Exercise Metabolism

Lactate Threshold

n Despite the continual production of some ATP via anaerobic glycolysis during submaximal exercise, blood lactate levels remain relatively constant

n However, as exercise intensity increases, a “threshold” is reached in which lactate begins to accumulate in the blood

n Lactate threshold

n Onset of blood lactate accumulation (OBLA)

Illustration of Lactate Threshold

Mechanisms to Explain the Lactate Threshold

Other Mechanisms to Explain Lactate Threshold

n Failure of the mitochondrial hydrogen shuttle to keep pace with glycolysis

n Excess NADH H+ in sarcoplasm favors conversion of pyruvic acid to lactic acid

n Type of LDH

n Enzyme that converts pyruvic acid to lactic acid

n LDH in fast-twitch fibers favors formation of lactic acid

Law of Mass Action: Effect of NADH H+ Accumulation on Lactate Production

The Cori Cycle: Removal of Lactate from the blood by the liver

Relationship Between VO2max and Lactate Threshold

n Lactate threshold is often expressed as a percent of VO2max

n ~30% of VO2max is walking

n In untrained people, the lac threshold is at ~50-60% of VO2max

n In trained people, the lac threshold is higher

- competitive marathoners run at 75-80% of VO2max – still below their lac threshold

- Alberto Salazar – an elite marathoner ran at 86% of his measured VO2max

n Both VO2max and lac threshold are used to evaluate aerobic and endurance capacity, fitness

Sources of Fuel During Exercise

n Carbohydrate

n Blood glucose

n Muscle & liver glycogen

n Fat

n Plasma FFA (from adipose tissue lipolysis)

n Intramuscular triglycerides

n Protein

n Usually a small contribution to total energy production (~5%) May increase ~15% late in prolonged (at least > 1 hr) exercise

n Blood lactate

n Gluconeogenesis via the Cori cycle

More on Lipids

n Lipids include:

n Triglycerides

n Phospholipids

n Cholesterol

n Lipoproteins (mix of tryglyceride, phospholipid, cholesterol, and protein)

n VLD LP (50% triglyceride, 23% cholesterol)

n LD LP (20% trigly, 47% cholesterol)

n HD LP (8% trigly, 30% cholesterol, 30% pro)

More on Lipids

n Lipoprotein lipase breaks down lipoproteins & makes FFA available

n Enzyme found in capillary walls, esp. in adipose & muscle tissue

n Second kind of lipoprotein lipase (L-HSL) found only in muscle, active during exercise (when insulin/glucagon ratio falls)

n L-HSL activity is increased by training – fat available quicker

Fuel Selection During Exercise – How do you know what is used?

n Fuels vary in the amount of O2 used and CO2 produced during their metabolism

n Respiratory exchange ratio (RER): VCO2/VO2

n From the RER, the % fat and CHO used for metabolism can be estimated

n Resting RER is ~0.75 – 0.80

Caveat to the use of RER to estimate % fat or CHO metabolism

n RER = respiratory exchange ratio – at the mouth

n RQ = respiratory quotient – the VCO2/VO2 at the muscle

n RER does not necessarily equal the RQ

n During steady-state in humans RER may equal RQ

n RER will not equal RQ during non-steady state or hyperventilation (measuring equipment makes many people hyperventilate at rest)

Exercise Intensity and Fuel Selection

n Low-intensity exercise (<30% VO2max)

n Fats are primary fuel

n High-intensity exercise (>70% VO2max)

n CHO are primary fuel

n “Crossover” concept

n Describes the shift from fat to CHO metabolism as exercise intensity increases

n Due to:

n Recruitment of fast muscle fibers

n Increasing blood levels of epinephrine (leads to cAMP and increased glycogenolysis)

n Inhibition of lipolysis by high blood lactate concentration

Illustration of the “Crossover” Concept

Exercise Duration and Fuel Selection

n During prolonged exercise, CHO metabolism gradually decreases while fat metabolism gradually increases

n Increased rate of lipolysis

n Breakdown of triglycerides into glycerol and free fatty acids (FFA)

n Stimulated by rising blood levels of epinephrine, norep, and glucagon

n Stimulated by decreasing blood levels of insulin

Shift From CHO to Fat Metabolism During Prolonged, Moderate Intensity Exercise

Effect of Exercise Duration on Muscle Fuel Source

Control of Glycogenolysis

n Breakdown of muscle glycogen is under dual control

n Epinephrine-cyclic AMP

n Ca++-calmodulin

n Delivery of glucose parallels activation of muscle contraction

n Glycogenolysis can still occur in presence of b-blocking agent

Control of Glycogenolysis

Interaction of Fat and CHO Metabolism During Exercise

n “Fats burn in the flame of carbohydrates”

n Glycogen is depleted during prolonged high-intensity exercise

n Reduced rate of glycolysis and production of pyruvate

n Reduced Krebs cycle intermediates

n Free Fatty Acids become Acetyl-CoA which is oxidized in the Krebs Cycle

n Reduced fat oxidation

Effect of Lactic Acid on FFA Mobilization – trapping of fatty acids inside lipocytes