Expert groups that develop sports nutrition guidelines typically make universal recommendations without specific adjustment for the sex of the individual athlete. The absence of sex-specific guidelines could mean that females do not have special considerations regarding exercise metabolism or nutritional requirements for sport. Alternatively, it could mean that there is no information on which the effect of sex can be based. It is of importance to differentiate between these two options since the absence of evidence for an effect is not the same as evidence for the absence of an effect. Three broad categories of studies would be useful in either confirming the suitability of present sports nutrition guidelines or developing separate recommendations for female athletes: (i) studies in which female subjects have been included within the group outcomes without distinguishing any differences based on sex; (ii) studies that focus on female subjects alone, and (iii) research in which direct comparisons have been made between the response of male and female subjects to a sports nutrition intervention. It is clear, notwithstanding the difficulties of undertaking such research, that these types of studies are under-represented in the literature and should be encouraged.

There are several issues that might explain or call for differences in recommendations for sports nutrition for female athletes. The first is the different hormonal environment and its changes over the menstrual cycle experienced by females. The small number of factors in which this has been investigated in terms of exercise metabolism (e.g., protein synthesis and carbohydrate metabolism) suggests that such differences, if they occur, are sufficiently subtle that they do not merit separate recommendations in terms of sports nutritional strategies. The second issue is the smaller body size or muscle mass of females. This issue is covered to some extent in that old sports nutrition recommendations for absolute nutrient amounts have been replaced with targets expressed relative to body mass (e.g., carbohydrate intake) or in terms of developing an individualized and tolerated plan (e.g., fluid intake during exercise). A final consideration is the observation that many female athletes consume diets that are low in energy availability; apart from impairing health and function, this can indirectly alter nutritional requirements.

An inadequate intake of energy in relation to the energy cost of exercise prevents the body from having sufficient energy to fuel the functions underpinning optimal health and performance. This situation, termed low energy availability, is frequently observed among female athletes related to their management of optimal body mass/physique. However, it can also occur when appetite or opportunities to consume food fail to adapt to an increase in training/competition load. The chapter on the female athlete triad (Chapter 9) describes the causes and outcomes of this syndrome in more detail. Here, we will quickly consider the effects of low energy availability on sports nutrition requirements. Although the clear objective is for female athletes to avoid scenarios of energy deficiency, there are some situations in which some reduction in energy availability may be required or tolerated. In such cases, some secondary nutritional issues should be considered.

An outcome of the reduction in energy intake below energy expenditure is an adjustment to physiological function to conserve energy and preserve against starvation. A reduction in metabolic rate ultimately reduces energy requirements, whereby in extreme cases, a female may regain energy balance (intake = expenditure) despite a low level of energy availability (where EA = intake minus the energy cost of exercise < level required for healthy function). In these cases, nutritional counseling of the female athlete may need to take into account a current energy requirement below predicted/healthy levels. A gradual intake in energy may be required to help restore energy availability, metabolic rate, and health. Even when this takes place, however, the female athlete will need to make sound dietary choices from a range of nutrient-dense foods to ensure that all requirements for micronutrients and beneficial food constituents are met from a restricted energy budget. The case for reduced EA and the protein and carbohydrate guidelines covered later in this chapter merit special comment.

Older studies of female athletes and carbohydrate loading made observations that they are less efficient at storing glycogen than their male counterparts (i.e., females store less glycogen from a given carbohydrate intake or fail to super-compensate muscle glycogen stores). More recent research has shown that this finding is related to energy deficiency and females can store glycogen effectively when they consume adequate energy intake. Alternatively, when carbohydrate intake is below the targets identified for optimal refueling, the addition of protein (∼20 g) to the meal/snack enhances glycogen storage. Meanwhile, although protein targets can be set for meals and snacks to optimize protein synthesis in response to exercise over the day, recent studies show that these targets need to be increased even when energy availability is reduced by amounts normally considered to be “safe” for weight loss (30 kcal/kg fat-free mass).