MECHANISM EXPLAINING WHY DIETARY SOYA PROTEIN VS. SKIM-MILK PROTEIN LOWERS FAT DIGESTION IN VEAL CALVES

The hypothesis was tested that, in veal calves, dietary soya protein v s. skim-milk protein withdraws bile acids from the process of fat dige stion in the intestine, leading to enhanced faecal excretion of bile a cids and reduced fat digestibility. Veal calves were fed control milk replacers containing milk proteins or test milk replacers containing s oybean protein. The finisher control diet contained milk protein only, of which 64% was casein. The finisher test diet contained milk protei ns, but not casein, and also a soya protein preparation, the preparati on 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 f ed soya protein, however this lowered body-weight gain did not reach s tatistical significance. Faeces were collected during week 15 of the t rial. The test diet with soya protein vs. the control diet with skim-m ilk 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 sig nificantly reduced by the feeding of the diet with soya protein instea d of that with skim-milk protein. In the light of earlier work in rabb its, it is proposed that in veal calves the feeding of soya protein, b ecause of its negligible degree of phosphorylation, raises the amount of insoluble calcium phosphate in the intestine lumen. As a result, mo re bile acids are bound so that less bile acids an available for fat d igestion and reabsorption of bile acids is depressed. This study may b e useful in attempts to improve the nutritional value of milk replacer s containing soya protein preparations. (C) 1997 Elsevier Science B.V.