Under-fueling in endurance runners refers to an inadequate supply of energy and/or carbohydrate relative to training demands. While some athletes intentionally restrict intake for weight goals, chronic energy deficit—often termed Relative Energy Deficiency in Sport (RED-S)—is a clinically important syndrome with broad consequences for metabolism, performance, endocrine function, bone health, immune competence, and thermoregulation. A runner who repeatedly runs “colder than usual,” feels unusually fatigued, develops persistent soreness, experiences frequent illness, or has impaired recovery may be showing early phenotypes of under-fueling.
At the physiologic level, the central mechanism is a mismatch between energy availability and expenditure. Energy availability is the dietary energy remaining for physiological functions after subtracting exercise energy cost (kcal/kg of fat-free mass/day). When energy availability drops below individual thresholds, the body downregulates nonessential systems to preserve survival. In RED-S, hormonal axes are affected: leptin and insulin dynamics shift, thyroid hormone conversion can decrease (leading to lower resting metabolic rate), and gonadotropin-releasing hormone signaling may be suppressed, contributing to menstrual dysfunction, reduced libido, or impaired spermatogenesis. In males and females, suppressed recovery and altered substrate utilization can reduce training adaptation and increase injury risk.
Thermoregulation is one reason under-fueling can present as feeling colder. With reduced glycogen stores and limited carbohydrate availability, runners have diminished capacity for rapid ATP regeneration and may rely more on fatty acid oxidation, which is slower and can be associated with less heat production at a given workload. Additionally, low glycogen and energy deficit can impair peripheral blood flow regulation and reduce the effectiveness of behavioral and autonomic responses to cold exposure. Sleep disturbance, which commonly co-occurs with inadequate intake, further worsens temperature perception and recovery.
Carbohydrate is particularly relevant. During endurance exercise, muscle glycogen availability supports sustained power output. When total daily carbohydrate intake is inadequate—or when pre-run and post-run fueling are inconsistent—performance may drop, perceived exertion rises, and recovery slows. Low energy availability also contributes to increased muscle protein breakdown and reduced synthesis, creating a cycle of prolonged soreness and delayed adaptation.
Clinical red flags often include repeated “off” days, unexpected fatigue, frequent minor infections, persistent injuries or slower healing, gastrointestinal symptoms, mood changes, irritability, reduced concentration, and in some athletes, disordered eating behaviors or fear of weight gain. Although “feeling cold” is not diagnostic alone, it can serve as a practical warning sign when combined with other indicators such as poor recovery, reduced libido, menstrual irregularities, or unexplained performance plateaus.
Diagnosis is primarily clinical and dietary. Clinicians assess training load, body weight trends, menstrual history (for women), injury pattern, sleep quality, gastrointestinal symptoms, and psychological factors. Dietary assessment using recalls, weighed food records, or validated apps can estimate energy and carbohydrate intake. Laboratory tests may support evaluation (e.g., iron status, thyroid function, vitamin D, markers of inflammation), but normal results do not exclude RED-S; the syndrome is defined by low energy availability and functional consequences.
Management focuses on restoring energy availability and optimizing carbohydrate timing. Practical interventions include:
1) Increase daily energy intake to match training demands, using a gradual approach to avoid gastrointestinal distress.
2) Prioritize carbohydrate: commonly 1–4 g/kg/day depending on training intensity, with larger amounts on hard or long sessions; for sessions over ~60–90 minutes, include carbohydrate during exercise (often 30–60 g/hour, individualized).
3) Ensure pre- and post-run fueling: pre-exercise carbohydrate with some fluid 1–3 hours before and post-exercise carbohydrate plus protein within a recovery window to support glycogen resynthesis and muscle repair.
4) Distribute intake across the day to avoid “all calories at night” patterns that may not support glycogen refueling.
5) Address micronutrient adequacy (iron, calcium, vitamin D, and electrolytes) and hydration.
6) Screen for eating disorders or anxiety around food; involve a sports dietitian and, when necessary, a mental health professional.
Athletes should also consider modifying training while correcting intake—reducing volume or intensity temporarily can help reverse energy deficit and reduce injury risk. Monitoring can include symptom tracking, recovery metrics, menstrual and libido changes, body composition trends, and performance markers.
If symptoms are persistent, severe, or accompanied by functional impairment (stress fractures, amenorrhea, recurrent illness, or significant mood changes), medical evaluation is warranted. Early recognition of under-fueling can prevent progression to more serious RED-S complications and supports sustainable performance.
Source: Runner’s World UK post about feeling colder than usual and other signs of being under-fueled
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