Does chronic low energy availability lead to mixed cortisol awakening responses, including a blunted CAR?

Low energy availability (LEA), a state where energy intake is insufficient to support both health and exercise demands, acts as a significant metabolic stressor that alters the hypothalamic-pituitary-adrenal (HPA) axis. Research into the cortisol awakening response (CAR)—the rapid increase in cortisol levels within 30–45 minutes of waking—reflects a complex and sometimes contradictory landscape in individuals facing chronic energy deficits.

Clinical and mechanistic findings

Research into the endocrine profiles of athletes and individuals with LEA highlights significant variability in how the HPA axis responds to long-term energy insufficiency:

• Evidence of dysregulation: Chronic LEA is a hallmark of Relative Energy Deficiency in Sport (RED-S). While the HPA axis initially responds to energy deficits with hyper-responsiveness (elevated baseline cortisol), chronic metabolic strain can lead to adaptive downregulation. Studies on female athletes with menstrual dysfunction—a clinical marker of chronic LEA—frequently show lower cortisol levels or flattened diurnal rhythms compared to eumenorrheic controls.
• Mixed results: Data remains inconsistent; while some studies of elite athletes show a 22% increase in systemic cortisol under LEA, others observe a blunted or entirely flattened CAR. This heterogeneity is often attributed to the severity and duration of the deficit, as well as the specific population studied (e.g., endurance athletes vs. individuals with eating disorders).
• Biological mechanisms: Mechanistically, a blunted CAR in chronic LEA is plausible as an energy-conservation strategy. Prolonged metabolic stress can reduce the pulsatility of hypothalamic corticotropin-releasing hormone (CRH), leading to a suppressed adrenal response. This mirrors HPA axis "exhaustion" seen in other chronic stress states, where the body limits its awakening cortisol surge to prevent further catabolism of tissues.

Practical considerations

The variability in research findings suggests that the cortisol awakening response is highly sensitive to external variables. Factors such as sleep quality, the timing of the last meal, and the intensity of training the previous day can all influence the CAR, making it difficult to establish a universal "LEA signature" for cortisol. Furthermore, many studies utilize single-point cortisol measurements rather than the detailed area under the curve (AUC) metrics required to fully characterize the CAR slope.

Bottom line

The claim that studies of the cortisol awakening response in chronic LEA show mixed results, including a blunted response, is well-supported by physiological evidence. While chronic LEA generally drives HPA axis dysregulation, the specific manifestation as a blunted CAR is a documented but inconsistent finding that likely depends on the degree of metabolic adaptation and individual stress resilience.