calcium: how much is too much?
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I EXPERIMENTAL NUTRITION I
CALCIUM: HOW MUCH IS TOO MUCH?
Recent evidence in pigs suggests that megadoses of calcium may decrease bone strength and
induce internal bleeding.
Key Words: excess calcium, bone strength, internal hemorrhage, osteoporosis, hypertension
Calcium has been very much in the limelight within the nutrition community in recent years. The typical Western-world diet is high in phos- phorus, but relatively low in calcium. Low di- etary ca1cium:phosphorus ratios have been associated with osteoporosis, colorectal cancer2 and, more recently, hypertensi~n,~.~ all of which are prevalent in Western countries. Because of epidemiologic and experimental associations between low calcium intakes and the disease states noted above, many adults have begun to supplement their diets with cal- cium pills, usually in the form of either calcium carbonate (e.g., oyster shell) or calcium lac- tate. The medical profession has been gener- ally supportive of this practice. Magazine, newspaper and even television advertising have also advocated calcium supplementa- tion. While most media emphasis has been placed on osteoporosis in postmenopausal women, hypertension in both men and women has been receiving increasing attention.
Prudent calcium supplementation of ones diet may be beneficial, particularly for those who consume minimal quantities of dairy prod- ucts. But as is the case with all over-the- counter nutrient supplements, be they min- erals, vitamins or amino acids, overdosing can occur. It is thus incumbent on nutrition scien- tists to provide evidence concerning possible contraindications of nutrient excesses, espe- cially concerning nutrients that are widely mar- keted and prominently available in drugstores, supermarkets and health-food stores. Among mineral elements, calcium, zinc and selenium certainly fall into this category.
For those taking calcium supplements, a dose of about 1,000 mg per day seems to be
most common. Many c the calcium supple- ments, moreover, contain added vitamin D to enhance gut absorption of calcium. There are few reasons to suspect that this dosage level of calcium could in any way be harmful, but what would happen if a person decided to con- sume 10,000 mg per day? Effects of these overdoses, with or without added vitamin D, are not well documented in humans, nor have there been definitive studies concerning sin- gle-time versus spaced dosing of high calcium intakes.
There is little question that excess dietary calcium can exacerbate phytate-induced bind- ing of trace elements, particularly zinc5p6 The scientific community is reluctant to give this the attention it deserves, however, for fear that those who supplement with calcium may begin megadosing with zinc. Excess zinc ingestion can markedly decrease the bioavailability of dietary iron and ~ o p p e r . ~ ~ ~ ~ *
Two recent reports involving pigs contribute information about some of the possible dele- terious effects of excess calcium ingestion. Because pigs are physiologically similar to humans, these studies are of particular inter- est. Hall et aL9 at the university of Kentucky fed growing pigs three levels of phosphorus (0.3, 0.6, 0.9 percent-representing deficient, adequate and excess) in the presence of three ca1cium:phosphorus ratios (1 :1, 2:1, 3:l) . Cal- cium carbonate and dicalcium phosphate were used as sources of calcium and phosphorus. In one experiment, all pigs on the 3:l regimen (i.e., 2.7 percent dietary calcium) died, and necropsy revealed severe internal bleeding. When a synthetic source of vitamin K was added to the diet in a subsequent trial, no deaths occurred from excess calcium inges- tion, and the increased clotting and prothrom- bin times observed in pigs consuming excess
NUTRITION REVIEWSNOL. 43, NO. 1 l/NOVEMBER 1985 345
calcium without supplemental vitamin K did not occur. Averaged across trials, blood clotting times for pigs not fed supplemental vitamin K were 150, 248 and 1,937 seconds at dietary calcium levels of 0.9 percent, 1.8 percent and 2.7 percent, respectively. It appears that ex- cess calcium in this study may have inhibited synthesis, reduced absorption or somehow in- activated vitamin K at the level of the gut.
Another recent pig study by Reinhart and MahanlO at Ohio State University showed that ca1cium:phosphorus ratios above 2:l (excess calcium but adequate phosphorus) resulted in decreased bone ash as well as reduced bone breaking strength. In these studies, diet, and therefore the calcium and phosphorus supple- ments, were available ad libitum. Also, excess calcium was fed in the presence of a slight, but not a great, excess of phosphorus. Certainly, the phosphorus levels in these studies did not approach the excessive levels typical of those consumed by the average human in Western- world countries. Nonetheless, the pig would appear to be a good model for studying excess calcium ingestion by man.
Future animal studies should be directed at simulating the single or twice-daily dosing done by humans. Moreover, the question of excess calcium per se should be separated from the effect of a high ca1cium:phosphorus ratio. Because chickens fed dietary calcium levels in excess of 2.5 percent exhibit nephrosis, nephrolithiasis, visceral gout, cal- cium urate deposits in the ureters, and 10 to 20 percent mortality, urinogenital-tract studies should be included.
As has been said many times, nutritional risk
and benefit cannot be assumed. Both must be demonstrated in biological experiments, and then the risk:benefit ratio can be evaluated for given populations. 0
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