Unbeknown to most sport and exercise practitioners, the concept of Calories stems from very early ‘science’. The classic ‘calories in versus calories out’ energy balance equation still forms the basis of many sports nutrition strategies today, and is generally taken as a static approach. Whilst it is clear that physical exercise (particularly when performed at high intensity) increases energy expenditure, and therefore increases the need for higher dietary caloric requirements, the energy balance equation is both complex and recognisably flawed, with notable errors when quantifying both energy intake and expenditure. Therefore, in order for a sport and exercise practitioner to optimally support their athlete’s potential, they are encouraged to critically evaluate these fundamental nutrition concepts on an individual (n = 1) basis, in line with the athlete’s lifestyle, behaviours and goals.

In this chapter, we explore how caloric expenditure and caloric intake are currently assessed, and in doing so, aim to demonstrate the complexity of human metabolism, which should be taken into consideration when working with athletes. This chapter also introduces contemporary views on energy balance, including some of the more likely contributors to metabolic flow and efficiency, and how this could apply within the sport and exercise domain. With current interest in minimising cases of Relative Energy Deficiency in Sport (REDs), this chapter will also address such concerns and the importance of adequate energy availability for training and performance.