ReviewAssociation of fructose consumption and components of metabolic syndrome in human studies: A systematic review and meta-analysis
Introduction
Fructose is a monosaccharide that naturally exists in fruits, honey, and some vegetables. These days, however, fructose is mostly ingested from industrial and commercial products such as soft drinks, sweetened beverages, and high-fructose corn syrup (HFCS) [1], [2]. Natural foods that contain fructose, such as fruits and vegetables, only contain small amounts; moreover their fructose is absorbed slowly. Thus, after consuming these foods, the rise in serum fructose concentration is negligible [3]. The use of HFCS in beverages and soft drinks is increasing by food manufacturers because of its low cost compared with sucrose, and the ease with which it can be added to food products [4]. Actually, the consumption of fructose through manufactured products is concerning. The average daily intake of total fructose in the United Stated increased 12 g/d between 1978 (37 g/d) and 2004 (49 g/d) [5], [6].
Fructose is more lipogenic than other carbohydrates, and unlike glucose it can be converted to glycerol-3-phosphate (required for tri-acylglycerol synthesis) without passing from the phosphofructokinase pathway, which is an important rate-limiting pathway of glycolysis. Therefore, high-fructose consumption can be the cause of triglyceride (TG) synthesis from unchecked pathways [7]. According to animal studies, consumption of a 60% total energy diet with fructose can induce obesity and some components of metabolic syndrome (MetS) such as insulin resistance, dyslipidemia, and hypertension [8], [9], [10].
The escalating trend in the prevalence of chronic diseases in industrialized and developing countries [11] and the increase in fructose consumption in the population’s diet is of concern. Researchers have proposed an association between fructose consumption and chronic diseases [12]. Therefore, many experimental and human studies are being conducted to assess the relationship of fructose consumption with the development of chronic diseases such as diabetes and MetS [4].
Studies of young healthy individuals have shown that fructose consumption (250 g/d) compared with the same amount of glucose significantly decreased insulin sensitivity [7]. There are fewer studies on the long-term effects of fructose consumption on human health than on the effects of glucose [3]. The adverse short-term effects of fructose seem to be dose-dependent. Short-term consumption of fructose in humans did not have adverse effects on health status, unless consumed in excessive amounts. Long-term consumption of fructose is associated with an increase in adiposity, dyslipidemia, and insulin resistance [3], [7].
Findings about fructose and health status are controversial. Some studies supported the hypotheses that fructose consumption leads to an increase in chronic disorders such as MetS [13], [14], [15], [16], whereas others did not confirm the positive association of fructose with the components of the disease [17], [18], [19], [20]. Some reasons may be suggested for the differences between the findings of various studies. Low fructose dose, for instance < 20 g/d, mostly showed improved or no effect on MetS parameters; however, high doses of fructose mostly increased the features of MetS [16], [20]. Different forms of fructose such as natural fructose, fructose alone, or fructose bonded with glucose in the form of sucrose, showed various findings [3]. Race, study design, animal or human studies, and different characteristics of participants such as sex, age, and body weight are other factors that influence differences in the findings obtained in various studies.
To help clinicians and researchers make decisions when treating their patients, it is necessary to summarize the controversial findings of various studies concerning the health consequences of fructose consumption.
This study aims to review the findings of human studies on the association of fructose consumption with various doses and different duration with MetS.
Section snippets
Literature search
The search was conducted using PubMed, Scopus, Ovid, ISI Web of Science, Cochrane library, and Google Scholar databases from January to May 2013. Keywords such as metabolic syndrome x, insulin resistance, blood glucose, blood sugar, fasting blood sugar, triglycerides, lipoproteins, HDL, cholesterol, LDL, blood pressure, mean arterial pressure, systolic blood pressure, diastolic blood pressure, hypertens*, waist circumference, and fructose, sucrose, high-fructose corn syrup, or sugar were used.
Fructose consumption and FBS
Individual study results and the overall summary results for the 10 clinical trials investigating the effects of fructose consumption on FBS are shown in Figure 2. When all 10 studies were analyzed, consumption of fructose was positively associated with an increase in FBS (summary mean difference, 0.307; 95% CI, 0.149–0.465; P = 0.002). There was no statistically significant heterogeneity among studies (I2 = 43.49%; Pheterogeneity = 0.075). The results for random-effect model were 0.357 (95%
Discussion
This meta-analysis showed that fructose consumption has significant adverse effects on FBS. Concerning other components of MetS, fructose consumption increased levels of TG and SBP and decreased levels of HDL-C. However, after excluding studies that led to heterogeneity, these adverse effects were not statistically significant. The present study is in agreement with some human and animal studies. Human studies indicated that fructose developed some of the features of MetS as
Conclusion
According to our meta-analysis, fructose consumption from industrialized foods, as sweetened beverages, is one of the causes of some chronic disorders as MetS in healthy adults. It provides further support to decrease consumption of fructose-sweetened beverages for the prevention of chronic disorders in a healthy population. However, several studies had investigated the effect of fructose in short-term. Long-term trials in the future about the effect of fructose consumption on chronic diseases
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