Substrate Utilization

Metabolism Metabolism is the sum of all chemical reactions with living cells to provide energy for vital processes. Aerobic Exercise At rest, 33% of the body's energy comes from carbohydrates, or glycogen, stored within the muscles and liver. 66% comes from fat. During aerobic work, 50-60% of the energy comes from fats Primarily carbohydrates are used during the first several minutes of exercise For an average fit person, it takes 20 to 30 minutes of continuous aerobic activity to burn 50% fat and 50% carbohydrate There is approximately a 7 fold increase of fat mobilization after 1 hour of exercise Proteins contribute less than 2% of the substrates used during exercise of less than 1 hour. Slightly more proteins are utilized as a fuel source during prolonged exercise. During the final moments of exercise lasting 3 to 5 hours, protein utilization may reach 5-15% of the fuel supply (Berg A & Keul J 1980; Cerretelli P 1977; Hood D & Terjung R 1990; Lemon P & Mullin F 1980; Lemon P & Nagle 1980) Protein can supply up to 10% of total energy substrate utilization during prolonged intense exercise if glycogen stores and energy intake is inadequate (Brooks, 1987) Training The more fit an individual, the more they utilize fats over carbohydrates Reaches steady state sooner, and stays there longer Sympathetic stimulation mobilizes FFA increased percentage of FFA uptake oxidized greater contribution from intramuscular triglyceride stores Lipolytic response to catecholamines is enhanced in trained subjects in both resting and exercised states Beta Adrenergic stimulation is responsible for much of the increase in lipolytic rate during exercise Basal lipolytic rate is primarily regulated through alpha adrenergic stimulation Carbohydrate Consumption On a low carbohydrate diet, you burn a higher proportion from fat Endurance can be reduced up to 50% until body adapts Adaptation to a low carbohydrate diet is possible if calories from protein and fat are sufficient If calories are not sufficient, lean tissue (muscle) is utilized by gluconeogenesis (conversion of protein to glucose) Exercise Duration & Intensity Low intensity, high duration aerobics Low intense exercise (<30% VO2 max) relies primarily on fat whereas high intense exercise (>70% VO2 max) primarily utilized carbohydrate. Higher proportion of fat is expended (not necessarily more fat) Lower intense submaximal exercise utilizes proportionally less carbohydrates During low intense exercise prolonged exercise (ie greater than 30 minutes), a gradual shift from carbohydrate to fat metabolism occurs (Ball-Burnett MH, Green H & Houston M, 1991; Gollnick & Saltin B, 1988; Ladu M, Kapsas H & Palmer W, 1991; Powers S, Riley W, & Howley 1980) High intensity, low duration aerobics More calories burned in less time More carbohydrates, or glycogen utilized Lactate threshold Sedentary: 70-75% max heart rate Trained: 80-90% max heart rate or higher Intense or prolonged exercise can rapidly deplete muscle glycogen Carbohydrates are used as a fuel source when more type II muscle fibers are recruited. Type II muscle fibers have an abundance of glycolytic enzymes but few mitochondreal and lipolytic enzymes. Increased blood levels of epiniphrine also increase the metabolism of carbohydrates. High levels of epinephrine increase muscle glycogen breakdown, glycolysis and lactate production (Brooks G & Mercier J 1994). Greater lactate production inhibits fat metabolism (Turcotte L, et al. 1995) More fat metabolized hours intense exercise (Mulla, et al., 2000) (Phelain, et al., 1997) Anaerobic Exercise Weight training, plyometrics, sprinting, or high intense interval training "It is known that the energy needs for sustaining maximal exercise of very short duration are largely met by the creatine phosphate breakdown such that its concentration decreases to almost zero at the end of maximal exercise leading to exhaustion. An almost complete creatine phosphate recovery is normally observed within rest periods lasting about 4 minutes following repeated maximal exercises of short duration." (Tremblay, et al., 1994) Primarily carbohydrates utilized (after limited ATP and CP stores) Fat is utilized many hours after anaerobic exercise See weight training and HIIT studies