1. Macronutrient Metabolism in Exercise and Training
Chapter 5
Macronutrient Metabolism in Exercise and Training

2. Fuel for Exercise
The fuel mixture that powers exercise generally depends on:
The intensity of effort
The duration of effort
The exerciser’s fitness status
The exerciser’s nutritional status

3. The Energy Spectrum of Exercise
ATP and PCr supply most of the energy for exercise.
ATP-PCr and lactic acid systems provide ½ of the energy required for intense exercise lasting 2 minutes.
Aerobic reactions provide the remainder of the required energy.

4. Aerobic Energy Transfer
Intense exercise
Intermediate in duration
5 to 10 minutes
Examples:
Middle distance running
Swimming
Basketball

5. Aerobic Energy Transfer without Lactate
Longer duration
Requires a steady energy supply
Examples:
Marathon running
Distance swimming or cycling
Jogging, hiking, or backpacking
For top performance in all-out 2-minute exercise, a person must possess a well-developed capacity for both aerobic and anaerobic metabolism.
When exercise progresses beyond several minutes, the aerobic system predominates, with oxygen uptake capacity becoming the important factor.

6. Anaerobic Energy Transfer
Supply energy for fast movements
Supply energy during increased resistance to movement
Short duration
Example:
Sprinting

7. Depicts the relative contributions of anaerobic and aerobic energy sources during various durations of maximal exercise.

8. Sources of Energy for ATP Synthesis
Sources of energy for ATP synthesis include:
Liver and muscle glycogen
Triacylglycerols within adipose tissue and active muscle
Amino acids within skeletal muscle donate carbon skeletons

10. Carbohydrate Use During Exercise
Muscle glycogen and blood glucose serve as primary fuels during intense anaerobic exercise.
Glycogen stores also play an important role in sustained high levels of aerobic exercise.
The liver releases glucose for use by active muscle as exercise progresses from low to high intensity.
Compared with fat and protein catabolism, carbohydrate remains the preferential fuel during high-intensity aerobic exercise because it rapidly supplies ATP during oxidative processes.

11. Carbohydrate Use During Exercise (cont.)
Carbohydrate availability in the metabolic mixture controls its use.
Carbohydrate intake affects its availability.
Exercise intensity impacts to what degree glucose and glycogen are used as a fuel source.

12. Intense Exercise Change in hormone release Glycogen phosphorylase
Early in activity:
Stored muscle glycogen is the primary contributor.
As duration progresses:
Blood glucose from the liver increases its contribution.
With strenuous exercise, neural–humoral factors increase hormonal output of epinephrine, norepinephrine, and glucagon and decrease insulin release.
Glycogen phosphorylase: augments glycogen breakdown (glycogenolysis) in the liver and active muscles.

14. Moderate and Prolonged Exercise
First 20 minutes
Glycogen stored in active muscles
Next 20 minutes
40-50% liver and muscle glycogen
Remainder from fat breakdown
As exercise continues
Glucose from the liver becomes major contributor
Fat use increases
During 90 minutes of strenuous exercise, blood glucose may actually decrease to hypoglycemic levels (<5 mg of glucose per 100 mL [dL] blood).

16. Glycogen Depletion Blood glucose levels fall.
Level of fatty acids in the blood increases.
Proteins provide an increased contribution to energy.
Exercise capacity progressively decreases.
The reduced power output level comes directly from the relatively slow rate of aerobic energy release from fat oxidation, which now becomes the primary energy source.
Severely lowered levels of liver and muscle glycogen during exercise induce fatigue, despite sufficient oxygen availability to muscles and almost unlimited potential energy from stored fat.
Known as “hitting the wall.”

17. Trained Muscle
Trained muscle has an augmented capacity to catabolize carbohydrate aerobically for energy.
Due to an increased oxidative capacity of the mitochondria and increased glycogen storage
Greater fat use during submaximal exercise, less reliance on muscle glycogen and blood glucose
This training adaptation represents a desirable response because it conserves the body’s limited glycogen reserves.

18. Gender Differences
Women derive a smaller proportion of energy from carbohydrate oxidation than do men during submaximal exercise at equivalent percentages of aerobic capacity.
Following aerobic exercise training, women show an exaggerated shift toward fat catabolism, whereas men do not.
This gender difference in substrate metabolism’s response to training may reflect differences in sympathetic nervous system adaptation to regular exercise (i.e., a more blunted catecholamine response for women).
The sex hormones estrogen and progesterone may affect metabolic mixture indirectly via interactions with the catecholamines or directly by augmenting lipolysis and/or constraining glycolysis.

19. Influence of Diet
A carbohydrate-deficient diet rapidly depletes muscle and liver glycogen.
Low carbohydrate levels profoundly affect both anaerobic capacity and prolonged, high-intensity aerobic exercise.
When carbohydrates are low, exercise intensity decreases to a level determined by how well the body mobilizes and oxidizes fat.
These observations pertain to both athletes and physically active individuals who modify their diets by reducing carbohydrate intake below recommended levels.

21. Influences of Diet
The following diets are counterproductive for weight control, exercise performance, optimal nutrition, and good health:
Starvation diets
Low-carbohydrate, high-fat diets
Low-carbohydrate, high-protein diets
These diets rapidly deplete muscle and liver glycogen.
A low-carbohydrate diet makes it extremely difficult, from the standpoint of energy supply, to engage in vigorous physical activity.

22. Fat as an Energy Substrate
Fat supplies about 50% of the energy requirement during light and moderate exercise.
Stored fat becomes more important during the latter stages of prolonged exercise.
During prolonged exercise, fatty acids provide almost 80% of the energy requirements.

24. Sources of Fat During Exercise
Fatty acids released from adipocytes
Delivered to muscles as FFA bound to plasma albumin
Circulating plasma triacylglycerol bound to lipoproteins as very low-density lipoproteins and chylomicrons
Triacylglycerol within the active muscle itself

25. Lipolysis Hormones activate lipase.
These hormones are secreted more during exercise.
Mobilization of FFAs from adipose tissue
Trained muscle has an increased activity of adipose tissue lipases.
The hormones epinephrine, norepinephrine, glucagon, and growth hormone activate hormone-sensitive lipase.

26. Hormones Hormones influence substrate: Availability
Mobilization from body tissue stores
Uptake at tissue site of utilization
Uptake within tissue itself
Trafficking among storage, oxidation, and/or recycling

27. Exercise Training and Fat Metabolism
Regular aerobic exercise:
Facilitates the rate of lipolysis
Increases the ability to oxidize long-chain fatty acids
Improves the uptake of FFAs
Increases muscle capillaries and the size and number of muscle mitochondria
Adaptations producing enhanced responsiveness of adipocytes to lipolysis allow the trained person to exercise at a higher absolute level of submaximal exercise before experiencing the fatiguing effects of glycogen depletion.

29. Protein Use During Exercise
Serves as an energy fuel to a much greater extent than previously thought
The amount depends upon nutritional status and the intensity of exercise training or competition.
This applies particularly to branched-chain amino acids that oxidize within skeletal muscle rather than within the liver.

30. Protein Use During Exercise (cont.)
Exercise in a carbohydrate-depleted state causes significant protein catabolism.
Protein synthesis rises markedly following both endurance- and resistance-type exercise.

31. Protein Requirements
Re-examining the current protein RDA seems justified for those who engage in heavy exercise training.
One must account for increased protein breakdown during exercise and the augmented protein synthesis in recovery.