Urinary response to an oxalic acid load is influenced by the timing of calcium loading in rats

Dietary intake of calcium or dairy products has been shown to decrease urinary oxalate excretion by limiting its intestinal absorption. However, not enough attention has been given to whether there is any benefit from altering the schedule of ingesting calcium and oxalate. Therefore, we investigated the effects of changes in the timing of calcium and oxalate loading on urinary oxalate excretion.

Male Wistar rats weighing 180 to 200 gm were fasted and randomized into several groups. They were then administered normal saline or oxalic acid with or without calcium or milk. Calcium or milk was given immediately, or 5, 10, 15 or 30 minutes before or after the oxalate load. All treatments were given via gastrostomy. Urine samples were collected by bladder puncture just before administration and at hourly intervals up to 5 hours afterward. Urinary oxalate was measured by capillary electrophoresis.

Urinary oxalate increased after the administration of oxalate alone, while it decreased when oxalate was combined with calcium or milk. Urinary oxalate showed a smaller increment when calcium or milk was given before than after oxalate loading, and it was much smaller when calcium or milk was given immediately before oxalate.

Prior calcium loading appears to have a positive influence on decreasing oxalic acid absorption from the intestinal tract. Therefore, calcium or dairy products should always be ingested before a meal rich in oxalate to prevent oxalate absorption and decrease urinary oxalate excretion.