Carbohydrates and Digestion

Canada's Food Guide

• US and Canadian guidelines differ
• Disregard information referring to MyPlate US guidelines
• The course follows Canada's Food Guide which is available for download

Carbohydrates

• Organic compounds composed of carbon, hydrogen, and oxygen
• Simple carbohydrates include monosaccharides and disaccharides
• Monosaccharides are single carbohydrate units like glucose, fructose, and galactose
• Disaccharides are two monosaccharide units bound together like sucrose, lactose, and maltose
• Complex carbohydrates are polysaccharides; many monosaccharide units joined together, such as starch and fiber

Dietary Carbohydrates

• Monosaccharides: glucose, fructose, galactose
• Disaccharides: sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose)
• Polysaccharides: starches and fiber

Function of Carbohydrates

• Carbohydrates are the most efficient form of energy for the body
• Fiber occurs naturally
• Supply naturally occurring sweetners
• The brain and nerve tissue function optimally on glucose from carbohydrates

Digestion and Absorption

• Mechanical digestion breaks food into smaller pieces in the mouth
• Amylase begins chemical digestion in the mouth breaking down starch into dextrin and maltose
• Stomach acid and enzymes halt amylase action in the stomach
• Intestinal enzymes and pancreatic amylase continue the breakdown of simple carbohydrates in the small intestine
• Brush border cells secrete specific enzymes for disaccharide hydrolysis in small intestine
• Maltase breaks down maltose into glucose
• Sucrase breaks down sucrose into fructose and glucose
• Lactase breaks down lactose into galactose and glucose
• Monosaccharides are then absorbed in the small intestine
• Most fiber continues to the large intestine, where bacteria digest some, and the rest exits the body

Digestion in the Small Intestine

• Intestinal enzymes and specific pancreatic amylase continue the breakdown of starch to monosaccharides
• Enzymes specific for disaccharides (lactase, sucrase, and maltase) are in the small intestine's brush border cells
• These enzymes hydrolyze disaccharides into monosaccharides
• Glucose, fructose, and galactose are transported to the liver
• The liver converts fructose and galactose to glucose

Storing Carbohydrates

• Glucose is converted into glycogen via glycogenesis
• Glycogen is the storage form of glucose

Metabolism

• The primary aspect of carbohydrate metabolism is blood glucose homeostasis
• Dietary starches and simple carbohydrates provide blood glucose after digestion and absorption
• Carbohydrate metabolites (lactic and pyruvic acid) are also used for energy
• Non-carbohydrates can also provide blood glucose
• Gluconeogenesis produces glucose from fat

Blood Glucose Regulation

• Insulin lowers blood glucose levels by enhancing the conversion of excess glucose to glycogen through glycogenesis
• Insulin eases the absorption of glucose into cells for energy use
• Glucagon stimulates the conversion of liver glycogen to glucose
• Somatostatin inhibits the function of glucagon and insulin

Absorption of Carbohydrates

• Carbohydrates require active absorption
• Absorptive cells in the small intestine take up the monosaccharides
• Glucose, fructose, and galactose enter the villi into the portal blood circulatory system, which transports them to the liver
• The liver takes the fructose and galactose and converts them into glucose
• Glucose can be used immediately for energy or stored as glycogen

Glycemic Index

• Ranking of food to the level that it raises blood glucose
• Factors include physical form, fat/protein/carbohydrate content, ripeness of fruits/vegetables, fiber content, and botanic variety of food

Sugar

• Sugar is sugar
• Sugar provides nutrient displacement, dental caries and can lead to obesity

Complex Carbohydrates: Starch

• Plant foods like grains, legumes, vegetables and fruit are broken down in the GI tract to provide glucose

Fiber

• Fiber is made of strings of simple sugars
• Cannot be broken down by human digestive enzymes
• Fiber is used for microbial fermentation which results in the synthesis of vitamins and short-chain fatty acids.
• Soluble dietary fiber dissolves in fluids
• Insoluble dietary fibers do not dissolve in fluids.
• The best source of fiber is whole grains, fruits and vegetables

Fiber Health Benefits

• Improves the physical function of the human body
• Prevents obesity, constipation, hemorrhoids, diverticular disease, and colon cancer

Fiber and Obesity

• High fiber foods help with weight control
• Fiber makes you feel more full
• Often replaces high fat and kcal foods

Fiber and Constipation

• Insoluble fiber (wheat bran and whole grains) lead to larger, softer stools
• This prevents constipation, hemorrhoids, and diverticular disease

Diverticular Disease

• Pockets develop on the outside walls of the large intestine
• Low bulk, secondary to low fiber, can weaken muscles leading to diverticula

Fiber and Cancer

• Diets high in fiber reduce the risk of colon cancer
• Reducing fat consumption also reduces the risk

Fiber and Heart Disease

• Lowers blood cholesterol and lipid levels
• Lowers the consumption of high-fat foods
• Soluble fiber binds to lipids and cholesterol and excretes them in the intestinal tract

Fiber and Diabetes

• Helps stabilize blood glucose levels
• Soluble fiber slows the absorption of glucose

Canadian Carbohydrate Information

• Recommended fiber intake: 25-38g a day
• Recommended carbs: 30-45g per meal

Fats

• Chemical group called lipids
• Includes fats (triglycerides) which are solids or oils and fat-related substances (phospholipids and sterols)

Functions of Lipids

• Food Functions: source of energy, provides palatability, satiety/satiation, food processing, and nutrients
• Physiologic Functions: stored energy, organ protection, temperature regulation, and insulation

Phospholipids

• Two fatty acids and one phosphate group
• Nonessential nutrient
• Form part of the cell membrane and work as emulsifiers
• Lecithin is the main phospholipid
• Lipoprotein- carriers or transports lipids
• Found in egg yolks and soybeans

Sterols

• Includes cholesterol
• Carbon rings intermeshed with sidechains of carbon, hydrogen, and oxygen
• Nonessential
• Provides basic material to form bile, vitamin D, sex hormones, and cells in the brain and nerve tissue
• Cholesterol in the skin is converted to Vitamin D with UV light exposure
• Dietary cholesterol accounts for 25% of cholesterol in the body; rest made in the liver

Types of Fats

• Trans fats are naturally found in some animal-based foods or produced industrially; some liquid vegetable oils contain small amounts
• Saturated fats are found in foods like meat, lard, shortening, tropical oils, and dairy
• Unsaturated fats are polyunsaturated and monounsaturated; found in nuts, seeds, avocado, fatty fish, vegetable oils, soft margarine, and soy products

Saturated and Unsaturated Fat

• Replace foods with mainly saturated fat to foods with unsaturated fat to promote cardiovascular health
• The type of fat consumed is more important than the total amount of fat consumed
• Replacing saturated fat with unsaturated fat decreases total and LDL-cholesterol
• Elevated LDL- cholesterol is seen in 1 in 5 adult Canadians which is a well-known risk factor for cardiovascular disease
• Omega 3 is a great example of an unsaturated fat that possesses many benefits!
• Health Canada banned the use of partially hydrogenated oils (PHOs), manufacturers can't add PHOs to foods sold in Canada

Fat in the Body

• Chemical digestion continues as CCK is released in response to fats entering the duodenum.
• CCK causes the release of bile, which emulsifies fats.
• The combined actions of chemical and mechanical digestion expose the most fat surface area to pancreatic lipase
• The process uses pancreatic lipase to turn Triglycerides into fatty acids, monoglycerides, and glycerol.
• Some fats are partially digested in the large intestine, then the rest pass through unchanged and exit in feces

Absorption of Fats

• Bile salts help move fatty acids from the lumen to the villi for absorption
• Micelles aid diffusion through the membrane wall
• After passing through the membrane wall fatty acids and glycerol combine back into triglycerides
• This allows them to travel through the lymphatic system to the blood circulatory system then to the hepatic portal system and liver
• Lipoprotein lipase then helps break the triglycerides back into fatty acids and glycerol at the cell membrane
• Fatty acids can either be used for fuel or reform them into triglycerides and store them for energy

Metabolism of Fats

• Liver cells produces ketones if fat catabolizes too quickly because of a lack of carbohydrates from the oxidation fo fatty acids
• Excessive ketones results in ketosis.
• Ketones are a product of fat metabolization

Cholesterol

• Cholesterol will be found in: Meat, Eggs, Fish & Dairy products
• Dietary cholesterol does not significantly raise blood cholesterol.
• Diets high in saturated fats WILL increase blood cholesterol levels

Energy Intake

• Fat is more efficient at being stored compared to carbohydrates and proteins
• This leads to more body fat with fat consumption

Test your knowledge of carbohydrates, including their formation, function, and dietary sources. Questions cover disaccharides, complex carbohydrates, fiber, and the roles of insulin and enzymes in digestion.