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A research study investigating the metabolic profile of children and young adults during and after moderate and vigorous exercise. The study aims to describe oxygen consumption pre-, during, and post-exercise, examine differences in Excess Post-Exercise Oxygen Consumption (EPOC) between moderate and vigorous intensity exercise, and explore differences in Respiratory Exchange Ratio (RER) between children and young adults. Hypotheses include differences in EPOC volume and duration, as well as RER values.
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By Justin Ross Bland A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Kinesiology 2012
By Justin Ross Bland INTRODUCTION: The majority of available literature on excess post-exercise oxygen consumption (EPOC) focuses on the adult population. No study has quantified EPOC in children after exercise lasting more just a few minutes. Previous research has shown fat oxidation, measured by respiratory exchange ratio (RER), to be greater in children than in adults at rest and during exercise. Few studies have investigated substrate utilization post- exercise in children. PURPOSE: To examine young adult-child differences in EPOC and substrate utilization following moderate and vigorous intensity exercise performed on the cycle ergometer. METHODS: 19 children (7 to 9 years old) and 22 young adults (20 to 23 years old) visited our laboratory on three separate occasions and completed an exercise trial each time: VO 2 max test, moderate exercise (MOD), and vigorous exercise (VIG). Maximum power output (MaxPO) during the VO 2 max test was used to determine workload for MOD (35% MaxPO) and VIG (70% MaxPO) exercise. MOD and VIG trials were randomized and counterbalanced. Participants rested for 30 minutes (min) prior to exercise; the last 20 min were used for baseline VO 2 measures. Tests were 2-min square-wave intervals lasting 1 hour for the MOD trial and 30 min for the VIG trial. Expired gases were captured 10 min prior to the cessation of exercise and for 20 minutes post-exercise. EPOC was examined using ANOVA and repeated measures ANCOVA to control for sex, fitness, body composition, and caloric intake. Substrate utilization was
iv To my loving and unbelievably supportive wife. “some day, you will get the best of me…”
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“The LORD is my rock, my fortress and my deliverer; my God is my rock, in whom I take refuge.” – Psalm 18: “Fear not, for I have redeemed you; I have summoned you by name; you are mine. When you pass through the waters, I will be with you…For I am the LORD, your God, the Holy One of Israel, your Savior.” – Isaiah 43:2a, 3a I wish to acknowledge my amazing advisor: Karin Pfeiffer, thank you for your patience as well as your display of grace and truth. I wish to acknowledge my family, thank you for your ceaseless prayers, love, and support. I wish to acknowledge a mentor: Joe Eisenmann, thank you for your wisdom and encouragement while “Running with Joe.” I wish to express thanks to my committee (Karin Pfeiffer, Jim Pivarnik, Lorraine Weatherspoon, and Kim Maier) for their hard work, dedication, and ease of access. I wish to acknowledge the funding from Michigan State University College of Education, which made this study possible. Finally, I wish to acknowledge all those who became my family in East Lansing, thank you for your love and support.
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Chapter 1 – Introduction Total daily energy expenditure (TDEE) results from a combination of factors: resting metabolic rate (RMR), thermic effect of food (TEF), and physical activity (PA). Each factor is further comprised of smaller elements. RMR is the energy expenditure due to basal metabolic rate (energy required for vital functions) and arousal metabolism (energy required for a general state of awareness). RMR is highly dependent on an individual’s fat free mass (FFM), sex, age, and hormonal balance^1. The TEF can contribute significantly to TDEE, accounting for 3 – 25 percent of energy consumed in a meal depending on nutrient composition. All other things being equal a high protein meal elicits the highest TEF, followed by a high CHO meal, with high fat meals being the lowest 2 - 4
. The high TEF from protein is due to the greater cost of metabolism and storage when compared to CHO and fat^5. Lastly, PA can significantly increase an individual’s metabolic rate above resting values, both during and after the activity. PA bouts provide a unique opportunity to study the metabolic adaptations of the human body; these adaptations provide insight into control of metabolism 6 , 7 . Pre-exercise metabolism Pre-exercise metabolic rate refers to the energy expenditure immediately prior to the exercise bout and has long been assumed to be slightly higher than RMR. The premise behind this assumption is that the human body, in anticipation and preparation of the work that is to come, increases sympathetic output, and as a result metabolic rate increases. One author, reviewing the literature of EPOC, suggests that pre-exercise measurements should be recorded with the understanding that RMR is not accurately
represented by pre-exercise metabolic rate and; therefore, if RMR is desired, measurements should be taken well in advance of the exercise bout^8. Other authors disagree. Thomas et al reported no significant differences between pre-exercise metabolic measurements and RMR during a control visit^9. Turley et al reported no significant difference between inpatient and outpatient RMR 10
. Against commonly held assumptions some research suggests that pre-exercise, inpatient metabolism is not significantly different from controls. Exercise metabolism During exercise, aerobic metabolism increases from commencement to completion of exercise, and there are subtle differences between the onset of exercise and the attainment of a steady state of exercise. As exercise commences, the fuel needed to produce power initially is supplied by stored ATP-PCr in the muscle and, moments later, the slightly slower metabolic process of glycolysis. Concurrently, oxygen consumption increases as the slower aerobic system is able to provide the majority of energy needed to perform sustained work. The beginning of exercise provides a unique insight into the kinetics of oxygen uptake and has been considered as an important window into the understanding of control of the aerobic energy system 6 , 11 . At the onset of exercise, the difference between the required energy to perform an activity and the produced energy by the aerobic system, as measured by oxygen consumption, has long been referred to as oxygen deficit 12 . The measure of the total volume of oxygen deficit, or accumulated oxygen deficit (AOD), has been used as a valid indication of anaerobic capacity^13.
maximal exertion in adults. Thus, observing steady state exercise has been important for the understanding of metabolism and in particular, fuel utilization. Post-exercise metabolism Recovery from exercise provides a view of the metabolic characteristics of an individual not seen during rest or exercise. Unlike rest, onset of exercise, or steady state exercise, the post-exercise period includes time of elevated metabolism with little skeletal muscle activity and was first published as a scientific observation in 1910 by A.V. Hill who later coined the phrase “oxygen debt” to describe the phenomenon^17 ,^18. The phrase “oxygen debt” was designed to imply excessive oxygen consumption after exercise was “making up” for the slower oxygen uptake kinetics at the onset of exercise. Thus, Hill believed that the oxygen debt was equal to the oxygen deficit at the onset of exercise. The implications of oxygen deficit and oxygen debt being equal have led some researchers to use oxygen debt as a measure of anaerobic capacity^19 -^21. However, the physiological differences between the onset of exercise and recovery from exercise led some researchers to reconsider the equality of oxygen deficit and oxygen debt. Thus, In 1984, Gaesser and Brooks addressed several faulty implications of the term “oxygen debt” and proposed a new term: excess post-exercise oxygen consumption (EPOC) citing that when compared to pre-exercise, recovery is characterized by: elevated body temperature, increased circulating catecholamines, increased uptake of calcium ions by the previously active muscles, continued liberation of fatty acids to fuel metabolism, and a host of other known and unknown factors which contribute to the uniquely elevated metabolic rate post-exercise 22
. Much of the literature concerning EPOC centers on the observation that
EPOC volume is dependent upon the intensity, duration, and mode of exercise 8 , 23 - 33
. In a review of the literature, EPOC is linearly correlated with the duration of the exercise and exponentially related with exercise intensity when exercising above 60% VO 2 max 8 . Controversy exists concerning the duration of EPOC. Some investigations have observed EPOC after aerobic exercise lasting 12 hours or more after cessation of the exercise bout 34 , 35 . Others have reported EPOC lasting between three and 10 hours 8 , 36 - 38 . Still others have observed the duration of EPOC to be less than an hour^33 ,^39 ,^40. Furthermore, at least one group of investigators found the duration of EPOC was as short as 13 minutes when exercising for 30-49 minutes at intensities below 60% of VO 2 max^41. Although controversy surrounds the duration of EPOC, intensity and duration of the exercise appear to be major contributors to the total volume of EPOC. Investigations into EPOC will contribute to the understanding of the body’s attempt to maintain homeostasis in the context of post-exercise. Furthermore, insights into the mechanisms of EPOC could provide information regarding metabolic control during recovery. The existence of EPOC has led some researchers to explore its potential impact on weight control in the adult population; however, the magnitude of the caloric expenditure from EPOC is not clear^26 ,^37 ,^42. Although the majority of the literature has concluded that EPOC contributes minimally to TDEE 8 , others have argued that more energy can be expended during EPOC than during the exercise itself^43. The accumulated effect of EPOC on energy expenditure with consistent exercise can contribute to weight balance, since even a seemingly negligible positive caloric balance, such as 25 kilocalories/day, can lead to obesity over time^44.
Substrate utilization The body preferentially uses different substrates (carbohydrate, fat, or protein) as fuel for metabolism depending on the needs of the individual. The body shifts from relying on carbohydrates (CHO) during moderate to vigorous exercise, to primarily fats oxidation during recovery in normal, healthy individuals 49
. Wong and associates observed that there were no differences in substrate utilization between the lean and obese men during a standard exercise bout; however, after exercise, obese adults showed a lower reliance on fat oxidation and a reduced EPOC when compared to their lean counterparts^49. It is of note that the degree of ability to utilize fats as a source of energy has been shown to be a significant component in the development of obesity 50 , 51 . Thus, it is possible that the suppressed fat oxidation in the obese individuals may only be observed during the post-exercise period of time. Substrate utilization, as estimated by RER, during rest and exercise is known to differ between children and adults^16. It is generally accepted that children have lower RER values than adults during rest and exercise, indicating that they rely more on fat oxidation for energy production 16 , 52 . Wong and Harber observed that after a 30-minute bout of cycling at ventilatory threshold RER was suppressed significantly below baseline in normal weight adults indicating a greater reliance on fat as a substrate for oxidative phosphorylation^49. Since skeletal muscle composition does not differ significantly between children and adults, it stands to reason that children would have a similar post- exercise response in RER. However, to our knowledge, no research studies have been published concerning the substrate utilization during recovery from submaximal exercise bouts in children when compared to adults. It is unknown if children utilize the same fuel
to recover from exercise as do adults. Therefore, more research is needed to investigate substrate utilization, particularly post-exercise, in children. Problem statements Investigations into exercise recovery contribute a unique view of metabolism and metabolic control in youth. However, little research is available concerning metabolism post-exercise in healthy children. Research on EPOC or “oxygen debt” available in the pediatric literature address exercise of differing intensity levels, acclimatization to altitude, and chronic diseases, making comparison across investigations difficult. In particular, it is unknown if EPOC differs as a function of moderate and vigorous intensity activities in which children are most likely to engage. Currently there is little research available to provide insight into a child’s metabolic profile, including oxygen consumption and substrate utilization, during recovery from moderate and vigorous exercise intensities. In addition, it is unknown if children continue to preferentially oxidize fats post-exercise when compared to adults. It is also unknown if children rely more on fat as a fuel after exercise than before exercise. It is important to first describe a normal response to exercise before researchers can identify anything deviates from normality. Once a normal response has been described, future research can begin to investigate different populations, specifically overweight and obese children.
Hypothesis B: Young adults would have a higher RER than children after vigorous exercise.
