Effect of rice hemicellulose and microcrystalline α-cellulose on selected minerals in the blood and feces of rats

In vitro mineral binding capacity of five fiber sources and their insoluble components for copper and zinc

Five fiber-rich food sources, wheat bran (WB), rice bran (RB), oat fiber (OF), apple fiber (AF), and tomato fiber (TF) and their isolated insoluble fiber fractions were evaluated in vitro for their binding capacity for zinc (Zn) and copper (Cu). Endogenous Zn concentrations of the fibers varied from 11.0 micrograms/g for OF to 136.0 micrograms/g for WB, whereas Cu concentrations ranged from 1.0 microgram/g for OF to 14.0 micrograms/g for WB. In all the fibers, total Cu bound was significantly higher than Zn. Total Cu bound ranged from 3687 micrograms/g for OF to 8019 micrograms/g and 8073 micrograms/g for WB and AF, whereas, bound Zn levels varied from 1213 micrograms/g for OF to 7121 micrograms/g and 7166 micrograms/g for WB and RB, respectively. Significantly more Zn and Cu were bound by the fiber fractions than the whole fibers, probably due to the exposure of more binding sites on the polymers during the fractionation process. Generally, the fiber components of all five fibers showed Cu and Zn binding capacities decreasing in the order; hemicellulose A > lignocellulose > lignin > cellulose. A strong correlation was seen between the combined effects of protein, hemicellulose, and lignin contents of the fibers versus total Zn binding capacity and a lesser correlation with Cu.

Effect of dietary rhubarb stalk fiber on the bioavailability of calcium in rats

Ground rhubarb stalk fiber containing, on a dry weight basis, 74% total dietary fiber (66% insoluble and 8% soluble) was prepared from rhubarb plants. This fiber source has been shown to have a pronounced lipid lowering effect in mice. Its high oxalic acid content may, however, be a limiting factor for its use. The present study was undertaken to determine the safe dose level of the fiber in terms of its effect on calcium bioavailability. Male weanling Sprague-Dawley rats were fed a semi-purified diet (AIN 76) for 4 weeks containing 0, 1, 3, or 5% rhubarb fiber, or pure oxalic acid equivalent to 5% rhubarb fiber. The fiber content of each diet was brought to 5% by adding cellulose, and the calcium level of all diets was the same. As the rhubarb stalk fiber content of the diet increased, a consistent trend of increasing total retention and apparent absorption of calcium was manifested. In contrast, however, cellulose, due to its increased effect on intestinal motility resulted in a decreased calcium bioavailability. Overall results suggest that an addition of rhubarb stalk powder up to the level of 5% in a diet does not have detrimental effect on calcium bioavailability.

Effects of dietary fiber and phytic acid on mineral availability

In general, it has been shown that dietary fiber may bind metallic cations in both in vitro and in vivo studies. However, there clearly are many unresolved questions on the effects of high-fiber diets on mineral availability. On one side, the effects of fiber on the utilization of nutrients vary greatly with the amount and type of fiber. In addition, there are many agents in both food and the digestive tract that may affect the mineral binding to fiber: some agents may inhibit binding, while others will enhance it. Also, there are several major difficulties in drawing conclusions from the in vitro and in vivo studies due to the different experimental conditions, methods used to follow the mineral balance, etc. Finally, it must be borne in mind that fiber and phytic acid occur together in fiber-rich diets and, thus, it is difficult to separate the effects of fiber and phytate in the utilization of most essential polyvalent metallic ions. The studies summarized in this review show that the recommendation for increasing dietary fiber in Western communities would not be expected to have any adverse effect on mineral absorption if we increase not only the intake of fiber, but also the dietary intake of other food components such as protein (both vegetable and animal protein) and ascorbic, citric, and oxalic acids (in fruits and vegetables). The adequate intake of minerals, fat, and simple sugars are maintained with this type of diet. The recommendations should be best interpreted in such a way as to prevent the consumption of excessive amounts of phytate, particularly for those whose mineral needs are great. Further studies are still needed in this field in order to understand the conflicting results published in the literature regarding the effects of fiber on the utilization of minerals; however, the studies reviewed in this article may give us an idea of the complexity of mineral availability in fiber-rich, phytate-rich diets.