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Article

The Retention and Distribution by Healthy Young Men of Stable Isotopes of Selenium Consumed as Selenite, Selenate or Hydroponically-Grown Broccoli Are Dependent on the Isotopic Form^{1, ,2}

United States Department of Agriculture, Agricultural Research Service, Grand Forks Human Nutrition Research Center, Grand Forks, ND 58202-9034

³To whom correspondence should be addressed at John W. Finley, Ph.D., USDA, ARS, GFHNRC, P.O. Box 9034, UND Station, Grand Forks, ND 58202-9034. Telephone: (701) 795-8366, FAX: (701) 795-8395, E-mail: jfinley{at}GFHNRC.ARS.USDA.gov

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Twenty-seven healthy young men were randomly assigned to diets that supplied low (32.6 µg/d) or high (226.5 µg/d) levels of selenium for a 105-d study. After consuming the diets for 85 d, subjects were fed a test meal that contained ⁷⁴Se in the form of selenite or selenate and ⁸²Se incorporated into hydroponically-raised broccoli. Urine, fecal and blood samples were collected daily. Isotope absorption was not different (P > 0.05) for selenate and Se in broccoli; Se absorption from selenite was highly variable and was not included in statistical analyses. Significantly more isotope was absorbed by subjects fed the high Se diet (P = 0.015). Urinary isotope excretion was greater when selenate was fed than when broccoli was fed (P = 0.0001), and consequently more Se from broccoli (as compared to selenate) was retained (59.2 ± 2.4 and 36.4 ± 4.6% for Se in broccoli and selenate, respectively; P = 0.0001). Despite the higher retention, less isotope from broccoli than from selenate was present in the plasma. Plasma proteins separated by gel permeation chromatography showed that most of the isotopes were distributed between two medium molecular weight peaks. Less isotope was found in plasma proteins of subjects fed the high Se diet, but the form of Se had no effect on isotope distribution. These results show that dietary Se intake alters the retention of stable isotopes of Se and that humans retain and distribute Se from broccoli in a different manner than Se from inorganic salts.

KEY WORDS: • selenium • stable isotope • absorption • broccoli • humans

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Selenium (Se) is an essential trace element that is incorporated into proteins (Burk and Hill 1993 ). The metabolism of Se depends on the chemical form of ingested Se, and absorbed Se will go predominately into one of three metabolic pools. Some Se will be used to produce selenoproteins, proteins that require Se for catalytic activity and incorporate Se as selenocysteine (SeCys)⁴ into the polypeptide chain by using UGA as the encoding codon (Burk and Hill 1993 ). Other forms of Se will go primarily into selenium-containing proteins, proteins that do not require Se for catalytic activity and apparently incorporate Se randomly by substituting selenomethionine (SeMet) for methionine. Finally, all forms of Se can go into a pool that can be methylated and excreted through the urine and lungs. Selenium consumed as a salt (selenite or selenate) will go into selenoproteins or be excreted (Whanger 1986 ). Much plant material contains Se in the form of SeMet, and SeMet will be incorporated into Se-containing proteins to a greater extent than will Se salts (Beilstein and Whanger 1986 , Butler et al. 1990 ), although it also can be degraded to selenide and then be incorporated into selenoproteins. Other forms of Se, especially methylated derivatives, apparently go primarily into the excretion pathway (Ip et al. 1991 ).

The metabolic pathway that Se follows determines its potential benefit to the animal. If the primary goal is to build up body stores of Se, then forms such as SeMet that can be incorporated into selenium-containing proteins are the most effective. Conversely, salt forms are more effective in increasing the amount and activity of selenoproteins. But another important function is its cancer-preventative ability, which apparently has no association with either selenoprotein or tissue Se status (Ip et al. 1991 ). Rather, it seems to be associated with activation of the excretory pathway.

Some plants contain Se in a form other than SeMet, and the metabolism of Se in those plants is unique. Garlic that is raised to contain large amounts of Se has potent anti-cancer properties (Ip et al. 1992 ) that are a consequence of the form of Se. Selenium in high-Se garlic is present as Se methyl-selenocysteine (SeMSC); the primary form of Se in broccoli raised to contain high amounts of Se is also SeMSC (Cai et al. 1995 ), although it is not known if SeMSC is the primary chemical form of Se in broccoli with lower total amounts of Se.

We recently showed in rats that Se from high-Se broccoli was metabolized differently than Se from selenite, selenate or SeMet (Finley 1998 ). Selenium from broccoli was not as effective as any of the other forms in its ability to replete Se-deficient tissues of Se and glutathione peroxidase (GSH-Px) activity. Also, more Se from broccoli was excreted in urine than Se from SeMet.

The purpose of the present study was to investigate in humans the metabolism of Se in broccoli. We used healthy young men to study the retention and distribution of Se from broccoli. The broccoli was grown hydroponically and contained lower total concentrations of Se than that used in the rat studies (Finley 1998 ) or in the study of Cai et al. (1995) but was enriched in a stable isotope of Se. The metabolism of the stable isotope of Se from broccoli was compared to the metabolism of Se stable isotopes consumed as selenate or selenite, and the absorption, retention and distribution of the isotopes were followed for 21 d.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Volunteers and diets.

Details of subjects, diets and clinical procedures were reported elsewhere (Finley and Penland 1998 ). In brief, 27 healthy young men (nonsmokers and not taking mineral supplements) between the ages of 18 and 45 y participated in this study. All subjects were informed in detail as to the nature of the research. The study was approved by the Institutional Review Board of the University of North Dakota and the U.S. Department of Agriculture and followed the guidelines of the Department of Health and Human Services and the Helsinki Doctrine regarding human subjects.

Subjects were divided into two groups, and each group consumed one diet for the duration of the study; 12 subjects consumed the low Se diet and 15 subjects consumed the high Se diet. The diets were based on a 3-d rotating menu cycle; they were not identical and were formulated to be either low or high in Se. The primary difference between the two diets was that the low Se diet did not contain as many wheat products as the high Se diet, and the low Se diet used meat (beef and pork) with low concentrations of Se. The diet low in Se was designed to provide 24.4 ± 1.8 µg Se/d and the adequate diet, 167.5 ± 10.0 µg Se/d, based on an intake of 10.5 kJ/d. Diets actually provided daily Se intakes of 32.6 or 226.5 µg Se/d based on subjects 14.1 kJ/d average intake. After consuming the diet for 85 d, subjects were fed a test breakfast that contained the stable isotopes. Total urine and feces and periodic blood samples were collected for the next 21 d. Because of limitations of the facilities, the study was carried out in two parts. Each part had an approximately equal number of men consuming each diet, and the first group of men completed the study before the second group began.

Stable isotopes.

Stable isotopes of Se were purchased in elemental form as ⁷⁴Se and ⁸²Se (Advanced Materials and Technology, New York, NY). Elemental ⁷⁴Se was converted to selenite or selenate (Finley et al. 1995 ) and was fed directly to subjects. Selenate labeled with ⁸²Se was put into a hydroponic nutrient solution for uptake and incorporation into broccoli. Broccoli was grown hydroponically by using standard procedures; the only alteration was that Se was not included in the plant nutrient solution from the beginning of growth (Vanderpool and Johnson 1992 ). Instead, a total of 2.1 mg of ⁸²Se was added to the hydroponic solution of each plant around the time of the development of the flowering head. The isotope was added to the nutrient solution in three increments: 0.42 mg was added after 50 d of growth, 0.63 mg was added after 76 d of growth and 1.05 mg was added after 97 d of growth. Simultaneous with the addition of the Se isotope, sulfur was lowered 20% by replacing MgSO₄ with Mg(NO₃)₂. During the time of isotope addition, the plants were allowed to take up most of the nutrient solution before the solution was replaced. These procedures allowed most of the stable isotope to be taken up and translocated into the broccoli flowering head. Mature broccoli heads were harvested, chopped, blended, frozen and lyophilized.

Test breakfast.

On d 85 of the study, and after an overnight fast, subjects were fed a test breakfast containing the stable isotopes. The composition of the test breakfast is given in Table 1. The test breakfast contained 100 µg of stable ⁸²Se in 33 g of hydroponically-raised broccoli and 11.5 µg of ⁷⁴Se as either selenite (first group of subjects) or selenate (second group of subjects). The salt form of the isotope was changed from selenite (given to the first group of subjects) to selenate (given to the second group of subjects) because stable isotope absorption, retention and distribution in subjects that consumed selenite were so variable. Following the test breakfast, subjects returned to their normal experimental diets.

Total urine and feces were collected for the subsequent 21 d. For the purpose of this report, percentage of absorption was defined as:

and percentage of retained Se was defined as:

where (intake) is the total amount of the particular stable isotope consumed in the test meal, (feces 20 d) is the total amount of the particular stable isotope voided into the feces during the 20-d collection period and (urine 20 d) is the total amount of the particular stable isotope found in the urine during the 20-d collection period.

Plasma kinetics.

Blood samples (10 mL) were collected at 0.33, 1, 2, 7, 15 and 21 d after the test meal. Blood was separated into plasma and erythrocytes and frozen at -20°C until analysis.

Gel permeation chromatography (GPC).

Aliquots of plasma (3 mL) were chromatographed on a Sephacryl S-200 column (1.5 x 100 cm); the buffer was 10 mmol/L Tris-HCl and 40 mmol/L ammonium acetate, pH 7.8; and the flow rate was 20 mL/h. Adjacent fractions were collected and combined and then digested with nitric acid (see below) in preparation for mass spectrometric analysis of stable Se. Chromatograms of the distribution of natural abundance and stable isotopic Se in plasma protein fractions separated by GPC were resolved into individual protein peaks by a peak analysis program (PeakFit; Jandel SPSS, Chicago, IL). The kinetics of incorporation of stable isotope into plasma proteins were determined for one subject on each diet by chromatographing plasma from all time-points. The effect of the isotope chemical form (in broccoli or as a salt), and the dietary Se intake on the distribution of stable Se among plasma proteins was compared in six subjects consuming the low and five subjects consuming the high Se diet.

Laboratory analyses—selenium status.

Samples containing stable isotopes were prepared and analyzed by inductively-coupled plasma mass spectrometry (ICP-MS) (Finley et al. 1995 ). Briefly, samples were introduced into the argon plasma by a custom-built hydride generator and the total amount of Se present, as well as the amount of each stable isotope, was calculated from ICP-MS intensity data. Quality control was maintained by running serum trace element standards (UTAK #66816; UTAK Laboratories Inc. Valencia, CA) after every eight samples. If the standard was out of range, the samples from that portion of the run were repeated. Samples were analyzed in duplicate, and the detection limit for this method was ~1 µg/L; the within assay coefficient of variation (CV) was 5.5% and the between assay CV was 2.8%.

Statistical analyses.

Data were analyzed by two-way analysis of variance with effects of dietary Se intake and chemical form of the isotope (SAS/STAT; SAS Institute, Cary, NC). Selenite data were excluded from the statistical analysis due to high variability. Differences were considered significant if P < 0.05.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Absorption and excretion.

The effect of dietary intake and chemical form of Se on the absorption, retention and excretion of stable isotopes of Se is presented in Table 2. The original design of the study was to feed equal numbers of subjects stable ⁸²Se incorporated into broccoli and stable ⁷⁴Se as selenite, which is the most common salt of Se used in previous human and animal studies. However, the subjects were divided into two groups, and results from the first group were analyzed before the second group began. A surprising result from the first phase of the study was the large degree of variation in the absorption of stable Se administered as selenite.

Absorption of stable isotopes of Se was significantly greater (P = 0.015) for subjects consuming the diet high in Se than for subjects consuming the low-selenium diet (Table 2) . Significantly more stable Se was excreted in the urine (P = 0.0003) of subjects who consumed the high Se diet than those who consumed the low Se diet (Fig. 1 ).Also, more stable Se was excreted into the urine from selenate than from broccoli (P = 0.0001). Consequently, retained Se was significantly greater (P = 0.0001) when the Se source was broccoli than when it was selenate.

View larger version (17K): [in this window] [in a new window]   Figure 1. The effect of chemical form of Se isotope (salts or Se in broccoli) and dietary intake of Se on urinary excretion of 27 young men fed stable isotopes of Se in different chemical forms. Twelve men consumed a diet providing 32.6 µg of Se/d (low Se diet) and 15 men consumed a diet providing 226.5 µg of Se/d (high Se diet). After 85 d of consuming the diet, stable isotopes of Se were ingested in the form of broccoli (n = 27) and either selenate (n = 13) or selenite (n = 14) (data not shown); urine, feces and blood samples were collected the subsequent 21 d. Values are means ± SEM. Significant effects of dietary intake of Se (P = 0.0003) and chemical form of Se isotope (P = 0.0001).

Maximal concentration of the isotope in plasma occurred within a few hours after the dose was given, and maximal concentrations were observed significantly earlier (Fig. 2 ;P = 0.03) in subjects consuming the high rather than the low Se diet. The maximal plasma concentration of isotope and area under the curve were significantly lower (P = 0.0001) when the isotope source was broccoli than when it was selenate.

View larger version (17K): [in this window] [in a new window]   Figure 2. The effect of chemical form of Se isotope (salts or Se in broccoli) on the appearance and retention of stable isotopes of Se in the plasma of 27 young men fed diets containing different amounts of Se and consuming stable isotopes of Se in different chemical forms. Twelve men consumed a diet providing 32.6 µg of Se/d (low Se diet) and 15 men consumed a diet providing 226.5 µg of Se/d (high Se diet). After 85 d of consuming the diet, stable isotopes of Se were ingested in the form of broccoli (n = 27) and either selenate (n = 13) or selenite (n = 14) (data not shown); urine, feces and blood samples were collected the subsequent 21 d. Values are means ± SEM. Significant effect of chemical form of Se isotope on maximal concentration (P = 0.0001) and area under the curve (P = 0.0001).

Natural abundance Se in plasma was primarily distributed between two medium molecular weight protein peaks (Fig. 3 A);a small low molecular weight peak was detected in some samples (data not shown). Serum albumin coeluted with the second medium molecular weight peak. Stable isotopic Se was also distributed between the same medium molecular weight peaks (Fig. 3B) .

View larger version (13K): [in this window] [in a new window]   Figure 3. The distribution of Se isotopes among plasma proteins of young that consumed either 32.6 µg of Se/d or 226.5 µg of Se/d and were fed stable isotopes of Se in the form of broccoli or selenate. Plasma samples (mL) were separated by gel permeation chromatography (GPC), and fractions were analyzed for stable Se. Plasma samples were collected 8 h after ingestion of stable isotope for six subjects consuming the low Se diet and five consuming the high Se diet. A. Distribution of natural abundance Se. B. Distribution of isotopic Se (data for 82Se and 74Se have been averaged). Legend: () = overall curve fit to all components; (···) = curve fit to individual peaks.

View larger version (22K): [in this window] [in a new window]   Figure 4. The effect of dietary Se intake by healthy young men (average dietary Se intake of 32.6 or 226.5 µg/d for the low and high Se groups, respectively) on the kinetics of Se isotope incorporation into plasma proteins of one subject consuming the low Se diet and one consuming the high Se diet; samples were separated by gel permeation chromatography (74Se and 82Se data are averaged). A. Kinetics of isotope incorporation into plasma proteins of a subject consuming the low-Se diet. B. Kinetics of isotope incorporation into plasma proteins of a subject consuming the high Se diet.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Many studies compared the metabolic fate of Se consumed as a salt (selenite or selenate) or SeMet (Butler et al. 1990 and 1991 , Finley 1989 , Salbe and Levander 1990 , Thomson et al. 1993 , Xia et al. 1992 ) and showed that the chemical form of ingested Se directly affects its metabolism. Selenium metabolism is also affected by whether the person or animal has been consuming less than adequate, adequate or luxuriant amounts of Se in their daily diet (Finley 1989 , Whanger and Butler 1988 ), and such differences also are partially related to the incorporation of Se into proteins. When animals are deficient in Se, some selenoproteins have priority for the incorporation of Se (Yang et al. 1989 ).

Thus, the interaction of chemical form and dietary intake of Se affects its metabolism and utilization by the body. One of the forms of Se used in the present study was Se incorporated into hydroponically-raised broccoli. Only one study has examined the chemical form of Se in broccoli, and that study used broccoli that contained 345 mg of Se/kg broccoli; selenium was found to be in the form of SeMSC. The broccoli used in the present study contained 3.0 mg of Se/kg broccoli (dry basis). Although 3.0 mg of Se/kg is substantially below the concentration used in the study by Cai et al. (1995) , it is higher than the Se concentration in broccoli purchased from grocery stores and analyzed by our laboratory [most values of grocery store broccoli are between 50 and 200 µg of Se/kg broccoli (dry basis); data not shown]. Possibly broccoli may synthesize different chemical forms of Se, depending on the total concentration of Se present so it cannot be definitively stated that the form of Se in broccoli used in the present study was SeMC. However, since the Se concentration in the broccoli of the present study was at least 15-fold greater than normal, it is likely at least a portion of the Se was present as SeMC.

Se-methyl selenocysteine has potent anticancer properties (Ip and Lisk 1995 ). Broccoli and related Brassica spp. vegetables also have well documented anticancer properties unrelated to Se (Doll and Peto 1981 ). Preliminary evidence by our laboratory now indicates that the Se in broccoli may also have substantial anticancer properties (data not shown).

Similar to previous studies (Butler et al. 1991 , Finley 1989 , Salbe and Levander 1990 , Thomson et al. 1993 , Whanger and Butler 1988 , Xia et al. 1992 , Yang et al 1989 ), both the form and amount of Se used in this study affected Se retention and distribution. Results are also consistent with a study that showed that Se-deficient rats repleted with Se from broccoli, Se salts or SeMet, distributed and retained the Se from broccoli differently than Se from selenate (Finley 1998 ). Selenium from broccoli was retained better than Se from selenate in the present study, and the reason was apparently the greater excretion of the Se from selenate. Despite the greater retention of Se from broccoli, less was found in the plasma than when selenate was the form of the isotope.

More Se isotope from selenate than from broccoli was excreted into the urine. Urinary excretion of Se from broccoli and selenate have not been directly compared, although the urinary excretion of SeMSC was studied. Slightly more Se as SeMSC is excreted into the urine of rats than Se from SeMet (Foster et al. 1986 ). Human urinary excretion of Se from selenate is faster and in greater quantities than urinary excretion of Se from SeMet (Finley 1998 , Thomson et al. 1978 ).

Selenium in urine is trimethyl selenide, and this compound may be demethylated sequentially to methyl selenol and then returned to the selenide pool where it may be used for the production of selenoproteins (Foster et al. 1986 ). High dietary Se can shift the equilibrium of this pathway toward excretion, whereas low dietary Se could conceivably shift the equilibrium toward the pool available for incorporation into selenoproteins. Assuming that a portion of the Se in the broccoli was SeMSC, and that it would enter the metabolic pathway as methyl selenol, there is evidence that this shift occurred. When dietary intake of Se was high, the urinary excretion of Se isotope from broccoli was 39% that of selenate. However, when dietary Se intake was low, the urinary excretion of Se from broccoli was 27% that of Se from selenate.

The retention of stable Se from broccoli and selenate in the plasma and urine demonstrate that the overall tissue distribution of Se from the two sources differs. We also wished to determine if the retention of Se at the molecular level was affected by the source. For this reason, we determined stable Se in plasma proteins separated by low-pressure GPC. Stable isotopes of Se were previously used in conjunction with GPC (Ducros et al. 1994 , Finley et al. 1995 ), and stable isotopes distribute among plasma proteins in the same manner as radioactive isotopes (Finley et al. 1995 ). Low-pressure GPC is not a precise enough method to separate pure proteins, but the pattern of two major Se-containing protein peaks was reported before. Deagen et al. (1991) used Sephadex G-150 and concluded the peaks to be selenoprotein P and albumin. Ducros et al. (1994) followed the time course of stable Se incorporation into proteins separated by GPC and showed Se incorporation into two primary peaks. Although they did not characterize the individual protein peaks, one protein peak eluted with albumin and took up Se initially; the other protein peak was identified as selenoprotein P and the maximal enrichment of stable Se in that peak occurred after 12 h. Albumin was also identified as the second protein peak in the current study.

Xia et al. (1992) showed that the daily intake of Se, as well as the chemical form, influenced the distribution of natural abundance Se among proteins in the plasma of Se-deficient Chinese men. We found in the present study that total natural abundance Se incorporation into protein fractions was directly related to the amount of dietary Se, and the amount of stable isotope in the peak was inversely related to dietary Se intake. However, the chemical form of the isotope did not affect Se distribution. The discrepancy between Xia et al. (1992) and the present study is probably a result of the much higher normal daily Se intake of North American men compared to Chinese men. Another cause of the discrepancy may be that the Chinese study examined the long-term accumulation of different sources of Se in a protein, whereas we studied the initial uptake of a tracer by subjects fed diets with similar sources of Se. These data also suggest that once Se from broccoli entered a tissue, it entered a pool exchangeable with Se that originated from other chemical sources. This implies that the control of plasma accumulation of Se from broccoli occurs prior to Se entering the Se pool that provides Se for selenoprotein production.

We are convinced that the variable results that we obtained with selenite were a consequence of its chemistry, and not an anomaly of our experimental procedure. The method used to prepare the selenite (Finley et al. 1995 ) was used by our laboratories, as well as our own. There were no problems with the selenite preparation; the Se concentration of the selenite was measured immediately prior to the test meal (after centrifugation to remove insoluble material), and the analyzed Se concentration was very close to the calculated concentration assuming 100% conversion to selenite. Thus, we believe the results to be a consequence of the difference in metabolism of selenite and selenate, and of the potential volatility of selenite in the test meal. In some individuals, the selenite remained in solution until it reached the absorptive area of the gut. Conversely, possibly conditions in the gut of other individuals resulted in the conversion of selenite to elemental Se, thereby rendering it insoluble and reducing absorption. Such potential problems should be carefully considered when designing future human Se studies.

In summary, this report shows that humans readily absorb Se from broccoli, but do not excrete it as well as Se from selenate. Selenium from broccoli is not distributed in the plasma the same as Se from selenate, although the distribution among plasma proteins is unaffected. Because broccoli is a common North American vegetable and because broccoli has many other nutritional benefits, the sum of these reports warrants continued investigation of human metabolism of Se from broccoli.

	ACKNOWLEDGMENTS

 
We gratefully acknowledge the assistance of Eugene Korynta in production of the hydroponic broccoli used in the study. Deborah Hoff for mass spectrometry analyses of stable isotopes and Mary Rydell in preparation of the manuscript.

	FOOTNOTES

 
¹ 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.

² Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the United States Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.

³ Abbreviations used: CV, coefficient of variation; GPC, gel permeation chromatography; GSH-Px, glutathione peroxidase; ICP–MS, inductively-coupled plasma–mass spectrometry; Se, selenium; SeCys, selenocysteine; SeMet, selenomethionine; SeMSC, Se methyl-selenocysteine.

Manuscript received August 25, 1998. Initial review completed September 16, 1998. Revision accepted January 5, 1999.

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