Evidence check: MC4R in the context of fatigue

MC4R (melanocortin 4 receptor) is a key regulator of energy homeostasis and appetite. Loss-of-function mutations are strongly linked to early-onset obesity, but the direct evidence for fatigue is limited. Fatigue may arise secondarily from obesity-related conditions such as sleep apnea, metabolic syndrome, or low-grade inflammation, which can be influenced by MC4R variants. Human observational studies and GWAS show associations with BMI and energy expenditure, but fatigue is not a primary endpoint. The evidence is moderate: MC4R's role in fatigue is indirect and context-dependent. Consumer DNA tests often overstate the link, marketing MC4R as a 'fatigue gene' without robust clinical validation. Effect sizes are small, and lifestyle factors (sleep, stress, nutrition) are far more impactful. A single gene test cannot diagnose or explain chronic fatigue. Clinical evaluation should consider multifactorial causes. Therefore, while MC4R is biologically plausible, its practical utility for fatigue management is currently low.