Low-calcium diet

Elizabeth Sellers and associates1 write about the adaptation of Inuit children to a low-calcium diet. Contemporary humans evolved in an equatorial environment, and there can be little doubt that populations living under radically different conditions have had to adapt in substantial ways. Nevertheless, 3 important errors in this article need clarification if we are to gain any insight into the character of the adaptation, at least with respect to calcium.

First, the magnitude of urinary calcium excretion, expressed in this paper as fractional micromoles per mole creatinine, is incorrect by 6 orders of magnitude. As reported by Sellers and associates,1 the urine of these children would have contained less calcium than distilled water. This might be taken as an indication of the adaptation the authors are seeking to define, except that the values reported are considered either at or above age-specific normal values in all of the 10 children studied. Therefore, the units for this test result are incorrect.

Second, the authors seem to have misinterpreted the data from the reference by Kuhnlein and colleagues2 when they state “With a traditional diet, Inuit children in northern Canada ingest only 20 mg of elemental calcium per day.” In the article concerned, traditional foods, providing 21 mg calcium daily (not the 20 mg cited), constituted only 17% of the total energy intake of the Inuit children studied. Had total energy intake come from traditional foods, total calcium intake would have been at least 120 mg/day. That is still not very much, but it is not safe to extrapolate from such a small proportion of the diet, since deriving total energy from traditional foods might well have involved a change in food types. This is strongly suggested by the standard deviation around the 21-mg average reported by Kuhnlein and colleagues,2 which was 400 mg. Thus, the intake data were severely skewed to the right, indicating that some of the children must have been getting 1000 mg calcium or more from traditional foods. Given these uncertainties, the article by Kuhnlein and colleagues2 provides no useful information about the calcium content of diets based completely on traditional foods.

The third error relates to the uncritical assumption that any adaptation at all would suffice to build an adult skeleton with a daily intake as low as the 20-mg figure mentioned by Sellers and associates.1 If all 20 mg could be absorbed and retained, and if dermal and excretory losses could be reduced to zero (both impossible conditions), total skeletal accumulation from birth to age 16 would produce a skeleton containing less than 120 g calcium. Thus, the premise that adaptation must be possible for such an intake is untenable. Whatever the basis for the error, the authors should have realized than any intake estimate as low as the one cited had to be incorrect.

Robert P. Heaney Creighton University Omaha, Neb.