Insulin Resistance in PCOS: The Metabolic Heart of the Syndrome

Introduction

Insulin resistance (IR) is the most important metabolic abnormality in PCOS and underlies many of its most serious long-term complications. It is present in 50–80% of women with PCOS — including lean women, who may have normal glucose levels despite significant underlying IR. Understanding insulin resistance explains not just why PCOS causes metabolic disease, but also why it causes excess androgens, anovulation, and many of its other features.

What Is Insulin Resistance?

Insulin resistance means that cells — particularly in muscle, liver, and adipose tissue — do not respond normally to insulin\'s signal to take up glucose. The pancreas compensates by producing more insulin (hyperinsulinaemia). Blood glucose levels may remain normal for years (compensated IR

How Insulin Resistance Causes Androgen Excess

The unique feature of insulin resistance in PCOS is that while the ovaries and adrenal glands are insulin resistant (like muscle and liver

Mechanisms:

• High circulating insulin directly stimulates ovarian theca cells to produce testosterone via the PI3K signalling pathway — the same pathway that mediates insulin\'s glucose-lowering action in muscle (and which is impaired in insulin resistance) but which remains intact in theca cells.
• Insulin suppresses hepatic SHBG (sex hormone-binding globulin) production — lowering SHBG raises free testosterone.
• Insulin amplifies LH-stimulated androgen production in the ovary.
• Insulin and IGF-1 (insulin-like growth factor 1
• elevated in hyperinsulinaemia) have additive effects on theca cell androgen synthesis.
• Elevated androgens from IR also worsen insulin resistance — creating a vicious cycle.

How Insulin Resistance Causes Anovulation

Hyperinsulinaemia impairs follicle maturation by:

• Amplifying LH\'s stimulation of thecal androgens
• which accumulates in the follicular fluid and arrests follicle development.
• Reducing granulosa cell FSH sensitivity
• impairing follicle selection and dominant follicle emergence.
• Promoting early follicle recruitment while preventing their maturation — resulting in the characteristic "string of pearls" of multiple arrested follicles.

Long-Term Metabolic Risks

Insulin resistance in PCOS drives an escalating metabolic risk profile over time:

• Impaired glucose tolerance: 30–40% of PCOS women by the third decade
• Type 2 diabetes: 40% of PCOS women by age 40 — 4–8 times the general population rate
• Metabolic syndrome: Approximately 40% of PCOS women
• Non-alcoholic fatty liver disease (NAFLD): Highly prevalent even in lean PCOS
• driven by hepatic insulin resistance
• Dyslipidaemia: Low HDL
• high triglycerides
• small dense LDL — atherogenic profile
• Cardiovascular disease: Risk is elevated
• though data on hard events (MI
• stroke) are mixed due to the younger age of the PCOS population

Diagnosing Insulin Resistance in PCOS

No perfect test exists for IR in clinical practice. Options:

• Fasting glucose and insulin (calculate HOMA-IR = fasting insulin × fasting glucose / 22.5): HOMA-IR >2.5–3.5 suggests IR
• though cutoffs vary by laboratory.
• Oral glucose tolerance test (OGTT): Gold standard for detecting impaired fasting glucose and impaired glucose tolerance. The 2023 PCOS Guideline recommends OGTT (not just fasting glucose) in all women with PCOS who are overweight
• or in all PCOS women if risk factors are present.
• HbA1c: Less sensitive for detecting impaired glucose tolerance; use as a supplement
• not substitute.
• AMH elevation is an indirect marker but not specific for IR.

Treating Insulin Resistance in PCOS

Lifestyle — First and Most Important

Even a 5–10% reduction in body weight through diet and exercise significantly reduces insulin resistance, lowers androgens, often restores ovulation, and reduces metabolic risk. A low-glycaemic index (GI) diet reduces postprandial insulin spikes; the Mediterranean diet has specific evidence for reducing IR in PCOS.

Metformin

Metformin (a biguanide) reduces hepatic glucose production and improves peripheral insulin sensitivity. It is the most prescribed insulin sensitiser in PCOS. Evidence supports its use for: metabolic risk reduction, improving cycle regularity, modest androgen reduction, and as an adjunct to ovulation induction (improving response to letrozole). It is not a first-line weight loss agent but may modestly support weight management.

Inositols

Myo-inositol and D-chiro-inositol are naturally occurring molecules involved in insulin signalling. Several RCTs show they improve insulin sensitivity, reduce testosterone, and improve cycle regularity in PCOS. They are available as supplements and are generally well-tolerated. The 2023 guideline notes emerging but insufficient evidence for a formal recommendation.

References: 2023 International PCOS Guideline; Diamanti-Kandarakis E, Dunaif A, Endocr Rev 2012; Balen AH, Polycystic Ovary Syndrome, CRC Press 2022.