Understanding the Key Differences Between Glucose and Fructose

Glucose is a monosaccharide that is synthesized by the body from carbohydrates, proteins, and fat, and is the body's preferred source of energy for cellular activities. Fructose is also a simple sugar that occurs naturally in several fruits and some vegetables. This Buzzle write-up enumerates the key differences between glucose and fructose.
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Difference between glucose and fructose
Did You Know?
The term 'fructose' was coined by English chemist William Miller in 1857. Commonly referred to as fruit sugar, fructose is the sweetest of all the sugars; it is 140% sweeter than table sugar.
The term 'monosaccharides' refers to the simplest form of sugars or carbohydrates. Both glucose and fructose are simple sugars or monosaccharides, with the same formula C6H12O6, but a different molecular structure. Table sugar, which is also called sucrose, contains fructose and glucose in equal proportions. Fructose is found in several fruits, berries, root vegetables, and honey. It can also be made from sugarcane, sugar beets, and corn. It must be noted that fruits often contain fructose, glucose, and sucrose. There's no denying the fact that increased intake of added sugars through processed food can make one susceptible to several health problems. Fruits are considered as a better alternative to sugary, processed food, as fruits also contain dietary fiber and nutrients. However, it must be noted that fructose is also present in fruit juices, sweetened beverages, soft drinks, etc., in the form of table sugar. High fructose corn syrup, agave nectar, molasses, maple syrup, etc., are rich sources of fructose.

Plants make glucose through the process of photosynthesis. Complex sugars and carbohydrates, whole grains, cereals, vegetables, fruits, etc., are some of the dietary sources of glucose. In case of maple sugar, honey, and invert sugar, it is the formed from the hydrolysis of sucrose. Basically, all dietary carbohydrates are absorbed in the intestines as monosaccharides, after salivary and pancreatic amylases act upon the starch and disaccharides. These carbohydrates are broken down into glucose, which is absorbed in the bloodstream. Glucose that is present in the blood is called blood sugar or blood glucose. Glucose is converted into energy, and the rest is stored as glycogen in the liver and muscle cells for everyday use, and the adipose cells and tissues for future use. It must be noted that excess of any form of sugar is bad for one's health.

Glucose vs. Fructose

Fructose, glucose, and galactose are hexoses, which means that they are monosaccharides with each carbon atom being bound to a water molecule. Unlike fructose and glucose, galactose is not naturally-occurring. They have the same molecular formula, which means that they are isomers.

Chemical structure of glucose and fructose

While glucose and galactose are aldoses (reducing sugars), fructose is a ketose (a non-reducing sugar). Fructose reacts with glucose to make the disaccharide sucrose. While glucose has a six member ring, fructose has a five member ring structure. In case of glucose, the carbon atom is attached to a hydrogen atom by a single bond and an oxygen atom by a double bond. In case of fructose, the carbon atom is attached only to an oxygen atom by a single bond. Thus, the molecular structure differs in terms of the bonding of the oxygen atom.

Sources of Glucose and Fructose

Fresh fruits such as apples, grapes, pears, cherries, pomegranate, kiwifruit, berries, watermelon etc., are rich in fructose, and so are dried fruits such as raisins, dried figs, dried peaches, dried apricots, and dried prunes. Some vegetables that are good sources of fructose include artichoke, asparagus, beetroot, broccoli, Brussels sprouts, burdock, chicory roots and greens, dandelion greens, okra, peas, shallots, onions, fennel, garlic, leek, legumes, etc. Sauces, canned fruit, baked goods such as candy and cookies, sodas, and beverages that are sweetened with fructose or glucose-fructose, honey, high-fructose corn syrup, agave syrup, etc., are all sources of fructose. On the other hand, glucose is present in plain dairy products such as milk, cheese, and yogurt. It is also present in fruits and grains. It must be noted that food products with added sugars will contain glucose, as well as fructose.

Metabolism

One of the major differences between fructose and glucose is the way in which these are metabolized in the body. While glucose relies on enzymes glucokinase or hexokinase for initiating metabolism, fructose relies on fructokinase. When we ingest complex carbohydrates, these are converted into glucose during digestion in the small intestine. Glucose is absorbed in the bloodstream, and its concentration is regulated mainly by insulin, along with glucagon and epinephrine. Glucose is used by cells and tissues for energy. The regulation of glucose requires insulin, which is a hormone secreted by the islets of Langerhans in the pancreas. It is insulin that allows the cells to absorb glucose from the bloodstream. If the concentration of glucose in the blood is too high, insulin is secreted by the pancreas. While some of the glucose is used for energy, the rest is stored in the form of glycogen in the liver for future use.

On the other hand, fructose is predominantly metabolized in the liver. It releases energy slower than glucose. Moreover, fructose does not cause insulin to be released, and does not stimulate the production of leptin, which is a hormone that helps regulate energy intake and expenditure. As per the National Institute of Health, short-term controlled feeding studies revealed that the intake of dietary fructose significantly increased postprandial triglyceride (TG) levels (a type of fat), but didn't have a significant effect on serum glucose concentrations. On the other hand, dietary glucose didn't effect the triglyceride levels, but affected the serum glucose concentration. This is mainly due to the fact that fructose is largely metabolized in the liver, whereas glucose is not removed by the liver, as it is first absorbed in the bloodstream and is later stored in the liver. However, many experts are of the opinion that large-scale trials need to be conducted to verify the aforementioned effects.

Health Risks

In case of healthy individuals, the concentration of glucose in the blood is around 0.1%. However, in case of individuals affected by diabetes, glucose levels in blood increase considerably. Though natural fructose is considered to be a slightly better choice for diabetics, it must be noted that fructose should not be taken in large quantities. Excessive consumption of fructose in the form of additive such as high-fructose corn syrup (55% fructose and 45% glucose) in soft drinks, pastries, and other processed food items, has been linked to gastrointestinal distress coupled with increased fat content of blood.

In a study published in the Journal of Nutrition and Metabolism in 2013, excessive fructose use was linked to metabolic syndrome, which puts one at a greater risk of cardiovascular disease and diabetes. Since manufacturers are not required to specify details about fructose concentration on the labels, consumers must try to cut down on the intake of processed food with added sugars. It is believed that consuming large amounts of high fructose sweeteners can lead to central obesity, low levels of HDL or good cholesterol, high levels of LDL or bad cholesterol, high triglycerides, and poor appetite (due to decreased leptin). Currently, studies are being conducted to substantiate these claims.

On a concluding note, the trend towards using high-fructose corn syrup that started in the 70s was mainly due to the fact that it is sweeter than glucose, which is why using it in smaller amounts would suffice. Moreover, it is cheaper. However, excess of anything is bad; more so in case of any form of added sugars, as these predispose one to serious medical conditions. Thus, the best way to lower the risk of obesity and diabetes would be to cut down on the intake of processed food, and consume whole grain, fresh fruits, and vegetables.
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Published: July 21, 2014
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