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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.

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Carbohydrate as a Fuel for Exercise: Importance and Dietary Recommendations

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  1. 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

  2. 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

  3. Dietary Carbohydrate • Optimum dietary CHO levels depend on: • Total energy intake • Body size • Health status • Duration, intensity, frequency, and type of exercise

  4. Function, Classification, and Dietary Sources of Carbohydrate

  5. 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)

  6. 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

  7. 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

  8. 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

  9. Structural Classification of CHO

  10. 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)

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. Glycemic Response to Carbohydrates

  17. 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

  18. 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

  19. 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

  20. 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)

  21. Video: GI vs. GL

  22. Carbohydrate Metabolism During Exercise

  23. 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)

  24. 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

  25. 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

  26. Figure 2.3 Gluconeogenesis Pathway

  27. 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

  28. 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)

  29. Figure 2.6 Cori Cycle

  30. 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

  31. Figure 2.7 Glucose-Alanine Cycle

  32. 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

  33. 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

  34. 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

  35. Hormonal Control of Carbohydrate Metabolism During Exercise

  36. 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

  37. 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

  38. 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

  39. Carbohydrate Reserves and Dietary Intake

  40. 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

  41. 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

  42. 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

  43. 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

  44. 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

  45. 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

  46. Carbohydrate Feeding Before Exercise

  47. 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

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