Calories
What is Energy Availability?
Energy availability (EA) is defined as dietary energy intake minus the energy expended in exercise normalized for fat-free mass (9). What does this mean exactly? EA is the amount of energy (from calories consumed) your body has left over after exercise to be used for normal physiological processes such as cellular maintenance and reproduction.
You can download and listen to this short presentation describing the concept of EA and how to calculate your energy needs to ensure that you are in the "adequate" or "optimal" range. (Once you download the presentation, go to the Slide Show tab and click Play from Start to enjoy the audio version of the presentation).
You can download and listen to this short presentation describing the concept of EA and how to calculate your energy needs to ensure that you are in the "adequate" or "optimal" range. (Once you download the presentation, go to the Slide Show tab and click Play from Start to enjoy the audio version of the presentation).
Causes of Low EALow EA usually results from not consuming enough calories to match the amount of energy expended during exercise. Disordered eating practices are a common cause of low EA but athletes who expend large amounts of energy during prolonged exercise training can become energy deficient without eating disorders or dietary restriction. Even the most well-intentioned athlete may have difficulty meeting their nutritional needs due to lack of knowledge, time and financial constraints.
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Effects of Low EA
According to the revised Female Athlete Triad position stand, the American College of Sports Medicine (ACSM) states that when EA is too low, physiological mechanisms reduce the amount of energy used for cellular maintenance, thermoregulation, growth, and reproduction (14). Effects of low EA include: menstrual dysfunction, difficulty maintaining muscle mass, difficulty recovering from injury, effects on bone health, and suppresses type 1 immunity.
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Lenka et al. (2013) found that 91% (n = 41) of female collegiate athletes were not meeting their energy needs. |
Menstrual Dysfunction
Back in 1998, a study was done that demonstrated the effects of low EA on reproductive function. This study separated healthy women into two groups of either adequate EA or low EA. Within the low EA group, the women were either low EA due to increased exercise or inadequate caloric intake. The results found that low EA caused a disruption in reproductive function regardless of the cause of low EA (i.e., exercise or caloric restriction). Therefore, this was the first study to show that low EA and not the "stress" of exercise causes the menstrual dysfunction (1).
The prevalence of secondary amenorrhea (no menstrual cycle for > 90 days) among female endurance athletes can be as much as 10 times higher than the general population. It has been reported to be as high as 65% in long-distance runners. Prevalence has been found to increase from 3 to 60% as training mileage increased from < 8 miles to > 70 miles per week and as body weight decreased from >60 kg to < 50 kg (1).
The prevalence of secondary amenorrhea (no menstrual cycle for > 90 days) among female endurance athletes can be as much as 10 times higher than the general population. It has been reported to be as high as 65% in long-distance runners. Prevalence has been found to increase from 3 to 60% as training mileage increased from < 8 miles to > 70 miles per week and as body weight decreased from >60 kg to < 50 kg (1).
Difficulty Maintaining Muscle Mass & Recovering From Injury
When EA is low, carbohydrate, protein, and nutrient intakes are probably also low. Not consuming enough carbohydrates means the body relies more on protein during exercise and therefore, there is not enough protein to maintain and repair muscle tissue (1). Additionally, reduced intakes of macronutrients can decrease the body’s ability to build bone, maintain muscle mass, repair damaged tissue, and recover from injury. Micronutrients are also necessary to build bones and muscle tissue, replace red blood cells, and provide co-factors for the energy-producing metabolic pathways (10). These dietary inadequacies can lead to increased risk of fatigue, muscle weakness, injuries and infections (1).
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Effects on Bone Health (Stress Fractures)
In athletic populations, incidences of stress fractures are generally highest among female runners and those athletes with low body fat may experience stress fractures linked to menstrual irregularities and/or low energy and nutrient consumption (22).
Sport types most at risk are those that favor or contribute to a lean build or thin frame. These include endurance sports and sports where athletes are judged based on their appearance. Endurance runners compared to athletes participating in moderate to high-impact, non-lean-build sports show a significantly higher prevalence of low bone mass (1). Athletes who are not menstruating are at risk first for osteopenia (low bone mineral density) and then osteoporosis. At least one study showed that 22 to 50 percent of amenorrheic (menstruation is absent or suppressed) runners and ballet dancers had varying degrees of osteopenia. In two studies of amenorrheic female distance runners between the ages of 20 and 30, 10 to 13 percent were diagnosed with osteoporosis (5). Menstrual irregularity, and the associated bone mineral loss, is associated with a greater incidence of stress fractures in runners. A study of female collegiate runners found a much higher percentage of stress fractures in runners reporting and irregular menstrual history compared to runners reporting regular menstruation (See chart to the right). |
The runners were divided into three groups according to their menstrual history: very irregular (0 to 5 menses/year), irregular (6 to 9 menses/year), and regular (10 to 13 menses/year). Forty-seven percent of the amenorrheal group, 20% of the one to five menses/year group, 10% of the irregular group, and 7% of the regular group admitted to an eating behavior disorder.
Barrow, G.W. & Saha, S. (1988). Menstrual irregularity and stress fractures in collegiate female distance runners. American Journal of Sports Medicine. 16(3), 209-216 |
Suppresses Type 1 Immunity (Defense Against Viruses)
Endurance athletes frequently suffer from upper respiratory tract infections caused by viruses (9). One study surveyed all Swedish team members participating
in the Olympic Games (2002, 2004) and found that those participating in
disciplines emphasizing leanness made more frequent attempts to lose weight,
trained longer, and reported almost twice as many illnesses, primarily upper respiratory tract infections,
during the preceding 3 months (9).
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Consuming sufficient energy and nutrients is vital for supporting immune function |
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