By Cami Salisbury, MS, RDN

You’re doing all the “right” things—training hard, eating healthy, and staying consistent—yet you’re exhausted, struggling with performance, dealing with frequent injuries, or maybe even noticing changes in your body composition that don’t seem to match your efforts. If this sounds familiar, it may have less to do with your effort and more to do with whether your body is getting the energy it needs. This is known as low energy availability.

What is low energy availability?

Think of food as fuel. Some of that fuel is used during exercise, and what’s left is what your body has available to run everything else—your brain, heart, hormones, bones, and immune system—while also supporting recovery, repair, and muscle growth after training. Low Energy Availability (LEA) happens when there isn’t enough fuel left to meet all these needs.¹ What seems like enough food may not be enough to keep up with your training demands.^1,2  Energy needs also vary from person to person depending on factors like lean body mass, training demands, and individual physiology.¹

There are plenty of ways this fuel gap can happen, and it isn’t always intentional:^1,8

• Consistently not eating enough (under-fueling)
• Dieting or restricting food to lose weight or change body composition
• Increasing training without increasing fuel intake
• Filling up on high-volume, low-calorie or very high-fiber foods
• Appetite changes, including hard exercise temporarily blunting hunger
• Busy schedules that make consistent fueling difficult
• Underestimating how much fuel your training requires

An occasional day of under-fueling isn’t necessarily a problem. But when the gap between what your body needs and what it receives becomes too large or continues over time, low energy availability can progress into Relative Energy Deficiency in Sport (RED-S).

RED-S describes the broader physical and psychological effects that can develop when low energy availability begins to negatively affect health and performance, potentially impacting multiple body systems in both males and females.^1,3

Despite the name, RED-S isn’t limited to elite athletes. It can affect anyone from recreational exercisers to competitive athletes, particularly those with high training demands, and the effects may show up in ways you wouldn’t immediately connect to under-fueling.^1,4 

When Under-Fueling Works Against Your Goals

It seems logical: if you want to lose fat or “get lean,” you eat less. While a moderate calorie deficit can support fat loss, consistently cutting back too much can leave your body without enough energy to meet its needs. When that happens, your body starts conserving energy, which can affect metabolism, muscle building and repair, hormones, bone health, and reproductive function.²

These changes can happen faster than you might think. In a study of trained women, just 10 days of low energy availability reduced resting metabolic rate, muscle protein synthesis, and key hormones—even with plenty of protein and resistance training.^2,5

Your training can take a hit, too. LEA and RED-S have been linked to reduced endurance, strength, power, coordination, and recovery, as well as increased injury risk.^1,4  In other words, your body may not have enough fuel to adapt to and recover from the training you’re asking it to do.

How RED-S Can Affect Your Health

Because your body is working with less energy, the effects of RED-S can show up across multiple systems:

Bone health: Your bones need adequate energy and nutrients to continually break down and rebuild. Chronic under-fueling can disrupt this process, increasing the risk of bone stress injuries and stress fractures.^1,2,4

Hormones and reproductive health: In females, under-fueling can contribute to irregular, lighter, or missing periods.^1,2 Hormonal birth control can make these changes harder to recognize because bleeding patterns may not reflect your natural menstrual cycle.⁷ In males, under-fueling can lower testosterone, potentially affecting sex drive, recovery, and muscle maintenance or growth. ^1,2

Immune system: Consistently low energy availability can affect immune function, potentially increasing illness and disrupting training consistency.^1,6

Heart: Heart rate, blood pressure, and cardiovascular function can change. In the context of chronic under-fueling, a low resting heart rate isn’t always a sign of improved fitness—it may also reflect the body conserving energy.^1,6

Gut: Bloating, constipation, stomach discomfort, and appetite changes can occur—and may make eating enough even harder.^2,6

Mental health: The relationship can go both ways. Stress, perfectionism, disordered eating behaviors, or body-image pressure can contribute to under-fueling, while inadequate energy availability can also affect mood, concentration, and decision-making.^1,6

Blood sugar: Under-fueling or cutting carbs can affect hormones involved in blood sugar regulation, but short-term studies haven’t found consistent changes, and the long-term effects aren’t clear. Blood sugar can be affected by many factors, so abnormal results should be discussed with your healthcare provider or dietitian in the context of your training and nutrition.^1,7

Signs You May Be Under-Fueling

You don’t have to experience every symptom—or look a certain way—to be under-fueled. Signs can vary from person to person, but some things to pay attention to include:

• Persistent fatigue or low energy
• Declining performance or workouts that suddenly feel harder
• Poor recovery and difficulty building or maintaining muscle
• Frequent injuries, especially bone stress injuries
• Getting sick more often
• Irregular or missing periods
• Changes in sex drive
• GI symptoms such as bloating or constipation
• Changes in mood, concentration, or motivation

None of these symptoms automatically means you have RED-S, but a pattern of symptoms—especially alongside a high training load or inadequate food intake—is worth discussing with a qualified healthcare professional.^1,7 

What You Can Do

Addressing under-fueling starts with giving your body enough energy to support both your health and your training.

• Close the fuel gap. Increasing food intake, reducing training load when appropriate, or a combination of both can help restore adequate energy availability.^1,8
• Don’t skip meals, especially around training. Steady fueling across the whole day beats trying to catch up at dinner.^1,8
• Eat enough of everything. Carbs, protein, fats, and colorful, nutrient-dense foods all play a role in supporting training, recovery, hormones, immunity, and bone health.^1,8
• Ask for backup. A sports dietitian, physician, mental health provider, or athletic trainer can help determine what’s going on and what needs to change, especially if several of these symptoms sound familiar.^1,7

The bottom line: Eating enough isn’t giving up on your goals—it’s what gives your body the fuel to train, recover, adapt, and actually reach them. Catching under-fueling early matters because many of the effects of RED-S can improve once adequate energy availability is restored, although recovery takes time and can vary from person to person depending on the individual and which systems have been affected.^1,2  If you’re concerned you may be under-fueling, a qualified healthcare professional, such as a sports dietitian, can help you better understand your individual needs and determine the right approach for you. 

References

1. Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee’s (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023;57(17):1073-1097.
2. Angelidi AM, Stefanakis K, Chou SH, et al. Relative Energy Deficiency in Sport (REDs): endocrine manifestations, pathophysiology and treatments. Endocr Rev. 2024;45(5):676-708.
3. Mountjoy M, Sundgot-Borgen J, Burke L, et al. The IOC consensus statement: beyond the Female Athlete Triad—Relative Energy Deficiency in Sport (RED-S). Br J Sports Med. 2014;48(7):491-497.
4. Gallant TL, Ong LF, Wong L, et al. Low energy availability and Relative Energy Deficiency in Sport: a systematic review and meta-analysis. Sports Med. 2025;55:325-339.
5. Oxfeldt M, Phillips SM, Andersen OE, et al. Low energy availability reduces myofibrillar and sarcoplasmic muscle protein synthesis in trained females. J Physiol. 2023;601:3481-3497.
6. Ackerman KE, Holtzman B, Cooper KM, et al. Low energy availability surrogates correlate with health and performance consequences of Relative Energy Deficiency in Sport. Br J Sports Med. 2019;53(10):628-633.
7. Stellingwerff T, Mountjoy M, McCluskey WT, et al. Review of the scientific rationale, development and validation of the International Olympic Committee Relative Energy Deficiency in Sport Clinical Assessment Tool: V.2 (IOC REDs CAT2). Br J Sports Med. 2023;57(17):1109-1118.
8. Torstveit MK, Ackerman KE, Constantini N, et al. Primary, secondary and tertiary prevention of Relative Energy Deficiency in Sport (REDs): a narrative review by a subgroup of the IOC consensus on REDs. Br J Sports Med. 2023;57(17):1119-1126.