Understanding Carbohydrates

What are Carbohydrates?

• One of the three macronutrients providing energy.
• Important for energy in all cells.
• Preferred energy source for nerve cells and red blood cells (glucose).
• Composed of carbon, hydrogen, and oxygen (CHO).
• Sources include fruits, vegetables, and grains.
• Glucose is the most abundant carbohydrate obtained from plants.

Simple vs. Complex Carbohydrates

• Simple carbohydrates, also known as sugars, contain one or two molecules.
• Complex carbohydrates contain three or more molecules, up to thousands, linked together.

Simple Carbs - Monosaccharides

• Monosaccharides include glucose, fructose, and galactose.
• Disaccharides contain two molecules. Examples are lactose, maltose, and sucrose.
• Humans consume D-glucose (dextrose) but do not consume L-glucose.
• D and L forms are enantiomers, which are mirror images of each other and non-superimposable.

Simple Carbs - Disaccharides

• Lactose (milk sugar) is composed of galactose plus glucose.
• Maltose is composed of glucose plus glucose, found in starch and is a byproduct of fermentation.
• Sucrose is composed of glucose plus fructose, found in sugarcane, sugar beets, and honey.

Complex Carbs - Oligosaccharides

• The simplest complex carbohydrates contain 3-10 monosaccharides.
• Raffinose contains galactose, glucose, and fructose, found in beans, cabbage, brussel sprouts, and broccoli.
• Stachyose contains galactose, galactose, and fructose; found in beans and legumes.

Complex Carbs - Polysaccharides

• Polysaccharides contain hundreds to thousands of molecules.
• Glycogen is the storage form of glucose in humans, and is stored in the liver and muscles.
• Glycogen is not found in food, so it is not a source of dietary carbohydrates.
• Starch is the storage form of carbohydrates in plants, which humans consume.
• Amylose is a straight chain form of starch.
• Amylopectin is a branched chain form of starch.
• Resistant starch is a fiber that is indigestible.
• Sources of starch include grains, legumes, fruits, and vegetables.

Complex Carbs - Fiber

• Fiber is a controversial topic because there is disagreement on its definition.
• Total fiber is the sum of dietary fiber and functional fiber.
• Dietary fiber consists of the non-digestible parts of plants.
• Functional fiber consists of non-digestible carbohydrates extracted from plants or manufactured and added to foods, and is referred to as 'novel fibers'.
• As of 2012, Health Canada defines dietary fiber as consisting of carbohydrates with a degree of polymerization of 3 or more that naturally occur in plant-based foods that are not digested or absorbed by the small intestine, including accepted novel fibers.

Soluble vs. Insoluble Fiber

• Another way to classify fiber is based on solubility.
• Soluble fiber dissolves in water and forms a viscous gel.
• Soluble fiber is fermentable, is digested by intestinal bacteria in the colon, and reduces the risk of cardiovascular disease and type 2 diabetes.
• Sources of soluble fiber include pectin, gum, and mucilage, found in citrus fruits, berries, oats, and beans.
• Insoluble fiber does not dissolve in water and is non-viscous; it is also non-fermentable, promotes regular bowel movement, and alleviates constipation and diverticulosis.
• Diverticulosis is a condition involving the formation of pouches (diverticula) in the colon wall.
• Lignins, cellulose, hemicellulose are insoluble fibers found in whole grains (including the entire seed - bran, endosperm, and germ).

Digestion

• The goal of digestion is to break down carbohydrates into monosaccharides that can be converted to glucose.
• Salivary amylase is the enzyme present in the mouth for carbohydrate digestion.
• Pancreatic amylase works in the small intestine to break down carbohydrates to maltose. Maltase, sucrase, and lactase then break down disaccharides.
• Glucose and galactose are absorbed in the small intestine via active transport which requires ATP, while fructose is absorbed via facilitated diffusion (concentration gradient).

Metabolism

• In the liver, monosaccharides are converted to glucose.
• Excess glucose is stored as glycogen in the muscle and liver, and is used between meals as a source of energy.

Excretion

• Bacteria in the large intestine break down soluble fiber, producing short-chain fatty acids (SCFA) that provide energy for colonocytes.
• Most fiber is excreted in feces.
• Health Canada accepts that fiber has an energy value of 2 kcal/g, while the US uses a value of 4 kcal/g.

Control of Blood Sugar

• Maintaining a steady supply of glucose is important for brain function and for RBCs to transport oxygen.
• Hormones involved in blood sugar control include: insulin, glucagon, epinephrine, norepinephrine, cortisol, and growth hormone.
• Insulin, from beta cells of the pancreas, stimulates the liver to store glucose as glycogen.
• Glucagon, from alpha cells of the pancreas, stimulates the release of glucose from the liver and the production of glucose from amino acids (gluconeogenesis).
• Epinephrine and norepinephrine, secreted by adrenal glands, stimulate glucose release and trigger the "fight or flight" response.
• Cortisol, secreted by adrenal glands, increases gluconeogenesis and decreases muscle use.
• Growth hormone decreases muscle use of glucose, increases fat use, and increases liver glucose output.

Glycemic Index (GI)

• Foods greatly influence fluctuations in blood glucose levels.
• High GI foods, such as pure glucose and white bread, raise blood sugar dramatically, triggering a large release of insulin and a dramatic fall in blood glucose.
• Low GI foods (high in fiber) moderate fluctuations in blood sugar by slowing the absorption of sugar into the blood through increased transit time.
• GI is a partial perspective because foods are eaten in combination.
• The GI of an individual food may not equate to its effect when part of a meal.
• Low GI foods may reduce colon cancer risk.

Glycemic Load

• Glycemic load takes into consideration the amount (in grams) of food consumed.
• Glycemic load is calculated by multiplying the glycemic index by the amount of carbohydrate in the food.

Summary of Benefits of Carbs

• Bulking effect promotes satiety.
• Help with control of blood glucose levels.
• Promote heart health by controlling cholesterol.
• Support colon health as short-chain fatty acids.
• Essential for brain function and red blood cell production.
• Spare proteins, preventing gluconeogenesis.
• Breakdown of tissues to obtain amino acids.

How Many Carbs?

• Recall the Acceptable Macronutrient Distribution Range (AMDR) of 40-65% of daily calories or 130 g, which supplies adequate glucose to the brain.
• Focus on fiber-rich carbs and avoid simple carbs.
• Limit energy from added sugars to no more than 10% of total energy.
• Excess sugar can lead to excess calories.

How Much Complex Carbs?

• The Adequate Intake (AI) for fiber: 25 g for women, 38 g for men, or 14 g per 1000 kcal.
• Consume in tandem with 3 L of fluid to help with GI movement.
• Typical intake in Canada is 4-11 g/day.

Sweeteners

• Nutritive sweeteners include sucrose, fructose, honey, and brown sugar.
• Also include sugar alcohols like mannitol, sorbitol, isomalt, and xylitol, which are found naturally and provide 3-4 kcal/g.
• Non-nutritive sweeteners include saccharin, aspartame, and sucralose, and may have a negative impact on the gut biome.

High Fructose Corn Syrup

• Fructose consumed in high fructose corn syrup has no special properties, and is all about the calories.

Total, Added, and Free Sugar

• Added sugars are sugars added to food during processing or preparation.
• Free sugars include added sugars and naturally occurring sugars in honey, syrups, fruit juice, and fruit juice concentrates.
• Total sugars include all sugars presented in foods, derived from any source.
• Include fruits, vegetables, unsweetened milk, and yogurt.