Carbohydrate as a Fuel for Exercise: Importance and Dietary Recommendations
This chapter explores the role of carbohydrates as a fuel for exercise and highlights the importance of consuming adequate levels of carbohydrates to support athletic performance and recovery. It also discusses the classification of dietary carbohydrates and their sources.
Carbohydrate as a Fuel for Exercise: Importance and Dietary Recommendations
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chapter2 Carbohydrate as a Fuel for Exercise chapter 2 Carbohydrate as a Fuel for Exercise Prof Jennifer Broxterman, RD, MSc FN3373: Nutrition for Physical Activity Lectures 2 & 3 Author name here for Edited books
Chapter 2 Introduction • Carbohydrate as a Fuel for Exercise: • Well-documented that CHO is important for athletic performance • High levels of stored glycogen before endurance exercise (esp. > 1hr) can help increase performance & reduce time to fatigue • High CHO post-exercise enhances recovery • Many athletes consume inadequate levels of CHO to support their training
Dietary Carbohydrate • Optimum dietary CHO levels depend on: • Total energy intake • Body size • Health status • Duration, intensity, frequency, and type of exercise
Function, Classification, and Dietary Sources of Carbohydrate
Function of Carbohydrates • CHO are: • Primary source of energy (1 of 3 macronutrients) • Provide the substrate necessary for glycogen replacement (substrate: glucose) • When consumed during exercise, help maintain BG levels & help prevent premature fatigue • CHO recommendations for active individuals: • Moderate training: 5-7 g/kg of BW • Heavy training: up to 10 g/kg of BW (Burke, 2007)
Classification of Dietary CHO • Different ways to classify CHO • Type of CHO found in the food • Level of commercial processing the food has undergone • BG or glycemic response to the CHO within the body
Structural Classification of CHO • Complex carbohydrates: long complex chains of sugars linked together • Initially believed that all complex CHO were digested more slowly than simple CHO • The term ‘complex carbohydrate’ only refers to the structure of the CHO, not to any digestive properties
Food Examples of Complex CHO • Nutritionists / dietitians generally consider the following foods “complex CHOs” because they are good sources of vitamins, minerals, and fibre • Vegetables & fruit • Whole grains (breads, cereals, pasta) • Legumes (beans, peas, lentils) • Primarily contain: starch and fibre
Structural Classification of CHO • Simple carbohydrates: primarily refer to processed foods or foods high in sugar • E.g. sweetener cereals, breakfast bars, candy, regular pop, desserts • Are generally low in vitamins, minerals, and fibre unless they are fortified • Primarily contain: mono-, di-, and oligo-saccharides (glucose, sucrose, fructose, and high-fructose corn syrup)
Primary CHOs & Sugar in the Diet • Monosaccharides: simplest form of sugar • Glucose: main CHO in the bloodstream • Main energy source in the cell • Stored in the liver, muscles, and other organs as glycogen • Rapidly absorbed from the gut through sodium-dependent glucose transporter • Fructose: simple sugar found in honey & fruit • Tastes sweeter than table sugar (sucrose) • Absorbed from the gut through the glucose transporter 5 (GLUT5) and must be transported to the liver for conversion to glucose • Galactose: simple sugar found in milk
Primary CHOs & Sugar in the Diet • Disaccharides: made up of 2 simple sugars • Sucrose: glucose + fructose • Common table sugar, extracted from sugar cane and beet sugar • Most common dietary disaccharide • Broken down into glucose and fructose in the gut prior to absorption • Lactose: glucose + galactose • Sugar found in milk products • Lactose intolerant (lacking the lactase enzyme), common in Asians, Native Americans, Hispanics, and blacks • Maltose: glucose + glucose • Primarily formed from the breakdown of starch • Rapidly digested to glucose and absorbed quickly into the body
Primary CHOs & Sugar in the Diet • Oligosaccharides: short chains of 3 to 10 monosaccharides linked together • Maltodextrin: • Glucose polymer manufactured as long starch units are broken into smaller groups • Sugar found in sports drinks and many processed foods • Rapidly digested to glucose and quickly absorbed • Corn syrup: • Sweet syrup made up of glucose and short-chain glucose polymers produced by enzymatic hydrolysisof corn starch • Rapidly digested and absorbed
Primary CHOs & Sugar in the Diet • Oligosaccharides: short chains of 3 to 10 monosaccharides linked together • High-fructose corn syrup: • Especially sweet corn syrup • 45% to 55% of the CHO is enzymatically hydrolyzed to glucose and fructose (has nearly 2x the concentration of mono- and disaccharides found in regular corn syrup • Predominant sweetener found in commercially sweetened foods
Primary CHOs & Sugar in the Diet • Polysaccharides: contain starch and fibre (“complex carbohydrates”) • Starch: found in plants, seeds, and roots • Made up of straight chains of glucose polymers called amylose and some branching chain polymers called amylopectin • Starch is digested into glucose • Starches high in amylopectin are more rapidly digested and absorbed than starches high in amylase • Dietary fibre: part of the plant that cannot be digested by human gut enzymes • Goes from the small intestine into the colon, where it is expelled as fecal material or fermented and used by gut bacteria as food • Soluble vs. insoluble fibre
Glycemic Response • Glycemic response: • Classify foods as producing a high, moderate, or low glycemic response • Glycemic response to both simple and complex CHO foods can vary greatly • Some complex CHO (i.e. high in starch) can be hydrolyzed and absorbed as quickly as simple sugars
Glycemic Response • High glycemic response: • Foods that produce a large and rapid rise in blood glucose and insulin • Can increase muscle glycogen more than foods that produce a low glycemic response
Glycemic Response • Glycemic index (GI): scale that ranks CHO-rich foods by how much they raise blood glucose levels compared to a standard food • Determined by feeding 50 g of a particular food and watching the blood glucose response over a 2 hr period • BG response is compared to a reference food (usually white bread or glucose), with a GI = 100 GI = BG area of test food x 100 BG area of reference food
Glycemic Response • Glycemic load (GL): accounts for both the amount and source of CHO in a meal • GL = (GI of a food or meal) x (g of available CHO in the food or meal)
Carbohydrate as a Fuel Source • Muscles use of CHO during exercise: • Amount of CHO required depends on the: • frequency, intensity, duration, and type of exercise • environmental conditions • CHO used during exercise comes from the following sources: • Endogenous production of glucose by the liver (gluconeogenesis) • Blood glucose • Muscle and liver glycogen stores • CHO consumed during exercise (exogenous CHO)
Figure 2.1 Crossover concept of fuel use during exercise: • Low-to-moderate intensity: CHO + lipids play major roles as energy substrates • Higher intensity (relative aerobic power = 60-65%): CHO becomes increasingly important • Lipids become important energy sources during recovery
Gluconeogenesis • Gluconeogenesis: endogenous glucose production • Metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates • One of the main mechanisms humans use to keep BG levels from dropping too low (hypoglycemia) • Main substrates during exercise: lactate, alanine, glycerol, pyruvate • Primarily come from the muscle • Small amounts of glycerol come from adipose tissue • Are transported to the liver for glucose production
Gluconeogenesis • Amount of gluconeogenesis that occurs during exercise is impacted by: • Available CHO reserves prior to exercise initiation • Amount of CHO provided during exercise • Type, duration, and intensity of the exercise bout • Exercise environment (e.g. temperature, altitude) • Level of endurance training
Gluconeogenesis Substrates • Lactate: • Primary source of lactate during exercise is from the metabolism of glucose to lactate (through gylcolysis) • Lactate is transporated to the liver for glucose production through the Cori cycle, or it may be used directly by adjoining cells as an energy source • As glycogen is depleted in the working muscles, non-working muscles can give up some of their stored CHO by releasing lactate • Ahlborg & Felig (1982) showed that lactate released from the arms increased both during and after 3-3.5 hr of leg exercise (cycling)
Gluconeogenesis Substrates • Alanine: • Primary amino acid released by working muscles during exercise • Alanine is synthesized as nitrogen (released from the breakdown of aa in the muscles) and is combined with pyruvate • Alanine is transported to the liver, where it is broken down into pyruvate and nitrogen • Pyruvate can be used as a gluconeogenic substrate • Nitrogen is converted into urea and eliminated through the kidneys • This pathway is called the glucose-alanine cycle
Gluconeogenesis Substrates • Glycerol: • Is the 3-carbon backboneof a triglyceride • Adipose tissue or muscletriglycerides can be brokendown to yield 3 FAs and glycerol • FAs transported to the muscles for energy production • Glycerol transported to the liver for gluconeogenesis
Gluconeogenesis Substrates • Pyruvate: • Final substrate used for gluconeogenesis • 3-carbon compound • Can leak from working cells into the blood and is transported to the liver to make glucose
Glycogenolysis • Glycogenolysis: the chemical process by which glucose is freed from glycogen • Liver glycogenolysis: Another source of BG during exercise is the breakdown of liver glycogen • Glucose from the liver can be released directly into the bloodstream helping to maintain BG levels during exercise (unlike muscle glycogen) • Liver glycogen can be depleted if exercise is strenuous and of long duration • Gluconeogenesis and consuming exogenous CHO (e.g. sports drinks, gels) become increasingly important to maintain BG levels
Hormones & Exercise • Hormonal changes: • Signal the body to break down stored energy for fuel, which can then be used by the working muscles for energy • Hormonal responses depend on 2 main factors: • Intensity and duration of the exercise • Individual’s level of physical fitness
Hormones & Exercise • Norepinephrine & Epinephrine: • Blood levels rise dramatically within minutes of the initiation of exercise • Stimulate the breakdown of stored fat (both adipose & muscle tissue) and CHO (both liver & muscle glycogen), making these fuels available to the working muscles • Insulin: • Levels decrease or are maintained at a low concentration during exercise • Acute & chronic exercise increases the sensitivity of the skeletal muscle to the action of insulin
Hormones & Exercise • Glucagon: • Released from the pancreas in response to the low BG levels that may occur with exercise • Potent stimulator of glycogenolysis and gluconeogenesis • Helps to maintain BG levels by increasing the release of glucose into the bloodstream • Cortisol: • Also stimulated gluconeogenesis and helps to mobilize free FAs and amino acids
Carbohydrate Reserves • Primary fuel sources during exercise: • Carbohydrate (glucose) & fat (fatty acids) • Relative amounts used depend on the exercise intensity and duration • CHO reserves: • Compared to fat & protein, the body’s CHO reserves are severely limited • Total amount of energy stored as glycogen ranges from 800-2000 kcal • Depends on the diet, size of athlete, fitness level, and time of day • CHO consumed during exercise can supplement these reserves
Total Body Glycogen Reserves • Total CHO storage: • Total glycogen found in the liver, muscle, and other organs is not much greater than the amount of CHO consumed on average each day • 2000 kcal/day 50% of kcal from CHO offers ~250 g of CHO • After a typical meal, approximately 25-33% of CHO consumed is converted to liver glycogen; about 33-50% is converted to muscle glycogen; and the remainder is oxidized for energy in the hours after eating
Liver Glycogen • CHO Reserves: • Primarily liver and muscle glycogen • Glycogen concentrations are highest in the liver • Amount in a typical liver weighing 1.5 kg after an overnight fast is ~4% of the liver’s total weight, or 60 g • After a meal, amount of glycogen can double to ~8% of the liver’s weight, or 120 g glycogen • Liver glycogen plays a major role in maintaining BG levels throughout the night • morning meal containing CHO is important to replenish glycogen stores
Muscle Glycogen • Storage of glycogen in the muscle: • Lower than that in the liver • Deliberate CHO loading is required to increase the amount to more than 2% of fresh weight of rested muscle (~400 g) • Absolute amount of glycogen stored in the muscle can range from ~300 to 400 g (1200 - 1600 kcal) in a 70 kg athlete
Muscle Glycogen • Use of muscle glycogen during exercise: • Depends on amount of glycogen available before exercise begins • Exercise intensity and duration • Environmental conditions • Whether or not exogenous CHO is consumed
Dietary Carbohydrate Intakes of Active Individuals • Dietary intake of CHO: • Active men & women usually report CHO intakes similar to weight-matched inactive individuals • 45-55% of total energy from CHO or ~5-6 g/kg BW per day • Appropriate for recreational athletes who exercise for 1 hr or less per day • May be too low for endurance athletes who engage in daily intense training and whose glycogen stores need to be replenished rapidly • May require up to 10 g CHO/kg BW for men and 6-8 g CHO/kg BW for women
Pre-Exercise & Between-Competition Meals • Goals of pre-exercise meal: • Promote additional glycogen synthesis • Supply the body with glucose for use during exercise • Minimize fatigue during exercise • Replenish liver glycogen, especially after an over-night fast • Timing: pre-exercise meal usually consumed 2-4 hours prior to the exercise event • Often can be safely eaten at late as 1 hour before exercise
