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Perit Dial Int 29(1): 89-101
2009
© 2009 International Society for Peritoneal Dialysis
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Clinical

ANTIOXIDANT STATUS OF PATIENTS ON PERITONEAL DIALYSIS: ASSOCIATIONS WITH INFLAMMATION AND GLYCOXIDATIVE STRESS

Isabella Sundl1, Johannes M. Roob2, Andreas Meinitzer3, Beate Tiran3, Gholamali Khoschsorur3, Bernd Haditsch2, Herwig Holzer2 and Brigitte M. Winklhofer–Roob1

Human Nutrition & Metabolism Research and Training Center Graz,1 Institute of Molecular Biosciences, Karl-Franzens University; Division of Clinical Nephrology and Hemodialysis,2 Department of Internal Medicine, and Clinical Institute of Medical & Chemical Laboratory Diagnostics,3 Medical University, Graz, Austria

Correspondence to: B.M. Winklhofer–Roob, Human Nutrition & Metabolism Research and Training Center Graz, Institute of Molecular Biosciences, Karl-Franzens University, Schubertstrasse 1, A-8010 Graz, Austria. brigitte.winklhoferroob{at}uni-graz.at


   ABSTRACT
TOP
 ABSTRACT
SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

{diamondsuit} Background: Patients on peritoneal dialysis (PD) frequently exhibit oxidant–antioxidant imbalance, advanced glycation end-product overload, and subclinical inflammation but the interrelations between these pathophysiological changes have not been fully elucidated.

{diamondsuit} Subjects and Methods: To study possible associations, a cross-sectional study of antioxidant status, glycoxidative stress, and inflammation, using HPLC and ELISA methods, was undertaken in 37 PD patients and age- and sex-matched healthy controls.

{diamondsuit} Results: Plasma ascorbate concentrations were low in patients not taking at least low-dose vitamin C supplements. In patients taking vitamin C supplements, there was a positive relation between ascorbate and pentosidine concentrations. Vitamin E and carotenoid concentrations were comparable between patients and controls, while lycopene and lutein/zeaxanthin concentrations were lower. Interleukin-6, C-reactive protein (CRP), and pentosidine concentrations were elevated in PD patients. β-Cryptoxanthin, lycopene, and lutein/zeaxanthin concentrations were inversely related to interleukin-6 concentrations. β-Cryptoxanthin concentrations were also inversely related to CRP concentrations. Pentosidine showed a low dialysate-to-plasma ratio, indicating low peritoneal clearance. Pentosidine concentrations increased with duration of PD therapy, while {alpha}- and β-carotene concentrations decreased. Malondialdehyde concentrations were elevated compared to controls but remained within the normal range. Retinol concentrations decreased with PD therapy and were inversely related to interleukin-6 and CRP concentrations.

{diamondsuit} Conclusions: Low-dose vitamin C supplements and a carotenoid-rich diet should be recommended for PD patients to maintain normal antioxidant status and efficiently counteract the chronic inflammatory response, rather than high doses of vitamin C, which could play a role as a precursor of pentosidine.

KEY WORDS: Advanced glycation end products; ascorbate; carotenoids; interleukin-6; malondialdehyde; oxidative stress; pentosidine.

Patients with end-stage renal disease (ESRD) exhibit an oxidant–antioxidant imbalance in favor of the former due to impaired antioxidant status on one hand and increased production of reactive oxygen species (ROS) on the other (1–6). In addition, accumulation of advanced glycation end products (AGEs) resulting from impaired renal function, and thus limited metabolism and excretion (7), and increased de novo synthesis of AGEs (8) is well described in patients with ESRD (9,10). Associations of oxidant–antioxidant imbalance, AGE accumulation, and inflammation with long-term complications and increased mortality in patients with ESRD have been demonstrated (1,4,9,10). In contrast, other studies showed that pentosidine concentrations were not associated with poor outcome in patients close to the start of dialysis therapy (11) and reported even a significantly better survival of hemodialysis (HD) patients with high total serum fluorescent AGE and N{epsilon}-(carboxymethyl)-lysine concentrations compared to those with low concentrations (12).

Peritoneal dialysis (PD) is a well-established renal replacement therapy used by approximately 15% of dialysis patients in the developed world (13). Duration of PD therapy is still limited because of progressive deterioration of the peritoneum within a relatively short period of time (14). This deterioration is due to chronic activation of macrophages and low-grade chronic inflammation on one hand (15,16) and bacterial peritonitis episodes as a major complication of PD treatment on the other (17).

Several factors appear to be pathogenically relevant to low-grade inflammation, including continuous exposure of the peritoneum to PD fluids that are hyperosmolar and have a low pH and a high glucose content. Peritoneal dialysis fluids also contain glucose degradation products (GDPs) (18), which are generated during heat sterilization or prolonged storage (19), particularly at high temperature (20). GDPs accelerate the production of AGEs in the peritoneal membrane and consequently exert adverse effects on the peritoneal membrane (18,19). AGEs are a heterogeneous group of compounds generated in a nonenzymatic reaction of reducing sugars, with amino groups of proteins forming Schiff bases and Amadori products followed by an Amadori rearrangement. One of these compounds, pentosidine, is produced through an oxidative pathway and has been proposed as a useful biomarker of in vivo glycoxidative stress (21).

In PD patients, increased ROS production is present due to uremia per se. Oxidative stress is further aggravated by chronic inflammation, diabetes, advanced age, and losses of small molecules, such as the antioxidant ascorbate, into the peritoneal cavity along a concentration gradient between plasma and PD fluids (1). Therefore, even in the presence of vitamin E status that is higher compared to healthy control subjects (1–5), PD patients still exhibit an oxidant–antioxidant imbalance (3,4,22), as evidenced by elevated plasma concentrations of malondialdehyde (MDA), an end product of the peroxidation of polyunsaturated fatty acids with three or more conjugated double bonds (23).

Generation of AGEs is not only accelerated in a pro-oxidant state but AGEs also induce an oxidative stress response, including lipid peroxidation (24,25). Furthermore, binding of AGEs to AGE receptors leads to the activation of redox-sensitive transcription factors, such as nuclear factor-kappa B (NF-{kappa}B), and consequently initiates gene transcription and subsequent protein synthesis of several proinflammatory cytokines, including interleukin-6 (IL-6) (21,26). These cytokines then activate an acute-phase response, leading to synthesis of C-reactive protein (CRP) in the liver (26).

The interactions between oxidant–antioxidant imbalance and AGE accumulation, and its effects on the inflammatory response, which in turn initiate a self-perpetuating vicious cycle, have not been fully elucidated in PD patients but could provide the basis for novel therapy concepts. Therefore, the aim of this investigation was to study associations and possible interactions between antioxidant status, biomarkers of oxidative and glycoxidative stress, and inflammation in PD patients. Focus was directed toward exogenous nonenzymatic antioxidants, including vitamins C and E, different carotenoids with and without provitamin A activity, and retinol, the preformed vitamin A, that have been shown to act as ROS scavenging antioxidants and exert anti-inflammatory effects as well as different effects on the immune response in different animal models and humans (27–29). Biomarkers of oxidative and glycoxidative stress included plasma MDA and pentosidine concentrations, while inflammation was assessed by IL-6 and CRP. In addition, we investigated the effects of duration of PD therapy and different PD fluids on these variables.


   SUBJECTS AND METHODS
TOP
 ABSTRACT
 SUBJECTS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
PATIENTS
In this cross-sectional study, 37 patients with ESRD on PD (25 males, 12 females; 29.8 – 82.7 years of age) regularly attending the PD Outpatient Clinic of the Division of Clinical Nephrology and Hemodialysis of the Department of Internal Medicine, Medical University of Graz, Austria, were enrolled. Duration of PD therapy was 1 – 66 months (median 7.46 months) and residual renal function (RRF), expressed as 24-hour urine volume, was 0 – 2500 mL/day (1313 ± 557 mL/day). Twelve patients took vitamin C supplements at doses of 20 – 300 mg/day (132 ± 77.1 mg/day), 1 patient took 43 mg/day, and another patient 269 mg/day RRR-{alpha}-tocopherol supplement, while 25 did not take any antioxidant supplements. Seven patients had type II diabetes; 5 of them were on insulin therapy. Medications taken by the patients included statins in 12, acetylsalicylic acid in 12, corticosteroids in 4, other immunosuppressive drugs in 4 patients, N-acetyl-cysteine in 1, and an antihistamine in another patient.

At the Division of Clinical Nephrology and Hemodialysis of the Medical University of Graz, Austria, PD treatment is individualized according to medical requirements and, as far as possible, also to preferences of the individual patients. Consequently, 30 study patients used continuous ambulatory PD (CAPD) and 7 were on automated PD (APD). Glucose-based PD solutions with glucose concentrations of 1.36% – 2.27% (Physioneal; Baxter, Vienna, Austria) were used by 25 of the 30 CAPD patients. These PD solutions were buffered with bicarbonate and lactate such that a physiological pH was achieved. Patients performed 2 – 4 exchanges per day with volumes of 1.5 – 2.5 L each. Eleven of these 25 patients used an additional 2.0 – 2.5 L polyglucose-based PD solutions with low pH (Extraneal; Baxter) during the nighttime. Three of the 30 CAPD patients performed 3 – 4 exchanges per day with 2.0 – 2.5 L each of 1.5% – 2.3% glucose-based bicarbonate-buffered PD solutions with physiological pH (Bica Vera; Fresenius Medical Care, Bad Homburg, Germany), while 2 patients performed 4 exchanges per day with 1.5 – 2.5 L of 1.5% glucose-based lactate-buffered PD solutions with physiological pH (Balance, Fresenius Medical Care). Patients on APD used 1.36% – 2.27% glucose-based bicarbonate- and lactate-buffered PD solutions with physiological pH (Physioneal, Baxter) in 3 – 7 cycles during the nighttime. All APD patients except 2 used polyglucose-based solutions with low pH (Extraneal, Baxter) as the last PD solution during the daytime.

To compare outcome variables in patients using different PD fluids, patients were grouped according to the types of PD fluids used: group A consisted of 19 patients that exclusively used glucose-based PD fluids with physiological pH; group B consisted of 13 patients that used glucose-based PD fluids with physiological pH during the daytime and polyglucose-based PD fluids with low pH during the nighttime; and group C consisted of 5 patients that used polyglucose-based PD fluids with low pH during the daytime and glucose-based fluids with physiological pH during the nighttime. Given that a recent publication showed differences in GDPs in PD fluids of different brands (20), an additional comparison was performed with patients grouped according to the brands of PD solutions used (groups D – G).

CONTROL SUBJECTS
Healthy volunteers living in the same area and matched for sex and age (range ± 2.63 years) with the study patients served as control subjects (n = 37). They had to pass strict inclusion/exclusion criteria to be classified as healthy, including anthropometric (body mass index < 30 kg/m2) and routine clinical chemistry variables (aspartate aminotransferase < 30 U/L, alanine aminotransferase < 35 U/L, gamma-glutamyltransferase < 38 U/L, creatinine 5.3 – 11.5 µmol/L, urate 1.7 – 7.5 mmol/L, albumin 35 – 53 g/L, CRP < 9 mg/L, glucose 3.9 – 6.3 mmol/L). Adherence to a special diet and/or taking vitamin supplements or medications were considered exclusion criteria.

ETHICAL CONSIDERATIONS
The study protocol was approved by the Ethics Committee of the Medical University of Graz, Austria, and informed consent was obtained from the PD patients and control subjects.

ANALYTICAL METHODS
Sample Collection: Plasma and Serum: Venous blood was drawn after an overnight fast into plastic tubes containing ethylenediaminetetraacetic acid (EDTA) to obtain plasma; tubes without any addition were used to obtain serum (BD Vacutainer, Belliver Industrial Estate, Plymouth, UK). Blood samples were protected from light using aluminum foil and centrifuged immediately at 1620g at 8°C for 10 minutes. Plasma was separated, divided into aliquots, collected in reaction tubes (Greiner Bio-One, Kremsmünster, Austria), and stored at –80°C until determination of plasma ascorbate, tocopherols, retinol, carotenoids, MDA, pentosidine, and IL-6. Routine clinical chemistry variables were determined in the fresh samples on the day blood was drawn.

Dialysate Effluents: In PD therapy, PD fluids are instilled into the peritoneal cavity and the used dialysate effluents are removed after several hours. In the present study, CAPD patients' overnight dwells, that is, dialysate effluents removed by the first exchange in the morning, were investigated using a standardized protocol. Ten minutes after start of the first morning exchange (which takes approximately 30 minutes) samples were collected into EDTA-containing plastic tubes (BD Vacutainer) and centrifuged immediately at 1620g at 8°C for 10 minutes. The clear supernatants (without cells) were divided into aliquots and stored at –80°C until analysis. Given that, in APD patients, dialysate effluents are removed several times during the night and thus no overnight dwells are available, dialysate effluents were not collected from APD patients.

Five-Day Food Records: For dietary assessment, 29 of the 37 patients completed and returned records of all food items and drinks consumed during a 5-day period with at least one weekend day included. On the basis of these food records, the amounts consumed and the relative distribution of energy derived from protein, fat, and carbohydrates were calculated along with the amounts of vitamins and trace elements. The software package of the Austrian food composition table Ernährungswissenschaftliches Programm (Dato Denkwerkzeuge, Vienna, 1997) was used.

Determination of Ascorbate: Ascorbate was determined using a method modified from Levine et al. (30) and Lykkesfeldt et al. (31). Briefly, immediately after blood collection and centrifugation, 200-µL aliquots of EDTA plasma or 500 µL of EDTA dialysate effluent were deproteinized with equal amounts of metaphosphoric acid (10%, wt/wt, 200 µL and 500 µL respectively) and brought to –80°C until analysis, such that less than 30 minutes elapsed since the venipuncture. Ascorbate content was determined using a reversed-phase column (Zorbax SB C18 Stable Bond 4.6 x 250, 5 µm; Agilent Technologies, Vienna, Austria) with an electrochemical detector (ESA Detektor Coulochem II, Chelmsford, United Kingdom) and a Model 5011 high-sensitivity analytical cell (ESA) at –200 mV (E1; upstream) and +150 mV (E2; downstream) respectively. The within-run coefficient of variation (CV) was 3% and between-day CV was 4%. Limit of detection was 1.14 µmol/L. Five patient plasma or dialysate effluent samples were processed along with both a standard solution and a control sample from a plasma pool obtained from a number of healthy subjects to be used for long-term quality control.

Determination of Carotenoids, Tocopherols, and Retinol: The determination of carotenoids, tocopherols, and retinol was performed as described by Aebischer et al. (32): a reversed-phase high-performance liquid chromatography (HPLC) method was used for simultaneous determination of carotenoids by a UV detector and tocopherols and retinol by a fluorescence detector. Separation of carotenoids, tocopherols, and retinol was achieved on an HPLC system (Hewlett Packard 1100A; Agilent, Vienna, Austria) using a reversed-phase column. All-trans and cis β-carotenes were measured and total β-carotene was calculated as the sum of all-trans and cis β-carotene. The within-run CV was 3.62% for {alpha}-carotene, 1.35% for cis β-carotene, 6.08% for all-trans β-carotene, 1.83% for lycopene, 1.01% for lutein/zeaxanthin, 0.54% for β-cryptoxanthin, 1.26% for {alpha}-tocopherol, 0.80% for {gamma}-tocopherol, and 0.87% for retinol. The between-day CV was 3.76% for {alpha}-carotene, 1.98% for cis β-carotene, 6.39% for all-trans β-carotene, 2.91% for lycopene, 1.80% for lutein/zeaxanthin, 2.27% for β-cryptoxanthin, 1.81% for {alpha}-tocopherol, 2.78% for {gamma}-tocopherol, and 1.40% for retinol. Limit of detection was 0.002 µmol/L for the carotenoids, 0.012 µmol/L for {alpha}- and {gamma}-tocopherols, and 0.017 µmol/L for retinol. Six patient plasma samples were processed along with a standard solution and two control samples from a plasma pool obtained from a number of healthy subjects to be used for long-term quality control.

Determination of MDA: MDA was determined using an HPLC method with fluorescence detection, as described by Khoschsorur et al. (33). The CV was 4% within runs and 3% between days (33).

Determination of Pentosidine: Pentosidine was determined using an HPLC method with fluorescence detection as described by Takahashi et al. (34) with modifications. Limit of detection was 9 nmol/L. For long-term quality control, the study samples were processed along with a standard solution and quality control plasma samples obtained from control plasma pools with low (pool A) and high (pool B) pentosidine concentrations. Pool A was obtained from a number of healthy subjects with normal pentosidine concentrations and pool B consisted of a mixture of samples from ESRD patients on HD and healthy subjects to obtain a pentosidine concentration about 3 times higher than that of healthy subjects. The pentosidine standard was a kind gift from Prof. Vincent Monnier and Dr. David Sell, Case Western Reserve University, Cleveland, OH, USA. The within-run CV was 2.5% and the between-day CV was 2.9%.

Determination of IL-6: Circulating IL-6 concentrations were measured in duplicate in serum samples using a specific chemiluminescent ELISA (QuantiGlo; R&D Systems, Wiesbaden–Nordenstadt, Germany) according to the manufacturer's instructions. The limit of detection as provided by the manufacturer for this method is 0.48 pg/mL.

STATISTICAL ANALYSIS
Where necessary, for all analyses, data were log transformed to obtain normal distribution. Comparisons of the different variables between PD patients and control subjects were made by t-tests (if normally distributed before or after log transformation) or by Mann–Whitney rank-sum tests (if data were not normally distributed, even after log transformation). Relations between different variables were studied using linear regression analysis. Multiple regression analysis was used for studying determinants of biomarkers of oxidative and glycoxidative stress. Comparisons between patients grouped according to types of PD solutions used (groups A – C and groups D – G) were made by one-way analysis of variance. All statistical analyses were performed using SigmaStat 3.1. All graphs were created using SigmaPlot 9.0 (both by SPSS, Erkrath, Germany).


   RESULTS
TOP
 ABSTRACT
 SUBJECTS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
COMPARISONS BETWEEN PD PATIENTS AND AGE- AND SEX-MATCHED CONTROL SUBJECTS
Routine Clinical Chemistry: As shown in Table 1, creatinine and urea concentrations were higher in PD patients compared to age- and sex-matched healthy controls. Cholesterol concentrations were lower and triglyceride concentrations were higher in patients than in controls. Patients on PD showed significantly higher total leukocyte but lower erythrocyte counts, lower hemoglobin, and lower albumin concentrations.


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  		TABLE 1 Characteristics of Patients and Age- and Sex-Matched Control Subjects (Mean±SD)

 

Vitamins and Carotenoids: Patients that did not take vitamin C supplements (n = 25) showed significantly lower plasma ascorbate concentrations compared to control subjects (p = 0.008), while patients that supplemented vitamin C (132 ± 77.1 mg/day) had plasma ascorbate concentrations that did not differ from those of control subjects (Table 2). As expected, plasma retinol concentrations were significantly higher in PD patients compared to control subjects (Table 2). Plasma {alpha}-tocopherol concentrations were also significantly higher in PD patients. This difference disappeared when {alpha}-tocopherol concentrations were standardized for the sum of cholesterol and triglyceride concentrations. Plasma {gamma}-tocopherol concentrations did not differ between PD patients and control subjects, whether corrected for the sum of cholesterol and triglycerides or not (Table 2). Plasma concentrations of {alpha}- and β-carotene and β-cryptoxanthin did not differ significantly between PD patients and control subjects, while lycopene and lutein/zeaxanthin concentrations were significantly lower in PD patients (Table 2).


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  		TABLE 2 Plasma Concentrations of Vitamins, Carotenoids, and Biomarkers of Oxidative and Glycoxidative Stress and Inflammation in Peritoneal Dialysis (PD) Patients and Age- and Sex-Matched Control Subjects (Mean±SD)

 

Biomarkers of Oxidative and Glycoxidative Stress and Inflammation: As shown in Table 2, plasma MDA concentrations were 1.5-fold higher (p < 0.001) in PD patients compared to control subjects. MDA concentrations in 36/37 PD patients were within the normal range (33,35). Patients on PD showed 4.9-fold increased plasma pentosidine concentrations (p < 0.001) compared to control subjects (Table 2). There was a positive relation between plasma pentosidine and serum creatinine concentrations (r = 0.42, p = 0.01). In PD patients, IL-6 could not be determined for 2 patients due to technical reasons. Both CRP and IL-6 concentrations were significantly higher in PD patients compared to control subjects (both p < 0.001; Table 2).

ESTIMATION OF DIETARY INTAKE
Five-day food records were completed by 29/37 patients, while 8 patients did not record dietary intake due to noncompliance with the study protocol. Total energy intake was 8325 ± 3012 kJ/day (1988 ± 719 kcal/day), with an energy distribution of 50.5% ± 6.16% from carbohydrates, 33.1% ± 4.91% from fat, and 13.4% ± 2.41% from protein. Micronutrient intake was 1.24 ± 1.04 mg/day vitamin A, 10.0 ± 4.73 mg/day vitamin E (excluding supplements; supplements were taken by 2 patients at a dose of 43 and 269 mg/day respectively), 121 ± 56.1 mg/day vitamin C (excluding supplements; vitamin C supplements were taken by 12 patients at a dose of 132 ± 77.1 mg/day), and 3.11 ± 1.45 mg/day β-carotene. There was a significant regression of plasma concentrations on dietary intake of β-carotene (r = 0.46, p = 0.01), while such relations were not found for retinol, ascorbate, or vitamin E.

ASSOCIATIONS OF ANTIOXIDANTS WITH BIOMARKERS OF OXIDATIVE AND GLYCOXIDATIVE STRESS
There was a significant linear regression of plasma pentosidine on plasma ascorbate concentrations in PD patients taking vitamin C supplements (r = 0.65, p = 0.02) (Figure 1) but not in PD patients that did not use vitamin C supplements, or in healthy subjects. Multiple regression analysis of plasma MDA showed a significant regression of MDA on plasma pentosidine (p = 0.04), ascorbate (p = 0.01), {alpha}-tocopherol (p = 0.02), ratios of {alpha}-tocopherol to cholesterol (p = 0.01), and β-cryptoxanthin (p = 0.05) but not on the other variables entered in the analysis ({gamma}-tocopherol, ratio of {gamma}-tocopherol to cholesterol, {alpha}-carotene, β-carotene, lycopene, lutein/zeaxanthin, age, duration of PD therapy).


Figure 1
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  		Figure 1 — Regression of plasma pentosidine on plasma ascorbate concentrations in supplement users (n=12).

 
ASSOCIATIONS OF RETINOL AND CAROTENOIDS WITH BIOMARKERS OF INFLAMMATION
Serum IL-6 concentrations showed a significant linear regression on CRP concentrations (r = 0.74, p < 0.001; y = –227 + 409 * IL-6 on log-transformed data). A possible impact of micronutrients on CRP and IL-6 concentrations was studied. Significant negative relations were found between IL-6 concentrations and β-cryptoxanthin, lycopene, lutein/zeaxanthin, and retinol concentrations (Figure 2) but not between IL-6 concentrations and {alpha}- and β-carotene concentrations. The regressions of IL-6 and β-cryptoxanthin also remained significant when age was included as an independent variable in multiple regression analysis (r = 0.61, p < 0.001). Serum CRP concentrations were inversely related with β-cryptoxanthin and retinol concentrations (Figure 3). There was no significant regression of either IL-6 or CRP on the antioxidants ascorbate and {alpha}- and {gamma}-tocopherol.


Figure 2
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  		Figure 2 — Regression of serum interleukin-6 (IL-6) concentrations on plasma β-cryptoxanthin, lycopene, lutein/zeaxanthin, and retinol concentrations (please note the log scale on the y-axis).

 

Figure 3
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  		Figure 3 — Regression of serum C-reactive protein (CRP) concentrations on plasma β-cryptoxanthin and retinol concentrations (please note the log scale on the y-axis).

 

RELATIONS BETWEEN PLASMA AND DIALYSATE IN CAPD PATIENTS
There were significant correlations between plasma and dialysate concentrations for ascorbate and pentosidine in CAPD patients (n = 30) (Table 3). In contrast, there was no correlation between plasma and dialysate MDA concentrations (Table 3). Retinol, tocopherols, and carotenoids, all of which are lipophilic compounds, were not detectable in dialysate effluent supernatants. Dialysate-to-plasma (D/P) ratios were calculated as surrogate markers of clearance efficiency by PD treatment in CAPD patients (n = 30). While ascorbate (D/P ratio 0.657 ± 0.135) and MDA (D/P ratio 0.485 ± 0.370) were efficiently cleared, pentosidine (D/P ratio 0.076 ± 0.078) accumulated (Figure 4).


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  		TABLE 3 Plasma and Dialysate Concentrations of Ascorbate, Malondialdehyde (MDA), and Pentosidine in CAPD Patients (n=30) (Mean±SD)

 

Figure 4
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  		Figure 4 — Dialysate-to-plasma ratios of ascorbate, malondialdehyde (MDA), and pentosidine in CAPD patients (n=30) (please note the log scale on the y-axis).

 

EFFECTS OF DURATION OF PD THERAPY AND TYPES OF PD FLUIDS
Plasma pentosidine concentrations increased significantly with duration of PD therapy (Figure 5). In contrast, plasma retinol and {alpha}- and β-carotene concentrations decreased with time on PD therapy (Figure 5). There were no relations between duration of PD therapy and the other variables studied, particularly not with biomarkers of inflammation. Plasma pentosidine was significantly related to serum creatinine concentrations (r = 0.42, p = 0.01). Residual renal function, expressed as 24-hour urine volume, did not show a relation with pentosidine, IL-6, CRP, MDA, time on PD, or age. When patients were grouped according to the different types of PD fluids used (groups A, B, and C, according to characteristics such as glucose/polyglucose, physiological/low pH) or according to the brands of PD fluids used (groups D – G), differences in plasma concentrations of antioxidants, MDA, pentosidine, IL-6, and CRP did not reach statistical significance (data not shown).


Figure 5
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  		Figure 5 — Regression of plasma pentosidine, retinol, and {alpha}- and β-carotene concentrations on duration of peritoneal dialysis (PD) therapy (please note the log scale on the x-axis).

 
EFFECTS OF AGE
Plasma retinol (r = –0.41, p = 0.01) and lycopene (r = –0.49, p = 0.002) concentrations decreased significantly with age. Circulating IL-6 concentrations increased significantly (r = 0.44, p = 0.008), while CRP concentrations did not change. There was a significant decrease in serum creatinine concentrations with age (r = –0.49, p = 0.002). There was no age effect on the other variables studied.

COMPARISONS BETWEEN PATIENTS WITH AND PATIENTS WITHOUT DIABETES MELLITUS
Fasting glucose concentrations were significantly higher in patients with diabetes mellitus compared to patients without, but significant differences were not found for plasma pentosidine, MDA, CRP, IL-6, or for the antioxidants {alpha}- and {gamma}-tocopherol, ascorbate, and carotenoids (data not shown).


   DISCUSSION
TOP
 ABSTRACT
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The current study is the first to comprehensively assess biomarkers of antioxidant status as well as oxidative and glycoxidative stress and inflammation in PD patients, and to elucidate the interrelations between these variables, along with associations with dietary intake. Overall, several biomarkers of antioxidant status of PD patients matched those of healthy control subjects.

Plasma ascorbate concentrations of patients taking vitamin C supplements (median 100 mg/day, range 20 – 300 mg/day) were comparable to those of healthy controls but were significantly lower in patients not taking vitamin C supplements. Vitamin C intake (diet plus supplements) was higher than the Dietary Reference Intakes (DRIs; 90 mg/day for males, 75 mg/day for females) (36) in all but 5 patients. Low plasma ascorbate concentrations in PD patients could be explained by removal of ascorbate by PD due to its low molecular mass (176 g/mol) and reduced tubular reabsorption due to severely impaired or even absent kidney function (1). In fact, patients lost, on average, 12.5 ± 8.42 mg ascorbate in their overnight dwells (equal to 15% of the DRI), as calculated individually for both male and female patients due to different DRIs as shown above. Given the significant relation between ascorbate concentrations in plasma and dialysate, one could extrapolate that even higher losses of ascorbate into dialysate could occur after intake of higher doses of vitamin C. Even in the presence of high losses (37), vitamin C supplements were not deemed necessary when patients consumed at least 60 mg vitamin C per day through diet (38). High-dose vitamin C supplements have not been recommended in PD patients due to the risk of hyperoxalemia (39,40); however, based on the results of the present and previous studies (1,2), a vitamin C supplement of 100 – 200 mg/day seems adequate to maintain plasma ascorbate concentrations at levels comparable to those of healthy subjects.

Interestingly, there was a significant positive correlation between plasma concentrations of pentosidine and ascorbate in vitamin C supplement users in the present study, while such a correlation was absent in patients not taking vitamin C supplements. It has been demonstrated that ascorbate and dehydroascorbate as well as glucose can act as precursors of AGEs both in vitro (41) and in vivo (42,43).

Accumulation of AGEs in ESRD has been described extensively (11,12,26,44). Plasma pentosidine concentrations were 4.9-fold higher compared to control subjects in the present study and 7-fold higher in another study on predialysis ESRD patients (11). Plasma pentosidine concentrations did not differ between PD patients with and without diabetes mellitus, which is in agreement with another study on predialysis ESRD patients (11).

A significant correlation between pentosidine concentrations in plasma and dialysate has also been observed by others (11). Compared to D/P ratios for ascorbate and MDA, D/P ratios for pentosidine were low, indicating low peritoneal clearance, probably because a high proportion of pentosidine is protein bound and therefore not readily accessible for excretion. Plasma pentosidine concentrations increased with duration of PD therapy due possibly to a decrease in RRF with time. Animal experiments showed that pentosidine is metabolized and excreted in the kidney, indicating that decreased renal function might lead to accumulation of pentosidine (7). This is underlined by the positive relation between plasma pentosidine and serum creatinine concentrations observed in the present as well as in other studies (45,46) and an inverse relation between glomerular filtration rates and plasma pentosidine concentrations in predialysis patients (11). In fact, pentosidine concentrations in plasma and peritoneal proteins were significantly lower in PD patients with RRF than in anuric patients (47).

Concentrations of AGEs also decreased rapidly upon restoration of normal kidney function after kidney transplantation (48). Given that RRF is an important determinant of AGE accumulation, and that advanced glycation and glycoxidation favor proinflammatory cytokine production and tissue damage through protein modifications, preservation of RRF is expected to contribute to better patient outcome, as reviewed recently (49). Unfortunately, data on residual clearance, in addition to urinary volume, could not be collected as part of the present study protocol.

Biomarkers of inflammation were elevated in PD patients but did not correlate with plasma pentosidine concentrations. This contrasts the positive relation found in predialysis ESRD patients with a higher prevalence of elevated CRP concentrations than seen in the present study (11).

Retinol concentrations were significantly higher in PD patients compared to healthy controls, which is a frequent finding in ESRD patients (2,50). Because of their low molecular weights, free and uncomplexed retinol-binding proteins are filtered by the glomerulus (51). Impaired glomerular filtration rate and reduced catabolism and excretion in ESRD are responsible for elevated retinol concentrations (52). The inverse relation between plasma retinol concentrations and biomarkers of inflammation observed in the present study could reflect an acute-phase response-induced decrease in retinol-binding protein synthesis (53). The inverse relation with time on PD should be further studied.

While previous data on carotenoid status in PD patients have been limited to β-carotene (5) or total carotenoids (1), this study investigated statuses of individual carotenoids. Patients showed comparable {alpha}- and β-carotene as well as β-cryptoxanthin concentrations, but lower lycopene and lutein/zeaxanthin concentrations compared to healthy controls. Both {alpha}- and β-carotene concentrations decreased with duration of PD therapy, while other carotenoids did not change significantly. The higher carotenoid status in the PD patients was significantly related to lower biomarkers of inflammation. Similar observations have been made in elderly women (54) and healthy male smokers (55).

Carotenoids act as ROS-scavenging antioxidants (28) and exert anti-inflammatory effects, improving cell–cell communication and intracellular signaling of the immune system (27,29). Increased proinflammatory cytokine concentrations and acute-phase responses are associated with increased mortality in ESRD patients on HD (9,56); therefore, any measures to avoid excessive generation of proinflammatory cytokines should also be taken in PD patients. Due to the facts that carotenoid status can be easily increased by moderate dietary modifications (57), and, unlike HD patients, PD patients do not have to restrict dietary potassium intake, a diet rich in fruits and vegetables should be recommended. Whether carotenoid supplements would exert more beneficial effects needs to be investigated.

Oxidant–antioxidant imbalance has been demonstrated in PD patients despite their having normal vitamin E status (4,5). Plasma {alpha}-tocopherol concentrations were even higher in PD patients compared to controls, which is in agreement with previous reports (22,58), while plasma {gamma}-tocopherol concentrations and plasma {alpha}- and {gamma}-tocopherol standardized for the sum of cholesterol and triglycerides did not differ. Interestingly, vitamin E intake was below the DRI of 15 mg/day (59), as also shown in another group of PD patients (58). Regardless of high plasma {alpha}-tocopherol concentrations, supplementation with vitamin E leads to a decrease in lipid peroxidation in both PD (60) and HD patients (61). This is in line with the findings that PD patients of the present study showed 1.5-fold higher (but still normal) MDA concentrations compared to control subjects, in contrast to 1.8-fold higher concentrations reported previously (3). Elevated MDA concentrations have been explained not only by lipid peroxidation but also by decreased renal clearance (62). However, the relatively high D/P ratio of the low molecular mass compound MDA (72 g/mol) of 0.49 observed in the present study could indicate efficient clearance by individualized PD treatment.

When comparing patients using different types or brands of PD solutions, we could not detect statistically significant differences in either antioxidant status and lipid peroxidation or in pentosidine concentrations. This is in contrast to the results of GDP formation in vitro in PD fluids exposed to 60°C for 1 day (20), which could be explained by the fact that the PD solutions used by the patients of the present study were not exposed to such high temperatures at any time. Lack of differences between patients using different PD solutions could be due to tailoring of the therapeutic regime according to the individual needs of patients for optimizing PD treatment, and/or the small number of patients using different types of PD fluids.

In summary, PD patients exhibit increased biomarkers of oxidative and glycoxidative stress and inflammation, while antioxidant status, with only a few exceptions such as ascorbate in the absence of supplementation as well as lycopene and lutein/zeaxanthin, is comparable to that of age- and sex-matched healthy controls. Inverse relations between carotenoid status and biomarkers of inflammation and a decrease in carotenoid status with duration of PD therapy might suggest that a diet rich in fruits and vegetables might be beneficial and should be maintained in PD patients to efficiently counteract the chronic inflammatory response, rather than taking high doses of vitamin C, which could play a role as a precursor of pentosidine.


   ACKNOWLEDGMENTS
 
This work was supported by the Austrian National Bank, Grant 8304, the Government of Styria, and the Commission of the European Communities, specific RTD progam "Quality of Life and Management of Living Resources," QLK1-CT-1999-00830, acronym VITAGE. It does not necessarily reflect its view and in no way anticipates the Commission's future policy in this area. Additional support by the Government of Austria (bm:bwk) and DSM Nutritional Products, Basel, Switzerland, is gratefully acknowledged.

The excellent assistance of Waltraud Wonisch, of the Division of Clinical Nephrology and Hemodialysis, Department of Internal Medicine, Medical University of Graz, Austria, is highly appreciated. The pentosidine standard was a kind gift from Prof. Vincent Monnier and Dr. David Sell, Case Western Reserve University, Cleveland, OH, USA.

Received 5 November 2007; accepted 1 May 2008.


   REFERENCES
TOP
 ABSTRACT
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Boeschoten EW, Scrijver J, Krediet RT, Schreurs WHP, Arisz L. Deficiencies of vitamins in CAPD patients: the effect of supplementation. Nephrol Dial Transplant 1988;2 : 87-93.
  2. Blumberg A, Hanck A, Sander G. Vitamin nutrition in patients on continuous ambulatory peritoneal dialysis (CAPD). Clin Nephrol 1983; 20:244 -50.[Medline]
  3. Ozden M, Maral H, Akaydin D, Cetinalp P, Kalender B. Erythrocyte glutathione peroxidase activity, plasma malondialdehyde and erythrocyte glutathione levels in hemodialysis and CAPD patients. Clin Biochem 2002; 35:269 -73.[Medline]
  4. Tarng DC, Chen TW, Huang TP, Chen CL, Liu TY, Wei YH. Increased oxidative damage to peripheral blood leukocyte DNA in chronic peritoneal dialysis patients. J Am Soc Nephrol 2002;13 : 1321-30.[Abstract/Free Full Text]
  5. Bonnefont-Rousselot D, Jaudon MC, Issad B, Cacoub P, Congy F, Jardel C, et al. Antioxidant status of elderly chronic renal patients treated by continuous ambulatory peritoneal dialysis. Nephrol Dial Transplant 1997; 12:1399 -405.[Abstract/Free Full Text]
  6. Handelman GJ. Evaluation of oxidant stress in dialysis patients. Blood Purif 2000;18 : 343-9.[Medline]
  7. Miyata T, Ueda Y, Horie K, Nangaku M, Tanaka S, van Ypersele de Strihou C, et al. Renal catabolism of advanced glycation end products: the fate of pentosidine. Kidney Int1998; 53:416 -22.[Medline]
  8. Miyata T, Horie K, Ueda Y, Fujita Y, Izuhara Y, Hirano H, et al. Advanced glycation and lipidoxidation of the peritoneal membrane: respective roles of serum and peritoneal fluid reactive carbonyl compounds. Kidney Int 2000;58 : 425-35.[Medline]
  9. Qureshi AR, Alvestrand A, Divino-Filho JC, Gutierrez A, Heimburger O, Lindholm B, et al. Inflammation, malnutrition, and cardiac disease as predictors of mortality in hemodialysis patients. J Am Soc Nephrol 2002; 13(Suppl 1):S28 -36.[Abstract/Free Full Text]
  10. Zimmermann J, Herrlinger S, Pruy A, Metzger T, Wanner C. Inflammation enhances cardiovascular risk and mortality in hemodialysis patients. Kidney Int 1999;55 : 648-58.[Medline]
  11. Suliman ME, Heimburger O, Barany P, Anderstam B, Pecoits-Filho R, Rodriguez Ayala E, et al. Plasma pentosidine is associated with inflammation and malnutrition in end-stage renal disease patients starting on dialysis therapy. J Am Soc Nephrol 2003;14 : 1614-22.[Abstract/Free Full Text]
  12. Schwedler SB, Metzger T, Schinzel R, Wanner C. Advanced glycation end products and mortality in hemodialysis patients. Kidney Int 2002; 62:301 -10.[Medline]
  13. Gokal R, Mallick NP. Peritoneal dialysis. Lancet 1999; 353:823 -8.[Medline]
  14. Maiorca R, Sandrini S, Cancarini GC, Gaggia P, Chiappini R, Setti G, et al. Integration of peritoneal dialysis and transplantation programs. Perit Dial Int 1997;17 (Suppl 2):S170 -4.[Medline]
  15. McGregor SJ, Topley N, Jorres A, Speekenbrink AB, Gordon A, Gahl GM, et al. Longitudinal evaluation of peritoneal macrophage function and activation during CAPD: maturity, cytokine synthesis and arachidonic acid metabolism. Kidney Int 1996;49 : 525-33.[Medline]
  16. Li FK, Davenport A, Robson RL, Loetscher P, Rothlein R, Williams JD, et al. Leukocyte migration across human peritoneal mesothelial cells is dependent on directed chemokine secretion and ICAM-1 expression. Kidney Int 1998;54 : 2170-83.[Medline]
  17. Topley N. Membrane longevity in peritoneal dialysis: impact of infection and bio-incompatible solutions. Adv Ren Replace Ther 1998; 5:179 -84.[Medline]
  18. Witowski J, Jorres A, Korybalska K, Ksiazek K, Wisniewska-Elnur J, Bender TO, et al. Glucose degradation products in peritoneal dialysis fluids: do they harm? Kidney Int Suppl2003; 84:S148 -51.[Medline]
  19. Miyata T, Devuyst O, Kurokawa K, van Ypersele de Strihou C. Toward better dialysis compatibility: advances in the biochemistry and pathophysiology of the peritoneal membranes. Kidney Int 2002; 61:375 -86.[Medline]
  20. Erixon M, Wieslander A, Lindén T, Carlsson O, Forsbäck G, Svensson E, et al. How to avoid glucose degradation products in peritoneal dialysis fluids. Perit Dial Int2006; 26:490 -7.[Abstract/Free Full Text]
  21. Singh R, Barden A, Mori T, Beilin L. Advanced glycation end-products: a review. Diabetologia2001; 44:129 -46.[Medline]
  22. Roob JM, Rabold T, Hayn M, Khoschsorur G, Resch U, Holzer H, et al. Ex vivo low-density lipoprotein oxidizability and in vivo lipid peroxidation in patients on CAPD. Kidney Int Suppl 2001; 78:S128 -36.[Medline]
  23. Janero DR. Malondialdehyde and thiobarbituric acid-reactivity as diagnostic indices of lipid peroxidation and peroxidative tissue injury. Free Radic Biol Med 1990;9 : 515-40.[Medline]
  24. Schmidt AM, Yan SD, Wautier JL, Stern D. Activation of receptor for advanced glycation end products: a mechanism for chronic vascular dysfunction in diabetic vasculopathy and atherosclerosis. Circ Res1999; 84:489 -97.[Abstract/Free Full Text]
  25. Bierhaus A, Hofmann MA, Ziegler R, Nawroth PP. AGEs and their interaction with AGE-receptors in vascular disease and diabetes mellitus. I. The AGE concept. Cardiovasc Res 1998;37 : 586-600.[Abstract/Free Full Text]
  26. Schwedler S, Schinzel R, Vaith P, Wanner C. Inflammation and advanced glycation end products in uremia: simple coexistence, potentiation or causal relationship? Kidney Int Suppl2001; 78:S32 -6.[Medline]
  27. Chew BP, Park JS. Carotenoid action on the immune response. J Nutr 2004; 134:257S -61S.[Abstract/Free Full Text]
  28. Krinsky NI. Actions of carotenoids in biological systems. Ann Rev Nutr 1993;13 : 561-87.[Medline]
  29. Hughes DA. Effects of carotenoids on human immune function. Proc Nutr Soc 1999;58 : 713-18.[Medline]
  30. Levine M, Wang Y, Rumsey SC. Analysis of ascorbic acid and dehydroascorbic acid in biological samples. Methods Enzymol 1999; 299:65 -76.[Medline]
  31. Lykkesfeldt J, Loft S, Poulsen HE. Determination of ascorbic acid and dehydroascorbic acid in plasma by high-performance liquid chromatography with coulometric detection—are they reliable markers of oxidative stress? Anal Biochem 1995;299 : 329-35.
  32. Aebischer CP, Schierle J, Schüep W. Simultaneous determination of retinol, tocopherols, carotene, lycopene, and xanthophylls in plasma by means of reversed-phase high performance liquid chromatography. Methods Enzymol 1999;299 : 348-62.[Medline]
  33. Khoschsorur GA, Winklhofer-Roob BM, Rabl H, Auer T, Peng Z, Schaur RJ. Evaluation of a sensitive HPLC method for the determination of malondialdehyde, and application of the method to different biological materials. Chromatographia 2000;52 : 181-4.
  34. Takahashi M, Kushida K, Kawana K, Ishihara C, Denda M, Inoue T, et al. Quantification of the cross-link pentosidine in serum from normal and uremic subjects. Clin Chem1993; 39:2162 -5.[Abstract]
  35. Winklhofer-Roob BM, Puhl H, Khoschsorur G, van't Hof MA, Esterbauer H, Shmerling DH. Enhanced resistance to oxidation of low density lipoproteins and decreased lipid peroxide formation during beta-carotene supplementation in cystic fibrosis. Free Radic Biol Med1995; 18:849 -59.[Medline]
  36. Food and Nutrition Board. Vitamin C. In: Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium and Carotenoids. Washington, DC: National Academy Press;2000 : 95-185.
  37. Sullivan JF, Eisenstein AB, Mottola OM, Mittal AK. The effect of dialysis on plasma and tissue levels of vitamin C. Trans Am Soc Artif Intern Organs 1972; 18:277 -82.[Medline]
  38. Shah GM, Ross EA, Sabo A, Pichon M, Bhagavan H, Reynolds RD. Ascorbic acid supplements in patients receiving chronic peritoneal dialysis. Am J Kidney Dis 1991;18 : 84-90.[Medline]
  39. Shah GM, Ross EA, Sabo A, Pichon M, Reynolds RD, Bhagavan H. Effects of ascorbic acid and pyridoxine supplementation on oxalate metabolism in peritoneal dialysis patients. Am J Kidney Dis1992; 20:42 -9.[Medline]
  40. Mydlik M, Derzsiova K, Svac J, Dlhopolcek P, Zemberova E. Peritoneal clearance and peritoneal transfer of oxalic acid, vitamin C, and vitamin B6 during continuous ambulatory peritoneal dialysis. Artif Organs 1998; 22:784 -8.[Medline]
  41. Grandhee SK, Monnier VM. Mechanism of formation of the Maillard protein cross-link pentosidine. Glucose, fructose, and ascorbate as pentosidine precursors. J Biol Chem 1991;266 : 11649-53.[Abstract/Free Full Text]
  42. Slight SH, Feather MS, Ortwerth BJ. Glycation of lens proteins by the oxidative products of ascorbic acid. Biochem Biophys Acta 1990; 1038:367 -74.[Medline]
  43. Ortwerth BJ, Olesen PR. Glutathione inhibits the glycation and crosslinking of lens proteins by ascorbic acid. Exp Eye Res 1988; 47:737 -50.[Medline]
  44. Weiss MF, Erhard P, Kader-Attia FA, Wu YC, Deoreo PB, Araki A, et al. Mechanisms for the formation of glycoxidation products in end-stage renal disease. Kidney Int 2000;57 : 2571-85.[Medline]
  45. Takahashi M, Oikawa M, Nagano A. Effect of age and menopause on serum concentrations of pentosidine, an advanced glycation end product. J Gerontol A Biol Sci Med Sci 2000;55 : 137-40.
  46. Sugiyama S, Miyata T, Ueda Y, Tanaka H, Maeda K, Kawashima S, et al. Plasma levels of pentosidine in diabetic patients: an advanced glycation end product. J Am Soc Nephrol1998; 9:1681 -8.[Abstract]
  47. Bayazit AK, Vogt BA, Dell KM, Davis ID, Noyan A, Anarat A, et al. Effect of peritoneal dialysis prescription on pentosidine in children. Pediatr Nephrol 2003;18 : 1049-54.[Medline]
  48. Makita Z, Radoff S, Rayfield EJ, Yang Z, Skolnik E, Delaney V, et al. Advanced glycosylation end products in patients with diabetic nephropathy. N Engl J Med 1991;325 : 836-42.[Abstract]
  49. Li PK, Cheng YL. Therapeutic options for preservation of residual renal function in patients on peritoneal dialysis. Perit Dial Int 2007; 27(Suppl 2):S158 -63.[Abstract/Free Full Text]
  50. Ha TK, Sattar N, Talwar D, Cooney J, Simpson K, O'Reilly DS, et al. Abnormal antioxidant vitamin and carotenoid status in chronic renal failure. QJM 1996;89 : 765-9.[Abstract/Free Full Text]
  51. Kanai M, Raz A, Goodman DS. Retinol-binding protein: the transport protein for vitamin A in human plasma. J Clin Invest1968; 47:2025 -44.[Medline]
  52. Mogielnicki RP, Waldmann TA, Strober W. Renal handling of low molecular weight proteins. I. L-Chain metabolism in experimental renal disease. J Clin Invest 1971;50 : 901-9.[Medline]
  53. Rosales FJ, Ross AC. Acute inflammation induces hyporetinemia and modifies the plasma and tissue response to vitamin A supplementation in marginally vitamin A-deficient rats. J Nutr1998; 128:960 -6.[Abstract/Free Full Text]
  54. Walston J, Xue Q, Semba RD, Ferrucci L, Cappola AR, Ricks M, et al. Serum antioxidants, inflammation, and total mortality in older women. Am J Epidemiol 2006;163 : 18-26.[Abstract/Free Full Text]
  55. Watzl B, Kulling SE, Moseneder J, Barth SW, Bub A. A 4-wk intervention with high intake of carotenoid-rich vegetables and fruit reduces plasma C-reactive protein in healthy, nonsmoking men. Am J Clin Nutr 2005; 82:1052 -8.[Abstract/Free Full Text]
  56. Kimmel PL, Phillips TM, Simmens SJ, Peterson RA, Weihs KL, Alleyne S, et al. Immunologic function and survival in hemodialysis patients. Kidney Int 1998;54 : 236-44.[Medline]
  57. Yeum KJ, Booth SL, Sadowski JA, Liu C, Tang G, Krinsky NI, et al. Human plasma carotenoid response to the ingestion of controlled diets high in fruits and vegetables. Am J Clin Nutr1996; 64:594 -602.[Abstract/Free Full Text]
  58. Nikolakakis N, Kounali D, Tornaritis M, Anastassou A, Papadakis E, Kassotakis G, et al. Adipose tissue fatty acid composition, serum lipids, and serum {alpha}-tocopherol in continuous ambulatory peritoneal dialysis patients living on the island of Crete. Perit Dial Int 1999; 19:154 -9.[Abstract/Free Full Text]
  59. Food and Nutrition Board. Vitamin E. In: Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium and Carotenoids. Washington, DC: National Academy Press;2000 : 186-283.
  60. Mydlik M, Derzsiova K, Racz O, Sipulova A, Boldizsar J, Lovasova E, et al. Vitamin E as an antioxidant agent in CAPD patients. Int J Artif Organs 2002;25 : 373-8.[Medline]
  61. Roob JM, Khoschsorur G, Tiran A, Horina JH, Holzer H, Winklhofer-Roob BM. Vitamin E attenuates oxidative stress induced by intravenous iron in patients on hemodialysis. J Am Soc Nephrol 2000; 11:539 -49.[Abstract/Free Full Text]
  62. Daschner M, Lenhartz H, Botticher D, Schaefer F, Wollschlager M, Mehls O, et al. Influence of dialysis on plasma lipid peroxidation products and antioxidant levels. Kidney Int1996; 50:1268 -72.[Medline]




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