Calcium and zinc balances during consumption of high and low oxalate-containing vegetables

Abstract

Three, 4-wk controlled feeding periods were used to assess calcium and zinc balance, and urinary hydroxyproline during consumption of a baseline diet, and the baseline diet with the addition of either a high-oxalate (spinach) or a low-oxalate vegetable (broccoli) in 8 premenopausal women. Calcium intake increased approximately linearly from baseline (795 mg) to the broccoli diet (905 mg) to the spinach diet (997 mg) and the corresponding mean calcium balances were 18, 19, and −65 mg/day, respectively. Mean apparent calcium absorption was significantly lower (p<0.01) for the spinach compared to the other dietary periods whereas no significant differences were observed in apparent zinc absorption between treatments. The finding that the addition of spinach was associated with a lowering of calcium balance in 6 of the 8 subjects, by an average of 124 mg/day, suggested that the utilization of calcium from other dietary sources may be impaired by the simultaneous ingestion of spinach. A significantly greater (p<0.05) urinary hydroxyproline/creatinine ratio during spinach ingestion supported the possibility of a higher rate of bone resorption during this period. Similar to the findings for calcium, neither the addition of spinach or broccoli to the baseline diet was associated with a significant improvement in zinc balance.