The effect of diet on retention by the rat of iron from a radiolabeled casein test meal

Previous work has shown that weanling rats fed a soy protein isolate (SPI)-based diet retained less iron from a casein test meal than did rats fed a casein-based diet. In the present work this diet effect was further investigated. In experiment 1 rats were fed either SPI- or casein-based diet (about 25 ppm Fe) for 13 d, fasted overnight, given a ⁵⁹Fe-labeled casein-based test meal, and refed either the same or the other diet for 7 d. ⁵⁹Fe retention was determined by the percent of ingested label retained after 7 d. SPI reduced ⁵⁹Fe retention when fed prior to the test meal (52.5 vs. 61.5%, P < 0.001) and also when fed following the test meal (50.8 vs. 63.2%, P < 0.001). In experiment 2a, exposure of SPI to steam at 108°C for 30 min did not lessen the observed diet effect. In experiment 2b rats were fed either SPI- or casein-based diet, and casein test meals were fed as above on days 14, 35 and 56. The SPI-based diet reduced ⁵⁹Fe retention from the casein test meal fed on d 14 (58.2 vs. 68.0%, P < 0.005), for the meal on d 35 there was no difference, and for the final test meal rats fed SPI retained more ⁵⁹Fe (53.0 vs. 40.2%, p < 0.01). In experiment 3 lactalbumin was shown to decrease ⁵⁹Fe retention to the same extent as SPI (54.9 and 53.4%) as compared to casein (73.3%), whereas neither rapeseed protein concentrate (68.8%) nor oxalic acid (72.2%) had any significant adverse effect. Clearly, protein source, duration of feeding, and diet before and after the test meal affect iron retention.