NUTRITIONAL-EVALUATION OF SWEET LUPIN (LUPINUS-ANGUSTIFOLIUS) - NET PROTEIN-UTILIZATION (NPU), NITROGEN-BALANCE AND FRACTIONATION STUDIES

The effects of raw sweet lupin (Lupinus angustifolius) meal and its fr actions on the growth and N utilization of rats were determined in two NPU and five N balance experiments. Sweet lupinseed grown in Western Australia, obtained as meal, unsupplemented (LMU), or fully supplement ed with required amino acids (360 g/kg diet) (LMFS) was tested. In add ition, six fractions were tested: aqueous non-dialysed extract at pH 7 .0 (LPAND), dialysed extracts soluble (LPAD) and insoluble at pH 7.0 ( LPADI), buffer-soluble extract at pH 7.0 (BUSOL), buffer-insoluble ext ract after dialysis at pH 7.0 (BUDI) and the residue (LMR) containing most of the material from meal insoluble in water and phosphate-citrat e buffer. All diets based on fractions contained the same amounts of e nergy and protein and were supplemented with amino acids, vitamins and minerals to target requirements. Body N and Lipid contents of rats fe d on LMU and LMFS were reduced significantly in comparison with rats f ed on positive lactalbumin (LACT) and non-protein diets (NPC) as negat ive controls. This was due in part to the lower retention of the absor bed N. As a result, the NPU and the biological value (BV) of sweet lup inseed proteins were less than expected. Urea-N outputs of the LMU- an d LMFS-fed rats were also elevated. In contrast, true N and DM digesti bilities of rats fed on LMU and LMFS were not significantly affected b y the difference in the energy content of the diet. The replacement of lactalbumin in the diet with LPAND (196 g/kg), LPAD (148 g/kg), LPADI (124 g/kg), BUSOL (136 g/kg) or BUDI (119 g/kg) reduced dry body weig ht, N and lipid contents, NPU and BV compared with those obtained from the LACT control, even though the N and DM digestibilities were not s ignificantly different. Inclusion of the residue fraction (170 g LMR/k g) had no apparent effect on any of the variables studied. Since sweet lupinseed had a small amount of nonreactive lectin and LMR had some u ndesirable side-effects in these rats, it appears that the low nutriti onal value of LMFS for rats (NPU 0.62) despite the very high level of digestibility of its N, results from disturbances in N metabolism, and particularly from the low retention value of the absorbed N.