Reading Passage: What is Energy Availability?

Module 3: What is Energy Availability?

In the context of Relative Energy Deficiency in Sport (RED-S), energy availability refers to the balance between energy intake and energy expenditure. Energy availability is calculated by subtracting the amount of energy expended through physical activity (energy expenditure) from the amount of energy consumed through food and drink (energy intake). Energy availability is the “amount of energy” that nets after the subtraction of these two values. Let's say an athlete consumes 3000 calories (kcal) per day and expends 1500 calories (kcal) through physical activity. To calculate their energy availability, we would subtract the energy expenditure (1500 kcal) from the energy intake (3000 kcal), resulting in an energy availability of 1500 kcal.

Energy availability (aka having energy leftover after exercise) is essential for normal metabolic processes in the body. The human body requires a certain amount of energy to function properly and carry out everyday tasks, such as breathing, circulating blood, and regulating body temperature. Energy availability plays a key role in fueling these basic metabolic processes.

When the body is in a state of energy balance, the amount of energy consumed through food and drink is equal to the amount of energy expended through physical activity and other metabolic processes. In this state, the body is able to carry out its metabolic functions efficiently, without any negative health consequences.

When energy availability is low (aka low energy availability or LEA), it means that the energy intake is not sufficient to meet the energy demands of the body. Athletes, rather than sedentary populations, more commonly have LEA due to their high energy expenditure. Low energy availability is the most important factor contributing to the health affects of RED-S. Research found that about 50% of athletes were at heightened risk for LEA which is a statistic that is difficult to ignore.

LEA may occur due to athlete drive to improve their body weight or composition for competitive purposes, to avoid gaining weight during periods of injury or illness, or due to disordered eating patterns. However, unintentional LEA can also occur due to factors such as large energy requirements during high-intensity training, combined with a preference for low-energy-density diets that are considered to be “healthy”. Other factors that can contribute to suboptimal energy intake include financial and time constraints, as well as cultural beliefs.

When energy availability is low, the body’s natural processes are not able to function properly and this can lead to a wide range of negative effects on the athlete's health and performance, potentially affecting every organ in the body in an irreversible manner. Even being in a state of low energy availability for a short period of time can trigger dysregulation in all body systems.

It's important to emphasize that low energy availability is not the same as low energy intake, in fact, some athletes may have a high energy intake but still have low energy availability because of high energy expenditure.

There are means of measuring energy availability but there is no standardized method. In addition, the tools required to do so are largely inaccessible to the general public. In practice, energy availability is often reported in terms of kilocalories per kilogram of lean body mass per day (kcal/kg LBM/day), which takes into account an individual's body composition. Lean body mass refers to the weight of the body minus the weight of fat tissue. So for example, if an athlete weighs 70 kg and has a body fat percentage of 20%, their lean body mass would be 56 kg (70 kg x 0.8). If their energy availability is 1500 kcal, their energy availability per kilogram of lean body mass would be 1500 kcal / 56 kg = 26.8 kcal/kg LBM/day.The range of energy availability that is considered safe for maintaining normal body functions varies depending on the individual and can range from 30-45 kcal/kg LBM/day.

It's important to note that this is just a general guideline and that each individual's energy needs can vary based on factors such as age, sex, body composition, and activity level. For example, a younger person may have a higher energy requirement than an older person due to their faster metabolism and growth-related demands. Similarly, males typically have higher energy requirements than females due to differences in body size, muscle mass, and hormone levels. Body composition also plays a role, as individuals with higher amounts of lean body mass typically have higher energy requirements than those with higher amounts of body fat. Finally, activity level can significantly impact an individual's energy needs. Athletes or individuals engaging in high levels of physical activity require more energy to fuel their exercise and maintain their basic metabolic processes compared to sedentary individuals. Therefore, the range of energy availability considered safe for maintaining normal body functions will vary based on these factors, and each individual's energy needs must be assessed on a case-by-case basis.

Overall, studies confidently say that maintaining energy availability at 45 kcals per kg of lean body mass per day or higher does not affect menstrual function, but when it falls below 30 kcals per kg of lean body mass per day, menstrual dysfunction occurs. However, some studies show conflicting results and indicate that there is no clear threshold for what specific value of energy availability causes menstrual dysfunction. Each woman may have her own energy availability threshold that disrupts normal body processes.

In summary, energy availability in the context of RED-S refers to the balance between energy intake and energy expenditure, and when it is low, it can lead to negative effects on an athlete's health and performance.