Effect of calcium, copper, and zinc levels in a rapeseed meal diet on mineral and trace element utilization in the rat.

Mineral and trace element interactions were studied in a balance trial with rats. Calcium, copper, and zinc were supplied to a rapeseed meal diet in a factorial design. Animals were fed ad libitum, and absorption, excretion, and retention of the elements were evaluated either as fractions of total intake or in relation to nitrogen retention to account for differences in food intake and lean body mass increment. The intrinsic content of minerals and trace elements was sufficient to support growth at a rate that could be expected from the rapeseed protein quality. However, when calcium was included in the diet, the intrinsic dietary level of zinc appeared to be limiting, despite the fact that the zinc level was twice the recommended level. Additional zinc supply reversed growth impairment. This calcium-zinc interaction is believed to be owing to the formation of phytate complexes. Calcium addition influenced the calcium, phosphorus, magnesium, zinc, and iron--but not the copper--balances. The addition of calcium reduced the availability of the intrinsic zinc, whereas no effect was seen in the zinc-fortified groups. The availability of intrinsic copper was in a similar way significantly impaired by addition of dietary zinc, whereas copper-supplied groups were unaffected by zinc addition. Intrinsic iron availability was also dependent upon zinc addition, although in a more ambiguous way. Thus, addition of extrinsic minerals to a diet high in phytate can result in significant impairments of growth and mineral utilization.