Nutrition
Volume 26, Issue 7 , Pages 842-848, July 2010

High-fructose diet elevates myocardial superoxide generation in mice in the absence of cardiac hypertrophy

  • Kimberley Mellor, B.Sc.

      Affiliations

    • Department of Physiology, University of Melbourne, Melbourne, Victoria, Australia
    • ,
  • Rebecca H. Ritchie, Ph.D.

      Affiliations

    • Heart Failure Pharmacology, Baker IDI Heart and Diabetes Institute, Melbourne, Victoria, Australia
    • ,
  • Greta Meredith, B.Sc.

      Affiliations

    • Department of Physiology, University of Melbourne, Melbourne, Victoria, Australia
    • ,
  • Owen L. Woodman, Ph.D.

      Affiliations

    • School of Medical Sciences, Royal Melbourne Institute of Technology, Melbourne, Victoria, Australia
    • ,
  • Margaret J. Morris, Ph.D.

      Affiliations

    • Department of Pharmacology, University of New South Wales, Sydney, New South Wales, Australia
    • ,
  • Lea M.D. Delbridge, Ph.D.

      Affiliations

    • Department of Physiology, University of Melbourne, Melbourne, Victoria, Australia
    • Corresponding Author InformationCorresponding author. Tel.: +61-3-8344-5853; fax: +61-3-8344-5897.

Received 20 October 2008; accepted 10 August 2009. published online 23 November 2009.

Article Outline

Abstract 

Objective

Dietary fructose intake has increased considerably in recent decades and this has been paralleled by an increase in the incidence of insulin resistance, especially in children and adolescents. The impact of a high-fructose diet on the myocardium is not fully understood. The aims of this study were to characterize the murine metabolic and cardiac phenotypes associated with a high-fructose diet and to determine whether this diet imparts differential effects with age.

Methods

Juvenile (4 wk) and adult (14 wk) C57Bl/6 mice were fed a 60% fructose diet or isoenergetic control (starch) diet for 6 wk.

Results

At completion of the dietary intervention (at ages 10 and 20 wk), fructose-fed mice were normotensive; hyperinsulinemia and cardiac hypertrophy were not evident. Interestingly, fructose-fed mice exhibited lower blood glucose levels (10 wk: 4.81±0.28 versus 5.42±0.31mmol/L; 20 wk: 4.88±0.30 versus 5.96±0.42mmol/L, P<0.05) compared with controls. Nicotinamide adenosine dinucleotide phosphate–driven myocardial superoxide production was significantly increased in fructose-fed mice at both ages (by approximately 29% of control at 10 wk of age and 16% at 20 wk, P<0.01). No increase in aortic superoxide production was observed. Fructose feeding did not alter gene expression of the antioxidant thioredoxin-2, suggesting an imbalance between myocardial reactive oxygen species generation and antioxidant induction.

Conclusion

These findings indicate that increased myocardial superoxide production may represent an early and primary cardiac pathologic response to the metabolic challenge of excess dietary fructose in juveniles and adults that can be detected in the absence of cardiac hypertrophy and hypertension.

Keywords: Cardiac hypertrophy, Oxidative stress, Insulin resistance, Fructose, Mice

 

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Introduction 

The increasing usage of fructose sweeteners in food [1] and the association between high-fructose intake and the development of insulin resistance [2], [3], [4] may be contributing factors to the escalating prevalence of metabolic syndrome and type 2 diabetes [5]. In particular, the increased incidence of insulin resistance in children and adolescents has been coincident with increased consumption of the high-fructose content foods that are commercially targeted to this age cohort. Unlike glucose, fructose bypasses the phosphofructokinase rate-limiting step in glycolysis, potentially resulting in unrestrained production of glycolytic endproducts and systemic metabolic dysregulation [6], [7]. In the heart, metabolic dysregulation can lead to pathologies such as oxidative stress and hypertrophy, compromising cellular structure and function [8], [9], [10], [11]. Thus, it is important to identify the mechanisms by which insulin resistance contributes to cardiac damage and dysfunction.

Although, the fructose-fed rodent is a recognized experimental model of systemic insulin resistance and metabolic dysregulation [2], [12], [13], [14], the reported phenotype is variable. Some rodent models of fructose-induced insulin resistance exhibit hypertension, but not all [14], [15], [16], [17], [18], [19], [20]. Fructose feeding is linked with dyslipidemia, but hyperinsulinemia and/or hyperglycemia are not always observed [14], [16], [21], [22], [23]. Some of these differences may be species specific (rat versus mouse) and others may arise due to differences in animal age and feeding duration. In addition, inadequately controlled dietary interventions in some previous studies may have contributed to the apparently inconsistent findings and the difficulty in identifying fructose-specific dietary impacts [2], [12], [14], [19], [22], [24], [25]. No studies of the age-dependent effects of fructose dietary intervention have been reported, and the possibility that a maturational window of fructose susceptibility exists has not been explored.

The cardiac effects of high-fructose intake have not been investigated in detail, and the few available studies have reported inconsistent findings in relation to cardiac hypertrophy and oxidative stress [12], [17]. Cardiac hypertrophy has been observed in response to fructose feeding, but only in the context of elevated blood pressure [17]. Thus, although a cardiac-specific detrimental effect of fructose is indicated, to date this has not been demonstrated to occur independent of pressure loading and may represent a secondary response to dietary intervention.

A causal role for oxidative stress in the development of cardiovascular complications in diabetes is increasingly recognized [26], [27], although a specific link between oxidative stress and insulin resistance is yet to be established. Myocardial expression of nicotinamide adenosine dinucleotide phosphate (NADPH) oxidase is markedly upregulated in insulin resistance, and antioxidant strategies attenuate cardiac hypertrophy and fibrosis in this setting [28]. Although increased production of reactive oxygen species (ROS) in the vasculature has been reported with fructose feeding [12], [20], it has not yet been determined whether evidence of myocardial oxidative stress can be detected with elevated fructose intake.

The goal of this study was to examine the age-dependent, cardiac-specific effects of elevated dietary fructose intake. Mice were treated for 6 wk, employing an isoenergetic intervention involving manipulation of only the dietary carbohydrate content. Cardiac growth and parameters relating to ROS generation and oxidative stress were examined. We observed evidence of myocardial metabolic stress associated with fructose feeding in the absence of hypertension and cardiac hypertrophy.

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Materials and methods 

Ethical approval 

All animals were cared for in accordance with the Australian Code of Practice for the Care and Use of Animals for Scientific Purposes and all procedures were approved by the animal ethics committee of the University of Melbourne.

Dietary treatments and in vivo measurements 

Male C57Bl/6 mice aged 4–5 wk (weaning) and 14–15 wk (adult) were housed in temperature-controlled conditions in a 12-h light/dark cycle. At the commencement of the dietary treatment period, mice were allocated to a control (54% starch, 10% sucrose, 7% total fat, 19.4% protein) or high-fructose (60% fructose, 3.6% starch, 7% total fat, 19.4% protein) diet (n=12/group). The diets were of equal digestible energy and micronutrient matched (Table 1) and based on the American Institute of Nutrition standard rodent growth diet (Specialty Feeds, Glen Forrest, WA, Australia). Food intake and body weight were monitored throughout the 6-wk treatment period. In the final week of treatment, blood pressure was measured on 3 consecutive days using tail cuff plethysmography [29]. After gentle heating by infrared lamp, the pressure cuff and pulse sensor were placed around the base of the tail and the pressure cuff inflated using a sphygmomanometer (Narco Biosystems, Sydney, NSW, Australia). The pulse was detected using a pneumatic pulse transducer (Narco Biosystems) and recorded using the MacLab data acquisition system (A.D. Instruments, Sydney, NSW, Australia). Determination of mean pressure for each animal was made by averaging three to four concordant measurements for each session and calculating the mean of three session averages [29].

Table 1. Diet composition: macro- and micronutrient components (Specialty Feeds, Glen Forrest, WA, Australia)
Control (AIN-93G)Fructose (SF03-018)
Digestible energy (MJ/kg)16.116.6
Components (g/kg)
Starch536.036.0
Fructose0600.0
Sucrose100.00
Casein (acid)200.0200.0
dl-Methionine3.03.0
Cellulose50.050.0
Canola oil70.070.0
Calcium carbonate13.113.1
Sodium chloride2.62.6
Potassium citrate2.52.5
Potassium dihydrogen phosphate6.96.9
Potassium sulfate1.61.6
Trace minerals1.41.4
Choline chloride (65%)2.52.5
Vitamin mix∗10.010.0
Total (100%)∼1000∼1000

∗Vitamin mix includes vitamins A (4000IU/kg), D3 (1000IU/kg), E (76mg/kg), K (1mg/kg), B1 (6mg/kg), B2 (6mg/kg), and B12 (102mg/kg).

Tissue harvest and plasma analysis 

At the completion of the 6-wk feeding period (ages 10 and 20 wk), mice were fasted overnight, heparinized (1000IU, intraperitoneally), and anaesthetized by sodium pentobarbitone (60mg/kg, intraperitoneally). A blood sample (10μL) was collected for measurement of blood glucose using a blood glucometer (ACCU-CHEK Advantage, Roche, Mannheim, Germany) and plasma insulin (Rat Insulin RIA kit, Linco Research, St. Charles, MO, USA) concentrations. Hearts were excised (excess tissue was dissected away) and weighed for determination of the cardiac weight index (heart weight in milligrams to body weight in grams). Ventricles were sectioned into tissue portions for gene expression analysis, assay of lipid peroxidation markers, and measurement of cardiac superoxide production as described below.

Detection of tissue superoxide production 

The NADPH-driven superoxide production, an estimate of NADPH oxidase activity, was measured in freshly dissected thoracic aorta and ventricular tissues using lucigenin-enhanced chemiluminescence [28], [30]. Small (1–2mm) fragments of ventricular tissue and aortic rings were incubated with 50μM NADPH in HEPES-Krebs buffer at 37°C for 45min. The tissues were transferred to 96-well opti-plates (Perkin Elmer Inc., Boston, MA, USA) containing HEPES-Krebs buffer, 100μM NADPH, and 5μM lucigenin (N,N′-dimethyl-9-9′-biacridianium, Sigma Aldrich, St. Louis, MO, USA). Luminescence generated by the reaction of tissue-derived superoxide and lucigenin (5μM) was counted at 20°C. Background chemiluminescence was measured in the 96-well opti-plate (dark-adapted for 12h) with HEPES-Krebs buffer, 100μM NADPH, and 5μM lucigenin before tissue addition and subtracted from the tissue fragment luminescence. Tissues were dried at 80°C for 48h and weighed for calculation of normalized superoxide production expressed as counts per second per milligram of dry tissue.

Real-time quantitative reverse transcription–polymerase chain reaction 

The RNA was extracted from frozen cardiac tissues using the TRIzol reagent (Invitrogen, VIC, Australia) according to the manufacturer's instructions. Total RNA was reverse transcribed to cDNA with the SuperScript III First-Strand Synthesis System (Invitrogen). Real-time reverse transcription–polymerase chain reaction was used to determine the relative gene expression levels of β-myosin heavy chain (cardiac prohypertrophic gene), the Nox2 subunit of NADPH oxidase (a major source of ROS), and thioredoxin-2 (a mitochondrial antioxidant) in ventricular tissue [28]. Primers and polymerase chain reaction cycling conditions were optimized and primer sequences for each gene were as follows: β-myosin heavy chain: 5′- TCTCCTGCTGTTTCCTTACTTGCTA-3′ and 5′- GTACTCCTCTGCTGAGGCTTCCT-3′; Nox2: 5′-GCTGCCAGTGTGTCGAAATCT-3′ and 5′-GCTACCATCTTATGGAAAGTGAGGTT-3′; thioredoxin-2: 5′-CAGCCTCTGGCACATTTCCT-3′ and 5′-GTTCGGCTTCTGGTTTCCTTT-3′; and 18S: 5′-TCGAGGCCCTGTAATTGGAA-3′ and 5′-CCCTCCAATGGATCCTCGTT-3′. The β-tubulin primers were purchased as a preoptimized assay (Entrez Gene ID 22153, QuantiTech Primer Assay, Qiagen Pty. Ltd., Melbourne, VIC, Australia). The comparative ΔΔCt method was utilized in analysis of the genes of interest as described previously [31], [32]. This method involves two comparative steps (i.e., two “delta” manipulations using base 2 logarithmic values). First, the threshold cycle number (Ct value) of the gene of interest relative to the Ct value of the housekeeper gene is calculated for each sample (this operation is termed ΔCt). Second, to reference the expression change in treatment groups to a selected control group, a second manipulation is performed (i.e., ΔΔCt calculation). Gene expression was normalized to the expression level of a housekeeper gene, 18S (β-myosin heavy chain, Nox2) or β-tubulin (thioredoxin-2) and presented relative to the 10-wk-old control group.

Markers of lipid peroxidation 

Ventricular lipid peroxidation was assessed by measurement of thiobarbituric acid-reactive substances. Ground ventricular tissue (15–30mg) was dissolved in 1.15% KCl (10μL/mg of tissue). Tissue homogenate (70μL) was added to 100μL of 8.1% sodium dodecylsulfate, 100μL of 20% acetic acid, and 100μL of 0.82% thiobarbituric acid freshly made on day of experiment. A standard curve was constructed over 0.1–2.5 nmol of 1,1,3,3-tetraethoxypropane. All samples, standards, and blanks were vortex-mixed and incubated at 100°C in a heat block for 45min. After cooling on ice, absorbance at 532nm was determined by spectrophotometry (DU800, Beckman Coulter Inc., Fullerton, CA, USA) and the concentration of thiobarbituric acid-reactive substances in the tissue was calculated using the standard curve.

Statistical analysis 

Data are expressed as mean ± standard error and were analyzed with SPSS 16 (SPSS Inc., Chicago, IL, USA). Two-way analysis of variance was performed using a univariate, general linear model. The two categorical independent variables were diet and age; interactions between these factors were also analyzed. P<0.05 was considered statistically significant.

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Results 

Systemic characteristics 

For each age group, mice were matched for body weight at the commencement of the feeding period. Food intake measured at 2-d intervals throughout the treatment period was not significantly different for the fructose and control diet groups (mean 2.75g/d). After 6 wk of treatment, the mean body weight in the fructose-fed group was 12% lower than in the control group for 10- and 20-wk-old animals (Table 2). As presented in Table 2, fasted blood glucose concentrations were approximately 11% lower in 10-wk-old fructose-fed mice and 18% lower in 20-wk-old mice compared with controls (diet, P<0.05). Plasma insulin levels after 6 wk of fructose feeding were not different from levels in control-fed animals and did not show a significant age-dependent difference (Table 2).

Table 2. Systemic characteristics of fructose-fed and control-fed mice at ages 10 and 20 wk∗
Initial body weight (g)Final body weight (g)Blood glucose (mmol/L)Plasma insulin (ng/mL)
(n=17–18)(n=17–18)(n=16–17)(n=14–17)
10 wk
Control15.1±0.724.9±0.55.42±0.310.18±0.02
Fructose14.3±0.521.5±0.6†4.81±0.28†0.20±0.02
20 wk
Control26.1±0.7‡28.7±0.5‡5.96±0.420.23±0.02
Fructose25.6±0.6‡25.5±0.7†, ‡4.88±0.30†0.19±0.02

∗Values are means±SEMs.

†P<0.05 compared with control.

‡P<0.05 compared with 10 wk.

Blood pressure and heart growth index 

As shown in Figure 1, an age-dependent elevation in blood pressure was apparent, but the fructose diet was not associated with a specific blood pressure effect. An age-dependent reduction in the cardiac weight index was evident in both diet groups (age, P<0.05), due to the proportionally greater increase in body weight relative to heart weight with age (Fig. 2A). No significant difference in cardiac weight index was detected between diets. The 6-wk dietary intervention did not significantly alter gene expression levels of β-myosin heavy chain, the prohypertrophic gene, in either age group (although a non-significant trend was evident for increased β-myosin heavy chain gene expression in younger mice). Thus, the high-fructose diet did not induce cardiac hypertrophy in these mice (Fig. 2B).

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  • Fig. 1 

    Mean systolic blood pressure in the final week of dietary intervention (n=11–12/group). The horizontal bars depict an age effect. Data are presented as mean±SEM. # Age factor effect, P<0.05, two-way analysis of variance. cont, control; fruc, fructose.

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  • Fig. 2 

    (A) Cardiac weight index. Heart weight (milligrams) to body weight (grams) ratio at completion of a 6-wk dietary treatment (n=17–18/group). The horizontal bars depict an age factor effect. (B) Gene expression level of β-MHC, a prohypertrophic gene in fructose-fed versus control-fed mice at 10 and 20 wk of age (normalized to 18S and determined relative to 10-wk-old control group, n=11–12/group). Data are presented as mean±SEM. # Age factor effect, P<0.05, two-way analysis of variance. β-MHC, β-myosin heavy chain; cont, control; fruc, fructose.

Parameters of oxidative stress 

The NADPH-driven superoxide production was measured in ventricular and aortic tissues in freshly harvested tissues using lucigenin-enhanced chemiluminescence. As presented in Figure 3A, the 6-wk period of fructose feeding was associated with a significant elevation in ventricular superoxide production in mice in both age groups, which was increased by approximately 29% in 10-wk-old mice and 16% in 20-wk-old mice. There were no significant differences in aortic superoxide production between fructose-fed and control-fed mice at either age, and no age-dependent differences between groups (10 wk: 2280±320 versus 2840±310 counts ∙ s−1 ∙ mg−1; 20 wk: 2760±390 versus 2840±380 counts ∙ s−1 ∙ mg−1). Average aortic levels of superoxide production were about five-fold greater than the levels measured in cardiac tissue (P<0.05).

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  • Fig. 3 

    Nicotinamide adenosine dinucleotide phosphate–stimulated superoxide (O2−) production (counts per second per milligram of dry tissue) in fructose-fed and control-fed mice at completion of the 6-wk dietary intervention. Superoxide production in (A) ventricular tissue (n=6 tissue fragments/animal, n=15–17/group) and (B) thoracic aorta (n=3 aortic rings/animal, n=16–17/group). Data are presented as mean±SEM. ∗ Diet factor effect, P<0.05, two-way analysis of variance. cont, control; fruc, fructose.

Fructose-fed mice at 10 wk showed a non-significant tendency for increased mRNA expression of the NADPH oxidase subunit Nox2 (by 16.8±9.5-fold of control, P=0.13). This was not observed in the 20-wk-old animals, where Nox2 gene expression was comparable for fructose- and control-fed mice (Fig. 4). A significant correlation was not observed between Nox2 gene expression and myocardial superoxide production (Pearson's coefficient −0.27, P=0.15). Gene expression of the mitochondrial isoform of the redox protein thioredoxin-2 was determined using real-time polymerase chain reaction. No effect of fructose feeding on myocardial redox protein thioredoxin-2 expression was observed for either age group (Fig. 5). The extent of myocardial lipid peroxidation (determined using thiobarbituric acid-reactive substances) was not significantly modified by diet at either age (Fig. 6), although age per se tended to elevate lipid peroxidation. Thiobarbituric acid-reactive substances levels increased in control-fed mice (18%) and in fructose-fed mice (11%) from 10 to 20 wk of age (P=0.09).

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  • Fig. 4 

    Nox2 gene expression normalized to 18S and determined relative to the 10-wk-old control group. Fructose-fed versus control-fed mice at 10 and 20 wk of age (n=9–10/group). Data are presented as mean±SEM. cont, control; fruc, fructose.

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  • Fig. 5 

    Thx2 gene expression normalized to β-tubulin and determined relative to the 10-wk-old control group. Fructose-fed versus control-fed mice at 10 and 20 wk of age (n=5–6/group). Data are presented as mean±SEM. cont, control; fruc, fructose; Thx2, thioredoxin-2.

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  • Fig. 6 

    Lipid peroxidation (ventricular TBARS) after 6-wk dietary treatment (n=13–15/group). Data are presented as mean±SEM. cont, control; fruc, fructose; TBARS, thiobarbituric acid-reactive substances.

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Discussion 

The key finding of the present study is that high dietary fructose intake in mice is associated with increased myocardial superoxide generation. In contrast, a fructose-induced increase in ROS generation was not observed in vascular tissue. Fructose-fed mice were normotensive, insulin levels were not elevated, and cardiac hypertrophy was not evident. Interestingly, blood glucose levels were lower in fructose-fed mice. Normal maturational differences in blood pressure and cardiac weight index were observed between 10- and 20-wk-old mice, but no age-dependent differences in other myocardial measures were detected. Thus, this investigation provides support for the proposition that fructose feeding is associated with a cardiac-specific metabolic stress response. Importantly, this increased capacity for ROS generation in the myocardium was observed with a relatively short dietary intervention of 6 wk and occurred in the absence of the complicating systemic factors of body mass increase, hyperinsulinemia, and/or hypertension. No evidence of increased susceptibility to juvenile fructose feeding could be detected in this experimental context.

This is the first study to investigate the direct effects of high-fructose intake on the heart in a mouse model and suggests that the murine myocardium may be compromised by dietary fructose-induced elevated ROS production. The fructose-induced increase in myocardial NADPH-driven superoxide production was evident in young and older animals, and although not statistically resolvable, the data suggest a more marked effect in the younger animals. A similar trend of increased Nox2 (a catalytic subunit of NADPH oxidase) gene expression in the young but not older mice was also observed. These findings suggest that the fructose diet–induced elevated superoxide production may be mediated in part by NADPH oxidase upregulation, which may be accentuated in younger animals, although in this dataset a significant correlation between these parameters could not be discerned. No changes in the expression of thioredoxin-2, an important mitochondrial antioxidant, were observed in the context of this upregulated ROS production. Thus, an imbalance between ROS generation and antioxidants could potentially render the heart vulnerable to oxidative damage and oxidative stress may represent an early manifestation of fructose-induced cardiac pathology from which further complications emerge. With this 6-wk dietary intervention, increased myocardial lipid peroxidation damage was not evident in fructose-fed mice. A more prolonged intervention might be expected to confer detectable peroxidation damage, possibly in association with impaired antioxidant defense, as has been reported in the rat [33].

It is notable that this effect of high-fructose intake on ROS generation was specific for the myocardium. The role of oxidative stress in the cardiovascular complications of type 2 diabetes has been previously demonstrated [27], [34]. Although the mean level of superoxide production in the aorta was approximately five-fold higher than in the myocardium, possibly reflecting the differential expression of NADPH oxidase subunits in the vasculature relative to the myocardium [35], an effect of fructose feeding was not detected. This finding indicates that, where vascular ROS production has been previously found to be increased with fructose feeding (in rats), coincident hypertension may be the predominant causative factor [12].

In this study cardiac weight indices and expression of the hypertrophic growth marker β-myosin heavy chain were not different in either age cohort after a 6-wk high-fructose feeding regime. A trend for increased β-myosin heavy chain expression was indicated in younger mice, suggesting a heightened susceptibility to growth induction. Previous studies with rats have reported cardiac hypertrophic effects in response to fructose feeding [17], [36]. Those previous studies reported an occurrence of hypertension coincident with hypertrophy, suggesting that the cardiac growth effects were secondary to the hypertension induced by fructose feeding rather than a direct effect of the diet.

Fructose feeding was not associated with hemodynamic volume load (these animals were not obese) or pressure overload. Older mice exhibited a small significant elevation of blood pressure. The capacity to detect this expected normal maturational pressure increase confirmed the sensitivity of the plethysmographic methodology employed. Previous studies in rodents have reported increases in [25], [37] and no change in [19] systolic blood pressure with fructose feeding. This discrepancy may reflect difficulties in diet matching in these studies, as discussed below. In the present study, daytime blood pressure measurements within a 2-h time window were taken. Interestingly, a recent telemetric study determined that nocturnal (i.e., active period) blood pressure was significantly elevated in fructose-fed mice after 10 wk of dietary treatment [14]. The possibility that some pressure variation over a 24-h period may occur in mice, particularly with extended duration feeding, remains to be evaluated. In this study, heart rate measurements were not obtained, and the possibility that fructose feeding may influence heart rate cannot be precluded. However, fructose feeding in rats is not associated with heart rate alteration, even when hypertension is evident [38].

Although fructose feeding in rodents is a recognized model of diet-induced insulin resistance, there has been little consistency in the composition of diets utilized. In some studies diet specification was not provided and in others diet caloric content and macronutrient types were not matched [2], [12], [14], [19], [22], [25]. In the present study, strict matching of diet composition was achieved, and all components of the two diets were matched, with only the type of carbohydrate manipulated. Surprisingly, body weight gain in fructose-fed mice was reduced. The basis for this is unknown and merits further investigation. Most pertinent to the present study is the confirmation that, with these specified well-matched diets, fructose feeding was not associated with body weight increase and therefore interpretation of the findings is not confounded by consideration of an obesity-related hemodynamic or endocrine effect [39].

Fructose feeding in the present study was not associated with hyperglycemia, but rather, surprisingly, with lower blood glucose levels. Previous studies of fructose feeding in rats and mice have reported no change or elevated blood glucose levels [24], [33], [40]. In the present study, fasted plasma insulin levels were not different in response to a high-fructose diet, consistent with a recent report in mice [14]. Studies in fructose-fed rats have shown a treatment duration-dependent influence on plasma insulin levels. With 4 wk of fructose feeding in rats, plasma insulin levels were significantly elevated, yet after 7 wk, insulin levels were reportedly not different [12]. The effects of a high-fructose diet on plasma insulin are controversial and the limited available evidence is not conclusive. Although the present study suggests an effect of high-fructose intake on systemic glucose homeostasis, there was no evidence of hyperglycemia. Thus, the cardiac-specific effects of high-fructose intake cannot be attributed to elevated plasma glucose or to hyperinsulinemia.

In summary, this study provides the first demonstration that high-fructose intake has a direct and selective cardiac effect inducing elevated myocardial ROS production. With a relatively short 6-wk dietary intervention, this indication of cardiac oxidative stress could be observed independent of systemic insulin dysregulation. It is evident that the myocardium responds to the environmental insult of a high-fructose diet in the absence of marked systemic pathology. Importantly, in the present study, the fructose-induced increase in cardiac superoxide production observed was apparent in the absence of hypertrophy and hypertension, demonstrating that the NADPH oxidase upregulation occurs independently of pressure loading and precedes any structural cardiac remodeling event. This investigation also provides suggestive evidence of increased susceptibility of younger hearts to the cardiac effects of high-fructose intake, and further work is required to explore this possibility.

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Acknowledgments 

The expert technical assistance provided by Bill Meeker, Jill Pavia, and Anh Cao is acknowledged. The contribution of Specialty Feeds (Warren Potts) in managing diet-batch consistency is appreciated.

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 This study was supported by Diabetes Australia, the National Heart Foundation, and the National Health and Medical Research Council, Australia.

PII: S0899-9007(09)00342-6

doi:10.1016/j.nut.2009.08.017

Nutrition
Volume 26, Issue 7 , Pages 842-848, July 2010

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