Free sugars overconsumption is associated with an increased prevalence of risk factors for metabolic diseases such as the alteration of the blood lipid levels. Natural fruit juices have a free sugar composition quite similar to that of sugar-sweetened beverages. Despite the similarity of fruits juices to sugar-sweetened beverages in terms of free sugars content, it remains unclear whether they lead to the same metabolic consequences if consumed in equal dose.
1. Introduction
The ingestion of free sugars may favor the overconsumption of energy, thus promoting the development of risk factors associated with metabolic diseases such as hypertriglyceridemia, hypercholesterolemia, and insulin resistance
[1,2,3][1][2][3]. Moreover, the current literature strongly suggests that ingestion of sugar-sweetened beverages increases the cardiometabolic risk and risk factors more than isocaloric amounts of complex carbohydrates
[4]. Free sugars are defined as any types of simple sugars (monosaccharides or disaccharides) that have been added to beverages or food products during their transformation or preparation by food industries or by the consumer per se, plus sugars naturally present in fruit juices, fruit juice concentrates, honey, and various syrups
[5]. Sugars that occur in the natural structure of entire fruits and vegetables as well as those from milk (lactose) are not categorized as free sugars
[5,6][5][6]. Added sugars include sugars and syrups that are added during the preparation or the transformation of food and beverages. Therefore, natural fruit juices do not contain added sugars, but on the basis of the above definition, they are a significant source of free sugars.
While it is mainly accepted that the overconsumption of sugar-sweetened beverages may lead to adverse effects on health
[3[3][4],
4], the evidence pertaining to the consumption of fruit juices is a matter of debate. This is reflected in the inconsistency between dietary guidelines that relate to the consumption of natural fruit juice. The World Health Organization (WHO) recommends reducing the intake of free sugars to less than 10% (and, ideally, less than 5%) of total daily energy intake, thus including sugars naturally present in fruit juices in the category of sugars whose consumption should be reduced. The 2015–2020 Edition of the Dietary Guidelines for Americans, a resource for health professionals and policymakers for the design and implementation of nutrition programs in the United States, recommends consuming less than 10% of calories per day from added sugars, thus not including sugars naturally present in fruit juices in the category of sugars whose consumption should be reduced
[7]. However, the American Academy of Pediatrics recommends limiting the consumption of fruit juice for children between the age of 1 and 3 years to 4 oz (120 mL)/day, for those from 4 to 6 years to 4–6 oz (120–180 mL)/day, and for those from 7 to 18 years to 8 oz (240 mL)/day
[8,9][8][9].
2. Added Sugars: Sucrose and High-Fructose Corn Syrup
Simple sugars have been a part of the human diet for millennia. They were provided mainly by fruits and honey until white sugar (sucrose) became a common consumer product in the 19th century
[10]. Nowadays, the worldwide consumption of sucrose is widespread, to the extent that it has tripled over the past 50 years
[11]. In the United States, 77% of all calories purchased from 2005 to 2009 contained sweeteners, of which corn syrup, cane sugar, High-Fructose Corn Syrup (HFCS), and fruit juice concentrate were listed as the most commonly used
[12]. Sucrose is naturally occurring in sugar cane and sugar beet and therefore extracted and purified directly from sugar cane or beet sap. In contrast, HFCS, which replaces sucrose in 40% of processed foods and beverages, is not naturally occurring
[13].
3. How Does Fructose Can Alter Lipemia?
3.1. Glucose Metabolism
Concerns about the ratio of fructose to glucose in beverages relate to the well-established differences between glucose and fructose metabolism. Glucose enters the enterocytes mostly by secondary active transport via sodium–glucose transporters (SGLT1) located in the apical membrane of the enterocytes. SGLT1 transporters have a high-affinity for glucose, but a low transportation capacity. Thus, under high concentrations of glucose in the lumen of the intestine, glucose also enters the enterocytes by facilitated diffusion via low-affinity, but high-capacity glucose transporters (GLUT2)
[26][14]. GLUT2 transporters are expressed to a lesser extent in the apical membrane of the enterocytes but can be rapidly translocated from the basolateral membrane to the apical membrane to enhance glucose uptake under high concentrations of intestinal glucose
[26][14]. Then, glucose exits the enterocytes to enter the bloodstream by facilitated diffusion via GLUT2 transporters located in the basolateral membrane (
Figure 1). Glucose is then transported to the liver by the portal vein. Hepatic glucose metabolism is regulated by insulin and hepatic energy needs. This allows much of ingested glucose arriving via the portal vein to bypass the energy-replete liver and to rapidly reach the systemic circulation
[27][15]. The first step of glycolysis, the responsible pathway for glucose metabolism, is the phosphorylation of glucose on its 6th carbon by the enzyme glucokinase (hexokinase). This step is then followed by an isomerization reaction resulting in fructose 6-phosphate (F6P). The major limiting step of glycolysis is the phosphorylation of F6P to fructose 1,6-biphosphate (catalyzed by phosphofructokinase), which will allow the molecule to be cleaved in two three-carbon units that can later be used to generate ATP
[28][16].
Figure 1. Absorption of fructose and glucose in the enterocytes. Glucose enters the enterocytes mostly by secondary active transport via sodium-glucose transporters (SGLT1) located in the apical membrane of the enterocytes. Under high concentrations of glucose in the lumen of the intestine, glucose also enters the enterocytes by facilitated diffusion via glucose transporters (GLUT2). Fructose enters the enterocytes through a specific fructose transporter (GLUT5). Then, both glucose and fructose exit the enterocytes to enter the systematic circulation by facilitated diffusion via GLUT2 transporters located in the basolateral membrane of the enterocytes. A small part of dietary fructose will be converted and released in the bloodstream by the enterocytes as glucose.
3.2. Fructose Metabolism
When fructose is ingested, it enters enterocytes through a specific fructose transporter (GLUT5), which is independent of sodium-glucose linked transporters and does not require ATP hydrolysis as opposed to SGLT1
[1]. Fructose will then enter the systematic circulation in a similar way to glucose, that is by facilitated diffusion via GLUT2 transporters (
Figure 1). A small part of dietary fructose will be converted and released in the bloodstream by the enterocytes as glucose
[1[1][17],
29], lactate
[1[1][17],
29], and fatty acids in chylomicrons
[30,31,32,33,34][18][19][20][21][22] (
Figure 2). Yet, the role of the enterocytes in determining the metabolic fate of fructose has not been clearly established.
Figure 2. Metabolic fate of dietary fructose. Dietary fructose is ingested and released by the enterocytes mostly as fructose but also converted and released as glucose, lactate, and fatty acids (in chylomicrons). Fructose spills over to the liver where it is phosphorylated as Fructose 1-Phosphate (F 1-P). The largest part of F 1-P will be metabolized and converted by the hepatocytes as glucose, which can be stored as glycogen or released in the bloodstream
[35][23]. Hepatocytes can also convert F 1-P into lactate and fatty acids. Fatty acids accumulate into the liver, consequently favoring the production and secretion of very low-density lipoproteins (VLDL), which leads to increased levels of circulating triglycerides and dyslipidemia.
The excessive amount of fructose will spillover to the liver where it is nearly all cleared from the portal blood after its first pass. Fructose will be rapidly phosphorylated (catalyzed by fructokinase C, a key enzyme in the metabolism of fructose
[36,37,38][24][25][26]) on its first carbon, resulting in fructose 1-phosphate (F 1-P) instead of F6P
[28][16] (
Figure 2). F 1-P has the capacity to bypass the first limiting step of glycolysis
[28][16] without being regulated by insulin nor inhibited by ATP production. F 1-P will mostly be metabolized by aldolase B into glyceraldehyde 3-P (G 3-P). Subsequently, G 3-P can be: (1) converted into pyruvate (resulting in acetyl-CoA production); (2) converted and released as lactate; or (3) converted to glucose (gluconeogenesis)
[1]. The largest part of G 3-P will be converted to glucose, which can be stored as glycogen or released as glucose 6-phosphate in the bloodstream
[35][23]. In fact, in the 1990s, isotope tracing with intravenously infused
13C-labeled fructose in humans showed that ~50% of a fructose load was converted and recirculated as
13C-labeled glucose
[39][27]. When released in the systematic circulation, glucose and lactate can be utilized as an energy substrate by the brain, heart, and muscle tissue
[35][23] (
Figure 2). As mentioned previously, a large part of F 1-P will be metabolized in G 3-P. Nonetheless, the excessive supply of fructose spilled to the liver has also been shown to simultaneously inhibit lipid oxidation
[40][28] and to enhance hepatic de novo lipogenesis (DNL)
[41,42][29][30]. DNL has the capacity to convert fructose, more precisely F 1-P, into fatty acids
[43][31], thus consequently increasing the intrahepatic lipid supply. Elevated levels of intrahepatic lipids content favor very low-density lipoproteins (VLDL) production and secretion
[44][32], which also leads to increased levels of postprandial triglycerides and dyslipidemia
[1,42][1][30] (
Figure 2). Increased levels of intrahepatic lipids are associated with hepatic insulin resistance
[45,46][33][34]. Of note, a systematic review indicated that fructose consumption in an energy-matched exchange for other carbohydrates (mostly glucose) induces hepatic insulin resistance
[47][35]. This suggests that the promotion of hepatic insulin resistance by fructose could not be attributed only to the excess of energy intake under hypercaloric diet conditions. Knowing that DNL is more strongly activated in the insulin-resistant liver
[48][36], fructose consumption has the potential to generate a vicious cycle that would further increase the intrahepatic lipid supply, thus amplifying VLDL-triglyceride production and secretion
[2]. Continued exposure to triglycerides promotes muscle lipid accumulation
[49][37], which may also promote whole-body insulin resistance
[50,51][38][39].
4. Sugar-Sweetened Beverages and Their Implication in Metabolic Health