Are You Training Hard but Underfueling? How Nutrition Affects Recovery, Strength, and Performance
Ground to Overhead Physical Therapy | Cary, Chapel Hill & Durham, NC
You're training consistently, hitting your protein goals, and following a structured program. But despite your efforts, progress has stalled, fatigue is accumulating, or injuries keep interrupting your training.
What if the missing piece isn't another exercise, supplement, or recovery tool?
What if you're not eating enough to support the demands you're placing on your body?
For CrossFit athletes, Olympic weightlifters, runners, triathletes, and obstacle-course racers, nutrition does more than fuel workouts. It supports muscle adaptation, bone health, recovery, and overall physiological function.
Research suggests that inadequate fueling can interfere with these processes, even when
training and protein intake are otherwise well managed.
What Is Low Energy Availability?
Low energy availability (LEA) occurs when insufficient energy remains to support normal physiological functions after accounting for exercise expenditure.
Researchers estimate energy availability using:
Energy Availability = (Energy Intake − Exercise Energy Expenditure) ÷ Fat-Free Mass
This distinction matters because eating a substantial number of calories doesn't necessarily mean you're eating enough for your activity level.
Consider an athlete lifting four days per week, running three days per week, and adding conditioning sessions. Their nutritional demands may be substantially greater than those of someone exercising three times weekly.
Underfueling can also occur unintentionally when training increases without a corresponding increase in food intake.
Importantly, low energy availability doesn't automatically mean someone has an eating disorder, is underweight, or has Relative Energy Deficiency in Sport (REDs).
REDs is a clinical syndrome associated with problematic, often prolonged or severe low energy availability. It can affect multiple physiological systems in both men and women.
How Common Is Underfueling?
A 2025 systematic review and meta-analysis published in Sports Medicine examined 59 studies investigating low energy availability and REDs.
Across 46 studies involving 6,118 athletes, approximately 44.7% were classified as having low energy availability.
Researchers also identified associations with reduced endurance performance, impaired training responses, and increased illness-related training absences.
However, these findings require context.
Different studies used different definitions and assessment methods. Estimating food intake and exercise expenditure can also introduce considerable measurement error.
Therefore, the 44.7% figure describes the athletes studied, not necessarily the prevalence across the entire athletic population.
Nevertheless, the research suggests that inadequate fueling deserves consideration when athletes experience unexplained declines in performance or recovery.
Can You Eat Enough Protein but Still Limit Muscle Growth?
Absolutely.
One of the most interesting studies examining this question was published in The Journal of Physiology in 2023.
Researchers randomized 30 trained women to 10 days of either low or optimal energy availability while completing supervised resistance and cardiovascular training.
Both groups received substantial protein intake, approximately 2.2 grams per kilogram of lean mass daily.
Despite similar protein provision, the low-energy-availability group demonstrated reduced muscle protein synthesis.
Researchers also observed unfavorable changes in lean mass and several metabolic and hormonal markers.
The takeaway: Adequate protein intake alone may not fully compensate for insufficient total energy intake.
This doesn't mean every athlete needs a calorie surplus to improve strength or muscle mass.
It does suggest that consistently restricting energy intake while training hard may interfere with the adaptations you're working to achieve.
What Does the Newer Research Show?
A 2026 systematic review published in the British Journal of Sports Medicine examined 21 studies involving 536 participants.
Researchers investigated how low energy availability affected neuromusculoskeletal responses to training.
Ten studies identified impaired lean-mass responses, while ten found little substantial change.
Six studies reported declines in functional or sport-specific performance among participants experiencing low energy availability.
Although these findings suggest underfueling can compromise training adaptations, differences in study design, energy-availability definitions, and training programs prevented a pooled meta-analysis.
The evidence is concerning but not entirely consistent.
For athletes trying to improve their squat, snatch, clean and jerk, running performance, or overall strength, nutrition deserves consideration alongside programming and recovery.
What About Bone Health and Injury Risk?
Bone stress injuries are especially relevant for runners and endurance athletes.
A prospective study published in the American Journal of Sports Medicine followed 259 physically active girls and women.
Researchers examined risk factors associated with the Female Athlete Triad, including menstrual dysfunction, reduced bone mineral density, and indicators of inadequate energy availability.
Twenty-eight participants developed bone stress injuries.
The likelihood of injury increased when multiple risk factors were present.
Certain combinations of risk factors were associated with bone stress injury rates approaching 30–50% within specific high-risk subgroups.
These findings don't establish that low energy availability independently causes every bone stress injury.
They demonstrate that nutritional, hormonal, skeletal, and training-related factors should be considered together.
Why This Matters for Physical Therapy
Imagine a runner recovering from a tibial bone stress injury.
A rehabilitation program might appropriately include progressive strengthening, walking, impact loading, and a gradual return to running.
But if inadequate fueling contributed to compromised bone health, exercise progression alone may not address every factor affecting recovery.
The same principle applies to athletes attempting to regain muscle mass and strength after injury or surgery.
Rehabilitation requires more than an appropriate exercise stimulus. The body also needs sufficient resources to respond to that stimulus.
Nutrition doesn't replace physical therapy. It supports the physiological environment in which rehabilitation occurs.
Are Carbohydrates Important for Recovery?
Carbohydrates are commonly associated with glycogen replenishment and endurance performance.
However, emerging research suggests carbohydrate availability may also influence bone metabolism.
A 2022 controlled study involving 28 elite racewalkers compared different dietary approaches over six-day intervention periods.
Researchers found that short-term carbohydrate restriction produced unfavorable changes in circulating markers associated with bone formation and resorption.
Low energy availability also affected certain bone turnover markers, although the responses differed between interventions.
Importantly, these were short-term biochemical measurements.
The study did not establish that carbohydrate restriction directly increases long-term fracture risk.
Still, it reinforces that both total energy and carbohydrate availability deserve attention, particularly among athletes completing prolonged or demanding training sessions.
Can Eating More Actually Improve Physiological Function?
The REFUEL randomized controlled trial provides important evidence.
Published in Human Reproduction in 2021, the study investigated whether increasing energy intake could improve menstrual function in exercising women experiencing menstrual disturbances.
Participants assigned to increase their intake consumed approximately 330 additional calories per day on average.
The increased-intake group demonstrated improved menstrual recovery compared with controls.
In a specified subgroup analysis, 64% of the increased-intake group showed improved menstrual function compared with 19% of controls.
These results provide direct evidence that increasing energy intake can improve certain consequences of inadequate fueling.
However, the additional 330 calories represented an average within this study—not a universal recommendation.
Individual needs vary, and treatment of suspected REDs requires appropriate clinical assessment.
How Can Athletes Recognize Potential Underfueling?
Low energy availability isn't always obvious.
Potential warning signs include:
Persistent fatigue or declining training performance
Difficulty recovering between sessions
Recurrent illness or bone stress injuries
Menstrual disturbances
Reduced libido or other hormonal concerns
Unexplained changes in energy, mood, or overall well-being
None of these symptoms independently confirms low energy availability or REDs.
Other medical conditions can produce similar symptoms.
The International Olympic Committee recommends a structured, physician-led assessment for suspected REDs, ideally involving a multidisciplinary healthcare team.
Five Practical Strategies to Support Recovery and Performance
1. Match Nutrition to Training Demands
A rest day, a heavy lifting session, and a three-hour trail run create different nutritional demands.
Consider how training duration, intensity, and frequency influence your energy requirements.
2. Don't Focus Exclusively on Protein
Protein is essential for muscle repair and adaptation, but carbohydrates, dietary fats, and sufficient total energy also matter.
Meeting your protein target doesn't automatically mean you're adequately fueled.
3. Fuel Longer and More Demanding Sessions
For prolonged endurance training or multiple daily workouts, appropriate carbohydrate intake can support performance and glycogen availability.
Individual fueling needs depend on training duration, intensity, and gastrointestinal tolerance.
4. Avoid Aggressive Calorie Restriction During Rehabilitation
Athletes often reduce food intake when injuries limit training.
While energy requirements may change, tissue recovery and rehabilitation still require adequate nutrition.
5. Evaluate Recovery Alongside Performance
Monitor patterns in training quality, fatigue, sleep, illness, and recovery.
If persistent symptoms develop, consider an assessment with a qualified sports medicine professional and registered sports dietitian.
The Bottom Line: Training Is Only Part of the Equation
Training provides the stimulus for adaptation.
Recovery provides the opportunity for that adaptation to occur.
Nutrition helps supply the energy and nutrients necessary to support the process.
Current research supports three important conclusions:
1. Inadequate energy availability can interfere with muscle adaptation and performance. However, the magnitude of these effects varies across studies.
2. Nutrition influences more than body composition. Bone health, hormonal function, and overall physiological well-being may also be affected.
3. Effective rehabilitation and performance training should consider fueling. Particularly when athletes experience recurrent injuries, persistent fatigue, or unexplained performance declines.
At Ground to Overhead Physical Therapy, we help CrossFit athletes, Olympic weightlifters, runners, triathletes, obstacle-course racers, and active individuals throughout Cary, Chapel Hill, Durham, Raleigh, and the surrounding Triangle area.
Our approach combines individualized orthopedic physical therapy, progressive strength and conditioning, and education to help athletes return to the activities they enjoy.
Whether you're recovering from an injury or preparing for your next competition, your rehabilitation and training should reflect your individual goals.
Train with purpose. Fuel appropriately. Recover effectively. Perform better.
Peer-Reviewed References
Mountjoy M, et al. (2023). 2023 International Olympic Committee's consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine.DOI: 10.1136/bjsports-2023-106994
Gallant TL, et al. (2025). Low Energy Availability and Relative Energy Deficiency in Sport: A Systematic Review and Meta-analysis. Sports Medicine.DOI: 10.1007/s40279-024-02130-0
DeJong Lempke AF, et al. (2026). Impaired neuromusculoskeletal response to training stimuli associated with low energy availability: a systematic review. British Journal of Sports Medicine.PMID: 41087038
Oxfeldt M, et al. (2023). Low energy availability reduces myofibrillar and sarcoplasmic muscle protein synthesis in trained females. The Journal of Physiology.DOI: 10.1113/JP284967
Barrack MT, et al. (2014). Higher incidence of bone stress injuries with increasing female athlete triad-related risk factors. American Journal of Sports Medicine.DOI: 10.1177/0363546513520295
Fensham NC, et al. (2022). Short-Term Carbohydrate Restriction Impairs Bone Formation at Rest and During Prolonged Exercise to a Greater Degree than Low Energy Availability. Journal of Bone and Mineral Research.DOI: 10.1002/jbmr.4658
De Souza MJ, et al. (2021). Randomised controlled trial of the effects of increased energy intake on menstrual recovery in exercising women with menstrual disturbances: the REFUEL study. Human Reproduction.DOI: 10.1093/humrep/deab149
Stellingwerff T, et al. (2023). Review of the scientific rationale, development and validation of the IOC Relative Energy Deficiency in Sport Clinical Assessment Tool: V.2. British Journal of Sports Medicine.DOI: 10.1136/bjsports-2023-106914
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