Insulin Resistance

Why This Matters

Insulin resistance is one of the most common metabolic disorders worldwide and is a major contributor to cardiovascular disease. Although it is often discussed in the context of type 2 diabetes mellitus, insulin resistance usually develops years before blood glucose levels become abnormal. During this prolonged period, it silently promotes dyslipidemia, hypertension, obesity, hepatic steatosis, chronic inflammation, endothelial dysfunction, and atherosclerosis. Recognizing insulin resistance early provides one of the greatest opportunities for preventing cardiovascular disease — it is the mechanism this book's main chapters place at the center of the whole argument, and everything in this section builds outward from it.

What Is Insulin Resistance?

Insulin is the principal anabolic hormone responsible for regulating glucose, lipid, and protein metabolism.

In insulin resistance, target tissues — including skeletal muscle, liver, and adipose tissue — respond less effectively to normal concentrations of insulin. To maintain normal blood glucose levels, pancreatic beta-cells compensate by secreting increasing amounts of insulin, resulting in hyperinsulinemia.

For many years, blood glucose may remain within the normal range while significant metabolic abnormalities are already present.

Effects on Lipid Metabolism

Insulin resistance profoundly alters lipoprotein metabolism.

The liver increases production of VLDL particles, leading to hypertriglyceridemia. Through the action of cholesteryl ester transfer protein (CETP), triglycerides are exchanged for cholesterol esters between VLDL and LDL or HDL. Hepatic lipase subsequently remodels these triglyceride-enriched particles, producing smaller, cholesterol-depleted LDL and HDL particles.

Although LDL cholesterol may appear normal, ApoB particle number is frequently increased because the liver produces more VLDL particles, each of which ultimately gives rise to an LDL particle. This is exactly why ApoB often provides a more accurate assessment of cardiovascular risk than LDL cholesterol in patients with insulin resistance, as Chapter 33 described.

Beyond Glucose

Insulin resistance affects nearly every organ system.

It contributes to fatty liver disease, visceral obesity, hypertension, polycystic ovary syndrome, elevated uric acid, obstructive sleep apnea, chronic kidney disease, and systemic inflammation. Collectively, these abnormalities form the basis of the cardiometabolic syndrome. Cardiovascular disease often begins long before diabetes is diagnosed.

Identifying Insulin Resistance

There is no single universally accepted clinical test for insulin resistance.

Instead, clinicians integrate metabolic findings such as central obesity, elevated triglycerides, reduced HDL cholesterol, increased waist circumference, impaired fasting glucose, elevated fasting insulin, metabolic syndrome, and family history. Additional tools — including the triglyceride-to-HDL ratio and advanced lipoprotein testing — may provide further insight in selected patients.

Treatment

The most effective treatment addresses the underlying metabolic disturbance rather than simply lowering blood glucose.

Weight reduction, regular physical activity, improved sleep, avoidance of tobacco, and dietary patterns that reduce postprandial hyperinsulinemia improve insulin sensitivity. When appropriate, medications that improve insulin sensitivity or reduce excess weight may provide additional metabolic and cardiovascular benefit. Treatment should focus on improving metabolic health rather than simply lowering blood glucose.

Clinical Relevance

Insulin resistance links obesity, dyslipidemia, hypertension, fatty liver disease, type 2 diabetes mellitus, and atherosclerotic cardiovascular disease.

Recognizing this common pathophysiologic pathway allows clinicians to intervene earlier, address multiple risk factors simultaneously, and reduce long-term cardiovascular risk.

Clinical Pearls

• Insulin resistance often precedes diabetes by many years.
• Hyperinsulinemia may be present despite normal blood glucose.
• ApoB frequently identifies increased cardiovascular risk when LDL cholesterol appears acceptable.
• Insulin resistance affects multiple organ systems, not glucose metabolism alone.
• Improving insulin sensitivity reduces cardiometabolic risk beyond glycemic control.

Common Misconceptions

Misconception: Insulin resistance is important only after diabetes develops.

Reality: It often begins years earlier and contributes to cardiovascular disease long before diabetes is diagnosed.

Misconception: Normal glucose excludes significant metabolic disease.

Reality: Many individuals with normal glucose concentrations already have substantial insulin resistance and hyperinsulinemia.

Misconception: Treatment should focus exclusively on lowering blood glucose.

Reality: Improving overall metabolic health — including insulin sensitivity, body composition, lipid metabolism, and blood pressure — is the primary objective.

Clinical Bottom Line

Insulin resistance is a central driver of cardiometabolic disease. By promoting dyslipidemia, inflammation, endothelial dysfunction, and accelerated atherosclerosis long before diabetes develops, it provides one of the earliest and most important opportunities for cardiovascular prevention.