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Subjects: Twelve normal subjects (6 men and 6 women) participated in the study. None of the subjects had a history of bone disease, peptic ulcer, enterectomy, regional enteritis, malabsorption, nephrolithiasis, liver cirrhosis, or renal disorder. The subjects had not taken calcium supplements or vitamin D preparations, nor received anticonvulsants, diuretics, adrenocorticosteroids, estrogens or any other drugs that could affect calcium metabolism during the month preceding the start of the study.

Methods: The subjects were divided into two groups; subjects of one group ingested coral-added crackers first (group A) and those of the other group ingested calcium carbonate-added crackers first (group B). After a subsequent 3-d wash-out period, the groups received the study regimens on a cross-over design. An additional group (group C) served as a control not ingesting crackers. Each 12-g piece of coral-added cracker contained 75 mg of calcium and 36 mg of magnesium.

Calcium and magnesium contents of a 12-g calcium carbonate-added cracker were 75 and 6mg, respectively. Each subject ingested seven pieces of either cracker each time in this study since, according to Harvey et al (8), oral ingestion of 500 mg of calcium suffices for adequate evaluation of intestinal calcium absorption by measurements of urinary calcium excretion.

The calcium intake and magnesium intake after the ingestion of 7 coral-added crackers were calculated to be 525 and 252 mg, respectively, and those after ingestion of 7 calcium carbonate-added crackers to be 525 and 42 mg, respectively. Controls (group C) did not ingest either cracker at all.

RESULTS: Calcium absorption: The group receiving coral-added crackers and that receiving calcium carbonate-added crackers were practically comparable with respect to urinary calcium excretion during 2-h pre-ingestion. Mean urinary calcium excretion after the ingestion of coral-added crackers was greater than that after calcium carbonate-added crackers by four determination methods.

Significant intergroup differences were noted in urinary calcium excretion during 4-h post-ingestion, increase in urinary calcium excretion (mg/dL OF) during the latter half of the post-ingestion observation period, and increase in urinary calcium excretion (A from control mg/mg Cr) during 4-h post-ingestion. However, significant difference was demonstrated for the first half of the post-ingestion observation period.

These findings were generally in line with the conclusions from absorption studies on calcium citrate versus calcium carbonate by Harvey et al (8) and Nicar and Pak (9) that calcium citrate was better absorbed. The males exhibited better calcium absorption from coral-derived calcium as compared with the females, though the subject sample sizes were small. Meanwhile, the increase in urinary calcium excretion during the subsequent 2-h period determined for reference, showed a plateau with no appreciable difference between the two cracker regimens. It was thus considered appropriate to assess the responses by analyzing two consecutive 2-h post-ingestion urine samples for the comparison based on urinary calcium excretion.

DISCUSSION: The assessments of calcium absorption from supplemented crackers demonstrated a better absorption of coral-derived calcium than that of calcium carbonate-derived calcium on the average. A laboratory study in rats to explore the ability to utilize calcium derived from Ryukyuan coral which contains calcium and magnesium at a ratio of about 2-to-1 has been reported by Suzuki et al (10). The investigators calculated the calcium balance from excretions in the feces and urine during the last 3 days of a 4-week rat feeding trial using coral.

They concluded that the efficiency of calcium utilization was satisfactorily greater with coral-derived calcium as compared to calcium carbonate-derived calcium, although the difference observed did not attain a level of statistical significance. Suzuki et al also described that their concurrent test with a fivefold increase in dietary magnesium intake (i.e., 0.25% as against 0.05%) demonstrated a marked increase in urinary calcium excretion; hence, a better calcium absorption in the group fed on high-magnesium (0.25%) diet.

The present study was conducted under conditions with a higher rate of magnesium content (6-fold difference) as compared to the above two laboratory studies of Suzuki et al, viz. a magnesium content of 36mg (0.3%) per 12-g coral-added cracker versus a magnesium content of 6 mg (0.05% ) per 12-g calcium carbonate-added cracker.

While Suzuki et al have given no account of the high efficiency of calcium utilization from coral in their article, it would be reasonable to assume that the high magnesium content has some bearing upon the intestinal absorption of calcium when viewed together with consideration of the present human trial data. However, it is of importance to mention that problems such as coral calcium solubility in gastric acid, absorption from the intestine and reabsorption from the renal tubules per se should be discussed. Additionally, the potential involvement of magnesium and further basic studies are needed.

The present data demonstrating the remarkably good absorption of calcium from coral containing calcium and magnesium in a ratio of 2-to-1 are of profound interest.

Study Authors: Kunihiko ISHITANI, Eiko ITAKURA, Shiro GOTO and Takatoshi ESASHI. Higashi Sapporo Hospital, Sapporo 003-8585, Japan.

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