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With this article, we start discussing one of the most important for low carb dieting food classifications - the Glycemic Index.
Glucose and sugars including glucose as a building block are important, though not exclusive, fuel for the body. They are also structural components of plants and this is an important fact to remember for better understanding of what the glycemic index is. Surprisingly enough, amount and ratios of the sugars are not yet available for most foods.
However, the chemical structures and dietary properties are relatively well known. What are these sugars?
First of all, glucose itself.
The word glucose is from the Greek gleucos, which means sweet wine.
Glucose can be found in fruit and vegetables and is responsible (together with fructose and sucrose) for their sweet taste. With the exception of banana, seeds, grain, legumes, and tubers, which contain mostly starch, foods containing glucose, fructose and sucrose in various ratios are the major dietary carbohydrate sources.
Glucose is easily soluble in water and is present in the diet as part of the disaccharides sucrose (glucose and fructose) and lactose (glucose and galactose).
In healthy humans, after overnight fasting, blood glucose levels lie between range 3.5 to 5.5 mmol/1iter. They rise after breakfast meal to almost 10 mmol/liter. Blood glucose level over 7.8 mmol /liter measured 2 hours after a drink containing 75 g glucose is a diagnostic criterion for diabetes.
Glucose is stored by the body as glycogen (animal starch) mostly in the liver and muscles. A 70-kg man may store around 500 grams of glycogen. Glucose can also be synthesized from certain ("gluconeogenic") amino acids in process of so-called de novo gluconeogenesis.
Oligosaccharides, from Greek oligo (few) are polymers or molecules usually containing 2 to 9 building blocks, sometimes up to 19.
The 2-unit, 3--unit and 4-unit forms (so-called dimers, trimers and tetramers) in which glucose molecules are joined in chains are called maltose, maltotriose and maltotetrose. These sugars are the products of starch digestion in the malting process.
Starches are yet larger molecules. Most starches are composed as mixtures of differently configured chains of simple sugars. Some of branched starches are more readily digested than others, which is also an important fact to remember. The major sources of dietary starch are cereals, grains, legumes, and tubers.
Resistant starches got their name due to their resistance to digestion. They are divided into three classes: RS1, RS2 and RS3.
- RS1 resists digestion due to large particle size or compact nature of food, or starch entrapment by dietary fiber.
- RS2 resist digestion because of more crystalline structure
- RS3 resist digestion because after cooking they form compact structure in which water is lost.
Resistant starches are rather seldom in Western diets. Coarser milling or increasing particle size of cereal grains such as whole-grain bread or bulgur wheat (RS1) may achieve increased starch resistance. RS2 are found in green banana, corn and legumes (peas, beans and lentils). RS3 are components of common foods such as potato, rice and bread. Resistant starches in this category are produced commercially from cornstarch by enzymatic treatment.
Cellulose is also a glucose polymer, but it may consist of as many as 10 000 glucose building blocks. Cellulose is both resistant to digestion and insoluble in cold or hot water, acids and alkali. It can be however digested by intestinal friendly bacteria. This intestinal microflora is able to manage the cellulose from vegetables much better than cellulose from grains.
Cellulose is a structural component of plant cell walls. In human nutrition, it forms an important part of the insoluble fiber component.
Unlike cellulose, they are readily soluble in water, making a highly viscous substance. The bigger and heavier the molecule, the greater the viscosity. Thus reduction of molecular weight by acid or enzymatic hydrolysis, which may also occur during food processing, may greatly reduce viscosity. The glucans are resistant to digestion by enzymes. However unlike cellulose glucans can be completely fermented by intestinal bacteria.
Beta-glucans are found in oats and barley, with only very small amounts in wheat. In oats, the beta-glucan is concentrated in the outer bran layer and may comprise 50% of the dietary fiber. In barley, the beta-glucan is less easy to achieve.
Hemicellulose shows some similarity with cellulose not because of similarities of the chemical structures but because hemicellulose is also insoluble in hot or cold water or hot acid. It is, however, soluble in alkali. Hemicellulose is found in cereals.
Glucose pathways in the body
Glucose is treated in the stomach by the gastric juices and passes into the duodenum where it is absorbed. The absorbed glucose goes into the bloodstream. The major part of the absorbed glucose is up-taken by muscle and fat tissue with the help of insulin.
Sucrose and lactose are split and then are treated by the enzymes sucrase-isomaltase and lactase. Sucrose intolerance is a very rare health condition, but lactose intolerance (hypolactasia) is common around the world with the exception of northern European nations. Thus, lactose malabsorption in the small intestine occurs and large amounts of lactose enter the colon resulting in gas production, and sometimes diarrhea.
Purified starch enters into digestion in the mouth under the action of salivary amylase.
The digestion stops due to the stomach's acidic environment but resumes in the duodenum under the action of pancreatic amylase resulting in the production of free glucose, maltose, and maltotriose. The absorption in the small intestine is theoretically considered to be complete.
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