Sports nutrition energy metabolism and exercise part 1

7950_S002.fm Page 125 Wednesday, June 20, 2007 6:01 PM Section Estimation of Energy Requirements 7950_S002.fm Page 126 Wednesday, June 20, 2007 6:01 PM 7950_C005.fm Page 127 Wednesday, June 20, 2007 5:54 PM Energy Expenditure of Athletes Robert G McMurray and Kristin S Ondrak CONTENTS I II Introduction 127 Methods for the Measurement of Metabolic Rate 128 A Direct Calorimetry 128 B Indirect Calorimetry .129 Closed Circuit Spirometry 130 Open Circuit Spirometry .131 C Doubly Labeled Water 133 III Energy During Sport and Physical Activity 134 A Use of Open Circuit Technology 134 B Use of Heart Rate To Estimate Energy Expenditure 135 IV Ergometers 135 A Cycle Ergometers 136 B Rowing Ergometers 138 C Treadmills .139 D Cross-Country Ski Ergometers 139 V Metabolic Rate During Swimming 140 VI Maximal Metabolic Rate .142 VII Anaerobic Threshold 144 VIII Economy of Human Movement 146 IX Resting Energy Expenditure 147 A Measurement of Resting Energy Expenditure .147 B Factors Influencing REE 148 X Daily Energy Expenditure of Athletes .149 XI Summary 151 References 152 I INTRODUCTION The measurement of metabolic rate can provide important information to athletes concerning their innate abilities and training programs This is particularly true for high-caliber endurance athletes, but is also true for the everyday fitness enthusiast, 127 7950_C005.fm Page 128 Wednesday, June 20, 2007 5:54 PM 128 Sports Nutrition: Energy Metabolism and Exercise since knowledge of metabolism also has health implications Typically, athletes are interested in four factors First, they want to know their maximal metabolic rate, an indicator of the body’s maximal ability to consume oxygen Maximal metabolic rate is also referred to as aerobic power or maximal oxygen uptake (VO2max) A higher VO2max means that the athlete can sustain a higher level of work without fatigue Second, athletes want to know their anaerobic thresholds (AT), or the level of metabolic rate at which there is a rapid rise in lactate concentration Athletes with high ATs can sustain a higher level of exertion before fatigue ensues Third, athletes are interested in their economy of motion Improved economy of motion is related to the mechanics of the activity, be it swim stroke, running stride, or paddling stroke Highly successful athletes are usually very economical in their use of energy Fourth, some athletes are interested in their resting metabolic rate, because this knowledge can assist with their weight-loss or -gain programs This chapter will explore the issues of the measurement of metabolic rate as they relate to the athlete The chapter starts with an introduction to the units used to define metabolic rate, followed by a discussion of the varying techniques to measure metabolic rate and their use for athletes Information on sports-specific means of measuring metabolic rate is presented followed by a more in-depth examination of the four metabolic factors of interest to athletes Finally, the chapter will conclude with some commentary on estimating of overall energy expenditure for athletes The terms metabolic rate and energy expenditure are used synonymously through the literature on metabolism In the English system of measurement, the basic unit of energy for humans is the kilocalorie (kcal) This is the amount of heat required to increase one kilogram of water one degree Celsius The scientific community, however, has placed considerable emphasis on the use of the kiloJoule (kJ) or mega-joule (mJ) over the kcal (S.I Unit: le Système International d'Unités).1 Conversion between units is simple: one kcal = 4.184 kJ or 0.00418 mJ Measuring calories or joules is difficult in humans and requires extremely expensive equipment Thus, oxygen uptake (VO2) is more commonly measured and converted to kcal, knowing that there are approximately 4.7–5.1 kcal per liter of oxygen, and then finally to kJ II METHODS FOR THE MEASUREMENT OF METABOLIC RATE The energy output of humans can be measured using direct and indirect calorimetry, as well as doubly labeled water.2–4 The advantages, disadvantages, and uses of the methods are the foci of what follows A DIRECT CALORIMETRY Direct calorimetry assesses heat production, typically requiring a small room with highly insulated walls.2,4,5 The walls of the unit contain a series of pipes through which water is pumped at a constant rate The heat generated by the subject is measured by the difference between the incoming and outgoing water temperatures, 7950_C005.fm Page 129 Wednesday, June 20, 2007 5:54 PM Energy Expenditure of Athletes 129 knowing the volume of water and rate of the water flow Oxygen is continuously supplied to the subject and carbon dioxide is removed by chemical absorbent Direct calorimeters range from suit calorimeters, like those used by astronauts, to small chambers and even larger rooms Using direct calorimetry to measure metabolic rate takes considerable time, as it takes a minimum of 30 minutes to equilibrate heat loss and heat production.4 The method is highly accurate, but is limited only to resting measures, those activities that have minimal range of movement, or overall energy use for extended periods of time (2–24 hours) The methods will not work for most sports or activities, in varying environments, or in large-scale population studies Also, most organizations not have the expensive, complicated facilities and equipment needed to use this method B INDIRECT CALORIMETRY Indirect calorimetry relies on the measurement of VO2 and is good for measuring metabolic rate over short periods of time for specific activities The underlying principle for indirect calorimetry is that oxygen is needed for the production of energy and the measurable end-product of metabolism is carbon dioxide production.2–4 Oxygen uptake and CO2 production are computed via mathematical computations, knowing the volume of air expired and inspired and expired oxygen and carbon dioxide content of that air (Table 5.1) This method is based on some assumptions.2 First, the individual is not in a starvation state and proteins make up only a very small portion of the energy and can therefore be ignored Second, the contribution of anaerobic metabolism to the energy production is quite small Finally, when using a combination of carbohydrates, fats, and proteins as the source of energy, approximately 4.82 kcal (20 kJ) of energy is liberated per liter of oxygen used.5 TABLE 5.1 Computational Model for Oxygen Uptake4 VE (STPD) = (273°C /(273°C + TV°C)) × ((BP−WVT)/760) VI = VE × ((1 − FEO2 − FECO2) /0.7904)) VO2 = (VI × FIO2) − (VE × FEO2) VCO2 = (VE × FECO2) − (VI × FICO2) RER = VCO2/VO2 RER on chart will give kcal/L oxygen (as well as percent carbohydrates & fats)12 kcal/min = (kcal/L O2) × VO2 (L/min) kJ/min = (kcal/min) × 4.184 kJ/kcal TV°C: temperature of the expired air volume BP: barometric pressure WVT: water vapor tension for the expired (47 mmHg) air at TV°C FI: fraction of inspired gas expressed as a decimal (O2 = 0.2093 & CO2 = 0.003) FE: fraction of expired gas (O2 & CO2) obtained from the gas meters RER: respiratory exchange ratio 7950_C005.fm Page 130 Wednesday, June 20, 2007 5:54 PM 130 Sports Nutrition: Energy Metabolism and Exercise For convenience, the 4.82 kcal/L O2 has been rounded to kcal or 21 kJ per liter of oxygen Indirect methods are less expensive, smaller, and more portable than direct calorimetry Since good agreement (