Inhibition of calcium absorption in ponies fed diets containing oxalic acid

The effects of dietary oxalic acid on absorption, excretion, and retention of calcium was determined in Shetland ponies in 2 experiments. In each experiment, the results of feeding a control diet consisting of oats, molasses, and a complete pelleted horse ration were compared with those of feeding the control diet plus 1% oxalic acid. The diets contained 0.6% calcium in the 1st experiment and 0.45% calcium in the 2nd experiment. Oxalic acid increased the fecal excretion of calcium and decreased calcium absorption. The endogenous fecal excretion of calcium determined in the 1st experiment, using 47Ca, was not affected by dietary oxalic acid, indicating that the increase in fecal calcium was due to unabsorbed dietary calcium. Ponies fed 1% oxalic acid in diets containing 0.45% calcium were in negative calcium balance. The ponies did not conserve calcium by reducing the urinary calcium excretion if they were fed oxalic acid. Magnesium balance was measured in the 3 ponies in the 1st experiment. Urinary magnesium excretion increased and magnesium retention decreased in response to oxalic acid intake.