Carbohydrate Types and Functions

Carbohydrate Types

• Complex carbohydrates are oligosaccharides (3-10 monosaccharides) and polysaccharides (> 10 monosaccharides)
• They are straight or branched chains of monosaccharides joined by condensation reactions.
• Examples: starch, glycogen.

Starch

• Amylose is a continuous single chain of glucose molecules joined by alpha (1-4) links.
• Amylopectin is a branched chain of glucose molecules joined by alpha (1-4) links with branches connected by alpha (1-6) links.
• Different types of amylose and amylopectin contribute to different characteristics (flavor, solubility, thickening power) in food science.
• From a nutritional perspective, starches are all the same and broken down to glucose.

Glycogen

• The storage form of carbohydrates (energy) in humans and animals.
• Highly branched chains of glucose (10-18 chains).
• High branching allows glycogen to bind with water (limited water storage).
• Fast hydrolysis allows for a quick energy supply.
• The body stores glycogen for approximately 12-18 hours.

Carbohydrate Structure - Simple and Complex

• Monosaccharides are made up of 1 sugar unit (single sugars).
• Disaccharides are made up of 2 sugar units (double sugars).
• Polysaccharides are made up of more than 2 sugar units.
• Simple carbohydrates provide a quick energy source and are digested quickly.
• Complex carbohydrates are digested slowly and provide a sustained release of energy.

Fibre

• Total fibre is made up of dietary fibre and functional fibre.
• Dietary fibre is edible, non-digestible and derived from plants.
• Functional fibre is isolated, extracted, and added to food or pills for health benefits.
• Fibre can be described by its solubility and digestibility:
  • Solubility: the extent to which a substance can be dissolved in water.
  • Digestibility: the extent to which a substance can be broken down in the digestive system.

Types of Fibre

• Soluble (Viscous) fibre:
  • Dissolves and swells in water, forming a gel.
  • Slows down digestion, delaying gastric emptying.
  • Helps to lower cholesterol levels.
• Insoluble (non-viscous) fibre:
  • Does not dissolve in water.
  • Provides roughage and bulk, helping to keep you regular.
  • Helps to prevent constipation.

Examples of Soluble and Insoluble Fibre

• Soluble Fibre: Fruits (pectins), legumes, oats, flaxseed, barley, rye, guar gum, acacia gum, carrageenan, psyllium, some hemicelluloses.
• Insoluble Fibre: Skins of fruits and vegetables, broccoli, celery, blackberries, raspberries, whole wheat pasta, brown rice, nuts and seeds.

Gluconeogenesis

• The body produces glucose from various 3C substrates including protein and pyruvate.
• Gluconeogenesis is the process of making glucose from non-carbohydrate sources (gluco = glucose; neo = new; genesis = making).

Problems with Insufficient Dietary Carbohydrate

• Ketosis: inefficient breakdown of fatty acids.
• Protein breakdown to produce glucose: occurs during starvation or insufficient dietary carbohydrate.
• Adults need a minimum of 100 g/day of carbohydrates to supply sufficient glucose for bodily functions.

Insufficient Glucose in the Body: Protein Breakdown

• When there is insufficient carbohydrate, the body depletes muscle glycogen stores.
• When glycogen stores are depleted, the body breaks down body protein to produce glucose.
• This protein breakdown can lead to problems with growth, maintenance, nutrient transport, hormone production, and immune system regulation.

The Protein-Sparing Action of Carbohydrates

• Diets providing enough carbohydrate "spare" protein, meaning they preserve the body's use of protein for its intended and essential functions.

Blood Glucose Regulation

• Diabetes mellitus: disease caused by either insufficient insulin production or decreased sensitivity of cells to insulin, leading to elevated blood glucose levels.
• Hypoglycemia: Low blood glucose levels, usually below 2.2 to 2.8 mmol/L.
• Blood-glucose response curve: shows changes in blood glucose levels after consuming food.

Hormones Involved in Blood Glucose Regulation

• Insulin: secreted by the pancreas in response to a rise in blood glucose levels.
• Glucagon: secreted by the pancreas in response to a decline in blood glucose levels.

Glycemic Response and Glycemic Index

• Glycemic response: How quickly and how high blood glucose rises after carbohydrates are consumed.
• Glycemic index: ranks foods based on their impact on the glycemic response.

Carbohydrates and Heart Disease

• Diets high in sugar or refined carbohydrates raise blood lipid levels (triglycerides), increasing the risk of heart disease.
• Diets high in whole grains can reduce the risk of heart disease.
• High-carbohydrate diets can increase levels of triglycerides and high-density lipoproteins (HDL) cholesterol.
• High serum triglycerides are higher when the primary carbohydrate in the diet is monosaccharides (simple sugars), especially fructose.
• Soluble fibre can bind to dietary cholesterol in the gut and be excreted via waste.

Indigestible Carbohydrates and Bowel Disorders

• Diets high in fibre or indigestible carbohydrates can relieve or prevent certain bowel disorders: hemorrhoids, diverticulosis, diverticulitis, and constipation.
• Epidemiological studies show lower incidence of colon cancer in populations with high-fibre diets.
• More research is needed on the effects of fibre supplements and the relationship between fibre intake and certain cancers.

Indigestible Carbohydrates and Other Conditions

• Viscous fibre can increase insulin tissue response and protect against diabetes.
• The influence of dietary fibre on obesity is unclear.
• Some fibre is fermented by gut bacteria, producing short-chain fatty acids (SCFA) which cannot be absorbed.
• Some fibre lowers blood cholesterol levels.