Energy Metabolism in Exercise

Major Fuels for Energy Metabolism

Glucose – from bloodstream and stored glycogen

FAs from bloodstream and stored TGs

The relative contributions of each depends on:

Intensity and duration of exercise

Muscle glycogen breakdown is most rapid during the first 5-10min of intense exercise (muscle relies on stored energy) Þ supplies glucose to working muscle and prevents hypoglycemia

As exercise intensity and duration of exercise Ý , rate of gluconeogenesis from Ala/lactate/pyruvate/glycerol Ý

Alanine can be broken-down by muscle protein or synthesized from BCAA (Val/Leu/Isoleu) along with pyruvate (C-source) Þ Ala then goes to liver and undergoes gluconeogenesis (helps prevent hypoglycemia)

Utilization of plasma glucose (from liver glycogen and gluconeogenesis), and FFA’s (from adipose TG’s) contributes up to 80% of total energy during prolonged effort (>1hr)

If exercise intensity is >75% maximal effort, FFA contribution decreases and CHO contribution increases

As muscle contraction increases, migration of GLUT4 to cell surface occurs

On the other hand, if exercise is prolonged at moderate intensity, as glucose reserves are depleted, there is increased reliance on fat

Oxygen supply to muscle is a major determinant of FA utilization since it must undergo b -oxidation

Diet consumed before the exercise

Diet rich in fat and protein Þ shifts fuel use toward fat utilization

Diet rich in CHO’s Þ increase conc. of glycogen in muscle Þ increase the maximal duration of sustained exercise

The state of physical training

Increased endurance capacity in trained state due to adaptations in muscle fibers and increased cardiovascular reserve Þ results in improved capacity to deliver and consume oxygen aerobically

Regular exercise increases muscle capillary density while training results in increased mito content and respiratory capacity of muscle fibers (burn more fat and spare more glycogen) Þ also Ý VO₂max, SV, CO and ß HR

exercise of same intensity does NOT alter the oxygen requirement; rather, the increased respir capacity of muscle shifts metabolism toward a greater utilization of FA’s for energy

results in less reliance on muscle glycogen, blood glucose and reduced lactate production
produces a glycogen sparing effect associated with improved endurance performance

Changes in Metabolism during Exercise

Blood Flow During Exercise

Increase O₂ uptake, increase blood flow, increase glucose uptake
Huge increases in amount of blood flow to the muscles along with heart, skin, etc (blood flow to brain remains constant)
Vasodilation of working muscles – humoral mechanism (CO₂, lactate, K, H build up) and symp adrenergic stimulation
Vasoconstriction of inactive tissue – sympathetic adrenergic constriction

Hormonal Response to Endurance Exercise (aid in preventing hypoglycemia)

Plasma Insulin falls and glucagon rises
Cortisol and Epinephrine are released from the adrenal gland
Rise in serum growth hormone
Stimulates adipose tissue lipolysis and glycogen breakdown in muscle and liver

Muscle Fiber Types and Adaptation to Exercise

Type I – slow oxidative – RED (due to increased myoglobin content)

slow contracting, fatigue resistance, utilize FA’s, have high #’s of mito

Type IIA – fast twitch oxidative – RED

greater speed of contraction, fewer mito

Type IIB – fast twitch glycolytic – WHITE

fastest speed and power of contraction, low #’s of mito, high anaerobic enzyme content, higher glycogen content

Wide spectrum of fiber types within the same muscle

Majority of people are 50:50 with respect to conc of type I to type II, whereas endurance trainers have a higher % of type II

result from natural selection rather than a training phenomenon Þ no evidence has shown conversion between types

however, there is conversion from type IIB to IIA in athletes adapted to strenuous endurance training (increased # of mitochondria also)

VO₂ Max

Determines the maximum amount of aerobic exercise we can do; the % of VO₂ max increases with exercise intensity

70% of VO₂ max is where the lactate threshold occurs Þ where we start seeing increased amounts of lactate in blood

Once 100% VO₂ max is reached, lactate production is directly proportional to exercise intensity

What effects our VO₂ max? – genetics, age, fiber type

With training, can increase VO₂ max 15 to 20% by increases mito # and blood volume (increases amount of O₂ consumed)