Milk and calcium prevent gastrointestinal absorption and urinary excretion of oxalate in rats

Dietary oxalate plays a very important role in the formation of calcium oxalate stones, and dietary intake of calcium may decrease oxalate absorption and its subsequent urinary excretion. The purpose of the present study was to determine the effect on urinary oxalate excretion of an acute oral calcium load, standard milk, or high-calcium & low-fat milk followed by a dose of oxalic acid. Male Wistar rats weighing 180-200 g were divided into 7 groups of 6 rats each. All animals were fasted for about 24 hours, anesthetized, and hydrated with normal saline at 3-4 mL/hour. Then the animals were given 1 mL of normal saline {Control}, 10 mg (111.1 mumol) of oxalic acid {Ox alone}, 2 mL of standard milk (calcium: 1.16 mg or 29 mu mol/mL) {NCa milk}, 2 mL of high-calcium & low-fat milk (calcium: 2.05 mg or 51.3 mumol/mL) {HCa milk}, equimolar calcium (4.44 mg or 111 mumol) followed by 10 mg of oxalic acid {Ca + Ox}, 2 mL of high-calcium & low-fat milk followed by 10 mg of oxalic acid {HCa milk + Ox}, or 2 mL of standard milk followed by 10 mg of oxalic acid {NCa milk + Ox}. All treatments were administered via a gastrostomy. Urine samples were collected by bladder puncture just before administration and at hourly intervals up to 5 hours afterwards. Urinary oxalate was measured by capillary electrophoresis, while urinary calcium, magnesium and phosphorus were measured by inductively coupled plasma spectrometry. Urinary oxalate excretion peaked at 1 hour in the Ox alone group, while it peaked at 2 or 3 hours in the Ca + Ox, HCa milk + Ox, and NCa milk + Ox groups. Urinary oxalate excretion decreased significantly when 10 mg of oxalate was administered immediately after the administration of equimolar calcium, high-calcium & low-fat milk, or standard milk. The cumulative urinary oxalate excretion over 5 hours was approximately 13.6%, 3.5%, 1.6%, and 2.4% in the Ox alone, Ca + Ox, HCa milk + Ox, and NCa milk + Ox groups, respectively. In conclusions, this study demonstrated that calcium salt, or dairy products containing calcium (especially high-calcium & low-fat milk) could decrease the gastrointestinal absorption and subsequent urinary excretion of oxalate.