## INTRODUCTION

Most individuals, including athletes, maintain a stable body mass (BM) over long periods of time, while paying little attention to the amount of energy consumed or expended each day. However, energy balance is of primary concern to athletes who want to alter BM and/or composition to improve their exercise performance or meet a designated weight requirement for their sport. When energy consumption is insufficient to match that expended, much of the effort of training can be lost, since both muscle and fat will be used for energy. In addition, if energy intake is limited or restricted, the ability to obtain other essential nutrients, such as carbohydrate (CHO), protein, fat, vitamins and minerals—which are necessary for optimal sport performance and good health—will also be compromised.

Many athletes, especially female athletes, feel pressured by their coaches, parents, peers and themselves to reduce BM. To maintain a low BM, these athletes restrict energy intake even though their energy expenditure (EE) is high. Athletes of any age must consume enough energy to cover the energy costs of daily living, the energy cost of their sport and the energy costs associated with building and repairing muscle tissue. Females of reproductive age must also cover the costs of menstruation and reproduction, whereas younger athletes must cover the additional costs of growth. This chapter will briefly review the dynamic nature of energy balance and the many factors, such as macronutrient balance, that contribute to energy balance in an athlete or active individual. Manipulating the energy balance equation for either gain or loss of BM for an individual athlete is covered in other sections of this book.

## ENERGY BALANCE

At first, the concept of energy balance appears straightforward and simplistic. For BM to be maintained, energy in (total kilojoules or kilocalories consumed and those drawn from body stores) must equal the energy expended. Under these conditions, an individual is considered to be in energy balance. This concept can be stated using the equation shown, where ‘energy in’ means ‘metabolisable energy’ (energy intake minus energy lost in faeces and urine). For most individuals, metabolisable energy is about 90–98% of energy intake (Murphy et al. 1993; Hall et al. 2011).

$Energy balance occurs when Ein = Eout$

where Ein = energy consumed (kJ/d or kcal/d) and Eout = energy expended (kJ/d or kcal/d).

However, the ability of the body to regulate body weight within a narrow range and maintain energy balance is more complicated than it initially appears. Energy balance is a dynamic process whereby altering one component of the energy balance equation (e.g. energy intake or composition or form of the diet) can affect the physiological and biological components of the other (e.g. EE) in unpredictable and unintended ways (Galgani & Ravussin 2008; Hall et al. 2011; ...

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