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Nutrient Requirements

Selenium Bioavailability from Buckwheat Bran in Rats Fed a Modified AIN-93G Torula Yeast–Based Diet^1,2

U.S. Department of Agriculture, ARS, Grand Forks Human Nutrition Research Center, Grand Forks, ND 58203

³To whom correspondence should be addressed. E-mail: preeves{at}gfhnrc.ars.usda.gov.

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Selenium (Se) is an essential nutrient for humans and animals. The Se RDA for adult humans is 55 µg/d; however, dietary amounts as high as 200 µg/d in the highly available form of selenomethionine in yeast were shown to reduce the incidence of certain cancers. A number of natural foods contain relatively high amounts of Se; for the most part, however, the availability of food Se for absorption and utilization is unknown. This experiment was conducted to determine the bioavailability of Se from a high-protein, high-fiber bran-isolate of buckwheat groats that contains Se. The method used was based on the ability of Se from buckwheat bran to restore Se-dependent enzyme activities and tissue Se concentrations in Se-deficient rats. The responses produced from buckwheat bran Se were compared with a standard response curve generated by feeding graded amounts of Se as sodium selenite (Na₂SeO₃; Na selenite) or selenomethionine (SeMet) in a newly reformulated AIN-93G-Torula yeast diet with a more balanced nutrient composition than older diets of this nature. Relative bioavailability was determined by using the slope-ratio assay method for enzyme data, or the parallel lines assay method for tissue Se concentration data. Results showed that Se availability from buckwheat bran based on the restoration of plasma Se was 70–80% as high as Na selenite or SeMet. However, when based on the restoration of muscle Se, buckwheat bran was 90% as high as Na selenite, but only 60% as high as SeMet. When using the ability of dietary Se to restore whole blood and liver glutathione peroxidase activity, buckwheat bran Se was 75–80% as high as Na selenite or SeMet. However, for the restoration of liver thioredoxin reductase, buckwheat bran Se was only 40% as high as Na selenite and 70% as high as SeMet. The relative bioavailability of Se from buckwheat bran with all variables considered was 73% whether measured against Na selenite or SeMet. Although some variables indicated low bioavailability of Se from buckwheat bran, other factors such as Se speciation in the bran, digestibility of the bran, the cooking process, and combinations with other foods in the diet should be considered and analyzed before firm conclusions can be reached.

KEY WORDS: • AIN-93G diet • buckwheat bran • rats • selenium bioavailability • torula yeast

Selenium (Se) is considered an essential nutrient in both humans and animals because a deficiency of the mineral results in severe disease conditions, e.g., Keshan disease in humans and white muscle disease in domestic livestock (1–3). The basis for Se essentiality is its role at the catalytic site of multiple selenoproteins, e.g., glutathione peroxidase (GPx1),⁴ thioredoxin reductase (TRR), and the iodothyronine selenodeiodinases (2). High Se intakes of 200 µg/d are associated with reduced risk of cancer, especially prostate cancer (4), which has led to proposals to increase the Se intake of populations at risk. Although numerous Se supplements are commercially available, the American Dietetic Association has recommended that nutrients should be consumed through foods whenever possible (5). However, food nutrients, including Se, must be bioavailable to be absorbed and utilized by the body for specific physiologic functions.

The bioavailability of Se is complicated because there are multiple naturally occurring chemical forms of this element in nature. These include Se salts, Se derivatives of sulfur amino acids, and methylated derivatives of selenoamino acids. The chemical form of Se partially dictates its metabolism and its ultimate biological action; therefore, unlike most other nutrients, the bioavailability of Se cannot be estimated solely by measuring absorption. Instead, bioavailability must be assessed by the ability of a particular Se compound to be transformed into a metabolically active form of Se (6). Functional bioassays, such as activity restoration of the selenoprotein GPx1 in Se-deficient laboratory animals, are among the most commonly used methods to assess the bioavailability of Se from food (6).

Assessing the ability of a Se source to restore GPx1 activity and other Se-requiring reactions first requires that the test animal be made Se deficient. Laboratory rats are the most commonly used animals for these studies, and the diet most often used to induce Se deficiency is based on Torula yeast as the protein source. This is because Torula yeast has a very low concentration of Se and it can be purchased readily as a dry powder. Dry Torula yeast contains 50% protein, and the essential amino acid profile is reasonably good, except for the sulfur amino acids and tryptophan. However, the yeast is a complex mixture of other organic compounds, some of which are essential vitamins and some are unknown.

Torula yeast also contains copious amounts of certain minerals. When the yeast is mixed into the diet at the usual 30% to obtain an adequate amount of protein, the contributions of Fe, P, K, Zn, Mn, Mg, and perhaps Na, without further supplementation, will exceed the NRC requirement of these nutrients for rodents by 1.5–4 times (7). In previous Se studies that used the Torula yeast formulations, premixes containing these and other minerals were added to the already excess supply (8–10). It is not known what adverse effects these additional minerals might have on physiological changes in experimental animals, or whether there are interactions between Se and the excess minerals that could alter the results of the experiment.

In an attempt to minimize these potential problems, we designed a Torula yeast diet for rodents that is built around the AIN-93G diet, the mineral concentration of the yeast, and the amino acid composition of its protein. Without Se supplementation, the diet has a very low Se concentration (<5.0 µg/kg) and a more balanced supply of other minerals that reduces the possibility of interactions with Se. We call the diet the Modified AIN-93G Torula Yeast Diet, or simply, AIN-93G-Yeast. We tested the diet at different concentrations of added Se by feeding it to weanling male rats and measuring the response of a range of variables dependent upon dietary Se. The diet was then used to determine the bioavailability of Se in a bran-isolate of buckwheat.⁵ This buckwheat bran contains a relatively high amount of protein (37%) and fiber (17%). Most U.S. buckwheat is grown in the northern plains area of the Dakotas and Minnesota, and portions of that region contain soils with very high concentrations of Se (11). Consequently, the bran isolated from buckwheat grown there has the potential to become a good source of Se in the human diet. However, the relative availability of Se for absorption and utilization from this product is not known. In this study, the relative bioavailability of Se was determined in buckwheat bran by comparing the response of 3 Se-dependent enzymes and Se concentrations of plasma and various organs with that found on a standard response curve produced by refeeding Se-deficient rats various dietary concentrations of Se as sodium selenite (Na₂SeO₃; Na selenite) or selenomethionine (SeMet).

	MATERIALS AND METHODS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
This study was approved by the Animal Use Committee of the USDA-ARS, Grand Forks Human Nutrition Research Center. The procedures followed the guidelines of the NIH for the experimental use of laboratory animals (12).

    Preparation of the standard response curve. The base formula of the AIN-93G diet (13) and the average mineral and amino acid content of Torula yeast were used to derive the basal diet composition, which is shown in Table 1. For the yeast to supply enough essential amino acids without excessive supplementation, it was fed at 30% of the diet. However, at this concentration, there were insufficient amounts of the sulfur amino acids and tryptophan (Table 2) to equal that recommended for rodents by the NRC (7); thus, a supplement containing L-methionine, L-cystine, and L-tryptophan was added to the diet (Table 2).

    Addition of buckwheat bran to the diet. Buckwheat bran was obtained from Minn-Dak Growers. The bran was mixed with water and cooked before it was incorporated into the diet. Briefly, 1.0 kg was added to 500 mL of deionized water and mixed to a dough-like consistency. The dough was placed in a shallow pan to a thickness of 3 cm and baked at 200°C for 30 min. The cooked material was crumbled into small pieces and dried in a forced-air oven at 60°C for 48 h. The dried pieces were ground in a centrifugal grinder to the consistency of whole-wheat flour; the resulting powder was mixed in a Hobart mixer, and stored at 4°C until mixed into the diet at 2 concentrations, 5 and 7.5% of the diet. By analysis, the cooked material contained 670 ± 6 µg Se/kg dry weight, and the 2 diets contained 36.0 ± 3.2 and 54.7 ± 2.7 µg Se/kg, respectively.

    Enzyme assays. GPx activities in whole blood and liver were determined by the method of Paglia and Valentine (15) with hydrogen peroxide used as the substrate. Activity in whole blood was expressed as units/mg hemoglobin and for liver as units/mg protein; one unit was defined as that amount of enzyme required to oxidize 1.0 µmol NADPH/min. TRR activity in liver was determined spectrophotometrically using the method of Hill et al. (16) as modified by Hintze et al. (17). Activity was determined by subtracting the time-dependent increase in absorbance at 412 nm in the presence of the TRR activity inhibitor, aurothioglucose, from total activity. A unit of activity was defined as 1.0 µmol thionitrobenzoate formed/(min · mg protein). Protein concentrations were determined by the Bradford method (BioRad).

    Mineral analyses. Small sections of various organs such as liver, kidney, brain, and testes were flash-frozen in liquid nitrogen and weighed. The samples were digested in Pyrex glass beakers with 10 mL of nitric acid (16.0 mol/L), 10 mL of magnesium nitrate solution (40% in deionized water), and 2.0 mL HCl (6.0 mol/L). The initial digestion step was done by refluxing the samples on a hot plate at 120°C for 48 h. The samples then were dried and ashed in a muffle furnace at 490°C for 12 h. The ash was diluted with 25 mL HCl (4.8 mol/L) and analyzed by hydride generation, atomic absorption spectrometry. All chemicals were of the highest quality and purity. Magnesium nitrate was used to prevent loss of Se during the high-temperature ashing step.

    Statistical analyses. The rate of weight gain for each rat was calculated by fitting a linear regression to each rat’s body weights. The slopes were then compared by one-way ANOVA. The enzyme and tissue responses to Na selenite or SeMet were compared by fitting a single nonlinear model and testing whether the 2 curves shared a common upper asymptote by using multiple regression techniques (18). Brain Se was modeled by using a linear model that tested whether the rate of increase differed between Na selenite and SeMet.

The bioavailability of Se in buckwheat bran was determined relative to either Na selenite or SeMet. The relations between the enzyme data and intakes of Na selenite or SeMet were linearized by using log-linear regression models and the slope-ratio method was used to estimate relative bioavailability. The tissue and plasma Se data, however, exhibited log-log relations with dietary Se; therefore, the parallel line assay method was used to estimate relative bioavailability for these data (19). For all regressions, only the standard response doses between 0 and 100 µg Se/kg diet were used because at doses > 100 µg Se/kg diet, the responses had reached a plateau and were not affected by increasing Se intakes. Therefore, the relative bioavailability estimates are applicable only for doses between 0 and 100 µg Se/kg diet. For each set of criteria evaluated, linearity of the regression lines was ascertained for each source of selenium separately. Then, a single multiple regression model was derived to determine the slope and intercept of the responses for buckwheat bran and either Na selenite or SeMet with the "no added selenium" group serving as the blank (20). The 95% CI for relative bioavailability were obtained by using Fieller’s method (19). Data are reported as means ± SD in the tables and as means ± SEM in the figures. Differences with P < 0.05 were considered significant. All statistical analyses were done by using SAS/Stat Version 9.1 (SAS Institute).

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The basal diet was very low in Se, containing 4.4 ± 0.7 µg/kg. The rate of gain during the rapid growth phase of the first 21 d was 6.5 g/d, which is a normal growth rate for Sprague-Dawley rats (Fig. 1). When the rats were supplemented with the highest amount of Se (172 µg/kg diet) as Na selenite beginning at d 37, the rate of growth over the remainder of the experiment seemed to be higher than that of rats without Se supplementation; however, the difference was not significant (P > 0.05). The growth rate tended to be lower (P = 0.10) in rats supplemented with SeMet, compared with rats supplemented with Na selenite. Feeding 7.5% buckwheat bran in the diet did not affect body weight compared with that in rats fed Na selenite (Fig. 1).

View larger version (5K): [in this window] [in a new window]   FIGURE 1 Body weight of rats fed Se-deficient and Se-adequate AIN-93G-Torula yeast diets for 87 d. Points represent the means ± SEM, n = 6.

View larger version (8K): [in this window] [in a new window]   FIGURE 2 Response curves of serum (A) and organ (B–F) concentrations of Se in rats fed Se as increasing concentrations of sodium selenite or selenomethionine in the diet. Buckwheat bran was fed at 2 concentrations, 5 and 7.5% of the diet, and the results compared with the response curve. Relative bioavailabilities calculated from these curves are found in Table 6. Points represent the means ± SEM, n = 6.

Dietary Se as SeMet increased muscle Se concentration to twice that in rats fed Se as Na selenite (P < 0.05) (Fig. 2D). Relative bioavailability of Se from buckwheat bran was 93% of that from Na selenite, but only 62% of that from SeMet. Se concentration in testis was at its maximum at 20 µg Se/kg diet (Fig. 2E). The values from different groups were highly variable, including the 2 buckwheat bran groups; therefore, no relative bioavailability values could be derived. Se in brain was not severely depleted by feeding Se-deficient diets, and increased only slightly (P = 0.08) upon refeeding Se (Fig. 2F). However, the rate of increase in brain Se was greater (P < 0.05) in rats fed SeMet than in those fed Na selenite. Relative bioavailability of Se from buckwheat bran could not be calculated by using this organ.

The activity of GPx1 in whole blood was slightly increased (P = 0.10) from the deficient baseline by feeding 20 µg Se/kg diet from either source (Fig. 3A). At 60 µg Se/kg diet, GPx1 activity had reached 60% of the maximal activity, which occurred at 120 µg Se/kg diet. The titration curve for liver GPx1 activity was similar to that for whole blood GPx1, except 20 µg Se/kg diet was not sufficient to give a significant difference from the baseline values (Fig. 3B). However, activity was maximal in rats fed between 60 and 120 µg Se/kg diet. The relative bioavailability of Se from buckwheat bran as estimated by the change in activities of these enzymes was 80%.

View larger version (7K): [in this window] [in a new window]   FIGURE 3 Response curves of blood (A) and liver (B) GPx1 activity and liver TRR activity (C) in rats fed Se as increasing concentrations of sodium selenite or selenomethionine in the diet. Buckwheat bran was fed at 2 concentrations, 5 and 7.5% of the diet, and the results compared with the response curve. Relative bioavailabilities calculated from these curves are found in Table 6. Points represent the means ± SEM, n = 6.

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
    Selenium bioavailability. The bioavailability of Se from buckwheat was estimated by comparing buckwheat-Se to Se supplied as Na selenite and as SeMet in its potential to restore tissue Se concentrations and selenoprotein activities in Se-deficient rats. Methods similar to this one were used previously to determine the bioavailability of Se from many foods including beef (21), Se-enriched yeast (22), Se-enriched algae (23), Se-enriched broccoli (24), and salmon (25). Na selenite and SeMet are metabolized differently and accumulate in different pools in rats (26). Beilstein and Whanger (27) showed that Se supplied as SeMet accumulated more in hemoglobin and muscle than Na selenite, presumably because of the ability of SeMet to randomly substitute for methionine. Conversely, consumption of Na selenite resulted in Se being deposited primarily as selenocysteine in selenoproteins (27). However, these same studies also established that the metabolic transformations of Se are tissue specific, and differences in deposition of Se were not noted in all tissues. A similar response was found in the present study; compared with Na selenite, SeMet resulted in a much greater deposition of Se in the muscle, and slightly greater deposition in kidney and brain, but differences in the other tissues analyzed were very small.

Selenoprotein production is carefully controlled, and as expected, selenoenzyme activity began to reach a plateau after a certain dietary concentration of Se had been reached. Studies in Se-deficient animals found that under most circumstances, Se from Na selenite and SeMet are equally effective for restoring GPx1 activity (28). Human studies confirm this; Whanger (29) reported that although more Se from Na selenite than SeMet was associated with GPx1 in Se-deficient Chinese men, both forms were equally effective for restoring GPx1 activity. Brown et al. (30) reported that a 28-d supplementation of SeMet to Europeans with low Se status was equally or more effective than Na selenite for the restoration of GPx1 in blood fractions. Thus, it was somewhat surprising to find that GPx1 activity in the blood was 10% greater in rats fed SeMet than in those fed Na selenite when rats were fed dietary concentrations of Se in excess of 100 µg/kg. This is not without precedent, however, because Xia et al. (31) reported that after 40 d of Se supplementation, plasma GPx1 activity was significantly greater in men fed SeMet than in those fed Na selenite, although, by d 70, the 2 groups did not differ.

Bioavailability of Se from buckwheat was determined by comparing the indicators of Se status in rats fed Se from buckwheat to those in rats fed SeMet or Na selenite. Two concentrations of Se from buckwheat were used, and visual examination of the repletion curves shows that for all variables except testicular Se, the concentration of Se from buckwheat fell within the linear response range of Se from Na selenite and SeMet. The data further show that for plasma, liver, and kidney, Se from buckwheat was less available than from either Na selenite or SeMet. In the muscle, Se from buckwheat was less available than Se supplied as SeMet, but was identical to that supplied as Na selenite. Buckwheat bran was adequate for maintaining Se concentration in brain and testes compared with SeMet and Na selenite. In addition, Se from buckwheat bran was less effective in aiding the recovery of blood and liver GPx1 and liver TRR activities than similar amounts of Se from either Na selenite or SeMet. When relative values for Se bioavailability in buckwheat bran were taken into consideration for all variables measured, the mean value was 73% whether compared with Na selenite or SeMet.

Determining the availability of Se from food by using a laboratory animal and then extrapolating that information to humans can be problematic. Obviously, the dietary conditions are not the same between animals and humans. It is likely that the buckwheat bran would be cooked before being consumed by humans, as was done in the current study. This heating process might render the bran more digestible and affect the bioavailability of Se. In addition, the speciation of Se in the bran and the combination of bran with other components of the diet might affect Se bioavailability. In the current study, although Se bioavailability from buckwheat bran seemed low as determined by certain variables in this study, it is unclear whether this was caused by low absorption or by poor retention of Se. Other factors must be considered and analyzed before firm conclusions can be reached.

    Modified AIN-93G Torula yeast diet. The AIN-93-based diets have been in use for nearly 12 y and have proven to be good choices for both short- and long-term studies with laboratory rodents (32). The AIN-93G diet gives excellent growth in young rats, and its components are easily manipulated to obtain the desired nutrient composition for nutritional studies. In the current study, the goal was to construct a Se-deficient diet with Torula yeast as the protein source, but with an amino acid and mineral composition that more closely matched that of the AIN-93G diet than has been used in the past. Because of the inherent nature of Torula yeast, the mineral content is very high. Thus, when used at 30% of the diet to obtain a reasonable amount of protein, it was impossible to match the mineral content of the completed diet with that of the AIN-93G. The P content of the diet was 1.5 times higher than that in the AIN-93G; thus, to prevent nephrocalcinosis in female rats, if used in such a study, extra Ca was added to the diet to obtain a Ca:P molar ratio of at least 1.3 to 1.0. Potassium, Mg, Zn, and Mn also were higher, even though none of these minerals was added to the diet in the mineral mix. Although we were unable to match the mineral content of the AIN-93G precisely, overall, the AIN-93G-Torula yeast diet has a much more balanced nutrient composition than most similar diets used in the past, and would make an excellent substitute for them.

	ACKNOWLEDGMENTS

 
The authors acknowledge the assistance of Denice Schafer and staff for care of the animals and Bill Martin and the Mineral Analysis Laboratory for sample preps and Se analysis.

	FOOTNOTES

 
¹ Supported in part by Minn-Dak Growers, Grand Forks, ND, and by the U.S. Department of Agriculture CRIS Project No. 5450-51000-035-00D.

² The U.S. Department of Agriculture, Agricultural Research Service, Northern Plains Area is an equal opportunity/affirmative action employer and all agency services are available without discrimination. Mention of trade names or commercial products in this article is solely for providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.

⁴ Abbreviations used: GPx1, glutathione peroxidase 1; Na selenite, sodium selenite (Na₂SeO₃); SeMet, selenomethionine; TRR, thioredoxin reductase.

⁵ Similar to the product called Farinetta^TM, produced by Minn-Dak Growers, Grand Forks, ND 58203. http://www.minndak.com/factsofbuckwheat.htm.

Manuscript received 30 June 2005. Initial review completed 27 July 2005. Revision accepted 10 August 2005.

	LITERATURE CITED

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