Lecture 2 exercise metabolism

Overview

Aerobic Exercise and Oxygen Consumption

Substrate Utilization– Respiratory Exchange Ratio

Anaerobic Exercise and the Lactate Threshold– Bicarbonate Buffer System– Definition, Possible Causes

Causes of Fatigue

Glycogen Depletion– Exercise Intensity– CHO intake

Aerobic Exercise and O2 Consumption

O2

CO2

Maximal Duration of Energy System

30 sec

1 min

3 min

5 min

2-3 hr

% C

on

trib

uti

on

ATP-PC

Glycolysis

Oxidative

10 sec

Aerobic Exercise and O2 Consumption

Oxidative metabolism of CHO and FAT requires O2, produces CO2

Indirect calorimetry - calculated energy expenditure based on gas exchange (VO2 and VCO2)

Must be primarily aerobic to be accurate– Anaerobic metabolism results in excess CO2 release

from buffer systems

Difference between inspired and expired air

Aerobic Exercise and O2 Consumption

Energy Expenditure (kcals)

Fitness Level

Contribution of CHO

Contribution of FAT

Energy Expenditure

Fitness Level: VO2max

Maximal Oxygen Uptake

Measure of Aerobic Fitness

Graded Exercise Test

Maximal Effort

VO2max= Inspired O2 – Expired O2

Bruce Protocol

Stage Speed Incline

1 1.7 10%

2 2.5 12%

3 3.4 14%

4 4.2 16%

5 5.0 18%

6 5.5 20%

VO2max Data

Substrate Utilization

Primary fuel source is CHO and Fat.

Protein can serve as a secondary fuel source.

Fat requires more O2 than CHO

Relative Contribution determined by the Respiratory Exchange Ratio (RER)

Respiratory Exchange Ratio (RER)

Non-invasive technique to determine relative Metabolic Contribution of Carbohydrate and Fat.

RER =VCO2

VO2

Also called Respiratory Quotient (RQ) during Steady State Exercise.

1.0 = 100% CHO, 0.7 = 100% Fat

CHO vs. FAT

6 O2 + C6H12O6 6 CO2 + 6 H2O + 32 ATPCHO (Glucose = C6H12O6):

23 O2 + C16H32O2 16 CO2 + 16 H2O + 106 ATPFAT (Palmitic Acid = C16H32O2):

Amount of O2 required is proportional to amount of C in the substrate!

RER = VCO2/VO2 = 6/6 = 1.0

RER = VCO2/VO2 = 16/23 = 0.7

Crossover Effect

% of max

% U

tiliz

ati

on

CHO

Fat35-40% of VO2 max

Oxygen Consumption Limitations

Oxygen Deficit– Beginning of Exercise, Exercise Transitions

Oxygen Debt/EPOC– End of Exercise

Lactate Production– High Intensity Exercise

Exercise Transition

Stage 1 Stage 2

Oxygen Deficit

Oxy

gen

Co

nsu

mp

tio

n

Stop

Predicted

O2 Deficit

Rest TransitionO

xyg

en C

on

sum

pti

on

Start

Actual

O2 Debt or EPOC

Possible Explanations for EPOC

Reform ATP, PC, and replace tissue O2 stores.

Removal of Lactic Acid [to liver (Cori Cycle) or Oxidation]

2Lactate (C3H6O3) + energy (from 16 ATP) glucose (C6H12O6)

2Lactate (C3H6O3) + 6O2 6CO2 + 6H2O + 619Kcal

Removal of excess CO2

Body Temp. and Catecholamines

Why is Lactate Produced during aerobic exercise?

Glycolysis

NADH

Mitochondria

Hydrogen Shuttle

Pyruvate

Lactate

Failure

Lactic Acid

Metabolic by-product of Anaerobic Glycolysis.

Immediately hydrolyzed into Lactate and H+

(acid)

Acid portion is removed from active tissue and buffered in the blood (bicarbonate system).

Lactate can be reformed into glucose in the Liver via Cori Cycle (gluconeogenesis).

Bicarbonate System

CO2+H2OH+

From Lactic Acid

HCO3+

Bicarbonate

H2CO3

Carbonic Acid

Lactate Threshold

Lactic acid accumulates with prolonged, high-intensity exercise

Lactate Threshold is the systematic rise in blood lactate concentration– Production exceeds clearance

Often used as a measure of aerobic fitness level

Lactate Threshold

LT

Exercise Intensity

Blo

od

Lac

tate

Does Lactic Acid Cause Fatigue?

• No, lactic acid DOES NOT directly cause fatigue!• Acidosis (H+) causes fatigue

• Inhibits PFK (rate limiting enzyme) and energy production

• Inhibits actin-myosin cross bridges for muscle contraction

• Benefits of Lactic Acid:• Maintains cytosolic redox potential• Can be converted to glucose and used for

energy production (Cori Cycle)

Cytosolic Redox Potential

Lactic Acid

Pyruvic Acid

NADH+H+

NAD+

Lactate Dehydrogenase

Pyruvic Acid accepts H+; is reduced by NADH forming a molecule of lactic acid.

C3H4O3 + NADH + H+ → C3H6O3 + NAD+

(Pyruvic Acid) (Lactic Acid)

Causes of Fatigue

Energy System Failure– PC Depletion– Glycogen Depletion

Metabolic By-Products – Pi (inorganic phosphate)– Heat and Muscle Temperature– Acidosis (H+)

Neuromuscular Fatigue– Peripheral (neural transmission) – Central (CNS)

QUICK CHECK

When _________ runs out, endurance exercise

simply can’t continue……

A. Steam

B. Muscle glycogen

C. The trail

….. unless ______ is ingested.

A. Really strong coffee

B. Air

C. Carbohydrates

SUBSTRATE USE IN PROLONGED EXERCISE

Coggan and Coyle, 1991

Fat: 100,000 kcals

40 kcals400

kcals

Liver glycogen: 200 kcals

Glycogen Depletion

Muscle Glycogen used for energy production (glycolysis, oxidative phosphorylation)

Depletion selective within muscle fiber

: type I to type II (intensity low to high)

Glycogen depletion does not directly cause fatigue

Glycogen Depletion and Exercise Intensity

CHO and Glycogen Storage

CHO and Glycogen Storage

CHO Loading

CHO during Exercise

Delays fatigue by:– Maintaining blood glucose levels (especially

important for prolonged exercise)– “Sparing” glycogen stores– Glycogen synthesis during low-intensity

exercise

6-8% CHO solution is ideal

~16g CHO/hour