Effects of dietary zinc levels, phytic acid and resistant starch on zinc bioavailability in rats

Summary Background Owing to its fermentability, it has been advocated that resistant starch (RS) has a positive effect on the absorption of minerals by increasing their solubility in the hindgut. In marginally zinc–deficient rats, the enhancement of zinc bioavailability by RS occurs mostly when the diet contains phytic acid. Aim of the study This study aims to investigate the effect of dietary zinc level and phytic acid on the cecal zinc pools and zinc bioavailability of rats fed RS. Methods Wistar rats (male, 3wk old) were divided into eight groups (n = 6), and fed diets containing either 5% cellulose (control fiber: insoluble and low fermentable) or 20 % RS (test fiber: soluble and fermentable), with or without the addition of 1% sodium phytate, at the 10 and 30 mg/kg dietary zinc levels, for 21 days. Results At 10 mg Zn/kg, RS increased femur zinc concentration only in the group receiving the phytate–containing diet, while at 30 mg Zn/kg it increased femur zinc concentration in rats fed both phytate–free and phytate–containing diets. The total content of zinc in the cecum was increased by the higher dietary zinc level and tended to be increased by the addition of phytate, which is assumed to impair zinc absorption in the small intestine. Feeding RS lowered cecal pH values, which correlated with increasing values of zinc solubility (r = –0.3471; P < 0.05). The later was, in turn, directly associated with zinc apparent absorption (r = 0.3739; P < 0.05). Conclusions The increase in zinc bioavailability by RS occurs when dietary zinc levels are adequate and/or zinc absorption is impaired in the small intestine, increasing the influx of unabsorbed zinc into the cecum and favoring the increase of zinc bioavailability when RS fermentation lowers the cecal pH.     

Enhancement of Cu bioavailability in the rat by phytic acid

The objective of this study was to evaluate the effect of phytic acid on copper (Cu) bioavailability. Male weanling rats were fed a Cu-deficient diet (less than 1.0 micrograms/g) for 4 wk and then were divided into 12 groups (n = 8) in a factorial design. Cu-deficient rats were then fed diets containing 1.4, 3.0, 5.2 or 10.5 micrograms Cu/g (CuCO3) and 0, 0.4 or 0.8% phytic acid as sodium phytate at each Cu level. All diets contained 30 micrograms Zn/g. After 3 d of Cu repletion, liver copper (LCu), liver Cu-Zn superoxide dismutase (LSOD) activity, serum Cu (SCu) and serum ceruloplasmin (CP) concentrations were determined. These parameters were used as indexes of Cu bioavailability. The addition of phytic acid to diets fed to Cu-deficient rats significantly enhanced Cu bioavailability compared to that of rats fed diets without phytic acid. Coefficients of determination (r2) were calculated for each response parameter versus dietary Cu concentration. The r2-values for pooled LCu and LSOD values were 0.31 and 0.30, respectively, between 1.4 and 5.2 micrograms Cu/g. At low dietary Cu concentrations, liver Cu parameters (i.e., LCu and LSOD) were more responsive indexes of Cu status than SCu and CP. Each index of Cu status was found to correlate with the other indexes of Cu nutriture. Phytic acid is postulated to enhance Cu utilization by its ability to bind other dietary components, such as Zn, that compete with Cu at the site of intestinal absorption.     

Fermentation of soybean flour with Aspergillus usamii improves availabilities of zinc and iron in rats

Soybean flour was fermented with Aspergillus usamii to improve the availabilities of dietary zinc and iron through the degradation of phytate. Three kinds of experimental diets that differed in protein sources were prepared: one consisting of 40% fermented soybean flour (RS diet), one consisting of 40% fermented soybean flour (FS diet), and one consisting of 20% regular soybean flour and 20% fermented soybean flour (RF diet). Zinc solubilities in the upper and the lower segments of the small intestine were higher in rats fed the FS diet than in rats fed the RS diet. The FS group showed higher solubility of iron in the lower small intestine than the RS group did. Zinc concentrations in the femur and plasma and iron concentrations in the liver and plasma were higher in the FS group than in the RS group. These results suggested that the fermentation of soybean flour improved the availabilities of dietary zinc and iron, which may be induced by increasing the solubilities of these minerals in the small intestine through the reduction of phytate content. Femoral and plasma zinc concentrations in the RF group were higher than in the RS group, but lower than in the FS group. No difference was noted in liver and plasma iron concentrations between the RF group and the FS group. Although phytase activity in FS degrades phytate in the RF diet, higher activity may be needed to degrade phytate completely.     

Inhibitory effects of phytic acid and other inositol phosphates on zinc and calcium absorption in suckling rats

While it is known that phytic acid, inositol hexaphosphate, has a negative effect on zinc and calcium absorption, the effects of inositol which is phosphorylated to a lesser extent are less known. We have prepared inositol triphosphate (IP-3), tetraphosphate (IP-4), pentaphosphate (IP-5) and hexaphosphate (IP-6) by hydrolysis of sodium phytate and separation by ion-exchange chromatography and have studied their effect on zinc and calcium absorption. Using a suckling rat pup model, we found that liver uptake of 65Zn after 6 h was 5% of the total dose from solutions of IP-6, 19% from IP-5, 28% from IP-4, 29% from IP-3 and 31% from ZnCl2 (control). Non-absorbed calcium was 17%, 1.4%, 0.5%, 0.5% and 0.5% of the given dose of 45Ca, respectively. Thus, at a high degree of phosphorylation (IP-6, IP-5), zinc and calcium uptake was inhibited, while no effect was observed for the other phosphates. Consequently, total "phytate" analysis, which includes inositol phosphates with varying degrees of phosphorylation, can give misleading information with regard to mineral availability. In addition, even limited dephosphorylation of inositol hexaphosphate can have a positive effect on mineral absorption.     

Studies on the bioavailability of zinc in man. III. Effects of ascorbic acid on zinc absorption

As ascorbic acid is known to enhance the absorption of dietary iron and to inhibit the absorption of dietary copper, studies were undertaken to examine the effect of ascorbic acid on the bioavailability of zinc in human subjects. The index of absorption was the change in plasma zinc concentration after a 110-mg aqueous dose of ZnSO4.7H2O (containing 25 mg of elemental zinc). Doses of 0.5, 1.0, and 2.0 g of ascorbic acid, representing a spectrum of Zn:ascorbic acid molar ratios from the dietary to the pharmacological range, failed to produce any significant change in the pattern of zinc absorption. Moreover, 2.0 g of ascorbic acid, equivalent to a Zn:ascorbic acid ratio of 0.145 failed to improve the absorption of 108 mg of elemental zinc incorporated into 120 g of black bean gruel. Ascorbic acid over a range of dosages commonly consumed by man had no demonstrable effect on the absorption of inorganic zinc.     

Degradation of phytic acid in cereal porridges improves iron absorption by human subjects

BACKGROUND: Phytic acid in cereal-based and legume-based complementary foods inhibits iron absorption. Low iron absorption from cereal porridges contributes to the high prevalence of iron deficiency in infants from developing countries. OBJECTIVE: The objective was to measure the influence of phytic acid degradation on iron absorption from cereal porridges. DESIGN: An exogenous phytase was used to fully degrade phytic acid during the manufacture of 9 roller-dried complementary foods based on rice, wheat, maize, oat, sorghum, and a wheat-soy blend. Iron absorption from the phytate-free and native phytate porridges prepared with water or milk (wheat only) was measured in adult humans with an extrinsic-label radioiron technique. Ascorbic acid was added to some porridges. RESULTS: When the foods were reconstituted with water, dephytinization increased iron absorption from rice porridge from 1.73% to 5.34% (P < 0.001), from oat from 0.33% to 2.79% (P < 0.0001), from maize from 1.80% to 8.92% (P < 0.0001), from wheat from 0.99% to 11.54% (P < 0.0001), from the wheat-soy blend without ascorbic acid from 1.15% to 3.75% (P < 0.005), and from the wheat-soy blend with ascorbic acid from 2.40% to 8.46% (P < 0.005). Reconstituting wheat porridge with milk instead of water markedly decreased or completely removed the enhancing effect of dephytinization on iron absorption in the presence and absence of ascorbic acid. Dephytinization did not increase iron absorption from high-tannin sorghum porridge reconstituted with water but increased iron absorption from low-tannin sorghum porridge by approximately 2-fold (P < 0.01). CONCLUSIONS: Phytate degradation improves iron absorption from cereal porridges prepared with water but not with milk, except from high-tannin sorghum.     

Assessment of the contents of phytic acid and divalent cations in low phytic acid (lpa) mutants of rice and soybean

The objective of the study was to investigate the influence of mutational breeding on the contents of nutritionally relevant minerals in low phytic acid (lpa) mutants compared to their wild-types. Three lpa rice (Os-lpa-XQZ-1, Os-lpa-XS110-1, Os-lpa-XS110-2) and two lpa soybean mutants (Gm-lpa-TW75-1, Gm-lpa-ZC-2) were analyzed regarding their contents of phytic acid, lower inositol phosphates and the minerals calcium, iron and zinc. Additionally, cadmium was included into the spectrum of divalent cations analyzed. The phytic acid reduction in lpa rice was consistently more pronounced in Os-lpa-XS110-1 than in Os-lpa-XQZ-1 and Os-lpa-XS110-2. However, only for Os-lpa-XQZ-1, levels of calcium, iron and zinc were found to be consistently increased, whereas the cadmium level was shown to be predominantly decreased compared to the wild-type. Investigation of the two lpa soybean mutants, characterized by absence (Gm-lpa-TW75-1) and accumulation (Gm-lpa-ZC-2) of lower inositol phosphates, respectively, revealed no consistently decreased or increased contents of calcium, iron, zinc and cadmium. The data do not change the view that lpa mutations do not result in systematic increases or decreases of mineral contents in these crops. However, on the basis of the molar ratios of phytate/minerals in lpa rice and soybean mutants which are considered as predictive parameters, an improved bioavailability of minerals in the lpa materials can be expected.     

Oral administration of soybean lecithin transphosphatidylated phosphatidylserine improves memory impairment in aged rats.

Soybean lecithin transphosphatidylated phosphatidylserine (SB-tPS) was prepared from soybean lecithin and L-serine by a transphosphatidylation reaction, and its effect on age-related memory impairment was evaluated in rats by the Morris water maze test. Continuous oral administration of SB-tPS (60 mg x kg(-1) x d(-1) for 60 d) to male aged rats (24-25 mo) significantly improved performance in the water maze escape test (P < 0.01 vs. control aged rats) similar to bovine brain cortex-derived phosphatidylserine, which restores cognitive function in patients with senile dementia. SB-tPS also increased acetylcholine release and the Na(+), K(+)-ATPase activity of the synaptosomes prepared from these aged rats to the level in young rats. The nootropic actions of SB-tPS in the present study can be partly explained by the changes in these biochemical activities.     

In vitro availability of zinc from infant foods with increasing phytic acid contents

An in vitro method was used to determine the availability of Zn from infant foods containing increasing amounts of phytate, and to quantify the effect of the phytate:Zn molar ratio on the availability. During the in vitro assay, digestive conditions of infants, younger and older than 4 months of age, were carefully simulated since the solubility of phytate-Zn complexes during digestion is pH dependent. Availability was measured with a continuous flow dialysis in vitro procedure with previous intralumen digestive stage. Zn concentrations were determined with flame atomic absorption spectrometry. Phytic acid content was measured with HPLC. Adding phytate to infant formula lowered Zn availability to 2.84 (sd 0.17) % when the phytate:Zn molar ratio increased to 2.2 as compared with cows' milk-based formula (6.65 (sd 0.55) %). Availability from vegetables (23.83 (sd 2.17) %) significantly decreased at a ratio > 7.9 (15.12 (sd 1.63) %). Zn availability from soyabean-based formula (2.26 (sd 0.36) %) was lower compared with cows' milk-based formula (6.65 (sd 0.55) %). Availability between soyabean- and cows' milk-based formula was similar when a phytate:Zn ratio of 2.2 (2.84 (sd 0.17) %) was obtained in the cows' milk formula. The negative effect of phytic acid on Zn availability was dependent on the type of the food and the phytate content, and should be considered when using soyabean-based formulas during early infancy.     

Dietary zinc improves and calcium depresses growth and zinc uptake in rats fed rice bran

Wistar rats were fed a basal stabilized-rice bran diet (B), and five additional ones, fortified to supply either 50 or 100% of zinc and calcium requirements, and a combination furnishing 100% of both minerals. Another diet supplemented with the complete mineral mix (MM) was also used. Results show that the low feed efficiency of B was maximized either by supplementation with MM, or by supplementing bran with 50% of the zinc requirement. Attempts to compensate for the low calcium content of bran by supplementation with 50 or 100% of the mineral requirement, did not increase either feed efficiency, growth or liver zinc. These data confirm that only about 11% of zinc contained in bran is nutritionally available and that supplementing with calcium is detrimental, indicating that this by-product is not a recommendable source of minerals.     

Effects of dietary phytic acid on lead and cadmium uptake and depletion in rats

Rats were given a semipurified diet supplemented with phytate (10 g/kg) or calcium (6 g/kg) and lead (200 mg/kg) or cadmium (5 mg/kg) for 4 weeks. Addition of phytate or calcium reduced the accumulation of lead in bone (P less than 0.001) and in blood and liver samples (P greater than 0.05). The greatest inhibition of tissue lead retention was evident when phytate and calcium were fed together. Cadmium accumulation was measured in the liver and kidney and was increased (P less than 0.05) by the addition of calcium. Phytate inhibited the increase in tissue cadmium promoted by calcium supplementation but did not otherwise influence tissue cadmium levels. In a further experiment, weanling rats were given diets supplemented with cadmium (5 mg/kg) or lead (200 mg/kg) for 4 weeks, and the accumulation of these elements in the body tissues was estimated in some animals. A phytate-supplemented (10 g/kg) or phytate-free semipurified diet (free of lead and cadmium) was then given to the remaining rats for 4 weeks. Phytate supplementation was found to have no significant effect on the rate of loss of lead or cadmium from tissues.     

Effect of dietary fiber on bioavailability of zinc and copper and histology in rats

Effects of prolonged use of dietary fiber on the absorption of zinc and copper were determined. In addition, differences in histopathology of small intestine, liver and kidney were evaluated. Four groups of rats were fed semipurified diets containing no fiber (C) 7.5% of either pectin (P,) guar meal (G.) or carrot (Ca). Radioisotopes with short half lives (^69mZn and ⁶⁴Cu) were used to measure absorption and endogenous losses of zinc and copper. Fiber did not adversely affect (P<.05) the absorption or endogenous excretion of either isotopes. At the end of the experimental period distribution of the absorbed zinc in gastrointestinal tract, liver and kidney was measured using ⁶⁵Zn as a tracer. The ⁶⁵Zn content of the intestinal tissues and liver was higher (P<.05) in the pectin group which also showed maximum incidence of blunting and fusion of the villi of distal ileum. The growth rate and weight of the rats on the pectin diet were significantly lower (P<.05) than on the control diet.     

Effects of ascorbic acid, phytic acid and tannic acid on iron bioavailability from reconstituted ferritin measured by an in vitro digestion-Caco-2 cell model

The effects of ascorbic acid (AA), phytate and tannic acid (TA) on Fe bioavailability from Fe supplied as reconstituted ferritin were compared with FeSO4 using an in vitro digestion-Caco-2 cell model. Horse spleen apoferritin was chemically reconstituted into an animal-type ferritin (HSF) and a plant-type ferritin (P-HSF) according to the typical ratios of Fe:P found in these molecules. In the presence of AA (Fe:AA molar ratio of 1:20), significantly more Fe was absorbed from FeSO4 (about 303 %), HSF (about 454 %) and P-HSF (about 371 %) when compared with ferrous sulfate or ferritin without AA. Phytic acid (PA; Fe:PA molar ratio of 1:20) significantly reduced Fe bioavailability from FeSO4 (about 86 %), HSF (about 82 %) and P-HSF (about 93 %) relative to FeSO4 and the ferritin controls. Treatment with TA (Fe:TA molar ratio of 1:1) significantly decreased Fe bioavailability (about 97 %) from both FeSO4 and the ferritin samples. AA was able to partially reverse the negative effect of PA (Fe:PA:AA molar ratio of 1:20:20) on Fe bioavailability but did not reverse the inhibiting effect of TA (Fe:TA:AA molar ratio of 1:1:20) on Fe bioavailability from ferritin and FeSO4. Overall, there were no significant differences in bioavailable Fe between P-HSF, HSF or FeSO4. Furthermore, the addition of AA (a known promoter) or the inhibitors, PA and TA, or both, did not result in significant differences in bioavailable Fe from ferritin relative to FeSO4. The results suggest that Fe in the reconstituted ferritin molecule is easily released during in vitro digestion and interacts with known promoters and inhibitors.     

Ingestion of water-soluble soybean fiber improves gastrectomy-induced calcium malabsorption and osteopenia in rats

OBJECTIVE: Total gastrectomy produces calcium malabsorption and osteopenia. We examined the effects of feeding water-soluble soybean fiber (WSSF), a highly fermentable dietary fiber with low viscosity, on calcium absorption and bone characteristics in totally gastrectomized rats. METHODS: Male Sprague-Dawley rats were assigned to two groups: sham operation and gastrectomy. After the postoperative recovery period, rats of each group were fed diets with or without WSSF (50 g/kg diet) for 4 wk. RESULTS: Net calcium absorption and femoral dry weight and calcium content and maximum breaking force were lower in the gastrectomy groups than in the sham groups. In the gastrectomy groups, calcium absorption and these femoral parameters were higher in rats fed the WSSF diet than in rats fed the WSSF-free diet. Feeding WSSF increased the concentrations of short-chain fatty acids and soluble calcium and decreased the pH in cecal contents. Net calcium absorption in gastrectomized rats correlated positively with the concentration of total short-chain fatty acids (the sum of acetic, propionic, and butyric acids) in cecal contents (r = 0.545, P < 0.05) and negatively with cecal pH (r = -0.628, P < 0.05). CONCLUSIONS: These findings indicate that ingestion of WSSF partly prevents the diminished calcium absorption after total gastrectomy in rats, resulting in the improvement of postgastrectomy osteopenia. The increase of calcium absorption may be associated with cecal fermentation of WSSF.     

Making bread with sourdough improves mineral bioavailability from reconstituted whole wheat flour in rats

OBJECTIVE: We compared the effects of different kinds of bread fermentation on mineral bioavailability. METHODS: Wistar rats were fed one of the following experimental diets for 21 d: control, reconstituted whole wheat flour (white flour plus bran), yeast bread, and sourdough bread. The apparent mineral absorption and intestinal fermentation were measured in each animal. RESULTS: Phytate contents in yeast and sourdough bread were lower than in reconstituted whole wheat flour (-52% and -71%, respectively). Total cecal pool of short-chain fatty acids, in particular the butyrate pool, was significantly increased by the ingestion of unrefined products. Calcium homeostasis was not modified by these nutritional conditions, whereas magnesium absorption was significantly greater in rats fed the control and sourdough diets than in those consuming whole wheat flour and yeast bread. Magnesium kidney excretion was slightly stimulated by sourdough bread. Compared with the control diet, iron balance was significantly reduced by reconstituted whole wheat flour diet. Yeast bread making counteracted the deleterious effects of whole wheat on iron absorption, whereas sourdough bread making enhanced iron absorption. Further, liver and plasma iron and transferrin saturation levels were lower in rats adapted to the flour diet than in other groups. Zinc absorption was strongly depressed in the presence of unprocessed reconstituted whole wheat flour in the diet, but yeast fermentation afforded a zinc assimilation comparable to the control diet, whereas the sourdough bread led to maximal zinc absorption. Copper absorption increased significantly when rats were fed the sourdough bread, whereas unprocessed whole flour depressed copper absorption (-41% versus control diet). CONCLUSION: Mineral bioavailability from reconstituted whole wheat flour can be improved by bread making. Although yeast fermentation minimizes the unfavorable effects of phytic acid, sourdough bread is a better source of available minerals, especially magnesium, iron, and zinc.     

Absorption of zinc from dietary casein phosphopeptide complex with zinc in rats given a soybean protein-based diet

The absorption of zinc (Zn) in the form of casein phosphopeptide (CPP) complex was compared with that of its chloride form in rats given a soybean protein-based diet. We prepared 67Zn complex with CPP (67Zn-CPP). Male rats were given a diet containing ZnCl2 in the preliminary period. After overnight food deprivation, they were fed a test meal labeled 67Zn-CPP or 67ZnCl2 (4 g Zn-free diet + 0.12 mg 67Zn) with 0.5 mg Dysprosium (Dy) as a fecal marker. Feces were collected for 5 d and analyzed for 67Zn isotopic enrichment and Dy concentration with an inductively coupled plasma mass spectrometer. Fecal Dy excretion closely paralleled that of 67Zn, and more than 89% of the administered Dy was recovered in the feces collected for 5 d. Moreover, the Zn source did not affect the excretion pattern of 67Zn. Since Dy and 67Zn were almost excreted within 3 d after the administration, the apparent absorption of 67Zn was calculated from the pooled data for 3 d. The source of Zn did not affect the apparent absorption of 67Zn. These results suggested that dietary Zn in the form of CPP complex did not show higher absorbability in rats.     

The anti-inflammatory action of fermented soybean products in kidney of high-fat-fed rats

Soybean has many compounds with a variety of biological properties that potentially benefit human health; among them, isoflavones have inhibitory effects on lipid oxidation in adipose tissue. In this study, we examined two Korean traditional fermented soybean products--doenjang (DNJ) and cheonggukjang (CGJ)--for their ability to suppress redox-sensitive nuclear factor κB (NF-κB) activation in the kidney of rats fed a high-fat diet. Sprague-Dawley rats, 4 weeks old, were fed soybean, DNJ, or CGJ (1?g/kg/day) with a 20% fat diet for 6 weeks. Body weight and food intake were carefully monitored. NF-κB-related activities of genes for inflammatory proteins, such as inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), and vascular cell adhesion molecule-1 (VCAM-1), were determined. The soybean products exhibited antioxidative action by maintaining redox regulation, suppressing NF-κB activation, and modulating the expression of genes for NF-κB-induced inflammatory proteins such as COX-2, iNOS, and VCAM-1. Based on these results, we conclude that Korean traditional soybean fermented products, especially CGJ, suppress the generation of reactive species, NF-κB activity, and NF-κB-related inflammatory genes.