Sources of Energy (ATP) During Exercise

By at December 11, 2010 | 3:04 pm | Print

Sources of Energy (ATP) During Exercise

Sources of Energy (ATP) During Exercise written by Josh Duncan

Athletes are always looking to increase performance and gain the competitive edge against other competitors whether it is by enhanced training capacity, improved exercise performance, reduced injury risk, enhanced immune system or just enhanced personal strength or endurance.  Research has shown that proper athletic training along with healthy nutrition practices can help athletes maximize training and yield results in performance (American Dietetic Association (ADA), 2009).   Energy and macronutrients are especially important in times of high physical activity to maintain body weight, replenish glycogen stores, spare protein deprivation and aid in muscle and tissue repair (ADA, 2009).  The importance and functions of carbohydrates, proteins and fats will be discussed along with how they are used to obtain optimal recovery of health throughout exercise.

An overall balance between energy expenditure and energy intake must be achieved in order for athletes to maintain strength, endurance, and performance (Quatromoni, 2008).   Adenosine triphosphate (ATP)  is the energy currency of the body in which power output and frequency of muscle contractions determines the demand of how much ATP is needed (Saris et al., 2003).  There are three different energy systems capable of producing ATP via anaerobic and aerobic pathways.

Of the three energy metabolism systems, the first energy system is the phosphagen system which is used for events lasting no longer than a few seconds and of maximum intensities (Powers & Howley, 2009).   The importance of the phosphagen system in athletics can be appreciated by activities of high intensity and short duration like the clean and jerk in weight lifting or the fast break in basketball. When the demand for energy persists and the ATP and creatine phosphate stores are depleted the glycolysis system is activated (Dunford, 2006).  The second energy system is glycolysis which involves the breakdown of glucose or glycogen to create ATP for energy.  Like the phosphagen system, glycolysis is also an anaerobic system and supports athletic events lasting 1-3 minutes, though neither system can sustain the amount of energy needed to continue muscle contractions for greater than 3 minutes.  The third and final energy system is an aerobic system thus requiring oxygen to continue to provide energy for muscle work.  This oxidative pathway fuels events lasting longer than 2-3 minutes and is the only pathway that can produce large amounts of ATP over time via the Krebs cycle and the electron transport chain (Powers & Howley, 2009).  A few examples of events in which the primary fuel comes from oxidative pathways includes a 1500 meter run, marathon, half-marathon and endurance cycling (ADA, 2009).

All three energy systems work together and no one pathway is ever relied on exclusively. The intensity, duration, type of activity, frequency, sex, and fitness level of an athlete determines when the transition from aerobic to anaerobic pathways will occur (Saris et al., 2003). Nutrient intake, energy stores, and a balanced diet also play an important role in which fuel is used and by what pathway it is created (Clark, 2003). 

The regulation of fuel selection during exercise is under complex control and is dependent upon diet, intensity, and duration (Powers & Howley, 2009). As intensities increase, carbohydrates are used as the major fuel source, but as duration continues there is a shift from carbohydrate to fat metabolism (ADA, 2009).  Excess intake of carbohydrates, fat, or protein can be burned aerobically or stored as fats (Dunford, 2006).All athletes need adequate carbohydrate intake because it is the energy “powerhouse” that fuels muscles as well as limits protein deprivation (Clark, 2003).  Carbohydrates are stored in the muscle and liver as glycogen and a small amount is circulating in the blood as glucose (Kundrat, 2005). Carbohydrate is a primary fuel during physical activity, thus adequate carbohydrate stores are critical for optimum athletic performance and recovery from exercise. (Fink, Burgoon, & Mikesky, 2009).  Low blood glucose & low muscle and/or liver glycogen concentrations can lead to fatigue during exercise thus, building up and maintaining glycogen store during training requires an adequate intake of carbohydrates and energy (Clark, 2003).  At a high intensity (70% of capacity) carbohydrate is the preferred substrate (ADA, 2009). Low levels of carbohydrate can lead to fatigue especially when exercise continues to the point that compromises liver and muscle glycogen stores (Powers & Howley, 2009).  

To perform optimally, adequate amount of carbohydrates need to be supplied to the body prior to exercise. The source, quantity, and timing of the carbohydrate can either lead to a high-energy performance exercise or a feeling of staleness and fatigue.  The pre-exercise meal provides energy when the athletes exercises hard for one hour or longer, prevents hunger and supplies blood glucose for energy for exercising muscles.  Consuming carbohydrates during exercise has been shown to help delay fatigue in short-duration and long- duration activities (Fink, Burgoon & Mikesky, 2009). During extensive exercise that lasts longer than 60-90 minutes, fluid and energy should be replaced to help to provide enough energy to maintain blood glucose levels and aid in sweat loss (Clark, 2003).  Carbohydrate intake during exercise has also been shown to improve endurance performance as well as in stop-and-go-sports (Dunford, 2006). It is recommended that athletes take in 30-60 grams of carbohydrates every hour as food and/or liquid (Kundrat, 2005). Consuming carbohydrates (1.0-1.5 g/kg per hour for 3-4 hours) immediately after exercise starting within the first 45 minutes, increases muscle glycogen synthesis (Stout, 2007).  Once the use of carbohydrates during exercise is understood we can look into how proteins are used as a source of fuel.

Protein is essential for many functions in the body like providing essential amino acids to the cells, develop new tissues for growth and repair, make important enzymes, hormones & antibodies, keep the cells in fluid balance, transport substances in the blood and provide small amount of energy for the cell (Kundrat, 2005). Protein is constantly being broken down, transformed, or rebuilt in the body. The consumption of adequate energy particularly carbohydrates is important in the sparing of amino acids to be used for protein synthesis not oxidized to be used as fuel for energy (ADA, 2009). 

The RDA (Recommended Dietary Allowance) for normal healthy individuals for protein is .8 g/kg/day.  Strength trained athletes may need additional amino acids to support muscle growth especially in the early phase of training (3-6 months) when the most significant gains in muscle size occurs (Powers & Howley, 2009). Endurance athletes need higher protein intake under certain circumstances like very high training volumes, when carbohydrate availability is limited and energy from amino acids is being used (Dunford, 2006).  Training intensity and duration increases protein requirements and utilization. Increasing duration of the exercise begins to deplete glycogen reserves in the liver and muscle, increasing protein utilization.  Unlike, endurance training, single session of resistance exercise, regardless of the length or intensity of the workout, do not appear to increase the use of protein during the workout itself. However, amino acid uptake after the resistance training does increase, indicating that the amino acids are being used for muscle repair and construction rather than for energy (Fink, Burgoon & Mikesky, 2009). 

Some studies have suggested that consuming protein with carbohydrate during exercise improves endurance performance but other studies have reported no benefits. Additional research is needed to address protein intake during exercise but it can be concluded that no established mechanism by which protein intake during exercise could improve performance (ADA, 2009).  Protein is also important nutrient for recovery to aid in the muscle recover process and helps promote skeletal muscle growth and/or report to tissue damage (Stout, 2007). After exercise, protein breakdown decreases while protein synthesis increases resulting in an anabolic state. Eating protein within 3 hours after exercise has been shown to stimulate muscle protein synthesis due to the increased blood flow and hormones (Fink, Burgoon, & Mikesky, 2009). Along with carbohydrates and proteins, there is another source to where the body gets its fuel, fat.

Fat is a major energy source, which helps meet daily energy demands and can be stored for extra energy to be used when the body needs extra fuel (Kundrat, 2005). They are important components of plant, animal and microbial cell membrane that aid in the delivery and absorption of fat-soluble vitamins like A, D, E & K.  Fat is stored in the body in the form of triglyceride which can be broken down during exercise to produce a fuel source of energy (Power & Howley, 2009).  In general, fat intake varies among athletes in different sports.  Endurance athletes tend to have a lower fat and higher carbohydrate intake than sprinters and short distance runners (Fink, Burgoon & Mikesky, 2009). Fat along with carbohydrate, is oxidized to supply energy to the exercising muscles, though the extent to which these sources contribute to energy depends on the duration and intensity of the exercise (Powers & Howley, 2009).  Fats are the primary source of energy at rest, during low to moderate intensities and in periods of recovery (Fink, Burgoon & Mikesky, 2009).   During endurance training, the body’s ability to utilize fats for energy improves which causes a shift in the crossover point enhancing the body’s ability to make more fatty acids available for work. Also, endurance training improves the working muscle’s capacity to oxidize the fats that are delivered (Dunford, 2006). Increased blood flow, improved fat mobilization into muscle cells, larger size and quantity of mitochondria, and increased number of enzymes is adaptations that help explain trained muscles utilization of fat for energy.  Recent research has not supported fat intake during exercise due to decreases in performance caused by gastrointestinal distress. (Fink, Burgoon & Mikesky, 2009). Fat intake after exercise is not as critical as carbohydrate or protein because of the body’s ample source of fat stores (ADA, 2009).

The ability of athletes to perform at peak levels can be limited to how quickly their muscles recover and repair themselves after workout.  Prolonged exercise has a traumatic effect on the body, making it necessary that adequate carbohydrates and consumed post exercise to restore muscle glycogen (Stout, 2007).  The timing and composition of the recovery meal or snack depends on the length and intensity of the exercise session and when the next intense workout will occur (ADA, 2007). Timing is essential because the muscle cells are highly insulin receptive after an exercise thus increasing the transport of glucose and protein into the muscle cell to promote glycogen and protein synthesis (Dunford, 2006).

Consumption of adequate calories of carbohydrates, protein, and fat is key for the best nutrition recovery.  A few examples of recovery meals or snacks would include: cereal with milk, an energy bar, chocolate milk, ham sandwich, pasta, or a steak.   Proper recovery nutrition has the potential to make a tremendous difference to help recover faster from workouts.  Consuming a carbohydrate- moderate meal with protein has been shown to reduce muscle damage, replenish glycogen faster, improve athletic performance, increase strength gains, and aid in fewer injuries (Dunford, 2006).  

It is important for everyone, especially competitive athletes, to know how the body makes its energy during exercise and where it gets the fuel to make that energy.  The body’s energy, or ATP, is able to be produced three ways by means of exercise: phosphagen system, glycolysis system, and the Krebs cycle.  The energy is also obtained from the macronutrients carbohydrates, proteins, and fats.  That is why it is important to replenish the body after exercise in order to rebuild muscle tissue for the next work out.

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American Dietetic Association. (2009). Position of the American Dietetic Association dietitians of Canada, and the American College of Sports Medicine: Nutrition and athletic performance. Journal of the American Dietetic Association, 109(3), 509-526.

Clark, N.  (2003). Sports Nutrition Guide Book. United States: Human Kinetics.

Dunford, M. (2006). Sports Nutrition: A Practice Manual for Professionals. United States:American Dietetic Association

Fink, H.H., Burgoon, L.A. & Mikesky, A.E. (2009). Practical Applications in Sports Nutrition.Boston: Jones & Bartlett Publishers.

Kundrat, S.  (2005). 101 Sports Nutrition Tips. United States: Coaches Choice.

Powers, S.K. &  Howley, E.T. (2009). Exercise Physiology: Theory and Application to Fitness and Performance. New York: McGraw-Hill.

Saris, H.M., Antoine, J.M., & Brouns, F. et. al. (2003). Physical performance and fitness. European Journal of Nutrition, 42 (1), 1-51.

Stout, A. (2007). Fueling and weight management strategies in sports nutrition. Journal of the American Dietetic Association, 107(9), 1475-1479.

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  1. Tony McGee, 3 years ago Reply

    Thanks for the great post. We really appreciate it.

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