Substrate Utilization

Metabolism | Aerobics | Duration and Intensity | Training | Diet | Anaerobics


Metabolism is the sum of all chemical reactions in 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 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)

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 at low intensities (not necessarily more fat)
    • Total rate of fat oxidation is greatest at higher intensities, below lactate threshold.
      • At 20% of VO2 max, approximately 60% of the energy comes from fat.
      • At 50% of VO2 max, only about 40% of the energy comes from fat.
      • Yet, absolute amount of fat metabolism is 33% higher during exercise at 50% VO2 max since the total energy expenditure is 250% greater than exercising at only 20% of VO2 max.
    • Untrained subjects
      • Greatest absolute fat metabolism during exercise occurs at 50% of VO2 max
      • Body weight: 89 kg; VO2 max: 4.0 L/min; lactate threshold: 60% of VO2 max.
    • 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 mitochondrial and lipolytic enzymes.
    • Increased blood levels of epinephrine 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)
      • lactic acid is an essential hydrogen ion acceptor in glycolytic metabolism.
    • More fat metabolized hours intense exercise (Mulla, et al., 2000) (Phelain, et al., 1997)


  • 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

Dietary Consumption

  • On a low carbohydrate diet, you burn a higher proportion of energy 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)
  • On a high carbohydrate diet, you burn a higher proportion of energy from carbohydrates

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)
  • All substrates are being utilized by the body at any given time, but the magnitudes can dramatically vary depending on the metabolic demands (eg: intensity, duration), fitness level, and available substrates
    • ATP and Creatine Phosphate stores are typically taxed after several seconds
      • Can be prolonged with intermittent activity such as swimming
    • Lactacid energy system are typically taxed around 45 seconds
      • Can be prolonged with intermittent activity such as swimming
      • Lactate / lactic acid is circulated and utilized as a fuel substrate from adjacent muscles
      • High lactate levels compromise aerobic performance (Simoes, Campbell, & Kokubun, 1998)
    • Primarily carbohydrates utilized (after limited ATP and CP stores)
    • More fat is utilized as a fuel substrate many hours after anaerobic exercise

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