To The Editor:
I wish to comment on some aspects of the section on calcium supplementation that appeared in the recently published Instructional Course Lecture "Osteoporosis: Management and Treatment Strategies for Orthopaedic Surgeons" (2008;90:1362-74) by Gehrig et al. After correctly pointing out the importance of calcium intake and the likely need for the use of supplements, the paragraph proceeds to make some misstatements with respect to calcium absorption from the carbonate salt.
Several different investigators have shown that acid production is not necessary for calcium absorption from salts such as calcium carbonate or calcium phosphate, as long as the salt is coingested with food (which is how calcium supplements should normally be taken)1,2. This has been demonstrated by such diverse methods as testing in patients with pentagastrin-resistant achlorhydria1 and testing in normal volunteers in whom the gastric pH was maintained at 7.4 by in vivo titration2.
Second, the incidence of kidney stones is reduced with use of all forms of calcium, citrate as well as the carbonate salts. The reason is not, as the paragraph states, because the citrate ion binds to oxalate in the gut; it does not. Rather, it is the calcium ion that binds to oxalate, and, so far as is known, the citrate and carbonate salts of calcium function equivalently in this respect. Large doses of calcium carbonate have long been part of the standard treatment for the syndrome of intestinal hyperoxalosis, in which the salt is used to reduce kidney calcification.
Both carbonate and citrate salts are good products, as long as they come from reputable manufacturers. The Creighton Osteoporosis Center prescribes both salts for their patients. It is good to have options, and it is not correct to assert that one salt is intrinsically better than the other, as Gehrig et al. contend. If anything, the scales might tip in favor of calcium carbonate because only half as many pills must be taken per day, a factor that improves patient compliance.