Carbohydrates

Carbohydrates:

Carbohydrates are sugar molecules; most abundant organic molecules in nature and one of the most essential macronutrient in foods. This important food nutrient that our body turns into glucose to provide us energy to function.

Carbohydrates are a primary fuel source for cells, especially the cells of the central nervous system and red blood cells. Muscle cells also rely on carbohydrates to fuel intense physical activity.   

Structure of carbohydrates:

The carbohydrates family includes sugar, starch, and fiber. Most forms of carbohydrates are carbon, hydrogen, and oxygen. The general formula for carbohydrates is (CH2O)n, where n represents the number of times the formula is repeated. The simpler forms of carbohydrates are called monosaccharides and disaccharides. The more complex forms of carbohydrates are called polysaccharides and typically contain many glucose molecules linked together. 

Monosaccharides:

Monosaccharides are single sugars with the general formula of (CH2O)6. They are a class of single sugars that are not broken down further during digestion. The common monosaccharides are glucose, fructose, and galactose. Each monosaccharide contains 6 carbon, 12 hydrogen, 6 oxygen molecule, but in slightly different configurations. Because each is a 6-carbon sugar., it is classified as a hexose.

Glucose is the most abundant monosaccharide, much of the glucose in our diets is linked together with additional sugars to form di- or polysaccharides. In the bloodstream, glucose is sometimes called "blood sugar".

The monosaccharides fructose is found in fruits, vegetables, honey, and high fructose corn syrup (it is sweeter; used to sweeten many food products, especially beverages and less expensive than table sugar). The presence of fructose in these products makes it a common sugar in our diets. 

Galactose is the third major monosaccharide of nutritional importance. Most of the galactose in our diets is found in combination with glucose. When galactose combines with glucose, it forms a disaccharide called lactose, which is found in milk and other dairy products.

The sugar alcohols, which are derivatives of monosaccharides, include sorbitol, mannitol, and xylitol. These are used primarily as sweeteners in sugarless gums and dietetic foods.

Two additional monosaccharides found in nature are ribose and deoxyribose. These are classified as pentoses because they contain 5 carbons. Ribose is part of ribonucleic acid (RNA), and deoxyribose is part of deoxyribonucleic acid (DNA).

Disaccharides:

Disaccharides are double sugars, made of 2 monosaccharides with the general formula of (CH2O)12. They are class of sugars formed by the chemical bonding of  2 monosaccharides, occurs in a condensation reaction. During this reaction, 1 molecule of water is formed by taking a hydroxyl group from 1 sugar and a hydrogen from the other sugar.

Alpha & Beta Bond Linkage:

Many carbohydrates contain long chains of glucose  with the individual monosaccharides bonded together by either alpha or beta bonds. Beta bonds differ from alpha bonds in that they cannot be easily broken down by digestive enzymes for absorption in the small intestine. Thus, foods that contain saccharide molecules linked together by beta bonds are often difficult or impossible for individuals to digest because they lack the enzymes necessary for breaking beta bonds apart.

The disaccharide Maltose contains 2 glucose molecules joined by an alpha bond. This sugar provide the energy for the plant to grow. Malting, the first step in the production of alcoholic beverages, such as beer, lets grain seeds sprout. Few other food products or beverages contain maltose.

Sucrose, common table sugar, is composed of glucose and fructose linked by an alpha bond. Large amount of sucrose are found naturally in plants, such as sugarcane, sugar beets, and maple tree sap. Brown, white, and powdered sugars are common forms of sucrose.

Lactose, the primary sugar in milk and milk products, consists of glucose joined to galactose by a beta bond. Many people face lactose intolerance because their body don't produce enough of the enzyme lactase, which is needed to break this beta bond. This can cause bloating, intestinal gas, cramping, and discomfort as the unabsorbed lactose is metabolized into acids and gases by bacteria in the large intestine.

Oligosaccharides:

They are complex carbohydrates that contain 3 to 10 single sugar units. Two oligosaccharides of  nutritional importance are raffinose and stachyose; 

Raffinose: Indigestible oligosaccharides made of 3 monosaccharides (galactose-glucose-fructose).

Stachyose: Indigestible oligosaccharides made of 4 monosaccharides (galactose-galactose-fructose).

They are found in onions, cabbage, broccoli, whole wheat, and legumes, such as kidneys beans and soybeans. The beta bonds in oligosaccharides cannot be broken down by our digestive enzymes.

Although many people have no symptoms after eating legumes, other experience unpleasant side effects from intestinal gas. An enzyme preparation, such as Beano, can help prevent these side effects if taken right before a meal. This enzyme preparation works in the digestive tract to break down many of the indigestible oligosaccharides.

Polysaccharides:

They are complex carbohydrates that contain hundreds of thousands of glucose molecules. The polysaccharides include some that  are digestible , such as starch and some that are largely indigestible, such as fiber.  The digestibility of these polysaccharides is determined mainly by whether the glucose units are linked together by alpha or beta bonds.

Digestible Polysaccharides; Starch & Glycogen

Starch, the major polysaccharide in our diets, is the storage form of glucose in plants. It is complex carbohydrate made of multiple units of glucose attached together in a form that the body can digest.

 There are 2 types of plant starch; amylose and amylopectin__ both of which are a source of energy for plants and for animals that eat plants. They are found in beans, potatoes, breads, pasta, rice, and other starchy products.

Amylose and amylopectin contain many glucose units linked by alpha bonds. The primary difference is that amylose is a linear, unbranched chain of glucose molecules that contains only 1 type of alpha bond (called as 1-4 bond), whereas amylopectin is a highly branched-chain structure that links glucose molecules using 2 types of alpha bonds (1-4 bonds link straight chains of glucose and 1-6 bonds link glucose at the branching points).

 The properties of amylopectin and amylose make them useful in food manufacturing. Food manufactures commonly use starches rich in amylopectin to thicken sauces and gravies. Amylopectin also is used in many frozen foods because it remains stable over a wide temperature range. Amylose-rich molecules can be bonded to each other to produce modified food starch, a thickener used in baby foods, salad dressings, and instant puddings. 

Glycogen, the storage form of carbohydrate in humans and other animals, also contains many glucose units linked together with alpha bonds. The structure of glycogen is similar to that of amylopectin, but it is even more highly branched. Liver and muscle cells are the major storage sites for glycogen. The amount stored in these cells is influenced by the amount of carbohydrate in the diet. 

Indigestible Polysaccharides: Dietary & Functional Fiber

Total fiber refers to the dietary fiber that occurs naturally in foods, as well as the functional fiber that may be added to food to provide health benefits. Currently, Nutritional facts include only dietary fiber and do not reflect any added functional fiber. 

Fibers are composed primarily of the non-starch polysaccharides cellulose, hemicellulose, pectins, gums, and mucilages. Lignins are the only non-carbohydrate components of dietary fibers. 

Insoluble & soluble fibers:

Cellulose, hemicellulose, and lignins form the structural part of the plant cell wall in vegetables and whole grains. Bran layers form the outer covering of all seeds; therefore, whole grains are good sources of fiber. Because of their chemical structure, these fibers do not dissolve in water. Thus, they are often referred to as insoluble fibers.

 On the other hand, pectins, gums, mucilages, and some hemicelluloses dissolve easily in water and are classified as soluble fibers. In water, they become viscous in consistency. This property makes them useful for thickening jam, jelly, yogurt, and other food products. They also occur naturally inside and around plant cells in oat bran, many fruits, legumes, and psyllium.

The physical properties of soluble and insoluble fibers provide health benefits. When consumed in adequate quantities, these fibers have been shown to lower blood cholesterol levels and blood glucose levels, thereby reducing risks of cardiovascular disease and diabetes. In addition, fiber can decrease intestinal transit time, thus reducing risks of constipation, diverticular disease, and colon cancer.

Food Sources of Carbohydrates:

Carbohydrates are found in a wide variety of foods. Foods such as table sugar, jelly, jam, fruit, fruit juices, rice, pasta, soft drinks, baked products, cereals, and breads are predominantly carbohydrates. Other food products such as dried beans, corn, peas, lentils, and dairy products(milk and yogurt), also are good sources of carbohydrate. Foods with little or no carbohydrate include meats, fish, poultry, eggs, vegetable oils, butter and margarine.

 Substances that impart sweetness to foods fall into 2 broad classes: nutritive sweeteners, which can be metabolized to yield energy, and non-nutritive sweeteners, which provide no food energy. The sugar in alcohols, sorbitol, mannitol, and xylitol, are nutritive sweeteners used in sugarless gum and candies. Non-nutritive sweeteners provide noncaloric or very low calorie sugar substitutes.

Recommended Intake of Carbohydrates:

Regular intake of carbohydrates is important; maximum 55% to 60% of carbohydrates should be taken in daily routine meal. 

➤The Healthy People 2020 goals related to carbohydrate intake include:

⦿ Increase the contribution of whole grains to the diets of the population aged 2 years and older.

⦿ Increase the contribution of fruits to the diets of the population aged 2 years and older.

⦿ Increase the variety and contribution of vegetables to the diets of the population aged 2 years and older.

⦿ Increase the contribution of dark green vegetables, orange vegetables, and legumes to the diets of the population aged 2 years and older.

⦿ Reduce consumption of calories from added sugars.

Functions of Carbohydrates in Body:

The digestible and indigestible carbohydrates in our diets have vital functions in our bodies. These diverse functions are critical to normal metabolism and overall health.

➤Digestible Carbohydrates:

Most of the digestible carbohydrates in our diets are broken down to glucose. As glucose, they provide a primary source of energy, spare protein from use as a energy source, and prevent ketosis.

Providing Energy:

The main function of  glucose is to act as a source of energy for body cells. Glucose also fuels muscle cells and other body cells, although many of these cells rely on fatty acids to meet energy needs, especially during rest and light activity. Glucose provides 4 kcal of energy per gram.

Sparing Protein From Use as an Energy Source:

The amino acids that makeup dietary protein are used to build body tissues and to perform other vital functions only when carbohydrate  intake provides enough glucose for energy needs. If you do not consume enough carbohydrate to yield glucose, your body is forced to break down amino acids in your muscle tissue and other organs to make glucose. This process is termed gluconeogenesis, which means the production of new glucose. However, when dietary carbohydrate intake is adequate to maintain blood glucose levels, protein is spared from use as energy.

Preventing Ketosis:

When carbohydrate intake falls below this level, the release of the hormone insulin decreases, resulting in the release of a large amount of fatty acids from adipose tissue to provide energy for body cells. These fatty acids travel in the bloodstream to the liver. The subsequent incomplete  breakdown of these fatty acids in the liver results in the formation of acidic components called ketone bodies, or keto-acids, and a condition called ketosis, or ketoacidosis. Ketone bodies include acetoacetic acid and its derivatives.

If the brain could not use ketone bodies, the body would be forced to produce much more glucose from protein to support the brain's energy needs. The resulting breakdown of muscles, heart, and other organs to provide protein for gluconeogenesis would severely limit our ability to tolerate starvation.

➤Indigestible Carbohydrates:

Although fiber is indigestible, it plays an important role in maintaining the integrity of the GI tract and overall health.. Fiber helps prevent constipation, and diverticular disease and enhances the management of body weight, blood glucose levels, and blood cholesterol levels.

Reducing Obesity Risk:

A diet high in fiber likely aids weight control and reduces the risk of accumulating body fat and becoming obese. The bulky nature of high fiber foods fills us up without yielding much energy. Fibrous foods also absorb water and expand in the GI tract, which may result in a sense of fullness and contribute to obesity.

Increase Blood Glucose Level:

When consumed in recommended amounts, soluble fibers slow glucose absorption from the small intestine and decrease insulin release from the pancreas. This contributes to better blood glucose regulation, which can be helpful in the treatment of diabetes. In fact, adults with high fiber diets are less likely to develop diabetes than are those with low fiber diets.

Reducing Cholesterol Absorption:

A high intake of soluble fiber inhibits the absorption of cholesterol and the reabsorption of bile acids from the small intestine, thereby reducing the risk of cardiovascular disease and gallstones. Overall, a fiber rich diet containing fruits, vegetables, legumes, and whole-grain breads and cereals is advocated as part of a strategy to reduce the risk of cardiovascular disease.

Promoting Bowel Health:

Fiber adds bulk to the feces, making them bowel movements easier. When adequate fiber and fluid are consumed, the stool is large and soft because many types of plant fibers absorb water. The larger size stimulates the intestinal muscles, which aids elimination. Consequently, less force is required to expel the feces.

When too little fiber is eaten, the opposite can occur; the stool may be small and hard. Constipation may occur, causing one to exert excessive force during defecation. Over time, excessive exertion can lead to the development of hemorrhoids. This high pressure from exertion also can cause parts of the large intestine wall to protrude through the surrounding bands of muscle, forming small pouches called diverticula (diverticula is in the large intestine, A low fiber diet increases the risk of developing diverticula. About one-third of people over age 45 have this condition, whereas two-thirds of people over 85 do). Fibrous material, feces, and bacteria can become trapped in diverticula and lead to inflammation.

Diverticular disease is asymptomatic in about 80%  of affected people. The asymptomatic form of this condition is called diverticulosis and 1 of the most common conditions in Western countries. If the diverticula become inflamed and symptomatic, the condition is known as diverticulitis. Intake of fiber then should be reduced to limit further bacterial activity and inflammation.

➤Carbohydrate Digestion & Absorption:

Enzyme made by the salivary glands, pancreas, and small intestine participate in the process of digestion. Most carbohydrate digestion and absorption take place in the small intestine.

Digestion steps:

1. Mouth:

Some starch is broken down to polysaccharide and disaccharide units by salivary amylase.

2. Stomach:

Salivary amylase is inactivated by the acidity in the stomach.

3. Pancreas:

Pancreatic amylase and dextrinase are secreted into the small intestine to break polysaccharides from starch into disaccharides.

4. Small Intestine:

Enzymes in the wall of the small intestine down the disaccharides into monosaccharides.

5. Liver:

The absorbed monosaccharides are transported to the liver by the portal vein.

6. Large Intestine:

Some soluble fiber is metabolized into acids and gases by bacteria in the large intestine.

7. Rectum & Anus:

Insoluble fiber escapes digestion and is excreted in feces.

Absorption:

With the exception of fructose, monosaccharides are absorbed by an active absorption process. Active absorption of glucose in the absorptive cells that line the villi in the small intestine. Glucose and sodium pass across the absorptive cell membrane in a carrier-dependent, energy-requiring process. Once inside the absorptive cell, glucose can exit by facilitated diffusion and enter the bloodstream. Sodium is pumped out of the absorptive cell to maintain a low concentration in the absorptive cell and a high concentration in the extracellular fluid.

Health Concerns related to Carbohydrates:

Very-High-Fiber Diet

High Sugar Diets

Lactose Intolerance

Glucose Intolerance

Regulation of Blood Glucose

Metabolic Syndrome

Hypoglycemia

Glycemic index load