Fructose

Fructose (or levulose) is a simple sugar (monosaccharide) found in many foods and one of the three most important blood sugars along with glucose and galactose. Honey; tree fruits; berries; melons; and some root vegetables, such as beets, sweet potatoes, parsnips and onions, contain fructose, usually in combination with sucrose and glucose. Fructose is also derived from the digestion of sucrose, a disaccharide consisting of glucose and fructose that is broken down by enzymes during digestion. Fructose is the sweetest naturally occurring sugar, estimated to be twice as sweet as sucrose.

Fructose is often recommended for, and consumed by, people with diabetes mellitus or hypoglycemia, because it has a very low Glycemic Index (GI 23) relative to cane sugar (sucrose). However, this benefit is tempered by concern that fructose may have an adverse effect on plasma lipid and uric acid levels, and the resulting higher blood levels of fructose can be damaging to proteins (see below). The low GI is due to the unique and lengthy metabolic pathway of fructose, which involves phosphorylation and a multi-step enzymatic process in the liver. See health effects and glycation for further information.

Structure

File:Fructose.svg

Structure formula of fructose

Fructose, or levulose, is a levorotatory monosaccharide with the same empirical formula as glucose (C₆H₁₂O₆) but with a different structure. Pure fructose has a sweet taste similar to cane sugar, but with a "fruity" aroma. Although fructose is a hexose (6 carbon sugar), it generally exists as a 5-member hemiketal ring (a furanose). This structure is responsible for the long metabolic pathway and high reactivity compared to glucose.

The first -OH points the opposite way from the second and third -OH.

Isomerism

D-Fructose has the same configuration at its penultimate carbon as D-glyceraldehyde. Fructose is sweeter than glucose due to its stereomerism structure.

• Alpha-D-Fructose-structure-corrected.png

  alpha-D-Fructose

• Beta-D-Fructose-structure.png

  beta-D-Fructose

• Alpha-L-Fructose-structure-correct.png

  alpha-L-Fructose

• Beta-L-Fructose-structure.png

  beta-L-Fructose

Health effects

Fructose depends on glucose to carry it into the blood stream via GLUT-5 and then GLUT-2 . Absorption of fructose without glucose present is very poor, and excess fructose is carried into the lower intestine where it provides nutrients for the existing flora, which produce gas. It may also cause water retention in the intestine. These effects may lead to bloating, excessive flatulence, loose stools, and even diarrhea depending on the amounts eaten and other factors.

Fructose has been hypothesized to cause obesity , elevated LDL cholesterol and triglycerides, leading to metabolic syndrome. Unlike animal experiments, some human experiments have failed to show a correlation between fructose consumption and obesity. Short term tests, lack of dietary control, and lack of a non-fructose consuming control group are all confounding factors in human experiments. However, there are now a number of reports showing correlation of fructose consumption to obesity, especially central obesity which is generally regarded as the most dangerous type. (Wylie-Rosett, 2004)(Havel, 2005)(Bray, 2004) (Dennison, 1997)

Fructose also chelates minerals in the blood. This effect is especially important with micronutrients such as copper, chromium and zinc. Since these solutes are normally present in small quantities, chelation of small numbers of ions may lead to deficiency diseases, immune system impairment and even insulin resistance, a component of type II diabetes (Higdon).

Fructose is a reducing sugar, as are all monosaccharides. The spontaneous addition of single sugar molecules to proteins, known as glycation, is a significant cause of damage in diabetics. Fructose appears to be as dangerous as glucose in this regard and so does not seem to be the answer for diabetes (McPherson et al, 1988) This may be an important contribution to senescence and many age-related chronic diseases (Levi & Werman 1998).

Fructose intolerance (Hereditary Fructose Intolerance, or HFI) is a hereditary condition due to a deficiency of liver enzymes that metabolise fructose. The deficient enzyme is Fructose-1-phosphate aldolase-B, this means that the fructose cannot be further metabolised beyond fructose-1-phosphate. This traps phosphates; which are needed to phosphorolyse glycogen phosphorolase to carry on to make glucose. Therefore glucose cannot be made through the breakdown of glycogen nor from gluconeogenesis, resulting in severe hypoglycaemia. If fructose is ingested, vomiting, hypoglycaemia and eventually kidney failure will follow.

Hereditary Fructose Intolerance should not be confused with fructose malabsorption or Dietary Fructose Intolerance (DFI), a deficiency of fructose transporter enzyme in the enterocytes, which leads to abdominal bloating, diarrhea and/or constipation.

Fructose malabsorption is a dietary disability of the small intestine in which the fructose carrier in enterocytes is deficient. Medical tests are similar as in lactose intolerance, requiring a hydrogen breath test for a clinical diagnosis.

Fructose Malabsorption is not to be confused with fructose intolerance or Hereditary Fructose Intolerance (HFI), a hereditary condition in which the liver enzymes that break fructose up are deficient. In patients with fructose malabsorption, the small intestine fails to absorb fructose properly. In the large intestine the unabsorbed fructose osmotically reduces the absorption of water and is metabolized by normal colonic bacteria to short chain fatty acids and the gases hydrogen, carbon dioxide and methane. The abnormal increase in hydrogen is detected with the hydrogen breath test.

There is no known cure, but an appropriate diet will help. However, it is very difficult for undiagnosed sufferers to see any relationship between the foods they eat and the symptoms they suffer, even if they keep a daily diet diary. This is because most foods contain a mixture of fructose and glucose. Foods with more fructose than glucose are a problem, as are foods with a lot of fructose (regardless of the amount of glucose). However, depending upon the sufferer's sensitivity to fructose, small amounts of problem foods could be eaten (especially when they are not the main ingredient of a meal).

Foods with a high glucose content actually help sufferers absorb fructose.

Commercial production

Fructose is used as a substitute for sucrose (common sugar) because it is less expensive and has little effect on measured blood glucose levels. Often Fructose is consumed as high fructose corn syrup which is corn syrup (glucose) which has been enzymatically treated, by the enzyme glucose isomerase, to convert a portion of the glucose into fructose thus making it sweeter. This is done to such a degree to yield corn syrup with an equivalent sweetness as sucrose by weight. While most carbohydrates have around the same amount of calories, fructose is sweeter, so manufacturers may use less fructose to get the same sweetness. The free fructose present in fruits, their juice, and honey is responsible for the greater sweetness of these natural sugar sources.

High fructose corn syrup

High fructose corn syrup (HFCS) is a newer and sweeter form of corn syrup. Like ordinary corn syrup, the high fructose variety is made from corn starch using enzymes. The production process of HFCS was developed by Japanese researchers in the 1970s. HFCS was rapidly introduced in many processed foods and soda drinks in the US over the period of about 1975–1985, and usage continues to increase as sugar use decreases at a nearly one to one level (Bray, 2004 & U.S. Department of Agriculture, Economic Research Service, Sugar and Sweetener Yearbook series, Tables 50–52.). There are three main reasons for this switch; first is cost, as HFCS is a bit cheaper due to corn subsidies and import sugar tariffs. The second reason is that it is a liquid which is easier to blend and transport. The third is that a product made with HFCS has a much longer shelf life. (White JS. 1992. Fructose syrup: production, properties and applications, in FW Schenck & RE Hebeda, eds, Starch Hydrolysis Products – Worldwide Technology, Production, and Applications. VCH Publishers, Inc. 177-200)

By increasing the fructose content of corn syrup (glucose) through enzymatic processing, the syrup is more comparable to table sugar (sucrose). This makes it useful to manufacturers as a possible substitute for sugar in soft drinks and other processed foods. Common commercial grades of high fructose corn syrup include fructose contents of 42%, 55%, or 90%. The 55% grade is most commonly used in soft drinks and equivalent to caster sugar.

Unlike sucrose, HFCS consists of a mixture of glucose and fructose, which doesn't require an enzymatic step to break it down before absorption in the intestine.

There are currently suspicions that over-consumption of HFCS may be a main contributor to the epidemic of diabetes in the US.

Comparison to other sugars

Sucrose (table sugar) is a disaccharide composed of one unit each of fructose and glucose linked together. Sucrose is 50% fructose, so HFCS may have a higher or lower fructose content than sucrose, with a corresponding change in sweetness. Sucrose is broken down during digestion into fructose and glucose through hydrolysis by the enzyme sucrase.

Honey is another product that is a mixture of different types of sugars, water, and small amounts of other compounds. Honey typically has a fructose/glucose ratio similar to HFCS, as well as containing some sucrose and other sugars.

Production

High-fructose corn syrup (HFCS) is produced by processing corn starch to yield glucose, and then processing the glucose to produce a syrup with a higher percentage of fructose. First, cornstarch is treated with alpha-amylase to produce shorter chains of sugars called oligosaccharides. Then, an enzyme called glucoamylase breaks the sugar chains down even further to yield the simple sugar glucose. The third enzyme, glucose isomerase, converts glucose to a mixture of about 42% fructose and 50–52% glucose with some other sugars mixed in. While alpha-amylase and glucoamylase are added directly to the slurry, glucose-isomerase is packed into columns and the sugar mixture is then passed over it. This 42–43% fructose glucose mixture is then subjected to a liquid chromatography step where the fructose is enriched to approximately 90%. The 90% fructose is then back-blended with 42% fructose to achieve a 55% fructose final product. Numerous ion-exchange and evaporation steps are also part of the overall process.

Sweetener consumption patterns

In the United States

File:Usda sweeteners.jpg

US sweetener consumption, 1966-2004 (cane and beet sugar are both pure sucrose)

The accompanying graph shows the consumption of sweeteners per capita in the United States since 1966. Since HFCS and sucrose (cane and beet sugars) provide almost identical proportions of fructose and glucose, no metabolic changes would be expected from substituting one for the other. However, it is apparent from this graph that overall sweetener consumption, and in particular glucose-fructose mixtures, has increased since the introduction of HFCS. Thus, the proportion of fructose as a component of overall sweetener intake in the United States has increased since the early 1980s. This would be true whether the added sweetener was HFCS, table sugar, or any other glucose-fructose mixture.

International markets

HFCS is produced in the industrialized countries.The production of HFCS is dependent on the agricultural, especially sugar, policy.

In Europe, due to the fact that HFCS (isoglucose) is under the adjustment of production, the greater availability of cane sugar over maize would make its production there uneconomical.

In Japan, HFCS consumption accounts for one quarter of total sweetener consumption.

Health effect controversy

The average American consumed approximately 19.2 kg of HFCS versus 20 kg of sugar in 2004.^{[citation needed]} Where HFCS is not used or rarely used, the sugar consumption per person can be higher than the USA; for example, the 2002 figures for some countries are: USA 32.4 kg, EU 40.1 kg, Brazil 59.7 kg, and Australia 56.2 kg.

One study concluded that fructose "produced significantly higher fasting plasma triacylglycerol 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". A study in mice suggests that fructose increases adiposity. However, these studies looked at the effects of fructose alone. As noted by the U.S. Food and Drug Administration in 1996, the saccharide composition (glucose to fructose ratio) of HFCS is approximately the same as that of honey, invert sugar and the disaccharide sucrose (or table sugar).

A more recent study found a link exists between obesity and high HFCS consumption, especially from soft drinks.

However, the obesity epidemic has many contributing factors. University of California, Davis nutrition researcher Peter Havel has pointed out that while there are likely differences between sweeteners, "the increased consumption of fat, the increased consumption of all sugars, and inactivity are all to blame for the obesity epidemic."

See also

• Fructose intolerance
• Fructose malabsorption
• Glycation
• High fructose corn syrup
• Hyperuricemia

References

ISBN links support NWE through referral fees

• Levi B, Werman MJ. Long-term fructose consumption accelerates glycation and several age-related variables in male rats. J Nutr 1998;128:1442-9. Fulltext. PMID 9732303.
• McPherson JD, Shilton BH, Walton DJ. Role of fructose in glycation and cross-linking of proteins. Biochemistry 1988;27:1901-7. PMID 3132203.
• Higdon, J., Linus Pauling Institute, Oregon State U. Chromium 2003
• Wylie-Rosett,Judith, et al, Carbohydrates and Increases in Obesity: Does the Type of Carbohydrate Make a Difference? Obesity Research 12:124S-129S (2004)[1]
• Havel, PJ, Dietary fructose: Implications for dysregulation of energy homeostasis and lipid/carbohydrate metabolism. Nutr Rev. 2005 May;63(5):133-57 [2]
• Bray, George A, Consumption of high-fructose corn syrup in beverages may play a role in the epidemic of obesity American Journal of Clinical Nutrition, Vol. 79, No. 4, 537-543, April 2004 [3]
• Dennison, Barbara Excess Fruit Juice Consumption by Preschool-aged Children Is Associated With Short Stature and Obesity, PEDIATRICS Vol. 99 No.1, January 1997, pp. 15-22 [4]

External links

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