A sugar is a monosaccharide if it is made up of a single sugar molecule; it is disaccharide if it is built by two sugar molecules; and a polysaccharide if it is composed with multiple sugar molecules.
Sugars that we ingest are broken, in the digestive system, into monosaccharides of glucose, fructose, and galactose. Almost all of the absorbed monosaccharides are first converted into glucose in the liver. This conversion is a very important task of the liver: 80% of the sugars passing into the blood are glucose. As a result, very limited amounts of fructose and galactose are present in the blood. Therefore, when blood sugar is mentioned, normally glucose is taken into consideration and the fructose and galactose levels in the blood are ignored. Glucose, which is also called grape sugar, is most abundantly found in grapes, while fructose is called fruit sugar, as it is plentiful in fruits, and galactose is named milk sugar after its dense presence in milk. The most important characteristic of fructose is that it is sweeter compared to other simple sugars.
Insulin is secreted from the pancreas in order to lower elevated blood glucose levels after digestion. Insulin functions in the transport of glucose from the blood into cells to provide necessary energy, therefore reducing blood sugar levels; furthermore, it also plays a role in the storage of excess glucose as glycogen, which is found primarily in the liver. Once glycogen storage limits are reached in the liver and muscles, glucose is then stored as fat. Fat tissue acts as sustenance during long fasting periods.
Differences between fructose, glucose, and galactose
Glucose and galactose are absorbed actively, depending on salt. They cannot be absorbed without salt while passing through the intestines. Salt is necessary for the absorption of glucose which is present in the starches of potatoes and other foods. Thus, when potato is consumed with salt, the transport of glucose into the blood is facilitated.
However, salt is not necessary in the case of fructose absorption. The intestinal absorption of fructose contained in fruit is delayed by fruit fibers, since these fibers prevent or balance the transport of fructose into the bloodstream. However, when fructose is ingested as a fruit juice, it is absorbed and joins the bloodstream much faster because of the lower fiber content.
A person feels full after a meal when neurons in the satiety center of the hypothalamus are stimulated by elevated blood glucose. Then, hunger center neurons are repressed, eliminating the feeling of hunger. Therefore, a person reduces their food intake during a meal as their blood glucose levels increase. Increased levels of amino acids and fatty acids in the blood also suppress hunger and stimulate fullness after meal. However, one important point is that fructose does not stimulate fullness in the brain. Therefore, if the blood fructose levels are elevated instead of glucose, a person cannot generate a sensation of fullness sensation. As a result, a person desires to intake more food during consumption of fructose. It is only possible for fructose to generate fullness after it has been converted into glucose by liver.
How to consume fruits?
We should prefer direct consumption of fruits instead of drinking natural or industrial fruit juices because of the high fructose content of fruits. The Prophet Muhammad, peace be upon him, consumed fruits before meals, the wisdom of which we learn only today. Fruits should be consumed at least an hour before or two hours after a meal, for sufficient time should be given for the fructose of an ingested fruit to be absorbed by the intestines and converted to glucose by the liver. Such practices will result in a reduced appetite and food intake. If fruit is consumed after a meal, a delay occurs in the conversion of fructose into glucose since the liver will be occupied by other biochemical processes, along with a full storage of nutrients; this will increase blood fructose levels and fail to reduce appetite. Fatty liver occurs as a consequence of high fat content of the blood. Arteriosclerosis and cirrhosis of the liver may be seen in people with a habit of excessive post-meal fruit consumption.
In a research carried out on laboratory animals, it was found that glucose induces fullness in the hypothalamus and suppresses food intake, whereas fructose was found to repress this effect of glucose, stimulating food intake.1 Insulin reduces the harms of accumulating sugar in the blood by increasing lipid synthesis. Insulin also takes place in leptin secretion from adipose (fatty) tissue. Leptin is important in the prevention of obesity; therefore, insulin helps in weight loss, too. The leptin hormone causes reduced food intake by stimulating nerve cells in certain parts of the hypothalamus.2 Fructose does not cause any leptin secretion because it does not stimulate an insulin release; therefore, it is not effective in generating a sense of fullness.
Ghrelin is a hormone secreted into blood by stomach cells during hunger. This hormone, which produces stomach acids, is enacted through the hypothalamus. It induces hunger, and therefore increases appetite. Insulin secretion increases along with the blood glucose levels during satiety. This eventually causes the increase of the leptin hormone, which also leads to a decrease in ghrelin secretion. As a result, fructose gets absorbed more than glucose in the intestines. Elevated fructose in the blood leads to insufficient or reduced insulin secretion. In this case, a person continues eating.
Fructose and diseases
Free circulation of lipids in the blood damages arteries and veins. For this reason, lipids are transported in "molecular vehicles" that are called as high, low, and very low density lipoproteins (HDL, LDL and VLDL). Neutral lipids (triglycerides) that are present on VLDL (very low density) vehicles are broken down with an enzyme. These lipids are then unloaded from the vehicles by cellular uptake and stored as fats. This transfer of lipids into adipose (fatty) tissue is enhanced via the insulin hormone. In the case of fructose intake, without its insulin secretion effect, lipids accumulate in the blood and liver and eventually prepare ground for liver damage and arteriosclerosis.As the result of a fructose based diet in laboratory animals, it was discovered that lipid production shifted from adipose tissue into the liver, therefore elevating the risk of high blood and liver fat levels.
There are two reasons for this shift. The first one is that fructose acts on the fat producing enzymes of the liver whereas it does not act likewise in adipose tissue.
Secondly, fructose plays an inhibitory role in the conversion of glucose into lipids in adipose tissue. Also, fructose consumption in humans has been linked to elevated blood fat levels.
Overconsumption of fructose causes increased liver fat synthesis. Phosphofructokinase is the limiting enzyme regarding the breakdown of glucose in the liver. This enzyme is regulated by citrates and ATP produced by glucose catabolism and the Krebs cycle, limiting glucose breakdown. However, there is no such limitation in fructose breakdown. Through fructose catabolism, glucose, glycogen, pyruvate, lactate, glycerol and the acyl part of acylglycerol are synthesized. This synthesis can not be limited. As a result of this excessive output and high amounts of triglycerides, VLDL is produced.3 It has been found that persons who consume two or more boxes of fructose sweetened beverages every day carry a 35% higher risk of heart disease.4
This isn't the only disease associated with fructose. In some studies on laboratory animals, it has been reported that a high fructose diet is associated with hypertension.5 A lot of research exists suggesting that excessive fructose consumption leads to insulin resistance in both the liver and peripheral tissues, which can often cause diabetes.6 In a recent study, it was claimed that excessive fructose intake poses risks for renal diseases leading to glomerular hypertension, renal damage, and inflammation and damage to renal tubules and tissues.7
In a study conducted on 21,483 Americans who were older than two years, daily consumption of 37 gr. of fructose (8% of total calorie need) was found to be elevated to 54.7 grams (10.2% of total calorie need) gradually between the years of 1988-1994, mostly consumed by younger people. Increased use of fructose syrup was linked to obesity during the last 35 years.8 Furthermore, in a study carried on 1,749 male and female children and teenagers, a positive relation was found between body mass index (BMI) and excessive consumption of carbonated beverages containing high fructose concentrations.9 There many studies that support this report.10Excessive fructose consumption is known to cause "metabolic syndrome" in which many diseases like obesity, arteriosclerosis, and diabetes emerge together.
Are fruit juices harmful?
Fructose syrup is being used at increasing rates in the food industry. According to the annual report of US Food and Drug Administration (FDA) for the year 2000, fructose syrups are sugar solutions containing approximately more than 50 % fructose. It is often synthesized by a conversion of corn starch into glucose by glucose isomerase.11 There is also a third syrup type containing 90% fructose, however this has limited uses.
The sweetness of fructose syrup is similar to that of table sugar. It prevents the dehydration of food with its hydrophilic character. It is mostly used in aromatic foods, especially carbonated beverages and fruit juices. It prevents the proliferation of microbes with its high osmotic pressure property and makes food more resistant against them. Syrups containing 42 to 55% of fructose are used in baked goods, cereal products, dairy products, processed foods, both carbonated and regular beverages, ice creams, and frozen desserts. High fructose syrups are used in foods to decrease water activity and prevent spoilage.
Fructose syrups have a very low ash level due to application of intense purification processes during production and product color is water-white. Therefore colors of fructose used industrial foods are white as well. Fructose syrups have a lower viscosity and density compared to glucose syrups and therefore it is runny like water and not as sticky.
How to consume sugars after a meal?
Especially after a fatty meal, our body seeks sugar. The reason behind this is the requirement of sugar for the storage of lipids into fat tissue. However, this sugar should absolutely be glucose instead of fructose. Therefore, some amount of sugar can be consumed to facilitate the removal of lipids from blood after meals. This is recommended to lower blood lipid levels. However, this should not be done with fruits but with natural sugars like grape molasses. A baklava or a dessert made with industrial sugars (fructose) will not be beneficial but harmful.
In conclusion, the consumption of corn-derived fructose syrup is gradually increasing in recent years. Fructose syrup is used both in various carbonated or regular soft beverages, and in desserts. The reason for our fructose syrup preference is that it helps preserve foods longer and it leads to food addiction because it enhances appetite due to its strong sweetness. Fructose syrup is synthesized by the conversion of natural glucose in corn into fructose by isomerase enzymes. In this sense, today's increased consumption of fructose is altering the existing sugar balance of natural food items. Overconsumption of fructose can pave the way to obesity, metabolic syndrome, arteriosclerosis, diabetes, hypertension, and arteriosclerotic heart and kidney diseases.
Arifagaoglu is a professor of medicine in Ankara, Turkey.
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