Carbohydrate

Carbohydrates are molecules containing carbon, hydrogen, and oxygen elements. They are formed as aldehyde or ketone derivatives of alcohols with multiple hydroxyl (-OH) groups, or through the hydrolysis of these derivatives. They serve as one of the body’s primary energy sources, contributing to energy needs and regulating blood glucose levels. In normal adults, 45-60% of daily energy requirements are met by carbohydrates. They are classified as simple or complex carbohydrates based on the number of sugar molecules they contain.

Carbohydrate Types and Metabolism

Carbohydrates perform several functions in the body, including providing energy, forming muscle and liver glycogen, regulating fat and protein metabolism, and meeting the energy needs of the nervous system. Carbohydrates are generally categorized into three major groups: monosaccharides, disaccharides, and polysaccharides.

Polysaccharides and disaccharides in the diet are broken down by specific glycosidases in the digestive tract. The enzymes involved in carbohydrate digestion are specific to the type of sugar and the number of glycosidic bonds. As a result of digestion, monosaccharides such as maltose, cellobiose, lactose, sucrose, glucose, fructose, and galactose are present in the lumen of the small intestine. Disaccharides are bound to appropriate transporters on the epithelial cells of the small intestine, hydrolyzed into monosaccharides, and then absorbed into the bloodstream through these cells.

When referring to blood sugar, glucose is generally understood. Certain processes in the body release glucose into the blood, while others remove it. Processes that release glucose into the blood include intestinal absorption, glycogenolysis, and gluconeogenesis. Processes that remove glucose from the blood include glucose oxidation, glycogenesis, liponeogenesis, the formation of other carbohydrates and monosaccharides, and glucosuria. Glucose is central to carbohydrate metabolism, and this process is regulated by hormones, primarily insulin and counter-regulatory hormones, as well as thyroxine and somatostatin.

Disorders of carbohydrate metabolism are classified into four groups: absorption, conversion, storage, and utilization problems. Absorption disorders cause abdominal pain, gas, and diarrhea due to disaccharidase deficiencies and intestinal issues. Conversion disorders affect fructose and galactose metabolism; examples include hereditary fructose intolerance, fructose-1,6-bisphosphatase deficiency, and galactosemia. Glycogen metabolism disorders arise from deficiencies in synthesis or breakdown enzymes and affect liver or muscle tissue. Mucopolysaccharidoses disrupt skeletal, organ, and mental development due to lysosomal enzyme deficiencies. Diabetes mellitus manifests as insulin deficiency or resistance. Glucose-6-phosphate dehydrogenase (G6PD) deficiency increases susceptibility to oxidants and leads to hemolysis.

Monosaccharides (Simple Sugars)

Monosaccharides are molecules composed of a single sugar unit containing carbon, hydrogen, and oxygen, and cannot be broken down into smaller compounds by hydrolysis. They are rapidly digested and cause a quick rise in blood glucose. Excessive intake may lead to fat formation and increased blood lipid concentrations. Monosaccharides are the simplest carbohydrates, containing hydroxyl groups and either an aldehyde or ketone group. They are classified by their reactive groups as aldoses (containing an aldehyde group) and ketoses (containing a ketone group); and by the length of their carbon chain as trioses, tetroses, pentoses, hexoses, and heptoses. The most common monosaccharides in nature and organisms are trioses, pentoses, and hexoses. Among hexoses, the most prevalent are glucose, fructose, galactose, and mannose.

• Glucose: Also known as dextrose or grape sugar. Normal blood level is 65-80 mg/100 ml. Found in grapes, grape-derived foods and beverages, and honey.
• Fructose: Known as fruit sugar or levulose. Found freely in fruits, molasses, and honey. In honey, glucose and fructose are present in equal proportions.
• Galactose: Bound to glucose in milk sugar (lactose). Obtained through the hydrolysis of lactose.

Disaccharides (Two-Sugar Carbohydrates)

Disaccharides form when two monosaccharides combine covalently with the loss of one water molecule.

• Sucrose: Formed by the linkage of glucose and fructose via a 1,4-glycosidic bond. Found in sugar beet and sugarcane. Commonly referred to as “sugar” in daily use.
• Lactose: Formed by the combination of one glucose molecule and one galactose molecule. Present at 7-8% in human milk and 4-5% in cow’s milk.
• Maltose: Formed by the combination of two glucose molecules. Found in limited amounts in cereals and legumes.

Polysaccharides

Polysaccharides are polymers formed by the linkage of many monosaccharides through O-glycosidic bonds.

• Starch: A polymer of glucose stored in granules in plant seeds, grains, and tubers. Constitutes 80-90% of total dietary carbohydrates.
• Glycogen: A polymer of glucose stored in human and animal tissues. Most abundant in the liver, muscles, and other tissues. Easily converted to glucose by enzymes that break down glycogen.
• Cellulose: A polymer consisting of approximately 3000 glucose molecules linked by glycosidic bonds. Provides structural support in plants, is insoluble in water, and absorbs 0.4 grams of water per gram. Functions as dietary fiber in the digestive tract and helps maintain bowel regularity.

Functions of Carbohydrates in the Body

• Energy Supply: One gram of carbohydrate provides approximately 4 kilocalories of energy. Muscles utilize carbohydrates more efficiently than fats.
• Protein Sparing: When carbohydrates are consumed in sufficient amounts, proteins are not used for energy, reducing protein requirements.
• Electrolyte and Water Balance: Assists in sodium absorption and maintains fluid-electrolyte balance in the body.
• Digestive Health: Non-digestible carbohydrates (fiber) regulate bowel movements and support colon health.
• Control of Ketone and Acid Levels: In the absence of sufficient carbohydrates, ketone and acid production increases, lowering blood pH and potentially leading to ketoacidosis.

Importance of Carbohydrates in Sports and Performance

• Muscle Glycogen: Muscle glycogen serves as the primary energy source during prolonged exercise. Low glycogen levels lead to early fatigue.
• Carbohydrate Loading: A high-carbohydrate diet can increase muscle glycogen stores to nearly twice the normal level. This method is used by long-distance runners, cyclists, and endurance athletes.
• Intake Before and During Exercise: Consuming carbohydrates 2 hours before and during exercise helps preserve muscle and liver glycogen. Consuming carbohydrates 15-45 minutes before exercise may cause hypoglycemia.

Glycemic Impact and Health

• Glycemic Index (GI) and Glycemic Load (GL): Determine the capacity of foods to raise blood glucose levels.
• Low GI and High Fiber: Support blood sugar control, enhance insulin sensitivity, and reduce inflammatory marker CRP levels.
• Dried Legumes: High in fiber, have a low glycemic impact, and reduce the risk of diabetes and metabolic diseases.

Sources of Carbohydrates

Carbohydrate-containing foods include table sugar, honey, jam, molasses, marmalade, sugary fruit drinks, soft drinks, chocolate, ice cream, sweets, flour and flour-based products (bread, phyllo, pasta, erişte), rice, bulgur, dried legumes, potatoes, vegetables, fruits, yogurt, and milk.

• Simple Carbohydrates: Foods such as honey, jam, sugar, and sugary drinks that cause rapid increases in blood glucose.
• Complex Carbohydrates: Foods such as whole wheat bread, bulgur, dried legumes, and vegetables that are digested slowly, leading to a more balanced rise in blood glucose. They provide sustained energy and support glucose balance.

Regular meal patterns and avoiding skipped meals are important for blood glucose control in individuals with diabetes. Excessive consumption of simple carbohydrates during meals can lead to hyperglycemia.

Carbohydrates in Energy Metabolism

• Muscle and Liver Glycogen: Determine training and endurance performance. Low glycogen levels during exercise lead to fatigue.
• Effect of Carbohydrate Intake: High carbohydrate intake rapidly replenishes muscle glycogen stores, while low carbohydrate intake limits glycogen resynthesis.

Recommended Intake

It is recommended that 45-60% of daily dietary energy come from carbohydrates. For example, an adult with a daily energy requirement of 2000 kcal should consume 225-300 grams of carbohydrates.

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