Mechanism explaining why dietary soya protein vs. skim-milk protein lowers fat digestion in veal calves

Abstract

The hypothesis was tested that, in veal calves, dietary soya protein vs. skim-milk protein withdraws bile acids from the process of fat digestion in the intestine, leading to enhanced faecal excretion of bile acids and reduced fat digestibility. Veal calves were fed control milk replacers containing milk proteins or test milk replacers containing soybean protein. The finisher control diet contained milk protein only, of which 64% was casein. The finisher test diet contained milk proteins, but not casein, and also a soya protein preparation, the preparation comprising 50% of total protein. After 26 weeks of feeding the milk replacers, body-weight gain was on average 3 kg lower in the calves fed soya protein, however this lowered body-weight gain did not reach statistical significance. Faeces were collected during week 15 of the trial. The test diet with soya protein vs. the control diet with skim-milk protein significantly reduced fat digestibility from 89.8 to 86.7% of intake. The feeding of soya protein produced an almost three-fold rise in bile acid excretion. Phosphate and calcium absorption were significantly reduced by the feeding of the diet with soya protein instead of that with skim-milk protein. In the light of earlier work in rabbits, it is proposed that in veal calves the feeding of soya protein, because of its negligible degree of phosphorylation, raises the amount of insoluble calcium phosphate in the intestine lumen. As a result, more bile acids are bound so that less bile acids are available for fat digestion and reabsorption of bile acids is depressed. This study may be useful in attempts to improve the nutritional value of milk replacers containing soya protein preparations.