Improvement in the nutritional quality of Bangladeshi rice-based diets is sought through increasing the amounts of vegetables, fish and legumes consumed. The aim of the present study was to compare the effects of rice-based diets on selected parameters of Ca and P availability and retention in young, growing rats. The study was designed as a randomised balance trial with five diets, eight animals per diet, and two balance periods of 1 week each. Apart from diet 1, which was a pure rice diet, the other four diets were composite diets and included the leafy vegetable, amaranth leaves (Amaranthus gangeticus), the small fish, mola (Amblypharyngodon mola), and lentils (Lens culinaris) in different amounts to simulate the average rural rice-based diet, the recommended diet, the recommended diet diluted with starch, and the recommended diet excluding amaranth leaves. The inclusion of amaranth leaves, mola and lentils significantly improved N and growth retention in the rats compared with the pure rice diet. However, a minor addition of amaranth (0·66 g/100 g DM) significantly reduced the fractional Ca absorption and retention. Femur bone mass and Ca and P densities were significantly lower in the rats fed the diets that included amaranth leaves. The observed inhibitory effect of the amaranth leaves on Ca absorption and utilisation was probably due to the oxalate content. It is concluded that the formulation of a recommended diet cannot be based only on nutrient content values of individual food components due to interactions between nutrients and anti-nutrients in the diet.

The bioavailability of intrinsic minerals in rapeseed (Brassica napus L.) meal was studied in growing, fistulated pigs. Furthermore, the effect on mineral availability of an extrinsic Ca supply to this diet, high in phytate, was observed. Comparisons between small intestinal and total gastrointestinal absorption were accomplished for minerals and other dietary components. N retention increased as the dietary Ca intake increased, but dropped, in general significantly, throughout the experimental period, indicating that factors other than protein were limiting for growth. The highest dietary Ca level increased the absorption and retention of Ca and Mg. In contrast, addition of CaCO3 did not influence the apparent absorption and retention of P, Fe, Zn, Cu and Mn. The majority of observed blood variables was not affected by the Ca content of the diet. Zn status-related variables were, however, thoroughly depressed by duration of the experiment and it seems probable that the amount of absorbed Zn was a factor limiting growth. Total gastrointestinal balances showed a Zn retention of 4.7 mg/d, which accounted for 13.6 % of that ingested. This low absorption of Zn may have been due to the high intrinsic content of phytate. Apparent absorption of organic matter, ash, N and S was significantly greater calculated from faecal contents than from ileal contents, indicating a disappearance of material in the caecum–colon section of the gastrointestinal tract. The minerals which were studied showed the opposite pattern: apparent small intestinal absorption was larger than total intestinal absorption, suggesting that the caecal–colon region takes part in mineral turnover.

The milk components lactose and casein enhance the apparent absorption of magnesium and possibly also of calcium, whereas phytate, which occurs in soya-bean products, has an inhibitory effect. This implies that soya-bean beverage v. cow's milk could lower bioavailability of Mg and Ca. This hypothesis was tested in two experiments with growing rats. Feeding soya-bean beverage v. cow's milk consistently lowered body-weight gain, enhanced bone turnover, as measured by increased plasma alkaline phosphatase (EC 3.1.3.1) activity and increased urinary hydroxyproline excretion, and decreased Mg and Ca concentrations in the femur. Because the mineral compositions of the soya-bean beverage and the cow's milk were different, the intake of Mg was higher in rats fed on soya-bean beverage, whereas that of Ca was higher in rats fed on cow's milk. Supplementation of the soya-bean beverage either with phosphorus and Ca or with P, Ca and methionine, to concentrations identical to those in milk, restored growth and bone mineralization. When using diets carefully balanced for Mg, Ca, P, sodium, potassium and methionine, soya-bean beverage v. cow's milk in the diets decreased apparent absorption and urinary excretion of Mg and Ca. Hydrolysis of lactose in milk decreased absorption and urinary excretion of Mg; it did not significantly affect Ca absorption but lowered urinary Ca excretion. The present study shows that soya-bean beverage v. milk depresses Mg and Ca bioavailability, as would be predicted on the basis of reported effects of their purified components.