $10 billion per year to the health care system (2). In Canada, the death rate from complications due to osteoporotic fractures, result in more deaths each year than the combined mortality rate for breast and ovarian cancer (3).
In addition to preventing osteoporosis a number of publications have addressed the possible role of calcium intake in the prevention of oral bone loss and periodontal disease, colon cancer, high blood pressure, and preeclampsia, a hypertensive disorder of pregnancy (2).
With the realization that it requires an intake of four eight ounce glasses of milk per day or five ounces of cheese to get 1,000 mg. of calcium, many people have looked to calcium supplements as a result of their inability to consume these quantities of dairy products every day of their lives. Although other foods contain respectable amounts of calcium a large percentage of people do not consume the recommended amount of calcium per day from food alone (4).
Due to heightened consumer demand for calcium supplements there are an ever-increasing number of different preparations of calcium supplements entering the marketplace. As the bioavailability (absorption ability) may vary from product to product it has become necessary to test the different types of calcium supplements to ensure that they are of sufficient quality to improve calcium nutrient status.
When comparing calcium supplements several factors need to be addressed. These include bioavailability (absorption ability or percent of calcium absorbed from each product), contamination with lead or other heavy metals, and to a lesser degree solubility (the degree to which there is separation of a compound into its components by chemical action) (1). As demonstrated by Heaney in 1990, the solubility of a calcium supplement at neutral pH does not necessarily reflect its bioavailability (5,6). This is an important finding because a number of calcium supplement manufacturers have wrongly proclaimed that calcium carbonate is not well absorbed due to its poor solubility (7).
On the contrary a number of well designed studies have shown calcium carbonate supplementation to be effective in improving bone mineral density in various groups and slowing the rate of bone loss in postmenopausal women (8,9,10,11,12). At the same time the more soluble forms of calcium such as calcium citrate, calcium citrate-malate and ossein-hydroxyapatite have also been shown to have a favorable effect on bone mineral density (13,14,15).
The available research is very supportive of the fact that various types of calcium supplements are effective at improving calcium nutritional status and reducing risk of osteoporosis (16,17,18,19). As for bioavailability, world renowned researcher Bess Dawson-Hughes (U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston) explains that overall, variation in the absorption of calcium from different sources is fairly small when compared with variation in other factors affecting absorption of calcium (20). Harvey et al compared calcium absorption after administration of different doses of calcium citrate and carbonate. With both calcium sources, urinary calcium rose rapidly (indicating excellent absorption from the intestinal tract into the bloodstream) with increasing calcium loads up to 500 mg. elemental calcium. With calcium loads between 500 and 2,000 mg. however, urinary calcium remained fairly constant for both supplements (21). Thus 500 mg. of calcium carbonate demonstrated the same bioavailability as calcium citrate. The most important point of this and other similar studies is that calcium supplements of any kind should not contain more than 500 mg. of elemental calcium. Adding more calcium does not further increase calcium absorption into the bloodstream (20). Therefore, if an individual required 1,000 mg. of calcium per day from supplementation they are best advised to ingest 500 mg. at two different times during the day. This approach increases total calcium absorption. The most accurate method available for assessment of calcium absorption is the double-isotope procedure first proposed by Bronner (22). Using this method R. Heaney et al tested the absorption efficiency of calcium from milk against calcium citrate-malate. Their results demonstrated that calcium absorption could vary approximately by 3 percent from day to day or week to week in the same individual. They conclude that calcium absorption in adults commonly averages 25-35 percent (23).
More recently L. Mortensen and P. Charles tested the bioavailability of calcium from milk against calcium carbonate. They demonstrated that calcium carbonate was at least as bioavailable as the calcium from milk. Interestingly, the addition of vitamin D to the calcium supplement further increased calcium absorption (1). Previous studies have established milk as the "gold standard" for calcium absorption studies (24) (25).
The net result is that calcium carbonate; calcium citrate, calcium citrate-malate, and calcium chloride have all demonstrated impressive bioavailability. However, under certain circumstance calcium citrate is a preferred form. On an empty stomach between meals calcium citrate is better absorbed than calcium carbonate (26).
Recker found that in subjects with decreased gastric acid secretion (as often occurs with aging) calcium carbonate is only bioavailable when it is taken with meals (27). Although the research by Bo-Linn et al demonstrates that calcium carbonate absorption is unrelated to the amount of gastric acid production (28), it is probably best to advise elderly subjects to ingest calcium carbonate supplements with meals (20).
In fact R. P. Heaney (a world authority on calcium supplementation from Creighton University; Department of Internal Medicine, Centers for Hard Tissue Research, Metabolic Research Unit) has stated that calcium taken with a meal is better absorbed than is calcium taken without a meal. Thus, taking calcium supplements at mealtime is the recommended regime (29).
The next point to consider when assessing calcium supplements is the calcium yield. For example calcium carbonate yields the most calcium of all supplements at 40% (40% calcium, 60% carbonate). The yield of calcium citrate is 21%; calcium lactate is 13% and calcium gluconate yields only 9%. From a practical standpoint the size of the calcium tablet must be considered. Thus, the maximum amount of elemental calcium that can be yielded from the largest possible calcium-citrate tablet is 350 mg. From calcium carbonate the total amount of elemental calcium can easily reach 500 mg. or more and still remain at a size that is easy to swallow (30). As there is little variation in bioavailability between different calcium supplements when taken with meals, calcium carbonate offers a total net amount of calcium to the body that is significantly greater than other forms of calcium supplements (20).
Studies with various groups (men, young women, postmenopausal women, elderly women) have all conclusively demonstrated that calcium carbonate improves bone mineral density and can slow bone loss later in life. In postmenopausal women, who are willing to exercise sufficiently calcium carbonate can even increase bone density at this stage of life.
Taken together these studies prove that calcium carbonate is both bioavailable and is utilized efficiently by bone tissue, reducing risk of osteoporosis in women and men (8,9,10,11,12,31).
The recent finding by Mortensen and Charles, that adding vitamin D (200 I.U. x 3 per day) to the calcium carbonate supplement further increased calcium absorption is an important discovery (1). In previous studies vitamin D supplementation in elderly people has been shown to reduce osteoporotic fractures, especially if consumed with a calcium supplement (32,33,34). As vitamin D is a fat-soluble nutrient it too, must be ingested with meals.
Thus, a multiple vitamin containing a daily dosage of 400 I.U. of vitamin D and 500 mg. of elemental calcium would benefit a great number of young, middle-aged and older individuals, who routinely ingest too little calcium and often demonstrate suboptimal vitamin D status. From a practical standpoint the form of calcium must include calcium carbonate in order to maintain a tablet size that is small enough to be ingested by the majority of people. Provided the calcium carbonate originates from refined sources and not oyster shell (which is actually mined from fossilized limestone quarries), there is no reason to fear high levels of aluminum or lead (35,2). Supplements such as dolomite and bone meal can also be contaminated with hazardous amounts of toxic heavy metals like arsenic and lead (2). For patients who require more calcium (500 mg.) than can be obtained from a multiple vitamin a number of stand-alone calcium supplements can be used. Between meals or at other meals calcium citrate, calcium citrate malate, calcium chloride or microcystalline hydroxyapatite concentrate (MCHC) can be taken as supplements to further enhance calcium nutritional status (36,15,7)
In summary the debate about which form of calcium supplement is most effective to use boils down to the following well established facts:
1. There are many forms of calcium that demonstrate excellent bioavailability (i.e. calcium citrate, calcium carbonate, calcium citrate-malate, calcium lactate, calcium gluconate, MCHC, and calcium chloride).
2. By weight calcium carbonate yields the most elemental calcium (40%) compared with other forms of calcium supplements. This form of calcium delivers the most elemental calcium within the smallest possible vitamin tablet.
3. When taken with meals the bioavailability between different types of calcium supplements does not vary appreciably. Typically, about 30% of the available calcium can be absorbed.
4. On an empty stomach (especially in people with reduced stomach acid production) calcium citrate or calcium citrate-malate is likely the preferred supplement from a bioavailability standpoint.
5. Taking calcium with sufficient vitamin D appears to further enhance calcium absorption and lower risk of osteoporotic fractures.
6. To include 500 mg. of elemental calcium in a multiple vitamin formula (including 400 I.U. vitamin D) calcium carbonate is a logical choice due to its compact size and its excellent bioavailability when consumed with a meal (which is the time when multiple vitamins must be consumed to offer any real benefit)
Copyright 1999 Dr. James Meschino D.C., M.S., N.D.
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