Refuel for Life:
The Importance of Energy Availability
Energy availability is a key feature of the Female Athlete Triad. Before we discuss energy availability, we first need to understand the basic concepts of energy expenditure and energy balance.
Total daily energy expenditure compromises the following components:
- Thermic effect of food (TEF) describes the energy cost of food digestion, absorption, transport, metabolism and storage within the body and accounts for ~6-10% of total daily energy intake.
- Non-exercise associated thermogenesis (NEAT) is the energy expended during all the activities of daily living (i.e. dressing, grooming, cleaning, working) that are not considered purposeful sport, exercise and/or physical activity.
- Purposeful exercise energy expenditure (EE) is often considered as activity in which heart rate (HR) is >55% of maximal HR. Purposeful exercise EE is highly variable dependent on the frequency, duration and intensity of the activity undertaken.
- Resting metabolic rate (RMR) is the energy expended to maintain body systems at rest. The utilisation of energy in this state is sufficient only for the functioning of vital organs including the heart, lungs, nervous system, kidneys, liver, intestines, muscles, bone, skin and reproductive organs. RMR may contribute >55% of total energy expenditure in exercising women.
Energy balance describes the weight stability that occurs when energy intake is equal to total energy expenditure and is under the premise that all physiological processes and body systems are working at full capacity. In contrast, energy availability describes the amount of dietary energy remaining, after accounting for exercise energy expenditure, for use by all other physiological processes and body systems. In healthy young women, energy balance (0kcal.day-1) occurs when energy availability, controlled for fat-free mass (i.e. heart, lungs, nervous system, kidneys, liver, intestines, muscles, bone, skin and reproductive organs) is 45kcal.kgFFM.day-1.
When energy availability is inadvertently or intentionally suboptimal, the human body is forced to prioritise available calories. Energy is drawn away from physiological processes and body systems that are not essential for immediate survival (e.g. growth, deposition of body fat stores and reproduction) and towards those essential for life (e.g. movement, breathing and keeping the body warm and heart pumping!) This compensation tends to restore energy balance and promote survival, but it does not improve energy availability.
Low energy availability in female athletes and exercising women
It is very common for women who are engaged in regular training for sport or exercise for enjoyment and health reasons to experience inadvertent energy availablity. Low energy availability develops when insufficient calories are consumed to meet the demands of exercise training. If the magnitude or duration of energy imbalance is too large then parameters of health, and possibly training and performance, is compromised. For some women, low energy availability is by accident, but for many others, exercising and restricting caloric intake is done with the aim to lose weight or maintain a particular weight.
Let’s look at the three different mechanisms through which low energy availability can occur:
a) Inadvertent: Female athletes and exercising women can develop low energy availability without consciously restricting their energy intake. They may simply fail to match their energy intake with the energy demands of their exercise training, particularly when exercise load is increased. Reasons for this include time constraints and food availability issues for sufficient re-fuelling between training sessions and/or a lack of appropriate nutritional knowledge. For women whose energy deficit is not intential, this is a matter of good sports nutrition.
b) Intentional: Some female athletes may be required or desire to lose weight to meet competition regulations (e.g. light-weight rowing), to improve movement economy (e.g. distance running) or for sport-specific aesthetic reasons (e.g. gymnasts). Other exercising women may require weight-loss to minimise the risk for chronic disease (e.g. diabetes, cardiovascular disease). Many more women, however, restrict their energy intake in their attempts to lose weight or to maintain a particular weight. By exercising and intentionally limiting energy intake, women may be exposing themselves to risks to reproductive, bone and cardiovascular health.
c) Disordered eating behaviours and eating disorders: Disordered eating behaviours can develop from a constant drive to be thin or achieve an inappropriately low body weight. This can be caused by, or lead to, poor body image and/or purposeful restriction of dietary intake. Disordered eating comprises various behaviours such as restrictive eating, fasting, skipping meals and over-eating/binge-eating following by purging with vomiting, diet pills, laxatives, diuretics, enemas and/or conducting more exercise than is required for their sport. The severity and frequency of disordered eating behaviours can range widely and so their presence does not necessarily mean the girl or woman has an eating disorder. Indeed, diagnosis of clinical eating disorders, such as Anorexia, Bulimia and Eating Disorders Not Otherwise Specified (EDNOS) must meet very specific behavioural, psychological and physiological criteria. Professional advice should be sought for girls and women struggling with body image concerns or disordered eating behaviours.
Energy Deficiency and Body Weight
Body mass is often maintained in energy-deficient female athletes and exercising women due to physiological adaptations that occur to conserve energy and restore energy balance. Reproduction is essential for species, but not individual, survival and pregnancy and lactation are highly energy-dependent processes. Thus, during a sufficiently large or prolonged energy deficit, reproductive function is compromised and menstrual disturbances develop. The energy conserved reduces relative resting metabolic rate (RMR), and thus total energy expenditure. As such, less dietary energy intake is required to maintain body weight. That is, female athletes and exercising women can be in an energy deficit and not lose weight. Indeed, reductions in RMR and maintenance of body weight are commonly observed in amenorrheic exercising women. Therefore, energy balance (defined as energy intake minus total energy expenditure) and body weight do not provide reliable information about energetic status as they assume that all body systems are functioning optimally. This is however, not necessarily the case and so weight-stable female athletes and exercising women cannot be presumed to be energy replete.
So, the key points here are: just because a female athlete or exercising woman looks very thin, it does not mean she has an energy deficit, or is experiencing some of the risks of the Female Athlete Triad. Conversely it is important for coaches, trainers or parents to consider although a female athlete or exercising woman looks a ‘healthy’ weight, it does not mean she is energy replete or not-at-risk for the Triad.