APPARENT ABSORPTION OF CA, P AND ZN, AND TRUE ABSORPTION OF ZN-65 IN RATS FED DIETS CONTAINING LUPIN (LUPINUS-ANGUSTIFOLIUS) SEED MEAL OR ITS FRACTIONS

The effects of lupin (Lupinus angustifolius, cultivar 'Unicrop') seed meal or some of its fractions on the apparent and true absorption of m inerals were studied in three experiments. Groups of four growing rats were pair-fed for 10 days (Experiments 1 and 2) or for 14 days (Exper iment 3) on diets which contained the same amounts of energy and prote in, and supplemented with amino acids and minerals to target requireme nts. In Experiment 1, the apparent absorption of Ca, P and Zn was comp ared in animals fed a control (lactalbumin-based, LA) and a lupin meal (LM) diet. In addition, three lupin fractions were tested, two protei n fractions (aqueous dialysed, soluble (ADS); aqueous dialysed. insolu ble (ADI)) and a residue (LR) containing most of the material from the meal insoluble in water and phosphate-buffered saline (PBS). In Exper iment 2, the true absorption of Zn-65 was determined in three groups o f rats given the following diets: LA (control), LM (lupin meal diet) a nd LR (diet containing the insoluble material). The inclusion of lupin protein fractions (ADS and ADI) in the diet had no significant effect on the apparent absorption of Ca, P or Zn. The apparent absorption of Ca was not affected by the inclusion in the diet of either lupin seed meal or its fractions but the apparent absorption of P was significan tly lower in rats fed whole lupin meal diets (50.8%) than in those giv en the control diet (63.7%). The apparent absorption of Zn was also si gnificantly lower in rats fed diets containing lupin meal (5.9%) or th e insoluble residue (6.5%) compared with that of control animals (26.8 %). The true absorption of Zn-65 was decreased in animals fed on diets containing lupin meal (22.2%) or insoluble lupin residue (30.9%) in c omparison with that of controls (41.9%). Growth was significantly reta rded in rats given a lupin based diet with limited Zn supply (12 mg Zn kg-1) compared with those given a lupin diet containing 40 mg Zn kg-1 (Experiment 3). The results suggest that insoluble non-starch polysac charides and phytate are mainly responsible for the lower availability of Zn in rats fed diets containing lupin meal as the only source of p rotein. Furthermore, growth can be compromised in animals fed on lupin -based diets when the total Zn supply of the diet is limiting.