Does low leptin from low energy availability suppress the HPG axis and cause fatigue and cognitive symptoms?

Leptin serves as a critical metabolic sensor that communicates energy status to the brain. When energy intake is insufficient to cover both the physiological demands of daily life and exercise—a state known as Low Energy Availability (LEA)—serum leptin levels drop significantly. This biological cascade has direct implications for hormonal health, energy levels, and cognitive function.

Clinical and mechanistic evidence

The suppression of the hypothalamic–pituitary–gonadal (HPG) axis is a primary physiological response to LEA. This mechanism is driven by the brain's "metabolic gatekeepers," where low leptin levels and increased hunger signals (like NPY/AgRP) inhibit kisspeptin neurons. Because kisspeptin is required to stimulate gonadotropin-releasing hormone (GnRH), its suppression leads to:

• Reduced LH/FSH pulsatility: Blunted secretion of luteinizing hormone and follicle-stimulating hormone from the pituitary.
• Hormonal deficiency: Lowered estradiol production, which can result in menstrual irregularities or functional hypothalamic amenorrhea.
• Metabolic slowing: Reductions in metabolic rate and bone turnover markers as the body prioritizes survival over reproduction.

Fatigue and cognitive symptoms

LEA is strongly associated with both subjective and objective impairments in energy and mental performance:

• Fatigue pathways: LEA is a hallmark of Relative Energy Deficiency in Sport (RED-S), which consistently elevates perceived fatigue. This is often accompanied by slower recovery times and a higher physiological strain during routine activities.
• Cognitive impact: Energy deficits can trigger hypothalamic–pituitary–adrenal (HPA) axis dysregulation, leading to elevated cortisol levels. Chronic hypercortisolemia is linked to deficits in working memory, executive function, and attention.
• Brain energy crisis: Inadequate carbohydrate intake—a common component of LEA—depletes glycogen stores, limiting glucose availability to the brain. This can result in reduced processing speed and impaired memory during periods of high demand.

Bottom line

Low leptin is a validated biomarker for energy deficiency that triggers a defensive shutdown of the reproductive system and metabolic rate. This state is scientifically linked to increased fatigue and cognitive deficits due to hormonal shifts and reduced fuel availability for the central nervous system.