Measurement of Energy
All food consumed in the diet is digested, metabolized, and then absorbed through various enzymatic and physiological processes. The resulting metabolites are then used to meet immediate energy needs or are stored for later use depending upon the nutritional and physiological state of the body. All processes of the body require energy. Energy is the capacity to do work, and without energy we would not be able to sustain life, let alone participate in competitive sport. Understanding basic principles of energy transfer and measurement is essential for understanding the weight loss recommendations and applications.
This chapter examines direct and indirect methods used to quantify energy intake and expenditure. Energy intake refers to the energy content of food and beverages consumed, and energy expenditure refers to the energy utilized by the body. Energy balance can be calculated by subtracting energy that is expended from energy consumed.
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Energy intake is a relatively easy concept to grasp (total energy of all foods and beverages consumed). Energy expenditure is somewhat more abstract and has three components including resting energy expenditure (REE), thermic effect of food (TEF), and energy expended through physical activity (PA). Sports nutrition professionals can measure energy expenditure using direct and indirect measures. The measurement method selected varies in terms of accuracy and validity, and the choice of the assessment tool is most often based on practical and logistical considerations. The fundamentals of energy balance and measurement of energy are essential to understanding how energy is utilized by athletes and allows the sports dietitian to make appropriate energy recommendations.
Measuring Energy Unit of Measurement
From the act of stretching first thing in the morning, to running a marathon, all bodily movements require energy. Energy is what fuels activity and exercise and comes from the calories that individuals consume; both quality and quantity of calories impact the energy available for sport. While the idea of energy may seem simple, actually measuring energy can be quite complex. As with any measurement, the appropriate unit must be identified. The International System of Units (SI units) has been developed by the scientific community as a set of standardized weights and measures that utilize the metric system. SI units establish the official units of energy used among most research communities globally, though this chapter also identifies units commonly used in the United States.
The SI unit of measurement for mechanical energy is the joule. Energy can also be understood and measured in terms of heat, which better represents how energy is transformed in living systems. A calorie (lowercase c) may be a familiar unit of measurement. It represents the amount of heat needed to raise the temperature of one gram of water by 1°C. Because this amount of heat is so small in relation to the amount of energy measured in food or expended during activity, using Calorie (uppercase C) or kilocalorie (kcal) is more practical. A Calorie, or kcal, equals 1,000 calories and is the amount of heat needed to increase the temperature of 1 kg (or 1 L) of water by 1°C. In practical terms, Calories, or kcal, is a measurement of the energy content of a food or liquid; kcal can also be used to determine the specific energy requirements of an activity. Scientific research still relies upon SI units (joules in the case of energy), so the following conversions may be helpful to know:
• 1 calorie = 4.184 joules
• 1 kcal = 1,000 calories
• 1 kcal = 4,184 joules, or,
• 1 kcal = 4.184 kilojoules
Throughout this text, kcal and calories will be used interchangeably, however, both reference 1,000 calories (lowercase c).
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