Michelle Stewart
Literature Review
Landis FN340
30 April 2015

Introduction
Fructose, commonly known as fruit sugar, is a major component of sweeteners such as table sugar, honey and high fructose corn syrup (HFCS)1. Fructose is a simple monosaccharide that has been used as a sweetener in food and drinks, and current estimations suggest that sweetener consumption in the U.S. has increased to an average of 477 kcal/person, or approximately 24% of a typical 2000 kcal/day diet2,3. An increase in the consumption of sweeteners containing fructose has occurred in parallel with the increasing prevalence of obesity, suggesting that increased consumption of fructose may contribute to the current epidemic of obesity-related metabolic disorders, including increased incidence of the metabolic syndrome2. The metabolic syndrome (MetS) consists of a syndrome of insulin resistance, dyslipidemia, abdominal obesity, and elevated blood pressure (BP), and often precedes the development of diabetes4. In some studies, humans and animals that have been administered fructose have developed these symptoms, however they have not been observed with glucose or starch-based diets. Fructose and glucose are metabolized quite differently, and it has been hypothesized that this differentiation is the key factor in the development of pathologies associated with MetS5. Additionally, ingestion of fructose does not stimulate the release of the hormones insulin and leptin, nor does it suppress the secretion of the hormone ghrelin as does glucose, and this has been proven to lead to overeating and obesity6. Furthermore, recent studies suggest that fructose has been demonstrated to increase circulating uric acid concentrations in both animals and humans. Evidence suggests that elevated levels of uric acid and increased activity of the liver enzymes gamma-glutamyl transferase (GGT) and alanine aminotransferase (ALT) are also associated with the metabolic syndrome. GGT and ALT are not only strong predictors of metabolic syndrome, but are considered to be markers for non-alcoholic fatty liver disease (NAFLD)2. Since similar effects do not occur following the intake of starch or glucose, it has been proposed that fructose-induced metabolic changes are not mediated by excessive sugar intake, but are specific to fructose1. Thus, the objective of this review is to compare the effects of fructose with that of glucose in the resulting developments of inflammatory biomarkers and obesity, which in turn can lead to the development of the metabolic syndrome.

Glucose and Fructose Metabolism
As mentioned in the introduction, glucose and fructose are metabolized quite differently. Because both glucose and fructose give the same number of calories/g, it was originally thought they could be interconverted instantly in the cells8. This is not the case, however. After oral consumption of glucose, the bolus enters the portal of circulation7. Approximately 20% of the glucose bolus enters the liver via the Glut2 glucose transporter, which is insulin dependent. The rest appears in the peripheral circulation, plasma glucose levels increase, and insulin is released by the pancreatic β-cells in response. Insulin binds to its liver receptor, which promotes the tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1). Increased kinase activity along with the transcription factor is responsible for insulin’s intracellular metabolic effects. During this process, gluconeogenesis is down-regulated and glycogen synthase is activated. This leads to the conversion of the majority of glucose molecules as hepatic glycogen storage. The small amount that enters glycolysis reaches the mitochondria as pyruvate and is converted to acetyl CoA. Any acetyl-CoA that cannot be oxidized for energy and exits the mitochondria is converted to free fatty acids (FFAs), and packaged into very low-density lipoprotein (VLDL) for hepatic export and storage in adipocytes. This VLDL can promote atherogenesis and obesity, but only a very small amount of glucose will find its way into VLDL. Thus, glucose contributes extremely slowly to cardiovascular disease (CVD) and other aspects of MetS. Fructose, on the other hand, is metabolized mainly in the liver. It is the only site of the Glut5 transporter, and as Glut5 has a high affinity for fructose, virtually all ingested fructose finds its way there. In contrast to the majority of glucose being converted to glycogen in the liver under the influence of insulin, fructose is not converted to glycogen directly. Rather, it is phosphorylated independently of insulin to fructose-1-phosphate (F-1-P) by the enzyme fructokinase. This is important to note because F-1-P is a substrate for aldolase, which suppresses the glycolytic pathway, and also acts to produce higher levels of adenosine triphosphate (ATP) and citrate, which results in the synthesis of fatty acids (FAs)8. Therefore, much of the fructose that is consumed gets converted to fat.
It is also important to note that when the liver receives glucose and fructose simultaneously, the glucose occupies the glycogenic pathway, forcing fructose down the lipogenic pathway, which exacerbates the rate of de novo lipogenesis (DNL) compared with fructose alone7.

Inflammation
Fructose consumption compared to glucose consumption has been shown to increase inflammatory biomarkers in animal and human subjects. In a randomized, single-blinded, controlled cross-over intervention trial, Jameel et al1 studied the acute effects of feeding fructose, glucose, and sucrose on blood lipid levels and systemic inflammation. The study focused on postprandial lipemia and low grade inflammation following a single source of sugary drink given as a sole source of energy after an overnight fast. Healthy male and female adults (n=14) between the ages of 18-60 were administered 3 different isocaloric sugary drinks on 3 separate occasions. During each visit a fasting blood sample was collected prior to supplementation, then 30, 60, and 120 minutes following intake of the sugary drink. Each one of the three sugary drinks contained 50 g of sugar (either fructose, glucose, or sucrose) dissolved in 300 ml of water. Participants remained in the research facility until the final sample was collected and were asked to limit physical activity during their time in the facility. Twenty four hour food recalls were collected and entered into a database to analyze daily energy and nutrient intake. The pro-inflammatory biomarker examined in this study was high-sensitivity C-reactive protein (hs-CRP), which is a pentameric protein found in plasma, the levels of which rise in response to inflammation. Fructose consumption was followed by an increase in hs-CRP level at 30 minutes when compared to glucose and again at 60 minutes, however at 120 minutes there were no differences between groups. The proposed mechanism of fructose-induced oxidative stress and inflammation markers potentially resulting in an increase synthesis of hs-CRP merits further investigation. Another study conducted to observe the inflammatory responses fructose may cause was conducted by Cox et al2. Comparing fructose and glucose consumption, they examined the effects that increased circulation of uric acid had on the prevalence of the biological markers ALT, GGT, and retinol binding protein-4, all of which are diagnostic markers associated with the metabolic syndrome. They conducted a parallel arm study with 3 phases: a 2 week in-patient baseline period, an 8 week outpatient intervention period, and a 2 week inpatient intervention period. During the first intervention period baseline data was collected. During the 8 week intervention the participants consumed either fructose (n=17) or glucose (n=15) sweetened beverages at 25% energy requirements with self-selected ad libitum diets. During the last intervention, glucose or fructose-sweetened beverages were consumed as part of an energy-balanced diet. Participants were aged 40-72 years with BMI’s of 25-35 kg/m2. At 10 wks of intervention fasting plasma uric acid levels were significantly increased from baseline in subjects consuming both fructose and glucose, however in participants consuming fructose-sweetened beverages the effects were significantly greater than those consuming glucose. 24-h serum uric acid levels were increased significantly from baseline in subjects consuming fructose but not in those consuming glucose. RBP-4 concentrations increased significantly in subjects consuming fructose and decreased significantly for those consuming glucose. Although RBP-4 concentrations decreased comparably in both men and women in the glucose group, the increase of RBP-4 from baseline levels in subjects consuming fructose was significantly greater in men (P=0.007) compared to women (P=0.908). Plasma GGT activity was significantly elevated compared with baseline values following 10 wks of fructose consumption but significantly decreased in subjects consuming glucose. Fasting activities of AST and ALT decreased slightly following 10 wks of fructose consumption, however these changes were not statistically significant. In subjects consuming glucose AST and ALT activities were decreased after 10 wks. Despite comparable weight gain, subjects consuming fructose primarily exhibited increases in visceral adipose tissue whereas subcutaneous adipose tissue was increased in subjects consuming glucose. Prolonged fructose consumption significantly increased uric acid concentrations. The mechanism by which fructose consumption leads to increased uric acid concentrations is thought to be initiated by the depletion of ATP and Pi resulting from the unregulated production of F-1-P and G-3-P from fructose, which bypasses the rate-limiting step of glycolysis, PFK. Fructose-induced depletion of ATP and Pi leads to a concomitant increase of purine nucleotide degradation and, subsequently, uric acid production. Furthermore, increases of circulating RBP-4 are strongly associated with insulin resistance in adipose tissue, which can be predictive of a diagnosis of metabolic syndrome. These results indicate that prolonged fructose consumption may contribute to the development of metabolic syndrome by increasing circulating concentrations of uric acid, GGT activity, and the production of RBP-4.
Triglyceride Levels and Dyslipidemia
In the aforementioned study conducted by Jameel et al1, acute fructose consumption resulted in a significant increase in the plasma levels of total cholesterol and the acute phase pro-inflammatory hs-CRP, however a significant change in triglyceride (TG) levels was not observed in participants. These findings are interesting, because many other studies provide conflicting results. In a randomized trial comprising 74 participants conducted Perez et al4, fructose ingestion resulted in a significant increase in fasting serum triglycerides from baseline. These findings fall in line with studies conducted by Stanhope et al9 and Silbernagel et al10, in which both observed a significant increase in plasma TG during fructose consumption, but not during glucose consumption. In all three of these studies, elevation of plasma TG seemed to be fructose-specific. Although studies conducted with human subjects may lend results that can be directly attributed to the biological processes and pathways for disease that they are designed to evaluate, animal models may be used to make inferences as well. A study conducted by Mamikutty et al3 was designed to establish a metabolic syndrome model by induction of fructose drinking water in male Wistar rats. The main aim of this study was to establish a metabolic syndrome rat model by consumption of fructose drinking water (FDW) with the consumption of normal diet. Eighteen male Wistar were acclimated for a 14 day period with free access to food and water. The rats were randomly divided into 3 groups (n=6) and fed standard rat chow. Each group was administered a different water type: normal tap water (C), fructose 20% drinking water (F20) and fructose 25% drinking water (F25). Physiological results showed the food intake was decreased significantly with consumption of FDW20 and FDW25 compared to C group. The higher total calorie intake of F20 group leads to significantly higher percentage in body weight gain, BMI, and abdominal circumference (AC) compared to F25. Both F20 and F25 showed significantly higher level of systolic blood pressure than the C group as well. Serum TG levels showed similar results, with F20 and F25 showing significantly higher levels than C. Depositions of abdominal adipose tissue were highest in F20. These depositions are important components in the development of dyslipidemia, hyperglycemia, and hypertension. Although metabolic syndrome criteria can be achieved by FDW 20% and 25%, FDW 20% proved to be more easily drunk, hence the total calorie intake was significantly greater in F20. This leads to higher obesity parameter which was the cause of the development of metabolic syndrome. In another animal study by Ock et al11, rats were fed ad libitum diets containing 70% glucose, or 70% fructose. The glucose-fed rats ate greater amounts of food than did the fructose-fed rats, however both groups gained weight in a steady manner. Despite the lower food intake, the rats fed on a fructose diet ad libitum exhibited over 2-fold elevated plasma TG concentrations. The relevance of these findings however, when compared to the overconsumption of fructose by human populations remains to be established. At present, consumption of fructose in the general population has been estimated to be approximately 20% to 25% of a normal 2000 kcal/day diet2,3, and this study deviates from that average in comparison.

Weight gain and Obesity
Another condition implicated in the cause of MetS is obesity. In many cases, individuals who display the conditions that comprise MetS are obese or overweight, however 20% of obese subjects are metabolically normal, where as many as 40% of normal-weight individuals manifest specific conditions of MetS12. In the study conducted by Stanhope et al9, the ability of fructose to induce features of MetS compared with glucose was independent of changes in weight. These findings raise the question of whether obesity is a cause of MetS or whether it is a marker for these conditions. Fructose has seen the same steady rise as obesity and has been implicated in the obesity pandemic. However, a direct link of fructose to the metabolic syndrome has not been studied until recently and is a topic that has become popular among researchers. As stated earlier, research suggests that there may be unique features involved in fructose metabolism that may predispose individuals to the development of MetS.
A study conducted by Ishimoto et al11 sought to link fructose consumption directly to MetS using mice that had been genetically altered to lack fructokinase isoforms A, or both A and C. Although fructose can be metabolized by hexokinase similarly to glucose, the affinity for fructose is much less and therefore most fructose is metabolized by fructokinase (ketohexokinase, KHK), an enzyme that is fructose-specific. Fructokinase exists in two alternatively spliced isoforms consisting of fructokinase C (KHK-C) and fructokinase A (KHK-A). KHK-C has a lower Km than does KHK-A, and as a result has a higher affinity for fructose. In the current study, 3 types of mice were analyzed: wild-type mice (WT), KHK-A/C knockout (KO) mice, and KHK-A KO mice. The mice were administered 15 or 30% fructose water (FW) or tap water for 25 weeks and all were on ad libitum diets. KHK-A/C KO mice diets were adjusted as well to correct energy balance as there was a loss of fructose in the urine.
Although mice are relatively resistant to the effects of fructose, the WT mice in this study showed many features of MetS, including increased body weight (BW), total body fat, epididymal fat, liver weight (LW), serum glucose, serum insulin, serum leptin, and serum LDL cholesterol. BP, serum TG, HDL cholesterol, and uric acid however were not different among WT groups. Fructose-fed mice also developed progressive hepatic steatosis, associated with increased hepatic TG content. This hepatic steatosis was associated with increased fatty acid synthase (FAS) mRNA expression and reduced serum ketones, consistent with both increased fat synthesis and decreased fatty acid oxidation.
Energy intake was of the same amount between WT mice receiving 15% fructose and KHK-A/C KO receiving 30% fructose, thus allowing the researchers to determine the role of KHK-A/C on features of MetS. Fructose-fed KHK-A/C KO mice were completely protected from the increases of BW, total body fat, serum insulin, serum leptin, serum LDL cholesterol, hepatic FAS expression, and hepatic steatosis. In contrast, glycogen staining of liver showed that fructose-fed KHK-A/C KO mice had much more glycogen accumulation than fructose-fed WT mice.
Comparisons between WT mice and KHK-A KO showed nearly identical energy intake and balance, however KHK-A KO mice showed significantly worse features of MetS. At 25 weeks, epididymal fat mass, serum insulin, serum leptin, intrahepatic triglycerides, and FAS expression were significantly higher in the KHK-A KO mice. In particular, heptic steatosis was much more severe than WT when tissue samples were analyzed. Although further studies are warranted to delineate the extent to which fructose metabolism has on the development of MetS, this study suggests that fructose metabolism, more than obesity, is a predictor of the metabolic syndrome.

Conclusion
Fructose intake from added sugars correlates with the epidemic rise in obesity, metabolic syndrome, and non-alcoholic fatty liver disease12. Fructose has also been shown to cause features of MetS in laboratory animals and in humans. Although the study by Jameel et al1 proved inconclusive for metabolic syndrome, studies conducted by Cox et al2 and Mamiktty et al3 conclude that high fructose consumption compared to glucose can lead to features the metabolic syndrome. Until recently, obesity has been a scapegoat for many of the pathologies associated with MetS, however further studies are necessary to determine whether these pathologies are directly related to obesity, or whether obesity itself is a condition and fructose metabolism is a root cause of its development. Studies conducted by Ishimoto et al12 and Stanhope et al9 suggest that fructose metabolism may play a larger role. More studies are warranted in this field of study, as fructose consumption is on the rise and proliferant in our food supply today. More insight into these areas could possibly lead to a positive change in the obesity epidemic we are experiencing on a global scale in the world today.

References
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12. Ishimoto T, Lanaspa M, Johnson R, et al. Opposing effects of fructokinase C and A isoforms on fructose-induced metabolic syndrome in mice. Proceedings Of The National Academy Of Sciences Of The United States Of America. March 13, 2012;109(11):4320-4325.