EX PHYS: interaction of substrates

Interactions of Substrates

•Substrate used depends on availability, amount of energy produced by substrate, intensity and duration of activity

•After meal, substrate most available preferentially metabolised; same if diet is high in substrate (i.e. high carbohydrate or high fat diet)

•2% of substrate use protein in exercise less than 60 min; 3-5 hours, 5-15%

•Anaerobic: ATP, PC, glucose (not protein or lipids)

•Aerobic: primarily carb and triglycerides

Exercise intensity: triglyceride or carbohydrate metabolism

•At rest, 33% of ATP is derived from carbs & 66% from fat

•As exercise intensity increases, % of ATP from carbs increases

•Maximal exercise intensity: 100% carbs as anaerobic metabolism becomes predominant source of energy at high intensity

•Factors resulting in this switch

•Carbohydrate metabolism produces more energy per liter of oxygen

•More fast-twitch muscle fibers are recruited, which are glycolytic

•Hormonal changes: increase in epinephrine stimulates glycolytic enzymes

•Increased lactate from glycolysis inhibit triglyceride metabolism by reducing triglyceride availability

Exercise duration: triglyceride or carbohydrate metabolism

•Even if same intensity is maintained, gradual shift from carb to fat metabolism in low-intensity, long-duration activity (e.g. jogging ≥30 min)

•Hormonal response (epinephrine, norepi, glucagon) - increases activity of lipases to breakdown triglycerides

•Insulin inhibits hormone-sensitive lipase, decrease free fatty acids and transports glucose into skeletal muscle; ingesting high-carb meal or drink therefore decreases triglyceride metabolism and increases carb metabolism

•Without consuming carbohydrates, decrease in insulin with exercise will cause shift towards lipid metabolism

•Long-duration, low-intensity activity >60 min (marathon) can cause depletion of glycogen stores & fatigue ("hitting the wall") triggers increase in fat metabolism

•Before 60 min, CHO drinks not likely to improve performance

Lactate Threshold

•Onset of blood lactate accumulation (OBLA): exercise intensity at which blood lactic acid concentration >4mM

•Lactate threshold: Exercise intensity at which blood lactic acid exceeds resting concentration

•Untrained: occurs at 50% to 60% of max oxygen consumption

•Endurance trained: 65% to 80% of max oxygen consumption (thus can exercise at higher intensity before reaching threshold)

•Lactate not cause acidity; acidity comes from ATP breakdown, which increases H+ concentrations; thus acidity itself hurts performance

•As lactate threshold increases, so does endurance performance

Aerobic Adaptations to Exercise

•Aerobic enzyme increase

•Often a biproduct of increased mitochondria & increased mitochondrial size

•Increases less clear for short-term sprints

•Increased substrate availability

•Increased intramuscular glycogen stores for aerobic and anaerobic use

•Increased intramuscular triglyceride stores for endurance training

•Improved substrate use during exercise (increased mitochondrial enzymes, increased capillary # and density, improved blood oxygen transport, higher cardiac output, faster removal of lactate)

•At a given submaximal intensity, greater reliance on lipid metabolism as compared to untrained individuals, thereby sparing glycogen and limiting fatigue at that pace

Adaptations to Aerobic Exercise

•Lactate threshold occurs at a higher percentage of maximal oxygen consumption with training

•Allows maintenance of higher intensity before switching to anaerobic metabolism (as compared to untrained)

•Due to increased ability to metabolize lipid, increased enzymes of the Krebs cycle and ETC, and increased capillary density and number

•Aerobic Metabolism at Its Best

•Increased mitochondrial density & volume

•Increased blood supply (increased cardiac output, increased capillaries around muscle fibers to transfer oxygen, increased myoglobin within muscle fibers)

Metabolic Recovery After Exercise

•After exercise, heart rate and breathing still elevated to enable recovery:

•Intramuscular PC stores increased using ATP from aerobic metabolism, blood acidity reduced

•Postexercise Oxygen Consumption

•Oxygen debt: oxygen taken in above resting values after exercise

•Steady-state: all energy needed is provided by aerobic metabolism

•Oxygen deficit: difference between amount of oxygen actually consumed during exercise and what would be consumed if energy demands met solely through aerobic metabolism (indirect measure of anaerobic energy)

Oxygen Debt Phases

•Rapid phase: 2-3 min

•Majority intramuscular PC resynthesized

•Slow phase: several hours

•Aerobic metabolism of intramuscular and blood lactate; 70% of the lactate aerobically metabolized, 20% synthesize glucose, and 10% synthesize gluconeogenic amino acids

•Restore muscle and blood oxygen stores

•Increased metabolic rate due to elevated hormones, body temperature, heart rate, breathing rate

•Excess Postoxygen Consumption (EPOC): improved description of oxygen debt as representing the energy "borrowed" from anaerobic metabolism to perform exercise

•Higher with anaerobic and intense exercise due to anaerobic metabolism, temperature, and hormones

causes of EPOC

- restore intramuscualr PC

- eoevated HR and ventalation effects

- aerobically metabolize lactate

- resynthesize glucose and glycogen from lactate

- restore blood and myoglobin oxygen

- effects of increased body temperature

- hormonal effects

Maximizing Recovery

•Active recovery: light-to-moderate aerobic exercise after exercise; decreases blood lactate faster than no recovery (passive recovery)

•Likely due to use of lactate for aerobic metabolism during recovery

•Cycling 30% to 45% maximal oxygen consumption, running and 55% to 60%; must be below lactate threshold

•Passive recovery results in higher muscle glycogen than active, likely due to use of lactate to resynthesize glycogen (instead of for energy needs of active recovery)

•No difference in lactate recovery in untrained vs trained, although individuals with higher maximal oxygen consumption may display faster decreases after some types of resistance exercise

Measuring Energy Production

•Direct Calorimetry: Measuring heat production to determine metabolic rate

•Measured in kilocalories (kcal) also known as Calories: heat required to raise temperature of 1g water by 1°C

•Calorimeter: airtight chamber surrounded by water jacket; heat from person increases temperature of water jacket, allowing calculation of energy expenditure

•Indirect Calorimetry

•Using oxygen utilized, CO2 produced, & their ratio to calculate metabolic rate; also provides insight into amount of carb vs fat being burned

Oxygen Consumption and RER

•Oxygen consumption (VO2) expressed as:

•L·min−1: don't carry body weight (rowing, cycling)

•mL·kg−1·min−1: running or cycling uphill w. body weight

•Respiratory exchange ratio (RER): ratio of oxygen used and CO2 produced during metabolism

•RER for carbohydrates: 6 CO2/6 O2 or 1, as 6O2 needed & 7CO2 made metabolizing glucose (C6H12O6):

6O2 + C6H12O6 --> 6CO2 + 6H2O + 30ATP

•RER for triglyceride (C16H32O2): 0.70 (16 CO2/23 O2)

23O2 + C16H32O2 --> 16CO2 + 16H2O + 129ATP

•RER indicates % carbs and fat; e.g. RER of 0.85 --> ~50% carb, 50% triglyceride, & approximately 4.86 kcal·LO2−1

•Less accurate for energy from anaerobic sources

•RER can exceed 1 at high intensities, not accurate for energy expenditure at that point

resting energy use

•Basal metabolic rate (BMR): metabolic rate in supine position, 12-18 hrs after meal, immediately after waking up in thermoneutral environment

•Accounts for 60-75% total calories per day

•Resting metabolic rate (RMR): metabolic rate 4 hrs after light meal, with 30-60 mins of resting quietly

•Factors: age, sex, body temperature, stress, body surface area

Endurance Event Metabolic Interactions

•Endurance events may use a high percentage of energy from anaerobic metabolism during periods of higher intensity (hills) or if the event is shorter duration (200m run)

•Crossover from anaerobic to aerobic metabolism takes place at about 15-30 seconds, and percent of energy from aerobic continues to increase over time

sources of energy

Carbohydrates:

•Blood glucose

•Muscle glycogen

Fats:

•Intramuscular Triglycerides (TRIG)

•Triglycerides in fat cells

•

TRIG + 3H2O -->(lipase) Glycerol + 3 Fatty acids

Regulation of Metabolism

•Factors increasing metabolic rate

-Increased temperature

-Small decreases in pH

-Decreases in ATP

-Increases in ADP and Pi

*note excesses of any of these will shut down metabolism. For instance, too much heat breaks down/denatures proteins/enzymes which would slow or shut down metabolism

Regulation of Carbohydrate Use

Blood glucose

•Increased ATP demand

•Increased blood glucose (insulin)

•Low muscle glycogen

Muscle Glycogen

•Increase ATP demand

•Low blood glucose (glucagon)

•Epinephrine

Regulation of Fat Metabolism - lipase activation/inhibition

•Activation

-Epinephrine

-Norepinephrine

-Glucagon

-Growth hormone

inhibition:

- lactate, ketones, insulin