Depicts the relative contributions of anaerobic and aerobic energy sources during various durations of maximal exercise.
The reduced power output level comes directly from the relatively slow rate of aerobic energy release from fat oxidation, which now becomes the primary energy source. Severely lowered levels of liver and muscle glycogen during exercise induce fatigue, despite sufficient oxygen availability to muscles and almost unlimited potential energy from stored fat. Known as “hitting the wall.”
This gender difference in substrate metabolism’s response to training may reflect differences in sympathetic nervous system adaptation to regular exercise (i.e., a more blunted catecholamine response for women). The sex hormones estrogen and progesterone may affect metabolic mixture indirectly via interactions with the catecholamines or directly by augmenting lipolysis and/or constraining glycolysis.
These observations pertain to both athletes and physically active individuals who modify their diets by reducing carbohydrate intake below recommended levels.
These diets rapidly deplete muscle and liver glycogen. A low-carbohydrate diet makes it extremely difficult, from the standpoint of energy supply, to engage in vigorous physical activity.
Section 4Macronutrient Metabolismin Exercise and Training
Fuel for Exercise The fuel mixture that powers exercise generally depends on: • The intensity of effort • The duration of effort • The exerciser’s fitness status • The exerciser’s nutritional status
Illustration of the contribution of COH, lipid, and protein during different exercise intensities: *Assume that little or no proteins are being used for energy. Rest Low-Intensity High-Intensity Mod-Intensity Long Duration Short Duration Long Duration Protein 2-5% 2-5% 2% 5-8% COH 35% 40% 95% 70% Lipid 60% 55% 3% 15%
1. Emphasize Carbohydrates in the diet:55-65% of total caloric intakeHigh quality carbohydrates (nutrient rich)Low glycemic COH are preferred 2. Storage of COHLiver glycogen COH Homeostasis During ExerciseMuscle glycogen COH from g.i. track Used for muscle In=Out contraction Liver glycogen Blood Glucose Muscle glycogen Adipose tissue ~400 kcal ~400 kcal ~1200 kcal ~a lot of kcal Converted to Fat
3. Use of COH during Training/Competition •Below 50% intensity--fat utilization •Above 50% intensity--primarily COH (intervals) •Depletion of glycogen stores within 2 hours headache, lightheadedness, nausea, fatigue, malaise •Training enhances ability to use COH •Training also enhanced the ability to use fat for energy Why is ability to use fat so important? It saves the COH… Estimation of energy available for muscle contraction: Fuel Depot Kcal Glycogen in muscle 480-1,000 Glycogen in liver 280-400 Adipose tissue 141,000- Body proteins ~24,000
4. Maintaining Glucose Levels During Exercise •Hepatic glucose production--”Feed forward mechanism” •Glucose feedings/drinks •Absorption -Start drinking early -Cold -Less than 8% (8 g of glu/oz of fluid) glucose *Maltodextrin drinks (Exceed, GatorLode, UltraFuel) -Adequate volume of fluid -Good tasting• Replenishing Glycogen Stores After Exercise •Immediately after Ex-High glycemic foods are okay •Thereafter: Avoid Glycemic Foods that produce an insulin response •Replenish Glycogen Stores, don’t feed Fat Stores• Carbohydrate Loading--Enhancing Glycogen Storage •Time to fatigue is related to glycogen stores •Repeated depletion during training--Increased storage
•Dietary Plan: 7 days before competition -depletion: Day 1-exhausting exercise to deplete stores Days 2 to 4-low COH diet -loading: Days 5-7 high COH diet, no depletion•If all goes well...can store 2x as much glycogen “Supercompensation” -normal: 2 g glycogen/100 g muscle -”loaded”: 4-5 g glycogen/100 g muscle•If all does not go well... -diarrhea/constipation/gas production -1 g glycogen stored in 3 g of water -fluctuations in plasma glucose, fatty acids, and cholesterol -difficulty training during low COH period
Glycogen Depletion Blood glucose levels fall. Level of fatty acids in the blood increases. Proteins provide an increased contribution to energy. Exercise capacity progressively decreases.
Nutritional Strategies to Enhance Fat Oxidation During ExerciseCarbohydrate stores are limited within the body, and fat depots represent an enormoussource of potential energy.However, fatty acid oxidation by muscle is limited, especially during exercise aboveabout 50% intensity. Adipose Tissue Blood Plasma Muscle Triglyceride Intra-muscular (~77,000 kcal) Triglyceride (~3,200 kcal) Glycerol FFA Glycogen (~2,000 kcal) Albumin FFA Fatty acids FFA Acetyl-CoA Kreb’s cycle & Electron Transport Liver Glucose Glycogen (~450 kcal) ATP (~1200 kcal) Oxygen
Processes that limit fatty acid oxidation during exercise: . External factors: -aerobic training status of the individual -habitual intake of fat -ingestion of COH and fat just prior to exercise -gender -intensity of exercise . Mobilization of fatty acids from adipose tissue: Lipolysis -cleavage of fatty acids from triglyceride is dependent on activation of the enzyme, hormone sensitive triglyceride lipase (HSL) in adipose tissue. -Epinephrine and glucagon activate HSL -Insulin and high blood glucose inhibit HSL . Transport of fatty acids across the sarcolemmal membrane into muscle: -Small fatty acids go into muscle by diffusion (8-12 C long) -Longer fatty acids require: Fatty acid binding proteins (FABP) Fatty acid translocases (FAT) Fatty acid transport proteins (FATP) *FABP is higher in slow twitch muscles and is enhanced by training . Transport of fatty acids across the mitochondrial membrane: -Carnitine palmitoyltransferase I takes FA across outer mitochondrial membrane -Carnitine palmitoyltransferase II takes FA across the inner mitochondrial membrane -Transport dependent activity . Oxidation of fatty acids: -Dependent on the availability of oxygen -Dependent on mitochondiral density -Dependent on plasma concentrations of epi, glucagon, insulin, and glucose -Dependent on exercise intensity
Strategies to Enhance Fatty Acid Oxidation During Exercise:1. Caffeine ingestion before and during exercise: stimulates lipolysis enhances FA oxidation decreases utilization of muscle glycogen How? Not sure... May be sympathomimetic (like epinephrine) May stimulate fat mobilization directly2. Fat feeding before exercise: enhances fat metabolism during exercise -probably by increasing FFA levels in the blood does not prolong exercise or spare glycogen3. Maintain low insulin levels prior to exercise avoid high glycemia foods that stimulate insulin and inhibit HSL pseudo-insulin resistance during exercise precludes this response4. Long, slow, gradual, and continuous warm-up prior to exercise. helps to maintain resting fatty acid levels during exercise5. High state of aerobic fitness. enhances oxygen delivery to cell enhances fatty acid deposits in muscle enhances blood flow to the cell increases density of fatty acid binding proteins (FABP), fatty acid translocases (FAT), and fatty acid transport proteins (FATP) enhances mitochondrial density5. Other unsuccessful things that have been tried: -high fat diets/high fat sports bars -high protein diets -L-carnitine supplementation
Nutrient Utilization During ExercisePercent contribution of aerobic and anaerobic energy pathways during exercise: Duration of Maximal Exercise Seconds Minutes Time 10 30 60 2 4 10 30 60 120 % Anaerobic 90 80 70 50 35 15 5 2 1 % Aerobic 10 20 30 50 65 85 95 98 99
Nutrient Related Fatigue: -Depletion of muscle glycogen and liver glycogen “bonking” “hitting the wall” “carrying the piano” “trip to Oz” -Possible reasons for fatigue and depletion:-Use of blood glucose as energy for the CNS -Use of glucose as a primer for fat metabolism-Significantly slower rate of energy release from fat compared to carbohydrate breakdown-*Hepatic glucose production When exercise begins – muscles take glucose from the blood (exercise stimulated glucose uptake). This could make an individual hypoglycemic if there were no compensatory mechanisms. At the onset of exercise a sympathetically-mediated feed-forward mechanism called hepatic glucose production prevents hypoglycemia during exercise (but it also speeds the use of liver glycogen stores).-Why fat metabolism is limited during exercise: -FFA mobilization from adipose tissue -FFA transport to muscle via blood -FFA uptake by muscle cells -FA mobilization from intramuscular fat -FA transport into mitochondria -FA oxidation in mitochondria
Preventing nutrient related fatigue:1.Optimize carbohydrate stores before exercising2. Optimize fat utilization during exercise -slow and gradual warm-up -continuous exercise -adequate cutaneous blood flow3. Glucose replacement during exercise4. Training -increases ability to utilize fats -increases glycogen storage capacity Effect of Training 100 60 30 70 10 50 Rest 25% 70% 100% % of Maximal Aerobic Capacity
Training-Induced Adaptations That Increase Lipid Metabolism: -facilitates lipolysis -increased capillary perfusion of muscle to deliver lipids -improved FA mobilization, transport, and oxidation -increased mitochondrial density -increased number of enzymes for β-oxidation
Protein Use During Exercise Serves as an energy fuel to a much greater extent than previously thought • The amount depends upon nutritional status and the intensity of exercise training or competition. • This applies particularly to branched-chain amino acids that oxidize within skeletal muscle rather than within the liver.
Protein Use During Exercise (cont.) Exercise in a carbohydrate-depleted state causes significant protein catabolism. Protein synthesis rises markedly following both endurance- and resistance-type exercise.
Protein Requirements Re-examining the current protein RDA seems justified for those who engage in heavy exercise training. One must account for increased protein breakdown during exercise and the augmented protein synthesis in recovery.
Gender Differences Women derive a smaller proportion of energy from carbohydrate oxidation than do men during submaximal exercise at equivalent percentages of aerobic capacity. Following aerobic exercise training, women show an exaggerated shift toward fat catabolism, whereas men do not.
Training-Induced Metabolic Adaptations Carbohydrate: Trained muscle has an augmented capacity to catabolize carbohydrate aerobically for energy (less lactic acid) Due to an increased oxidative capacity of the mitochondria and increased glycogen storage Greater fat use during submaximal exercise, less reliance on muscle glycogen and blood glucose Lipids: • Increases the ability to oxidize long-chain fatty acids • Improves the uptake of FFAs • Increases muscle capillaries and the size and number of muscle mitochondria• Protein: One must account for increased protein breakdown during exercise and the augmented protein synthesis in recovery. American diet provides a heartily sufficient reserve
Influences of Diet The following diets are counterproductive for weight control, exercise performance, optimal nutrition, and good health: • Starvation diets • Low-carbohydrate, high-fat diets • Low-carbohydrate, high-protein diets