Carbohydrates

Carbohydrates in Nutrition for Sport & Exercise

Overview

  • Carbohydrates are the primary fuel for muscle contraction.

Agenda

  • Classifying carbohydrates

  • Digestion and absorption

  • Carbohydrate metabolism

  • Carbohydrate recommendations

Carbohydrate Classifications

  • Monosaccharides: Simple sugars consisting of single molecules

    • Examples: Glucose, Fructose, Galactose

    • Functions: Used to supply energy; absorbed into the bloodstream from the digestive tract; converted to glucose in the liver.

  • Disaccharides: Formed from two monosaccharide molecules

    • Examples: Sucrose, Lactose, Maltose

  • Polysaccharides: Complex carbohydrates made up of many monosaccharide units

    • Types:

    • Starch: Digestible

      • Dietary sources

    • Fiber: Indigestible

    • Glycogen: Storage form in animals

Polysaccharides

  • Starch: Composed of glucose polymers

    • Structure:

    • Amylose: Long, unbranched chains of glucose linked by alpha-1,4 glycosidic bonds.

    • Amylopectin: Branched structure with alpha-1,4 and alpha-1,6 glycosidic bonds.

  • Fiber: Composed of glucose linked by beta-1,4 glycosidic bonds, making it resistant to digestion.

Fiber Intake Recommendations

  • Recommended Dietary Allowance (RDA):

    • Males: 38 g

    • Females: 25 g

Simple vs. Complex Carbohydrates

  • Classification generally does not reflect physiological effects; for sports nutrition, carbohydrates are classified by their oxidation rate.

  • Faster oxidation rates indicate energy that can be quickly utilized during exercise.

Classification by Oxidation Rate

  • Faster Carbohydrates:

    • Examples: Glucose, Maltose, Amylopectin

    • Approximate oxidation rate: 1.0 g/min (60 g/h) for fast digestion.

  • Slower Carbohydrates:

    • Examples: Fructose, Galactose, Amylose

    • Approximate oxidation rate: 0.6 g/min (35 g/h) for slow digestion.

Carbohydrate Usage During Exercise

  • Glucose and galactose can be efficiently utilized during exercise, but fructose can lead to gastrointestinal distress due to incomplete absorption.

Oxidation Rates During Cycling Example

  • Glucose and galactose were tested at varying concentrations (8% solution), showing performance and oxidation rates.

Carbohydrate Blends and Performance

  • Results from a study showed that glucose and fructose blends enhance time trial performance, leading to improved competition results.

Digestion and Absorption of Carbohydrates

  • Digested carbohydrates are broken down into monosaccharides (glucose, galactose, fructose) in the gastrointestinal (GI) tract by enzymes such as lactase, sucrase, and maltase.

  • The structure of the small intestine facilitates absorption with microvilli and enterocytes assisting in nutrient uptake.

  • Transport Mechanisms:

    • Sodium-glucose transporter 1 (SGLT1) for glucose and galactose absorption.

    • GLUT5 for fructose transport.

Carbohydrate Metabolism

  • Glycogen stored in the liver and muscle plays a critical role in energy metabolism.

    • Enzymatic Actions:

    • Glucose enters muscle or liver via facilitated transport. Once inside, it is phosphorylated by hexokinase to glucose-6-phosphate, making it unable to leave the cell, except in liver/kidney where glucose-6-phosphatase reverses this.

Muscle Glycogen

  • Trained athletes maintain higher glycogen stores compared to untrained individuals.

  • Glycogen helps maintain race pace, although it does not directly correlate to speed improvement.

Regulation of Blood Glucose Levels

  • Insulin and glucagon are hormones responsible for maintaining blood glucose homeostasis.

  • Insulin promotes glucose uptake and storage, while glucagon increases glucose release into the blood.

Glycemic Index

  • The glycemic index ranks how carbohydrates affect blood glucose and insulin levels:

    • High GI Foods (70 and above): Rapidly increase blood glucose (e.g., white bread, rice).

    • Medium GI Foods (56 to 69): Moderate response (e.g., sponge cake, whole wheat bread).

    • Low GI Foods (55 and under): Slow response (e.g., apples, beans).

General Carbohydrate Recommendations

  • Training Intensity-Based Recommendations:

    • Skill-based training: 3-5 g/kg/day

    • Moderate to high-intensity training: 5-7 g/kg/day

    • High-volume endurance training: 6-10 g/kg/day

    • Extreme endurance training: 8-12 g/kg/day

Specific Recommendations Based on Exercise Type

  • Low-Intensity Activities: 3-5 g/kg, e.g. Curling, Golf

  • High-Intensity Short-Duration: 5-7 g/kg, e.g. Sprints, Weight Lifting

  • High-Intensity Long-Duration: 6-10 g/kg, e.g. Marathon running, Triathlons

Carbohydrate Loading

  • Carbohydrate loading aims to super-saturate glycogen stores and enhance endurance performance, generally increasing time to fatigue by approximately 20% in endurance events over 90 minutes.

After Training

  • Post-exercise carbohydrate intake is crucial for replenishing glycogen stores.

  • Recommended intake: 1.2 g/kg/hr for up to 4 hours post-exercise.

Factors Influencing Glycogen Synthesis

  • Timing of carbohydrate intake: Maximized when consumed within 2 hours post-exercise.

  • Type of carbohydrate: High-glycemic index foods are absorbed faster.

  • Ingestion of protein: Enhances glycogen storage post-exercise.

  • Caffeine presence: May improve glucose delivery to muscles.

Training Considerations

  • Training in a glycogen-depleted state can elevate fat oxidation rates and mitochondrial function, though may impair exercise intensity.

FODMAPs and Gluten-Free Considerations

  • FODMAPs can induce gastrointestinal distress; examples include high lactose products, fruits like apples, and certain nuts.

  • A gluten-free diet has no substantial benefits for non-Celiac athletes unless they have specific sensitivities.

Study Questions

  • Understand and differentiate between monosaccharides, disaccharides, and polysaccharides.

  • Explain carbohydrate classification based on oxidation rates and their practical implications for athletes.

  • Discuss how glucose, fructose, and galactose are absorbed using specific transporters.

  • Define and differentiate hyperglycemia and hypoglycemia, including hormone regulation of carbohydrate metabolism.