Mineral balance of rats fed on diets containing sweet lupin (Lupinus angustifolius L.) or its fractions

Abstract

Metabolic balance studies were conducted in growing rats to investigate the effect of Lupinus angustifolius (sweet lupin) seed meal and its fractions on the balance of calcium, phosphorus, magnesium, potassium, sodium and apparent absorption of zinc. Diets contained: unsupplemented lupin seed meal (LMU); lupin seed meal, fully supplemented with essential amino acids to fulfil the target requirements for growing rats, at 360 g kg−1 diet (LMFS); aqueous non-dialysed and soluble protein fraction (LPAND); aqueous lupin seed meal extract, dialysed and soluble at pH 7.0, at 148 g kg−1 (LPAD); aqueous extract, dialysed and insoluble at pH 7.0, at 124 g kg−1 (LPADI); buffer dialysed and soluble at pH 7.0, at 136 g kg−1 (BUSOL); buffer dialysed and insoluble at pH 7.0 at 119 g kg−1 (BUDI); lupin meal residue after aqueous and buffer extraction, at 170 g kg−1 diet (LMR). Rats were pair-fed for 10 days with the above diets which had been supplemented with essential amino acids up to the target requirements for growing rats. As the diets differed in mineral composition, an increase in intake resulted in higher apparent absorption and faecal excretion. The inclusion of LMU and its protein and fibre (LMR) fractions in the diet had significant effects on the mineral content of the faeces and on mineral balance. The relatively low concentration of phytates in seed meal and the LPAND fraction (5.3 and 3.5 g kg−1, respectively) and its absence in LPAD, LPADI, BUSOL, BUDI and LMR fractions could not adequately explain the increased mineral excretion. The results suggest that lupin protein is involved in the elevated loss of minerals associated with long-term effects of this seed as a protein source in non-ruminants. The fibre fraction (LMR) was regarded as a modest metal binder compared with the proteins and may be used as a new source of dietary fibre; however, further work is warranted.