Carbohydrates: Sugar, Starches, and Fibre

CARBOHYDRATES: Sugar, Starches, and Fibre

Chapter Overview

  • Publication Information: Copyright ©2020 John Wiley & Sons, Inc.

Basic Questions to Consider

  • What is a carbohydrate (carb)?

  • What is sugar?

  • Initial associations with these terms?

Carbohydrates in the Modern Diet

  • Foundational Role: Carbohydrates are essential to human diet and health.

  • Consumption Statistics: Carbohydrates represent the largest macronutrient consumption in most diets.

Definition and Structure

  • What is a Carbohydrate?

    • Compounds that consist of carbon, hydrogen, and oxygen (CHO).

    • Energy Cycles:

    • Through photosynthesis, plants combine carbon dioxide, water, and energy from the sun to form glucose.

Dietary Recommendations in Canada

  • Canada’s Food Guide advises the following:

    • Increase consumption of unrefined carbohydrates.

    • Emphasis on whole foods.

    • Decrease intake of refined carbohydrates and added sugars from foods.

    • Negative health results associated with high intake of refined products.

Types of Carbohydrates (CHO)

  • Simple CHO:

    • Composed of sugars.

  • Complex CHO (Polysaccharides):

    • Includes starch and fibre.

Carbohydrates in Food

  • Unrefined Foods:

    • Naturally occurring carbohydrates with minimal processing; retains nutrients.

  • Refined Foods:

    • Processed foods with removed coarse parts, leading to loss of fibre, vitamins, minerals, and phytochemicals.

Free and Added Sugars

  • Definition of Free Sugars:

    • Mono- and disaccharides added to food during processing/preparation (table sugar, high-fructose corn syrup).

    • Also includes natural sugars in honey, syrups, and juices.

  • Concept of Empty Calories:

    • Nutritional value is minimal despite contributing energy.

Monosaccharides

  • Definition:

    • Simple carbohydrates representing single sugar units.

    • Components:

    • Mono = 1

    • Saccharide = sugar.

Disaccharides

  • Definition:

    • Consists of two sugar units.

    • Components are as follows:

    • Glucose + Glucose = Maltose:

      • Formed during starch digestion (example: sweet taste of chewed bread due to saliva action).

    • Glucose + Fructose = Sucrose:

      • Table sugar, only sweetener labeled as "sugar."

    • Glucose + Galactose = Lactose:

      • Milk sugar found in dairy products.

Complex Carbohydrates

  • Oligosaccharides:

    • Short chains of less than ten monosaccharides.

  • Polysaccharides:

    • Long chains of monosaccharides, significant for storage.

    • Types include starch and fibre.

Starch as a Complex Carbohydrate

  • Description:

    • Major digestible form of complex carbohydrates in the diet.

    • Plant storage form composed of 300 to thousands of sugar molecules, classified as:

    • Linear (amylose)

    • Branched (amylopectin).

  • Sources:

    • Starchy vegetables

    • Grains

    • Legumes.

Glycogen as a Complex Carbohydrate

  • Definition:

    • Highly branched chains of glucose; animal storage form of carbohydrates.

  • Sources and Functions:

    • Liver Glycogen:

    • Maintains blood glucose levels.

    • Supplies energy.

    • Muscle Glycogen:

    • Supports muscle contraction and is utilized during high-intensity exercise or endurance activities.

Whole Grains vs. Refined Grains

  • Whole (Unrefined) Grains:

    • Contains bran layers (fibre and vitamins), germ (vegetable oils and vitamin E), and endosperm (starch and some protein).

  • Refined Grains:

    • Processed from the endosperm and predominantly consist of starch. May be enriched with nutrients but lack essential nutrients lost during processing.

Indigestible Carbohydrates

  • Types Include:

    1. Fibre

    2. Oligosaccharides

    • Bonds are unbreakable by human enzymes.

    1. Resistant Starches

    • Changes in structure due to cooking and cooling.

Benefits and Implications of Indigestible Carbohydrates

  • Effects on Human Health:

    • Affects gastrointestinal motility, intestinal microflora, nutrient absorption, and gas production.

Soluble Fibre

  • Definition:

    1. Dissolves in water.

    2. Sources:

    • Oats, oat bran, legumes, barley, psyllium, pectin-rich fruits.

    1. Types:

    • β-glucan, inulin, mucilage.

    1. Health Benefits:

    • Reduces cholesterol absorption.

    • Slow absorption helps regulate glucose levels.

    • Promotes growth of beneficial gut flora.

Insoluble Fibre

  • Definition:

    1. Does not dissolve in water.

    2. Sources:

    • Whole grains, brown rice, bran cereals, vegetables, and fruits.

    1. Types:

    • Lignin, cellulose.

    1. Health Benefits:

    • Increases stool bulk and reduces intestinal transit time.

    • May lower the risk of colon cancer.

    • Stimulates intestinal motility.

Negative Consequences of High Fibre Consumption

  • Consequences Include:

    • Indigestible carbohydrates move to the large intestine where fermentation occurs, producing gas.

    • Quick increases in fibre intake can cause bloating, gas, and diarrhea if fluid intake is not adequate.

    • Very high fibre intake may reduce mineral absorption.

  • Recommendation:

    • Gradually increase fibre intake and fluid intake.

Digestion and Absorption of Carbohydrates

  1. Salivary Amylase: Converts starch to polysaccharides.

  2. Inactivation of Salivary Amylase: Occurs once food reaches the stomach.

  3. Pancreatic Amylase: Major contributor to carbohydrate digestion.

  4. Brush Border Enzymes:

    • Include maltase, sucrase, and lactase which facilitate disaccharide digestion.

  5. Fibre: Not digested; contributes bulk to stool.

Post-Absorption Process

  1. Monosaccharides (simplest form of carbohydrates) are transported to the liver.

  2. Conversion of Fructose and Galactose: Both are converted to glucose upon entering the liver.

  3. Energy Management:

    • Glucose released into the bloodstream if energy is needed.

    • Excess glucose is stored as glycogen in the liver and muscles.

CHO Metabolism Overview

  • Definition: Conversion of glucose to an energy form used by cells.

  • Cellular Respiration:

    • Involves metabolizing glucose in the presence of oxygen to produce ATP (adenosine triphosphate).

    • Requires Acetyl-CoA, involved in various metabolic processes.

Energy Sources When Dietary CHO Intake is Low

  • When carbohydrate intake is low, the body utilizes:

    • Glycogen (limited storage).

    • Fat stores (from adipose tissue).

    • Body protein (muscle tissue).

CHO and Fat Metabolism

  • Relationship:

    1. Fat metabolism requires carbohydrates.

    2. Insufficient carbohydrates hinder fat breakdown, resulting in liver-producing ketones.

Ketones/Ketone Bodies

  1. Acidic molecules generated during low carbohydrate intake.

  2. Utilized by certain tissues (brain, heart, kidney, muscle) for energy, particularly in states of starvation or diabetes.

  3. Ketosis: Elevated ketone levels lead to potential health risks.

  4. Ketoacidosis: Excess acid in blood from increased ketones, posing dangerous health risks.

Protein Metabolism When CHO Intake is Low

  1. Proteins are broken down for energy if necessary.

  2. Protein can be converted into:

    • Ketone bodies (ketogenic) or glucose (glucogenic).

  3. Disadvantages:

    • Proteins are essential for vital bodily functions (e.g., hormone and enzyme production).

    • Carbohydrates spare protein usage by providing primary energy source.

CHO Metabolism When CHO Intake is High

  • Adapts body to use more glucose rather than fat:

    • Excess fat circulates in the blood and can lead to storage in adipose tissues.

    • Elevated glucose can lead the liver to convert glucose into acetyl-CoA, resulting in fat synthesis if sustained.

Blood Glucose Regulation

  1. Control Mechanism: Managed by the liver and pancreas through hormones.

    • Insulin: Released in response to rising blood glucose levels, facilitating glucose cellular uptake, glycogen storage, and fat synthesis.

    • Glucagon: Released when blood glucose declines, promoting the breakdown of liver glycogen and glucose release into the blood.

Blood Glucose Fluctuations

  • When Blood Glucose Increases:

    • Insulin secretion triggers glucose uptake into cells and promotes energy storage.

  • When Blood Glucose Decreases:

    • Glucagon secretion stimulates the release of glucose from liver stores and promotes gluconeogenesis.

Summary of Insulin and Glucagon Actions

  • Insulin Effects:

    • Increases blood glucose uptake into cells when glucose levels rise.

  • Glucagon Effects:

    • Increases blood glucose levels by promoting glucose release from liver when glucose levels fall.

Glycemic Response and Index

  • Definitions:

    • Glycemic Response: Rate and extent blood glucose rises after carbohydrate consumption.

    • Glycemic Index: Classifies foods based on how they affect the glycemic response.

Abnormal Blood Glucose and Diabetes

  • Consequences of Elevated Glucose Levels:

    • Damage to blood vessels, increasing risks of heart disease and stroke.

    • Leading cause of adult blindness, renal failure, and nontraumatic amputations.

Types of Diabetes

  1. Gestational Diabetes:

    • Occurs during pregnancy and increases risk of developing type 2 later.

  2. Type I Diabetes:

    • Autoimmune condition causing destruction of insulin-producing pancreas cells.

  3. Type II Diabetes:

    • Characterized by insulin resistance and decreased insulin production; most common form.

Type 2 Diabetes Characteristics

  • Prevalence: Most common type (90% of cases), now manifesting in youth due to obesity and lifestyle factors.

  • Risk Factors: Includes family history, inadequate lifestyle habits, and higher rates in Indigenous communities due to genetics and socio-economic conditions.

Diabetes Treatments

  • Type 1: Primarily treated with insulin.

  • Type 2: Managed through weight management, exercise, dietary changes, and potentially medications.

Lactose Intolerance

  1. Deficiency of the lactase enzyme results in undigested lactose moving to the large intestine.

  2. Causes water retention and fermentation by bacteria, leading to gas and bloating.

  3. Symptoms include bloating, flatulence, cramping, and diarrhea.

Rationale for Low-Carbohydrate Weight Loss Diets

  • High carbohydrate foods stimulate insulin release.

  • Insulin promotes energy storage, primarily as fat.

    • Low-carbohydrate diets lead to reduced insulin levels and increased ketone production, which may suppress appetite.

Carbohydrates and Heart Disease

  • High sugar and refined carbohydrate diets raise blood lipids, increasing heart disease risk.

  • Higher intake of whole grains can reduce the risk of heart disease.

Implications of Fiber Intake on Health

  • Bowel Disorders:

    • High-fiber diets may alleviate or prevent conditions like hemorrhoids, diverticulosis, and constipation.

    • Associated with lower colon cancer risk in populations consuming abundant fibre.

Carbohydrates and Dental Health

  • Bacterial plaque build-up linked to carbohydrates can lead to dental caries.

    • Recommendations:

    • Reduce sugary foods.

    • Chew sugarless gum containing xylitol.

    • Maintain oral hygiene through toothbrushing.

Choosing Healthier Carbohydrates

  • Canada's Food Guide Recommends:

    • Consumption of whole foods, including whole grains, vegetables, fruits, and healthy protein sources.

    • Preference for complex carbohydrates that include fibre, essential nutrients, and minimal processing.

Recommendations for Healthy Carbohydrate Intake

  • Recommended Daily Allowance (RDA):

    • Minimum 130 g/day for adults and children (necessary for brain function).

  • Acceptable Macronutrient Distribution Range (AMDR):

    • 45% to 65% of total energy from carbohydrates.

    • Source quality should be prioritized over quantity; limit refined sugars.

Increasing Fiber Intake Recommendations

  • Adequate Intake for Fiber:

    • 38 g/day for men and 25 g/day for women.

    • Practical examples to meet these requirements:

    • 2 slices of 7-grain bread = 6.5 grams.

    • ½ cup blueberries = 4 grams.

    • Salad with various ingredients = 9 grams.

Calculating Carbohydrate Intake

  • Process Includes:

    1. Determine Total Energy: Measured in kilocalories for the day.

    2. Calculate Energy from Carbohydrates:

    • Carbohydrates provide 4 kcalories/gram.

    • Total energy from carbohydrates = grams of carbohydrate × 4 kcalories/gram.

    1. Calculate Percentage of Total Energy:

    • % Energy from carbs = (Energy from carbohydrates / Total energy) × 100.

Reading Food Labels for Sugar and Fiber Content

  • Understanding Label Claims:

    • Sugar-Free: Contains less than 0.5 g sugars/serving.

    • Reduced Sugar: 25% less sugar than a reference product.

    • Source of Fiber: At least 2 g fiber/serving; high source requires at least 4 g/serving.

Non-nutritive Sweeteners

  • Overview:

    • Also known as alternative sweeteners, provide sweetness with fewer calories.

    • Examples include:

    • Saccharin (tabletop sweetener).

    • Aspartame (used in uncooked foods, Health Canada ADI 40 mg/kg).

    • Sucralose (heat stable, ADI 9 mg/kg).

    • Acesulfame K (heat stable, ADI 15 mg/kg).

Alternative Sweeteners: Sugar Alcohols

  • Characteristics:

    • Not absorbed in the small intestine, providing less energy than sugar.

    • Examples include mannitol, sorbitol, lactitol, and xylitol.

    • Products using sugar alcohols can claim health benefits,

    • High consumption may lead to gastrointestinal discomfort (diarrhea).