The Physiologic Consequences of Low Energy Availability
Considerations for all active humans - not just athletes
For many years, the concept of low energy availability (LEA) was primarily associated with the Female Athlete Triad. The Triad’s counterpart, Relative Energy Deficiency in Sport (RED-S) came on the scene in 2014, primarily as a way to give a new name to a problem affecting all athletes, regardless of gender.
Like all research, the concept of energy balance has some limitations to consider. Energy availability as a whole is tough to measure and evaluate. LEA affects a number of systems in the human body, including endocrine, musculoskeletal, and immune, and is further influenced by a host of factors that are physical, mental and behavioral in nature.
Common terms + definitions to help you understand the nuance
Exercise Energy Expenditure (EEE): The amount of energy used during exercise
Energy Availability (EA): The energy left over after exercise used to maintain physiological function
Non-Exercise Activity Thermogenesis (NEAT): The energy used by the body for other movement activities that aren’t part of an exercise routine (e.g. walking, talking, biking to work, playing with your pet)
Dietary Induced Thermogenesis (DIT): The energy acquired from food
Resting Metabolic Rate (RMR): The amount of energy your body uses at rest to maintain life
Total Daily Energy Expenditure (TDEE): The amount of calories burned in a day regardless of method
To evaluate energy availability, the following equation is used:
(Energy Intake - Exercise Energy Expenditure) / Fat Free Mass
Limitations in Analysis
Although assessment of energy expenditure can be done with a bit more certainty, caloric intake is often approximated. Energy Intake (EI) is typically monitored via self-report and it’s easy to see how food logs can be wildly inaccurate. Have you ever “modified” a food log because you were shocked once you saw it written down? Perhaps you forgot a day, tried to remember what you had for lunch yesterday, and gave it your best guess. Furthermore, most energy related studies have been conducted in a sedentary population and average a mere 3-5 days in duration to ensure adherences, accuracy, and safety. A full picture of food intake alone could show dramatically different fueling processes outside of 3-5 days.
So, you can imagine how this data may not accurately show the full picture of someone who is highly active, yet has consistently under-fueled and over-performed for months and even years on end.
Body composition analysis to determine fat mass and fat free mass requires access to expensive laboratory equipment, such as a DXA scanner. While this may be a more costly analysis, access to high quality equipment can give a valid and reliable measure of one’s body composition.
Energy Balance vs Energy Availability
This image provides a visual of how energy balance and energy availability are defined. Energy balance accounts for all other forms of energy expenditure; energy availability focuses on the effects of exercise energy expenditure.
Systemic Effects
In females, LEA can result in amenorrhea and low bone mineral density (BMD). However, it’s important to note distinctions in diagnoses of both these conditions.
LEA-driven amenorrhea is accompanied by anovulation and a defective luteal phase, but can be hidden by oral contraceptive use.
BMD deficits generally occur after chronic, long term bouts of LEA. Lower BMD is associated with a greater prevalence of stress fractures.
Hormonal abnormalities can be noted as well. LEA lowers levels of leptin, which helps regulate metabolism. T3 levels, a thyroid hormone responsible for the regulation of many body functions, have shown to decrease in as few as two days of LEA.
Two other important hormones, growth hormone and insulin-like growth factor 1 are also negatively affected by LEA. IGF-1 is released from the liver. When growth hormone becomes resistant, the body produces more of it in an effort to elicit a metabolic response. As growth hormone increases, IGF-1 decreases. This cyclic relationship can have a deleterious effect on muscle function and repair.
LEA also causes the body to rely on fat oxidation to fuel exercise. In as few as 5 days in a low energy state (nutrition deficit + increased exercise), the body begins to reduce carbohydrate fuel sources in an effort to preserve its glycogen stores.
However, the most important fact here is that short term studies only provide so much information. Short term clinical markers may still appear normal, even in the presence of LEA. There are other markers, such as cholesterol and estradiol, that may be altered after long-term LEA. Clinical markers can be difficult to detect with accuracy due to the influence of exercise levels, oral contraceptive use, and even timing.The duration of LEA is another serious consideration, as it may be predictive of one’s ability to reverse the negative outcomes. The longer someone is in an energy deficit the tougher it may be to come out of it without lasting consequences.
Risk Considerations
You might be thinking, “I’m not an athlete, so this doesn’t affect me.”
That’s not entirely true.
LEA is a risk with any type of weight loss program. When energy intake (food) is restricted to elicit a calorie deficit, restriction must be carefully monitored in order to prevent long-term hazardous outcomes. Whether EI is restricted in response to dietary modification or through disordered eating, the spectrum and potential interrelatedness of these two should not be overlooked.
LEA can also pose a risk for those who are avid exercisers. Even if you don’t consider yourself an athlete in the traditional sense, recreational athletes and those training at moderate to high intensities 5-7 days a week should still be mindful of their EA. This group will often prioritize their training amidst their busy schedule and allow their nutritional intake to remain inconsistent. Working, training, and eating well while managing all the other necessary aspects of life is really hard - especially if you’re a parent.
The bottom line: You have to fuel for the exercise and activities you’re doing. For individuals trying to lose weight, I recommend working with a Registered Dietician to ensure a safe reduction in EI to avoid LEA. For athletes and the general populous with a rigorous training schedule, I also recommend meeting with an RD to ensure that macronutrient intake, quality, and timing are tailored to your specific needs.
References
Areta, J. L., Taylor, H. L., & Koehler, K. (2021). Low energy availability: history, definition and evidence of its endocrine, metabolic and physiological effects in prospective studies in females and males. European Journal of Applied Physiology, 121(1), 1–21. https://doi.org/10.1007/s00421-020-04516-0
Heikura, I.A., Stellingwerff, T. and Areta, J.L. (2022), Low energy availability in female athletes: From the lab to the field. European Journal of Sport Science, 22, 709-719. https://doi.org/10.1080/17461391.2021.1915391
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