Chapter 7

Carbohydrate Structure & Function

• Carbohydrates are the body’s primary fuel source and are categorized as sugars and starches, or simple and complex carbohydrates.
• Monosaccharides: Single sugar units (glucose, fructose, galactose) that are the absorbable form of carbohydrates for the body.
• Disaccharides: Pairs of sugar units (sucrose, maltose, lactose).
• Polysaccharides: Long glucose chains (starches, fibers, glycogen).
• Oligosaccharides: A small group of carbohydrates with 3 to 10 sugar units that are mostly indigestible.

Types of Carbohydrates

• Natural sugars: Found in fruit and milk; are generally considered healthier options because foods containing them also include other nutrients.
• Added sugars: Sugars that are not naturally occurring in food groups, such as syrups and caloric sweeteners found in candy and sweetened beverages.

Importance of Fiber

• Soluble fiber: Dissolves in water to form a gel and aids in digestion, heart health, and glucose control.
• Insoluble fiber: Does not dissolve in water and promotes regularity in the colon.
• Functional fiber: Derived from isolated, indigestible carbohydrates and may provide some benefits of natural fiber; examples include polydextrose and inulin.

Polysaccharide Structures

• Amylose: Straight-chain digestible form of starch containing glucose molecules.
• Amylopectin: Branched-chain digestible form of starch containing glucose molecules.
• Fiber: A block structure that is indigestible.

Glycogen: Glucose Storage

• Glycogen: The storage molecule of glucose in animals. It has a highly branched structure that allows enzymes to break it down rapidly during glycogenolysis.

Glycemic Index & Insulin Sensitivity

• Amylopectin digests faster than amylose, leading to a greater spike in blood sugar levels.

• Insulin sensitivity is important for maintaining blood glucose levels and can be improved by managing sugar and carbohydrate intake and engaging in regular physical activity.### Glycogen and Its Functions

• Glycogen is the storage form of carbohydrate in animals, primarily found in muscle and liver cells.

• Glycogenolysis is the breakdown of glycogen into glucose units, providing energy.

• Glycogenesis is the process of forming glycogen from glucose.

• Muscle glycogen is not readily available for blood glucose maintenance due to the absence of glucose 6-phosphatase enzyme, unlike in liver cells.

• Liver glycogen is essential for maintaining stable blood sugar levels.

• Muscle glycogen primarily fuels muscular work.

• Average muscle glycogen storage is around 15 grams per kilogram (6.8 g/lb) of muscle tissue.

• Muscle mass contributes significantly to total glycogen stores, averaging between 250 to 600 grams (1,000 to 2,400 kcal) in adults.

• Liver glycogen storage is approximately 50 grams per kilogram (22.7 g/lb.) of liver tissue, totaling about 75 to 125 grams (300 to 500 kcal) in adults.

• Blood glucose stores are relatively small, ranging from 3 to 5 grams in smaller individuals to 10 to 15 grams in larger individuals.

• An average adult stores around 2,000 to 2,500 kcal of glycogen, with endurance athletes potentially storing more than 3,000 kcal.

Carbohydrate Classifications

• Carbohydrates are classified based on the number of sugar units they contain.
• The body digests and absorbs carbohydrates as monosaccharides, the simplest form.
• These monosaccharides are the building blocks for larger carbohydrate structures like starches and sugars.

Carbohydrate Roles In The Body

• The primary role of dietary carbohydrates is to provide energy.
• Excess carbohydrates are stored as glycogen in muscle and liver.
• Liver glycogen helps regulate blood glucose and insulin metabolism, impacting diabetes and metabolic syndrome.
• Further excess carbohydrates can be converted to triglycerides (fats).
• Balanced glucose and insulin levels regulate other hormones like testosterone and thyroid hormones.
• Carbohydrates are crucial for complete fat metabolism. Insufficient carbohydrates result in the production of ketones from incompletely metabolized fat fragments.
• In low-carbohydrate states, the body can produce glucose from non-carbohydrate sources like amino acids through gluconeogenesis, primarily derived from muscle tissue.
• Adequate carbohydrates spare muscle breakdown during energy demands.
• Soluble and insoluble fibers offer numerous health benefits including weight loss, cholesterol reduction, improved digestion, and cancer prevention.

Carbohydrate Digestion and Absorption

• Digestion begins in the mouth with salivary amylase, an enzyme from the salivary glands.
• Chewing and saliva facilitate digestion by increasing surface area.
• Bolus refers to the chewed food mass ready for swallowing.
• In the stomach, food mixes with gastric juices and the high acidity continues digestion.
• Chyme is the partially digested food mass passed from the stomach to the duodenum.
• In the duodenum, sodium bicarbonate from the pancreas neutralizes the acidic chyme, allowing pancreatic amylase to continue digestion in the jejunum.
• Further breakdown occurs in the jejunum, producing disaccharides and monosaccharides, which then move to the ileum for absorption.
• Disaccharides are further broken down to monosaccharides within the brush border of the intestinal wall.
• Villi and microvilli within the small intestine increase surface area for nutrient absorption.
• Brush border cells release enzymes to digest disaccharides into monosaccharides.
• Once absorbed, monosaccharides enter the hepatic portal vein for transport to the liver.
• Glucose and galactose are absorbed faster than fructose.
• Glucose is used directly for energy, while fructose is partially oxidized for energy (35-45%).
• Lactose intolerance results from lactase deficiency or insufficiency, leading to undigested lactose, causing bloating, diarrhea, gas, nausea, and abdominal pain.
• Lactose intolerance is often genetically inherited and can develop with age.
• Individuals with lactose intolerance can avoid dairy, consume lactose-free products, or take lactase enzyme supplements.

Carbohydrate Fate After Absorption

• After meeting immediate energy needs, surplus glucose and fructose replenish liver and muscle glycogen stores.
• Glucose molecules form glycogen chains, while a portion of fructose is converted to glucose for glycogen synthesis.
• Excess fructose is either oxidized or converted to lactate (30%) or triglycerides (fats).