Evidence check: HbA1c in the context of fatigue

HbA1c is a well-established biomarker for average blood glucose over 2–3 months. Elevated HbA1c (>6.5%) indicates diabetes or prediabetes, which can cause fatigue through hyperglycemia, dehydration, or insulin resistance. Conversely, very low HbA1c (<4.5%) in diabetics may reflect frequent hypoglycemia, also linked to fatigue. However, fatigue is highly nonspecific: sleep deprivation, stress, thyroid disorders, iron deficiency, or depression are more common causes. A normal HbA1c does not rule out metabolic issues, especially in early prediabetes. The evidence for a direct causal link between HbA1c and fatigue is moderate and context-dependent. Clinical decisions should consider repeated measurements, fasting glucose, and symptoms. Persistent fatigue warrants comprehensive medical evaluation beyond HbA1c alone.