Very Low Body Fat in Athletes: Hormones and Performance

Achieving very low or essential body fat levels—typically around 4–6% for males and 10–13% for females—is common in physique, combat, and gravity-dependent sports [4]. While lower nonessential fat mass can optimize power-to-mass ratios, cardiorespiratory efficiency, and joint mechanics [12, 14], prolonged or extreme energy restriction induces substantial physiological stress. Extreme leanness driven by low energy availability (LEA) triggers profound alterations across endocrine signaling, metabolic rate regulation, and neuromuscular performance [4, 19].

Energy Availability and the Etiology of RED-S

Energy availability (EA) reflects the residual dietary energy available for basic physiological processes once the energetic cost of exercise is removed, calculated as:

$$\text{EA}=\frac{\text{Energy Intake (kcal)}-\text{Exercise Energy Expenditure (kcal)}}{\text{Fat-Free Mass (kg)}}$$

Optimal physiological functioning in healthy adults requires an EA of approximately 45 kcal/kg FFM/day [19]. When intake is suppressed to achieve extreme leanness, athletes frequently fall below clinical LEA thresholds, defined as <30 kcal/kg FFM/day for females and <25 kcal/kg FFM/day for males [19]. In collegiate athletes, being significantly below ideal body weight serves as a strong predictor of elevated risk for Relative Energy Deficiency in Sport (RED-S), even after adjusting for body fat percentage [24].

Under chronic LEA, the body shifts energy away from reproduction, growth, thermoregulation, and tissue turnover to preserve vital cellular processes [19, 23]. In weight-category combat sports, rapid weight loss (RWL) and chronic energy cuts drop EA drastically; for instance, a 5-week combat camp showed EA reductions from 42 kcal/kg FFM/day to 22 kcal/kg FFM/day during fight week, inducing systemic endocrine and metabolic strain [16, 17].

Endocrine Disruptions at Very Low Body Fat

Severe energy restriction and depletion of adipose reserves disturb multiple hormonal axes [4, 23]:

• Hypothalamic-Pituitary-Gonadal (HPG) Axis: Reduced gonadotropin-releasing hormone (GnRH) pulsatility dampens luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release [23]. In males, contest preparation leads to significant drops in total and free testosterone alongside increases in sex hormone-binding globulin (SHBG) [3]. In a case report of a natural male bodybuilder reaching stage leanness, total testosterone fell to a clinically low 7.2–9.1 nmol/L [1]. In females, HPG suppression manifests as functional hypothalamic amenorrhea (FHA) and anovulation [19, 23].
• Thyroid Function and Thermoregulation: Active triiodothyronine (free T3) and free T4 drop significantly [4, 5]. Free T3 has been documented dropping to clinically suppressed levels (2.7 pmol/L) at peak leanness, correlating directly with reductions in core oral temperature (r = 0.674) and resting heart rate (r = 0.560) [1].
• Somatotropic and Anabolic Axis: Circulating insulin-like growth factor 1 (IGF-1), its binding protein IGFBP-3, and resting insulin decline markedly during deep caloric restriction, with reductions in IGF-1 closely tracking fat-free mass (FFM) catabolism [3].
• Stress and Satiety Peptides: Extreme leanness elevates systemic cortisol and ghrelin while suppressing leptin due to adipose depletion, exacerbating mood disturbances, fatigue, and appetite disinhibition [3, 4, 23].

Metabolic Adaptations and Energy Expenditure

When athletes achieve low body fat percentages, resting energy expenditure (REE) decreases beyond what is predicted by the reduction in body mass alone—a phenomenon known as adaptive thermogenesis [1, 4].

In natural bodybuilding preparations, measured REE can decline by over 500 kcal/day (REE ratio ~0.78) alongside a shift in the respiratory exchange ratio from 0.95 to 0.85 [1]. Similarly, simulated fight camp weight cuts in female combat athletes induced a 253 kcal/day reduction in resting metabolic rate (RMR), concurrent with signs of renal strain and hypoglycemia [16, 22]. In female athlete triad cohorts, over 53% of participants presenting with LEA demonstrate suppressed RMR [22].

Neuromuscular and Aerobic Performance Outcomes

Reducing excess nonessential adipose tissue provides distinct mechanical advantages in endurance, sprint, and bodyweight-supported athletic tasks by improving the power-to-mass ratio and minimizing physical resistance to joint articulation [12, 14, 15]. For instance, elite CrossFit athletes with lower body fat percentages demonstrate higher relative ${\text{VO}}_2\text{max}$ and superior anaerobic Wingate power compared to alpine athletes with higher adiposity [11].

However, when body fat is reduced to essential thresholds under severe caloric restriction, neuromuscular performance suffers [1, 16]:

• Strength Reductions: Absolute strength across primary compound lifts (squat, bench press, deadlift) declines during contest prep [1, 3].
• Power Output Losses: Weight-cycling combat athletes can experience up to a 27% drop in maximal cycling power output when rapid weight loss strategies are compounded with chronic restriction [16].
• Neuromuscular Fatigue: Glycogen depletion, suppressed muscle protein synthesis, and elevated fatigue scores limit force generation and work capacity under extreme caloric deficits [3, 16].

Dietary Refeeding and Physiological Recovery

Mitigating lean tissue loss during fat loss and restoring physiological homeostasis post-deficit require targeted nutritional strategies [4, 8].

Intermittent Refeeds and Diet Breaks

Structuring temporary increases in carbohydrate intake attenuates metabolic and endocrine decline during fat loss [6, 8]:

• 2-Day Carbohydrate Refeeds: Implementing a 2-day consecutive carbohydrate refeed during a 25% caloric deficit significantly preserved FFM (0.4 kg loss vs. 1.3 kg loss in continuous restriction) and mitigated RMR suppression (-38 kcal vs. -78 kcal) without compromising total fat loss [8].
• Diet Breaks: Introducing 1-week diet breaks at maintenance intake during an 8-week restriction period preserved behavioral eating control by preventing increases in disinhibition on the Three-Factor Eating Questionnaire compared to continuous restriction [7].

Post-Restriction Recovery Timelines

Restoring suppressed hormonal and metabolic profiles requires a sustained return to energy balance or surplus [4, 5]. Longitudinal tracking of natural physique competitors reveals that fat mass (+3.4 kg) and fat-free mass (+2.7 kg) rebound rapidly within the first 6 weeks post-competition [5].

By 12 to 23 weeks post-competition, thyroid hormones (FT3, FT4), testosterone, cortisol, insulin, and IGF-1 normalize to baseline reference ranges [3, 5]. Larger post-restriction increases in energy intake accelerate the recovery of adjusted resting metabolic rate ($+2.4\text{ kcal}\cdot {\text{kg}}_{\text{FFM}}^{-1}\cdot {\text{day}}^{-1}$), underscoring that reversing the endocrine and metabolic consequences of extreme leanness requires adequate caloric and nutritional rehabilitation [5, 22].