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Fructose

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Fructose

D-Fructose
D-Fructofuranose
Ball-and-stick model of D-fructose
Identifiers
CAS number  YesY
PubChem
ChemSpider  YesY
UNII  YesY
EC-number
KEGG  YesY
ChEBI  YesY
ChEMBL  YesY
ATC code V06
Jmol-3D images Image 1
Properties
Molecular formula C6H12O6
Molar mass 180.16 g mol−1
Density 1.694 g/cm3
Melting point 103 °C (217 °F; 376 K)
Hazards
LD50 15000 mg/kg (intravenous, rabbit)[3]
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY   YesY/N?)

Fructose, or fruit sugar, is a simple ketonic monosaccharide found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847.[4][5] The name "fructose" was coined in 1857 by the English chemist William Miller.[6] Pure, dry fructose is a very sweet, white, odorless, crystalline solid and is the most water-soluble of all the sugars.[7] From plant sources, fructose is found in honey, tree and vine fruits, flowers, berries, and most root vegetables.

Commercially, fructose is frequently derived from sugar cane, sugar beets, and corn. Crystalline fructose is the monosaccharide, dried, ground, and of high purity. High-fructose corn syrup (HFCS) is a mixture of glucose and fructose as monosaccharides. Sucrose is a compound with one molecule of glucose covalently linked to one molecule of fructose. All forms of fructose, including fruits and juices, are commonly added to foods and drinks for palatability and taste enhancement, and for browning of some foods, such as baked goods.

About 240,000 tonnes of crystalline fructose are produced annually.[8]

Chemical properties

Figure 1: Relationship between the acyclic and the cyclic (hemiketal) isomers of fructose
d- and l-isomers of fructose (open-chain form)

Fructose is a 6-carbon polyhydroxyketone. Crystalline fructose adopts a cyclic six-membered structure owing to the stability of its hemiketal and internal hydrogen-bonding. This form is formally called D-fructopyranose. In solution, fructose exists as an equilibrium mixture of 70% fructopyranose and about 22% fructofuranose, as well as small amounts of three other forms, including the acyclic structure.[9]

Reactions

Fructose and fermentation

Fructose may be anaerobically fermented by yeast or bacteria.[10] Yeast enzymes convert sugar (glucose, or fructose) to ethanol and carbon dioxide. The carbon dioxide released during fermentation will remain dissolved in water, where it will reach equilibrium with carbonic acid, unless the fermentation chamber is left open to the air. The dissolved carbon dioxide and carbonic acid produce the carbonation in bottled fermented beverages.[11]

Fructose and Maillard reaction

Fructose undergoes the Maillard reaction, non-enzymatic browning, with amino acids. Because fructose exists to a greater extent in the open-chain form than does glucose, the initial stages of the Maillard reaction occur more rapidly than with glucose. Therefore, fructose has potential to contribute to changes in food palatability, as well as other nutritional effects, such as excessive browning, volume and tenderness reduction during cake preparation, and formation of mutagenic compounds.[12]

Dehydration

Fructose readily dehydrates to give hydroxymethylfurfural ("HMF"). This process, in the future, may become part of a low-cost, carbon-neutral system to produce replacements for petrol and diesel from plants.[13]

Physical and functional properties

Sweetness of fructose

The primary reason that fructose is used commercially in foods and beverages, besides its low cost, is its high relative sweetness. It is the sweetest of all naturally occurring carbohydrates. In general, fructose is regarded as being 1.73 times as sweet as sucrose.[14][15] However, it is the 6-membered ring form of fructose that is sweeter; the 5-membered ring form tastes about the same as usual table sugar. Warming fructose leads to formation of the 5-membered ring form.[16]

Figure 2: Relative sweetness of sugars and sweeteners

The sweetness of fructose is perceived earlier than that of sucrose or glucose, and the taste sensation reaches a peak (higher than that of sucrose) and diminishes more quickly than that of sucrose. Fructose can also enhance other flavors in the system.[14]

Fructose exhibits a sweetness synergy effect when used in combination with other sweeteners. The relative sweetness of fructose blended with sucrose, aspartame, or saccharin is perceived to be greater than the sweetness calculated from individual components.[17]

Fructose solubility and crystallization

Fructose has higher solubility than other sugars as well as other sugar alcohols. Fructose is, therefore, difficult to crystallize from an aqueous solution.[14] Sugar mixes containing fructose, such as candies, are softer than those containing other sugars because of the greater solubility of fructose.[18]

Fructose hygroscopicity and humectancy

Fructose is quicker to absorb moisture and slower to release it to the environment than sucrose, glucose, or other nutritive sweeteners.[17] Fructose is an excellent humectant and retains moisture for a long period of time even at low relative humidity (RH). Therefore, fructose can contribute a more palatable texture, and longer shelf life to the food products in which it is used.[14]

Freezing point

Fructose has a greater effect on freezing point depression than disaccharides or oligosaccharides, which may protect the integrity of cell walls of fruit by reducing ice crystal formation. However, this characteristic may be undesirable in soft-serve or hard-frozen dairy desserts.[14]

Fructose and starch functionality in food systems

Fructose increases starch viscosity more rapidly and achieves a higher final viscosity than sucrose because fructose lowers the temperature required during gelatinizing of starch, causing a greater final viscosity.[19]

Many artificial sweeteners are not suitable for home-baking, but, with a little adjustment, many traditional recipes can be prepared using fructose.[20]

Food sources

Crystalline fructose

Natural sources of fructose include fruits, vegetables (including sugar cane), and honey.[21] Fructose is often further concentrated from these sources. The highest dietary sources of fructose, besides pure crystalline fructose, are foods containing table sugar (sucrose), high-fructose corn syrup, agave nectar, honey, molasses, maple syrup, fruit and fruit juices, as these have the highest percentages of fructose (including fructose in sucrose) per serving compared to other common foods and ingredients. Fructose exists in foods either as a free monosaccharide or bound to glucose as sucrose, a disaccharide. Fructose, glucose, and sucrose may all be present in a food; however, different foods will have varying levels of each of these three sugars.

The sugar contents of common fruits and vegetables are presented in Table 1. In general, in foods that contain free fructose, the ratio of fructose to glucose is approximately 1:1; that is, foods with fructose usually contain about an equal amount of free glucose. A value that is above 1 indicates a higher proportion of fructose to glucose, and below 1 a lower proportion. Some fruits have larger proportions of fructose to glucose compared to others. For example, apples and pears contain more than twice as much free fructose as glucose, while for apricots the proportion is less than half as much fructose as glucose.

Apple and pear juices are of particular interest to pediatricians because the high concentrations of free fructose in these juices can cause diarrhea in children. The cells (enterocytes) that line children's small intestines have less affinity for fructose absorption than for glucose and sucrose.[22] Unabsorbed fructose creates higher osmolarity in the small intestine, which draws water into the gastrointestinal tract, resulting in osmotic diarrhea. This phenomenon is discussed in greater detail in the Health Effects section.

Table 1 also shows the amount of sucrose found in common fruits and vegetables. Sugarcane and sugar beet have a high concentration of sucrose, and are used for commercial preparation of pure sucrose. Extracted cane or beet juice is clarified, removing impurities; and concentrated by removing excess water. The end-product is 99.9%-pure sucrose. Sucrose-containing sugars include common table white granulated sugar and powdered sugar, as well as brown sugar.[23]

Table 1. Sugar content of selected common plant foods (g/100g)[24]
Food Item Total
Carbohydrate
including
"dietary fibre"
Total
Sugars
Free
Fructose
Free
Glucose
Sucrose Fructose/
Glucose
Ratio
Sucrose
as a % of
Total Sugars
Fruits              
Apple 13.8 10.4 5.9 2.4 2.1 2.0 19.9
Apricot 11.1 9.2 0.9 2.4 5.9 0.7 63.5
Banana 22.8 12.2 4.9 5.0 2.4 1.0 20.0
Fig, dried 63.9 47.9 22.9 24.8 0.9 0.93 0.15
Grapes 18.1 15.5 8.1 7.2 0.2 1.1 1
Navel orange 12.5 8.5 2.25 2.0 4.3 1.1 50.4
Peach 9.5 8.4 1.5 2.0 4.8 0.9 56.7
Pear 15.5 9.8 6.2 2.8 0.8 2.1 8.0
Pineapple 13.1 9.9 2.1 1.7 6.0 1.1 60.8
Plum 11.4 9.9 3.1 5.1 1.6 0.66 16.2
Vegetables              
Beet, Red 9.6 6.8 0.1 0.1 6.5 1.0 96.2
Carrot 9.6 4.7 0.6 0.6 3.6 1.0 77
Corn, Sweet 19.0 6.2 1.9 3.4 0.9 0.61 15.0
Red Pepper, Sweet 6.0 4.2 2.3 1.9 0.0 1.2 0.0
Onion, Sweet 7.6 5.0 2.0 2.3 0.7 0.9 14.3
Sweet Potato 20.1 4.2 0.7 1.0 2.5 0.9 60.3
Yam 27.9 0.5 tr tr tr na tr
Sugar Cane 13 - 18 0.2 – 1.0 0.2 – 1.0 11 - 16 1.0 high
Sugar Beet 17 - 18 0.1 – 0.5 0.1 – 0.5 16 - 17 1.0 high
^A The carbohydrate figure is calculated in the USDA database and does not always correspond to the sum of the sugars, the starch, and the "dietary fibre".

All data with a unit of g (gram) are based on 100 g of a food item. The fructose/glucose ratio is calculated by dividing the sum of free fructose plus half sucrose by the sum of free glucose plus half sucrose.

Fructose is also found in the synthetically manufactured sweetener, high-fructose corn syrup (HFCS). Hydrolyzed corn starch is used as the raw material for production of HFCS. Through the enzymatic treatment, glucose molecules are converted into fructose.[23] There are three types of HFCS, each with a different proportion of fructose: HFCS-42, HFCS-55, and HFCS-90. The number for each HFCS corresponds to the percentage of synthesized fructose present in the syrup. HFCS-90 has the highest concentration of fructose, and typically, is used to manufacture HFCS-55; HFCS-55 is used as sweetener in soft drinks, whereas HFCS-42 is used in many processed foods and baked goods.

Carbohydrate content of commercial sweeteners (percent on dry basis)

Sugar Fructose Glucose Sucrose (Fructose-Glucose) Other sugars
Granulated sugar 0 0 100 0
Caramel 1 1 97 1
HFCS-42 42 53 0 5
HFCS-55 55 41 0 4
HFCS-90 90 5 0 5
Honey 50 44 1 5
Maple syrup 1 4 95 0
Molasses 23 21 53 3
Tapioca Syrup 55 45 0 0
Corn syrup 0 35 0 0

Data obtained from Kretchmer, N. & Hollenbeck, CB (1991). Sugars and Sweeteners, Boca Raton, FL: CRC Press, Inc.[23] for HFCS, and USDA for fruits and vegetables and the other refined sugars.[24]

Cane and beet sugars have been used as the major sweetener in food manufacturing for centuries. However, with the development of HFCS, a significant shift occurred in the type of sweetener consumption. As seen in Figure 3, this change happened in the 1970s. Contrary to the popular belief, however, with the increase of HFCS consumption, the total fructose intake relative to the total glucose intake has not dramatically changed. Granulated sugar is 99.9%-pure sucrose, which means that it has equal ratio of fructose to glucose. The most commonly used forms of HFCS, HFCS-42, and HFCS-55 have a roughly equal ratio of fructose to glucose, with minor differences. HFCS has simply replaced sucrose as a sweetener. Therefore, despite the changes in the sweetener consumption, the ratio of glucose to fructose intake has remained relatively constant.[25]

Figure 3: Adjusted consumption of refined sugar per capita in the US

Fructose digestion and absorption in humans

Figure 4: Hydrolysis of sucrose to glucose and fructose by sucrase
Figure 5: Intestinal sugar transport proteins

Fructose exists in foods either as a monosaccharide (free fructose) or as a unit of a disaccharide (sucrose). Free fructose is absorbed directly by the intestine. When fructose is consumed in the form of sucrose, it is digested (broken down) and then absorbed as free fructose. As sucrose comes into contact with the membrane of the small intestine, the enzyme sucrase catalyzes the cleavage of sucrose to yield one glucose unit and one fructose unit, which are then each absorbed. After absorption, it enters the hepatic portal vein and is directed toward the liver.

The mechanism of fructose absorption in the small intestine is not completely understood. Some evidence suggests active transport, because fructose uptake has been shown to occur against a concentration gradient.[26] However, the majority of research supports the claim that fructose absorption occurs on the mucosal membrane via facilitated transport involving GLUT5 transport proteins. Since the concentration of fructose is higher in the lumen, fructose is able to flow down a concentration gradient into the enterocytes, assisted by transport proteins. Fructose may be transported out of the enterocyte across the basolateral membrane by either GLUT2 or GLUT5, although the GLUT2 transporter has a greater capacity for transporting fructose, and, therefore, the majority of fructose is transported out of the enterocyte through GLUT2.

Capacity and rate of absorption

The absorption capacity for fructose in monosaccharide form ranges from less than 5 g to 50 g (per day?) and adapts with changes in dietary fructose intake.[27] Studies show the greatest absorption rate occurs when glucose and fructose are administered in equal quantities.[27] When fructose is ingested as part of the disaccharide sucrose, absorption capacity is much higher because fructose exists in a 1:1 ratio with glucose. It appears that the GLUT5 transfer rate may be saturated at low levels, and absorption is increased through joint absorption with glucose.[28] One proposed mechanism for this phenomenon is a glucose-dependent cotransport of fructose. In addition, fructose transfer activity increases with dietary fructose intake. The presence of fructose in the lumen causes increased mRNA transcription of GLUT5, leading to increased transport proteins. High-fructose diets (>2.4 g/kg body wt) increase transport proteins within three days of intake.[29]

Malabsorption

Several studies have measured the intestinal absorption of fructose using the [30] Exercise immediately after consumption can exacerbate these symptoms by decreasing transit time in the small intestine, resulting in a greater amount of fructose emptied into the large intestine.[35]

Fructose metabolism

All three dietary monosaccharides are transported into the liver by the GLUT2 transporter.[36] Fructose and galactose are phosphorylated in the liver by fructokinase (Km= 0.5 mM) and galactokinase (Km = 0.8 mM). By contrast, glucose tends to pass through the liver (Km of hepatic glucokinase = 10 mM) and can be metabolised anywhere in the body. Uptake of fructose by the liver is not regulated by insulin. However, insulin is capable of increasing the abundance and functional activity of GLUT5 in skeletal muscle cells.[37]

Fructolysis

The initial catabolism of fructose is sometimes referred to as fructolysis, in analogy with glycolysis, the catabolism of glucose. In fructolysis, the enzyme fructokinase initially produces fructose 1-phosphate, which is split by aldolase B to produce the trioses dihydroxyacetone phosphate (DHAP) and glyceraldehyde [1]. Unlike glycolysis, in fructolysis the triose glyceraldehyde lacks a phosphate group. A third enzyme, triokinase, is therefore required to phosphorylate glyceraldehyde, producing glyceraldehyde 3-phosphate. The resulting trioses are identical to those obtained in glycolysis and can enter the gluconeogenic pathway for glucose or glycogen synthesis, or be further catabolized through the lower glycolytic pathway to pyruvate.

Metabolism of fructose to DHAP and glyceraldehyde

The first step in the metabolism of fructose is the phosphorylation of fructose to fructose 1-phosphate by fructokinase, thus trapping fructose for metabolism in the liver. Fructose 1-phosphate then undergoes hydrolysis by aldolase B to form DHAP and glyceraldehydes; DHAP can either be isomerized to glyceraldehyde 3-phosphate by triosephosphate isomerase or undergo reduction to glycerol 3-phosphate by glycerol 3-phosphate dehydrogenase. The glyceraldehyde produced may also be converted to glyceraldehyde 3-phosphate by glyceraldehyde kinase or further converted to glycerol 3-phosphate by glycerol 3-phosphate dehydrogenase. The metabolism of fructose at this point yields intermediates in the gluconeogenic pathway leading to glycogen synthesis as well as fatty acid and triglyceride synthesis.

Synthesis of glycogen from DHAP and glyceraldehyde 3-phosphate

The resultant glyceraldehyde formed by aldolase B then undergoes phosphorylation to glyceraldehyde 3-phosphate. Increased concentrations of DHAP and glyceraldehyde 3-phosphate in the liver drive the gluconeogenic pathway toward glucose and subsequent glycogen synthesis.[38] It appears that fructose is a better substrate for glycogen synthesis than glucose and that glycogen replenishment takes precedence over triglyceride formation. Once liver glycogen is replenished, the intermediates of fructose metabolism are primarily directed toward triglyceride synthesis.

Figure 6: Metabolic conversion of fructose to glycogen in the liver

Synthesis of triglyceride from DHAP and glyceraldehyde 3-phosphate

Carbons from dietary fructose are found in both the free fatty acid and glycerol moieties of plasma triglycerides. High fructose consumption can lead to excess pyruvate production, causing a buildup of Krebs cycle intermediates.[39] Accumulated citrate can be transported from the mitochondria into the cytosol of hepatocytes, converted to acetyl CoA by citrate lyase and directed toward fatty acid synthesis.[39][40] In addition, DHAP can be converted to glycerol 3-phosphate, providing the glycerol backbone for the triglyceride molecule.[40] Triglycerides are incorporated into very-low-density lipoproteins (VLDL), which are released from the liver destined toward peripheral tissues for storage in both fat and muscle cells.

Figure 7: Metabolic conversion of fructose to triglyceride in the liver

Fructose synthesis

The body is capable of synthesising fructose from glucose via the polyol pathway. This pathway is used by the seminal vesicles to secrete fructose into semen where it is a major source of energy for sperm.

Potential health effects

Digestion

Fructose absorption occurs in the small intestine via the GLUT-5[41] (fructose only) transporter, and the GLUT2 transporter, for which it competes with glucose and galactose. Over-consumption of fructose, inhibition of GLUT2 by other phytochemicals, such as flavonoids,[42] or other issues, may result in delivery of unabsorbed fructose into the large intestine, where it may provide nutrients for the existing gut flora.

Metabolism

Excess fructose consumption has been hypothesized to be a cause of insulin resistance, obesity,[43] elevated LDL cholesterol and triglycerides, leading to metabolic syndrome.[44] In preliminary research, fructose consumption was correlated with obesity.[45][46] A study in mice showed that a high fructose intake may increase adiposity.[47]

Although all simple sugars have nearly identical chemical formulae, each has distinct chemical properties. This can be illustrated with pure fructose. A journal article reports that, "...fructose given alone increased the blood glucose almost as much as a similar amount of glucose (78% of the glucose-alone area)".[48][49][50][50][51]

In Wistar fatty rats, a laboratory model of diabetes, 10% fructose feeding as opposed to 10% glucose feeding was found to increase blood triglyceride levels by 86%, whereas the same amount of glucose had no effect on triglycerides.[52] Neither glucose nor fructose influenced insulin or blood sugar in this model. The authors concluded "These results show that in genetically obese, diabetic rats feeding fructose and glucose is associated with an increase in hepatic lipogenic enzyme activities and triglyceride production, and suggest that fructose stimulates triglyceride production but impairs triglyceride removal, whereas glucose stimulates both of them."[52]

Another study in humans concluded that fructose and sucrose are metabolized similarly,[53] whereas a different analysis "produced significantly higher fasting plasma triglyceride values than did the glucose diet in men" and "...if plasma triacylglycerols are a risk factor for cardiovascular disease, then diets high in fructose may be undesirable".[54]

In a meta-analysis of clinical trials with controlled feeding—where test subjects were fed a fixed amount of energy rather than being allowed to choose the amount they ate—fructose was not an independent factor for weight gain. In one study of a diet with excessive calories, fructose consumption was associated with weight gain.[55]

Fructose is a reducing sugar. The spontaneous chemical reaction of simple sugar molecules binding to proteins is known as glycation. Showing potential cause of skin and bone damage in a rat model of diabetes, investigators suggested "that long-term fructose consumption negatively affects the aging process."[56] Another study using human proteins showed that the glycation caused by fructose appears to be equivalent to that caused by glucose and so does not seem to be a better answer for diabetes for this reason alone, save for the smaller quantities required to achieve equivalent sweetness in some foods. It also found evidence for glycation of human lens proteins caused by fructose.[57]

Compared with sucrose

Fructose is often recommended for diabetics because it does not trigger the production of insulin by pancreatic β cells, probably because β cells have low levels of GLUT5,[58][59][60] although the net effect for both diabetics[61] and non-diabetics[62] is debated. Fructose has a very low glycemic index of 19 ± 2, compared with 100 for glucose and 68 ± 5 for sucrose.[63] Fructose is also 73% sweeter than sucrose at room temperature, so diabetics can use less of it. Studies show that fructose consumed before a meal may even lessen the glycemic response of the meal.[64]

Studies that have compared high-fructose corn syrup (an ingredient in nearly all soft drinks sold in the US) to sucrose (common table sugar) find that most measured short-term physiological effects are equivalent for blood glucose, insulin, leptin, and ghrelin levels.[65][66] This is not surprising, since sucrose is a disaccharide that digests to 50% fructose and 50% glucose, whereas the high-fructose corn syrup most commonly used in soft drinks is 55% fructose and 41% glucose (4% other sugars). The difference between the two lies in the fact that HFCS contains little sucrose, the fructose and glucose being independent moieties. Studies that compare the long-term effects between sucrose and fructose have yet to be conducted.

Liver function

While a few other tissues (e.g., sperm cells[67] and some intestinal cells) do use fructose directly, fructose is metabolized primarily in the liver.[68]

Compared with consumption of high glucose beverages, drinking high-fructose beverages with meals results in lower circulating insulin and leptin levels, and higher ghrelin levels after the meal.[69] Since leptin and insulin decrease appetite and ghrelin increases appetite, some researchers suspect that eating large amounts of fructose increases the likelihood of weight gain.[70]

Excessive fructose consumption may contribute to the development of non-alcoholic fatty liver disease.[71][72]

Gout

A 2008 study found a substantial risk of incident gout associated with the consumption of fructose or fructose-rich foods.[73] Cases of gout have risen in recent years, despite commonly being thought of as a disease of the Victorian era, and it is suspected that the fructose found in soft drinks (e.g., carbonated beverages) and other sweetened drinks is the reason for this.[74][75]

Glycemic index

Fructose has the lowest glycemic index (GI = 19) of all the natural sugars. In comparison, ordinary table sugar (sucrose, which is half fructose) has a GI of 65 and honey (usually about 50% fructose content) has a GI of 55.

Appetite control

A preliminary human study indicated that fructose may not influence metabolic activity or blood flow in brain regions regulating satiety ("fullness"), and so may promote overeating.[76][77]

See also

References

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  3. ^ http://chem.sis.nlm.nih.gov/chemidplus/rn/57-48-7
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Gillespie, David, 'Sweet Poison - Why Sugar Makes Us Fat', Viking, Penguin Books 978 067 007247 7 First published 2008 http://www.penguin.com.au/products/9780670072477/sweet-poison-why-sugar-makes-us-fat

External links

  • What is HFI? at Boston University
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