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Articles

Selenium and Zinc Status Are Suboptimal in a Sample of Older New Zealand Women in a Community-Based Study¹

Department of Human Nutrition and ^* Department of Preventive and Social Medicine, University of Otago, Dunedin, New Zealand

²To whom correspondence should be addressed. E-mail: christine.thomson{at}stonebow.otago.ac.nz.

	ABSTRACT

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The importance of selenium and zinc in the immune functioning of the aged is widely recognized. Seniors in New Zealand are at particularly high risk of low selenium status because of the low selenium soil environment. The zinc status of the New Zealand elderly has never been assessed. In this cross-sectional study, the biochemical selenium, zinc and lipid levels, physical functional capacity and dietary intakes of 103 randomly selected free-living New Zealand women (mean age ± SD, 75 ± 3 y) were assessed. Among nonusers of selenium supplements (n = 80), 80% [95% confidence interval (CI): 70; 88%] had plasma selenium levels (0.85 ± 0.23 µmol/L) below 1.00 µmol/L [10% below mean plasma selenium necessary for full expression of glutathione peroxidase (GPx) activity in New Zealand subjects]. Plasma selenium was strongly correlated with GPx: r = 0.56; P < 0.0001. For nonusers of zinc supplements (n = 88), serum zinc concentrations were 12.4 ± 1.4 µmol/L, with 12% (95% CI: 6; 21%) having levels below the cut-off value (10.7 µmol/L). Estimated mean daily selenium and zinc intakes were 34 ± 10 µg and 8.7 ± 2.0 mg, respectively. Subjects in the highest tertile of a functional capacity index had higher biochemical zinc and selenium values than those in the lowest tertile (P < 0.05). The correlation between plasma selenium and GPx indicates that selenium intake in these women is still insufficient for full expression of GPx activity. Lower serum zinc levels also appear to be prevalent. Because a suboptimal trace element status may be more common among those with a poor physical functioning, promotion of the consumption of nutrient dense foods or supplements to improve selenium and zinc status of elderly women in New Zealand may be beneficial.

KEY WORDS: • zinc • selenium • elderly women • nutrient intake • biochemical indices • New Zealand

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Both aging and undernutrition influence immune function in humans. The effects appear to be cumulative, but it has been suggested that the role of nutrition is more important than age in the elderly population (1) . Of all the nutrients involved in immunocompetence, selenium and zinc are the two trace elements that have been most frequently investigated in older adults (2 –13) . Selenium’s role in immune function, as well as cancer prevention, is related to its physiologic requirement as an active component in various enzymes involved in redox reactions protecting membranes from oxidative damage (14 ,15) . Zinc’s role is attributed to a decline in both antibody and cell-mediated responses (16 ,17) , both of which have been reversed by supplemental zinc at physiologic levels (18 ,19) .

The selenium status of different populations has been investigated in traditionally low selenium countries such as New Zealand, Italy, France and Finland. In a 1996 report, Thomson and Robinson (20) concluded that the selenium status of New Zealanders had improved since the first reports in the early 1970s. The lack of correlation between blood selenium and glutathione peroxidase activity (GPx)³ noted in the New Zealand studies conducted after 1990 indicated that, at least for this enzyme, selenium intake was close to that required for maximum activities of GPx, a measure also used to determine selenium requirements. The improved selenium status was attributed to the importation of Australian wheat and breakfast cereals with a higher selenium content, together with an increased consumption of fish and poultry. The subjects in these studies were healthy adults, aged 19–50 y; only one early study had investigated elderly people (21) , which was unfortunate. Seniors are especially prone to low selenium status because intakes of selenium are strongly correlated with those of total energy and protein (22) . More recent studies in younger New Zealand adults still indicate an insufficient selenium intake for full expression of GPx and do not confirm the earlier findings of 1996 (23 , Paterson and Thomson, unpublished results).

Similarly, both dietary and biochemical data suggest that the current Western diets of the elderly may induce a considerable risk of zinc deficiency. The etiology of suboptimal zinc intake in older people has been related to low energy intakes and changes in eating habits that may reduce the amount and bioavailability of dietary zinc (24) , e.g., decrease in flesh foods concomitant with an increase in cereal consumption. Flesh foods are rich sources of readily available zinc, whereas cereals contain high levels of phytate, a potent inhibitor of zinc absorption. In a recent study of 330 New Zealand women, aged between 18 and 40 y, the prevalence of low serum zinc concentration was higher than in earlier studies overseas (25) . No data on the zinc status of older New Zealand adults are available, but the above-mentioned trend could be even more prevalent in this country’s elderly population.

The detrimental effect of a marginal selenium status on immune function may be further aggravated by inadequate zinc intake. Therefore, the objective of this study was to investigate the current selenium and zinc status of New Zealand seniors as a pilot for investigations of the effect of inadequate selenium and zinc status on the immune function and infectious illness in the elderly.

	SUBJECTS AND METHODS

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Survey design and study population

A cross-sectional survey was conducted from June to August, 2000 on 103 women aged between 70 and 80 y, living within the Dunedin urban area (South Island of New Zealand), randomly selected from the 1998 electoral roll. In addition to the age requirement, inclusion criteria were as follows: noninstitutionalized, ambulatory and no terminal illnesses. In total, 250 persons were sent an information letter, which was followed up by a telephone call for screening purposes. Of those approached, 103 women agreed to participate, met the inclusion criteria and gave written informed consent. Eighty-seven women were classified as nonresponders of whom 64 agreed to answer a short nonresponders questionnaire by telephone; 37 women were not traceable and 23 were not eligible or had died. This gave an overall response rate of 54% for those who were eligible and traceable. The study was approved by the University of Otago Human Ethics Committee.

Experimental measures

A fasting blood sample, anthropometric measurements and a limited set of physical functioning measures were collected during a morning clinic session in the Department of Human Nutrition, University of Otago. Instructions were provided on how to fill out a self-administered food-frequency questionnaire (FFQ). During a subsequent home visit, the food-frequency list was checked and additional questionnaires focusing on nutritional and health related issues were administered.

    Biochemical analyses. A fresh 3-mL EDTA blood sample from fasting subjects was used for a complete blood count. For selenium, 0.5 mL EDTA plasma was used for analysis using flow injection hydride generation atomic absorption spectrometry (AAS) (Perkin-Elmer Model 3100; Perkin-Elmer, Norwark, CT), as described by Tiran et al. (26) , with a between-run CV of 6%. An external normal range quality control standard (Bi-Level whole blood Trace Element Toxicological Control, Product #44522, Utak Laboratories, Valencia, CA) gave a mean of 2.80 ± 0.38 µmol/L (n = 10) (assigned value of 2.47 µmol/L). GPx activities in whole blood were measured via a modification of the coupled-enzyme procedure with glutathione reductase (27 ,28) , automated on the Cobas Fara autoanalyzer (Roche Diagnostic Systems, Somerville, NJ) (between-run CV of 10%). For serum zinc analysis, a 0.5-mL trace element–free sample was analyzed via AAS in our trace element–free laboratory, using a modification of the method described by Smith and colleagues (29) . Serial replication of aliquots of a pooled serum sample and quality control sera were used to check the precision and accuracy of the analytical method. The CV for zinc in a pooled serum sample was 2.0%. Values for the quality control sera (Bovine serum reference material no. 1598, National Institute of Standards and Technology, Gaitherburg, MD) were 13.75 ± 0.45 µmol/L (CV 3.2%) compared with the certified value of 13.69 ± 0.09 µmol/L. Serum lipids (total and HDL cholesterol, triglycerides) were measured using a 0.5-mL serum sample for enzymatic determination (Roche Diagnostics GmbH, D-68289 Mannheim, Germany) on the Cobas Fara autoanalyzer. Within-run CV for cholesterol (total and HDL) and triglycerides were 4%. LDL cholesterol was calculated using the Friedewald formula (LDL cholesterol = total cholesterol - HDL cholesterol - triglycerides/5). C-reactive protein (CRP) and -1-glycoprotein as measures of positive acute phase proteins and albumin as a measure of protein status and negative acute phase protein were analyzed in 150 µL serum via kinetic turbidimety using the Behring Turbitimeter (Behringwerke AG, Marburg, Germany), with between-run CV of 2.7–4%.

    Anthropometry. Weight was measured to the nearest 0.1 kg using an electronic scale (Seca Alpha, model 770, Hamburg Germany) with subjects wearing light clothing. Height was measured to the nearest 0.001 m with a stadiometer and used with weight to calculate body mass index (BMI) in kg/m². Height and weight were measured in duplicate and means were used in further analyses.

    Physical functioning measurements. Habitual gait speed was measured in seconds by the timed "Up and Go" test, which is the time taken by an individual to stand up from a standard arm chair (seat height 45 cm), walk a distance of 3 m, turn, walk back to the chair and sit down again. The participant walked through the test three times with the first time considered as a practice. The average of two measures was calculated and used in further analyses (30) . Hand-grip strength of the dominant hand (average of two measures) was measured with a hand-grip strength dynanometer (Smedley’s TTM, Tokyo, Japan). Quadriceps muscle strength of the dominant leg was measured with the subject sitting on a table with the lower legs hanging down. A wooden "shoe" attached to the subjects own shoe was used to affix weights. Subjects were then asked to stretch their leg into the horizontal position. Weights were increased gradually with 0.5-kg increments from 1.25 to 5.00 kg maximum, with only the last increment (0.75 kg) as an exception. These measurements were conducted to generate an overall physical functioning score as an index of biological age.

    Estimated dietary intake. Habitual dietary intake was estimated over the previous year by a validated self-administered FFQ. Details and the results of a validation study in older adults are described elsewhere (31 ,32) . The FFQ included a list of 120 food and beverage items plus qualitative questions concerning food preparation and cooking practices. For certain fruits and vegetables, the questionnaire took seasonality into account. Selection of food items was based on earlier diet surveys of the New Zealand population. In response to the question "How often do you usually eat these foods?," subjects were asked to circle one of the available options, i.e., never or rarely, about monthly, once or twice a week, three or four times a week or about daily. Standard portion sizes were compiled from estimates of the average amounts consumed by older subjects in earlier surveys (31) . Questionnaires were completed at home and took 30–40 min on average. Intakes of energy and selected nutrients were calculated from the FFQ using the 1988 New Zealand Food Composition Table (New Zealand Institute for Crop & Food Research, Palmerston North, New Zealand). Food composition data of 29 foods were added to the original table from the 1996 New Zealand Food Composition Table . Foods were grouped into eight categories (i.e., meat, poultry and fish; cereal products, legumes and nuts; dairy products; vegetables; fruits; sugar products; beverages; other) and consumption from each group was reported in g/d.

    Other questionnaires. Information on age, marital status, educational level, social contacts, diseases, medication and dietary supplement use, smoking habits, changes in food habits, self-rated health and appetite was collected using an interviewer-administered questionnaire. The capacity to perform activities of daily living (ADL) was assessed as self-rated disabilities in 21 items (35 ,36) .

    Statistical analyses. Statistical analyses were performed using SPSS for windows, version 9.0 (SPSS, Chicago, IL). The normality of variables was examined by visual inspection. The results of variables assumed to be normally distributed are expressed as mean ± SD; those for which this assumption was not appropriate were expressed as the median, and 25th and 75th percentile (P₂₅, P₇₅). Unpaired t tests were performed to compare the study group to the nonresponders. Associations between continuous variables were explored using Pearson’s correlation coefficients. Subjects were divided into tertiles on the basis of an overall physical functioning score calculated from the sum of the physical functioning measurements (i.e., handgrip and quadriceps strength, ADL and "Timed Up & Go" results). Equal weighting was given to each of these measures. Univariate analyses were carried out using one-way ANOVA to test for differences in mean levels of plasma selenium, whole-blood GPx, serum zinc and other biochemical and dietary parameters of interest across the three physical functioning categories. Multivariate analyses were then carried out using analysis of covariance to adjust for possible covariates, i.e., dietary intake, protein status and infection measures. Post-hoc comparisons used the Bonferroni method to adjust for multiple testing.

	RESULTS

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Several characteristics of the study sample were compared with those of the nonresponders (Table 1 ). The only difference was that the latter group reported fewer diseases (P < 0.05). Important reasons for not participating were as follows: no interest/dislike, illness, no time, illness of family members. The study population was predominantly Caucasian with only one participant having a 50% Maori descent. The most common diseases (95% CI) were the following: arthritis (35; 54%), hypertension (13; 29%), cardiovascular diseases, strokes, angina (13; 28%), and osteoporosis (10; 24%). The mean BMI was 26.7 kg/m², with 21% of the women having a BMI >30 kg/m² and 5% <20 kg/m².

Twenty-three subjects reported taking one or more supplements regularly, including selenium (n = 2), zinc (n = 5) multivitamin/mineral supplements (n = 8), kelp (n = 3) and garlic (n = 12). Supplements contained between 15 and 75 µg selenium and 5–25 mg zinc. However, incomplete data were available on the selenium and zinc content of all of the supplements. Therefore it was not possible to estimate the effect on overall selenium and zinc intakes. For this reason, all supplement users were grouped together for the purpose of statistical analysis.

The percentage of women falling below two thirds of recommended dietary intakes was particularly high for selenium (95% CI: 66; 83%) and zinc (95% CI: 32; 51%) intakes (Table 2 ). Ten percent (95% CI: 5; 17%) of the women had energy intakes <5.2 MJ, whereas 29% of the women (95% CI: 20; 38%) had energy intakes <6.3 MJ, a level below which one or more micronutrient deficiencies appear to be common (37) .

View this table: [in this window] [in a new window]   Table 3. Plasma selenium, serum zinc, C-reactive protein (CRP), -1-glycoprotein, albumin and blood lipids, whole-blood glutathione peroxidase (GPx) activity and complete blood count of the study sample of elderly women1

View larger version (17K): [in this window] [in a new window]   Figure 1. The relationship between plasma selenium concentration and glutathione peroxidase (GPx) activity in elderly women who were users (n = 23) and nonusers (n = 80) of selenium supplements.

Means for biochemical indices of three subgroups derived from the tertiles of a composite physical functioning score are summarized in Table 4 . Individuals had significantly higher serum selenium and zinc concentrations when in the highest compared with lowest tertile for physical functioning performance. After adjusting for serum albumin and -1-glycoprotein in multivariate analysis, the difference was still significant for zinc (P < 0.04) and there tended to be a difference for selenium (P < 0.07). No differences were observed among the tertile groups with respect to the dietary intake results or serum lipid concentrations

	DISCUSSION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Selenium

Several studies of selenium intakes and selenium status in the New Zealand population have been reported (20) , but only one early paper described the selenium status in the New Zealand elderly population (21) . A comparison of the group of older Dunedin adults studied in 1976 (21) with our group revealed that plasma selenium levels increased from 0.47 to 0.85 µmol/L. The correlation coefficient between plasma selenium and whole-blood GPx decreased from 0.82 (P < 0.001) to 0.56 (P < 0.0001), indicating that fewer subjects had suboptimal selenium status. At higher plasma selenium concentrations, GPx activities reached a plateau, indicating that the selenium requirement for full expression of that enzyme had been met. As more subjects reached these concentrations, there was a weakening of the correlation coefficient. However, the relatively strong and significant correlation found in the present study of Dunedin elderly women indicated that a large proportion still had suboptimal selenium status. These women had significantly lower plasma selenium and whole-blood GPx activities compared with a younger group of New Zealand women (n = 15, 18–40 y) measured at the same time with the same analytical method [plasma selenium: 0.86 ± 0.22 vs. 1.01 ± 0.22 µmol/L (95% CI of the difference: -0.28; -0.03 µmol/L) and whole-blood GPx: 19.1 ± 4.5 vs. 20.7 ± 6.1 U/g hemoglobin (Hb) (95% CI of the difference: -4.3; 1.1 U/g Hb)] (Corpeleijn and Thomson, unpublished results). In addition, the older women had low plasma selenium levels in comparison with other ambulatory elderly populations living in traditionally low selenium countries such as France and Italy (4 ,7 ,40 –42) . Institutionalized or hospitalized elderly in those countries had either higher (11) or lower selenium levels (43 ,44) than our subjects.

The increase in selenium status of the adult population of New Zealand is most likely due to the importation of Australian wheat and wheat products, supplementation of stock feed resulting in increased concentrations in meat and poultry, and the increase in consumption of fish, poultry, nonrefined cereal and legumes. In the 1996 report (20) , it was argued that supplementing wheat for human consumption with selenium-containing fertilizers, as has been practiced in Finland, or recommending dietary selenium supplements may not be necessary for New Zealand. Increased intake of the foods that contain high levels of selenium seemed to be sufficient at that stage. Nevertheless, a different approach may be necessary for New Zealand’s older population. Furthermore, in the South Island, 100% New Zealand grown wheat is used in bread-making, whereas in the North Island, between 50 and 100% imported wheat is used (Ministry of Health, personal communication). This places the elderly living in the South Island of New Zealand at particular risk. Because reductions in total food intake, especially flesh foods, are very common among the aged due to reduced physical activity and declining appetite, the contribution of particular selenium-rich foods to the increase in selenium status may also be rather limited. Indeed, from our data (Table 2) as well as from data of the New Zealand Mosgiel study (31) , it is evident that usual daily selenium intake in the older population of New Zealand’s South Island is compromised. Both our study as well as the Mosgiel study (31) used the New Zealand Food Composition Tables for calculation of nutrient intakes. In those tables, estimates of the selenium content of wheat products are based on a national average figure, and because of the inconsistent use of imported wheat throughout the country, the selenium intakes in our South Island women may be overestimated.

Zinc

The average serum zinc concentration of the elderly women in this study was 12.4 µmol/L (range, 9.2–16.6 µmol/L); 12% were classified as mildly zinc deficient on the basis of a fasting serum zinc concentration of <10.7 µmol/L. These average serum zinc concentrations were among the lowest reported for seniors in Western countries, which have ranged from 11.0 to 16.0 µmol/L (2 ,5 ,8 ,45 –48) . Indeed, the fasting serum zinc levels of the women in the present study fell between the 15th and 25th percentile of the second National Health and Nutrition Examination Survey distribution for white American females aged between 65 and 74 y (38) . In contrast, the estimated average daily zinc intakes of the women in this study (i.e., 8.7 mg/d) were similar to intakes reported for elderly women overseas, which ranged from 7 to 11 mg/d (48 –50) . They were also comparable to intakes for elderly women previously reported in New Zealand (i.e., 8.6 and 8.9 mg/d), using different assessment methods (24-h dietary recalls) (31 ,51) , confirming our results.

The reasons for the apparent low biochemical zinc status despite comparable zinc intakes in our study compared with others are not clear and reflect the well-known difficulties in the assessment of zinc status, particularly in elderly populations (52) . The high fiber intakes (26 mg/d) and low percentage of zinc contributed by animal products (i.e., 25%) in our study compared with others (19 mg/d and 61%) (2 ,51 ,53) suggest that dietary patterns might compromise dietary zinc bioavailability in our population. Indeed, the serum zinc levels of our seniors were comparable to those of younger New Zealand women, whose serum zinc levels were lower than reported previously for young New Zealand women, apparently because of changes in food selection patterns (i.e., decreased red meat and increased cereal consumption) (25) . Alternatively, population differences in rates of infection, hypoalbuminemia, muscle wasting, the use of diruetics and chronic diseases might contribute to the interpopulation differences noted via their influence on serum zinc (52) . Notwithstanding, the average serum zinc levels in our population did not change, and the percentage with low serum zinc levels declined from 11.8 only to 10.4 and 10.7%, after removing subjects with low plasma albumin (<35 g/L) and elevated CRP (>0.005 g/L), respectively. Hence, these factors probably contributed little to the interpopulation differences in average serum zinc levels noted. On the other hand, interstudy differences in blood sample collection and analytical procedures might have confounded comparisons. For example, adventitious contamination of samples, hemolysis and delays in the separation of serum/plasma could all have systematically increased serum/plasma zinc levels in other studies compared with ours (52) . Carefully controlled and standardized conditions similar to those used in our study are essential to ensure comparability. Despite these concerns, the interpopulation comparison of average serum/plasma zinc concentrations is the recommended approach for evaluating the zinc status of a population, when a zinc supplementation trial has not been conducted (54) . Therefore, without a doubt, our serum zinc results suggest that the zinc status of these elderly Dunedin women was marginal. This is of concern for this elderly population, given the requirement for zinc for optimal immune function (16) .

It was also of interest to note the discrepancy between dietary and biochemical estimates of risk for low zinc status in our population. Based on two thirds of the Australian recommended intakes, the dietary data suggested that 42% of the women were at risk of low zinc intakes per se. By contrast, only 1% were at risk when two thirds of the UK recommendations for daily zinc intake was used. Neither of these estimates was in accordance with the biochemical estimate of 12% at risk, highlighting the need to resolve the apparent controversy in zinc intake recommendations, especially for the elderly.

Study sample

Another consequence of the limited age range of our population was the difficulty of studying age as a factor of influence. We deliberately selected an age range from 70 to 80 y because above this age the likelihood of pathologic processes interfering with trace element status is high. We calculated a biological age factor through a set of physical functioning measurements that had potentially more variance and was potentially more meaningful than chronological age. It is remarkable that in this relatively small group of older women, a tendency was observed for those women who were classified in the lowest tertile, that is with a poorer physical functioning and/or a biologically older constitution, to have a higher risk of biochemical selenium and zinc deficiencies. This difference could be explained in part by individual differences across the tertiles in infection or protein status measures. This trend warrants further investigation in a larger group of elderly people. Countries in which trace element deficiencies are common should be aware of the possible increased severity of problems in their frailer persons.

A high proportion of our elderly women reported suffering from arthritis, which has been associated with low levels of plasma selenium and zinc (55 ,56) . However, controlled clinical trials with selenium have shown variable effects on rheumatic symptoms (57 ,58) . It is not clear whether the low selenium and zinc status of our elderly women contributed to the high incidence of arthritis, and further investigation is warranted.

We aimed to recruit a representative sample of an older female population with an adequate representation of different socioeconomic classes. The response rate of 54% was relatively high in an elderly population, given the respondent burden. No evidence was found that responders differed markedly from the nonresponders. Therefore, we think it is justified to extrapolate our findings to the older female population living in urban areas of New Zealand’s South Island.

Suboptimal zinc or selenium status can reduce resistance to infectious diseases (17 ,59 ,60) , as well as the onset of numerous other health problems such as cancer (61 ,62) and cardiovascular diseases (63) . It appears that the zinc and particularly the selenium status of urban elderly women living in the South Island of New Zealand are suboptimal. Conventional methods to increase selenium intakes may have been successful for younger New Zealanders, but do not seem adequate for the aged. Other strategies, such as the promotion of nutrient-dense foods or trace element supplements for South Island seniors should be considered. The efficacy and effectiveness of such interventions, however, must be investigated in future trials.

	ACKNOWLEDGMENTS

 
We are grateful to our research assistants Fiona Simpson and Eva Corpeleijn, to our laboratory technicians, Steve Tiszavari and Jody Joseph, and to our departmental nurse, Margaret Waldron.

	FOOTNOTES

 
¹ Supported by funds available from the Bristol Meyers/Squibb Mead Johnson Award and the Dutch Foundation Doctor Catharina van Tussenbroek.

³ Abbreviations used^: AAS, atomic absorption spectrometry; ADL, activities of daily living; BMI, body mass index; CI, confidence interval; CRP, C-reactive protein; FFQ, food-frequency questionnaire; GPx, glutathione peroxidase activity; Hb, hemoglobin; P₂₅, P₇₅, 25th, 75th percentile.

Manuscript received March 27, 2001. Initial review completed May 2, 2001. Revision accepted July 5, 2001.

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