Effect of indigestible polysaccharides on protein digestibility and nitrogen retention in growing rats

The effect of five indigestible polysaccharides (IP) of different origin and chemical structure on N balance and dry matter digestibility was studied in growing rats. The diets contained 10% of gum guar (GG), carob bean gum (CBG), Na-alginate (Na-A), agar-agar (AA) or carrageenan (C), respectively. Dry matter digestibility and apparent protein digestibility were significantly decreased below control values by all 5 IP. N retention was significantly lower after ingestion of AA or C. In the rats receiving GG, CBG or Na-A urinary N excretion was reduced, thus compensating for increased fecal N losses by these animals. The different effects of the 5 substances on N balance are attributed to their partial breakdown through the action of intestinal microflora and/or their ability to partially inhibit the activity of proteolytic enzymes in the gastrointestinal tract. Trypsin inhibition in vitro was observed for C only. Suitability of the various IP as bulking agents in reducing diets is discussed.