Acanthosis Nigricans
Cutaneous marker of insulin resistance
What it is
Acanthosis nigricans (AN) is a clinical sign characterised by hyperpigmented, velvety, thickened skin that appears typically in friction areas and skin folds: the neck, armpits, groin, and sometimes under the breasts, at the elbows, or behind the knees. The skin takes on a thick, dark (brown to black) appearance with a slightly rough, velvety texture.
It is a clinical examination finding, visible to the naked eye, that strongly suggests significant insulin resistance. It does not constitute a diagnosis on its own, but its presence should prompt a full metabolic workup (blood glucose, insulin, HOMA-IR, OGTT).
Mechanism: hyperinsulinaemia and keratinocytes
Acanthosis nigricans is caused by hyperinsulinaemia. When insulin is produced in excess (to compensate for insulin resistance), it stimulates IGF-1 receptors (insulin-like growth factor-1) on keratinocytes — the cells of the outer skin layer. This stimulation leads to accelerated keratinocyte proliferation, producing the characteristic thickening and pigmentation.
Prevalence in PCOS
Acanthosis nigricans is observed in 40 to 50% of obese people with PCOS. Its prevalence is lower in lean PCOS, although it can still appear in this context if hyperinsulinaemia is present. In African American and Hispanic populations, it is more common and may be more pronounced.
Course and treatment
Acanthosis nigricans is potentially reversible if insulin resistance improves. Approaches that have shown improvement include:
- Weight loss (5 to 10% of body weight is often sufficient)
- Metformin — documented improvement after 3 to 6 months
- Inositols (myo-inositol, D-chiro-inositol) — reduce hyperinsulinaemia
- Regular physical activity — improves insulin sensitivity
Topical cosmetic treatments (lightening creams) do not address the underlying cause — they may temporarily improve appearance, but AN returns if insulin resistance is not treated.
Key takeaways
- Dark, velvety, thickened skin in folds (neck, armpits, groin)
- Strong visual marker of insulin resistance
- Caused by hyperinsulinaemia stimulating IGF-1 receptors on keratinocytes
- Present in 40-50% of obese people with PCOS
- Potentially reversible with weight loss, metformin or inositols