Metabolism is the sum of all chemical reactions with living cells to provide energy for vital processes.
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)
The more fit an individual, the more they use fats over carbohydrates in the diet
Reaches steady state sooner, and stays there longer
Sympathetic stimulation mobilizes FFA
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)
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
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)
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