Strontium balance in breast-fed babies

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Abstract
The Ca and Sr intakes and the urinary and fecal outputs have been measured over 3 days in 12 babies from the 6th day after birth and in 3 babies who were 6 wk. old. The mean Sr:Ca ratio in the human milk received by the younger group of babies was 0.24 [plus or minus] 0.02 x 10-3 and in the urine of the newborn 1.16 [plus or minus] 0.16 x 10-3. For the babies 6 wk. old, this milk ratio was 0.26 x 10-3 and that for urine 0.58 x 10-3. The mean negative balance for Sr was - 1.7 [mu]g/kg day for babies between 6 and 8 days old, although the Ca balance was always positive. The 6-wk.-old babies were in positive balance for Sr as well as for Ca. The negative balance for Sr was smaller than that obtained in an earlier study. Improvements in the assay of Sr, particularly in urine, are described and make the new results more reliable. The negative Sr balance observed in young babies is not inconsistent with the results obtained for the Sr:Ca ratios in bones from stillborn babies or those dying in the 1st few days after birth. A possible reason for a high urinary output of Sr in the newborn is discussed.