Hormones

Insulin

The master nutrient storage and signaling hormone

Plain English

Insulin is a hormone released by the pancreas after you eat, primarily in response to rising blood glucose. Its job is to act as a key that unlocks cells so they can absorb glucose from the blood for energy or storage. Without insulin, glucose stays trapped in the bloodstream, which is the core problem in diabetes.

The Mechanism

When you eat carbohydrates or protein, blood glucose rises and the beta cells of the pancreas secrete insulin in proportion to the size of the rise. Insulin travels through the bloodstream and binds to receptors on muscle, fat, and liver cells, triggering the transport of glucose transporter proteins (GLUT4) to the cell surface. These transporters pull glucose into the cell, where it is used for energy immediately or stored as glycogen in muscle and liver, or converted to fat for longer-term storage.

Insulin also signals cells to take up amino acids for protein synthesis and suppresses fat breakdown, which is why it is considered the primary anabolic storage hormone. The pancreas releases insulin in two phases: a rapid first phase within minutes of eating (using pre-stored insulin) and a slower second phase that ramps up over the following 30 to 90 minutes in response to sustained glucose elevation.

The liver is the primary target for insulins glucose-lowering effect: high insulin suppresses hepatic glucose production, preventing the liver from releasing glucose into the blood at the same time glucose is arriving from a meal. When insulin sensitivity declines, this suppression fails, and the liver continues releasing glucose even as blood glucose is already elevated. This hepatic insulin resistance is one of the earliest signs of metabolic dysfunction, typically developing years before fasting glucose rises above clinical thresholds.

Why It Matters

Fasting insulin tells you how hard your pancreas is working. High effort means early resistance.

Insulin is not the enemy, but chronically elevated insulin is. When cells stop responding to insulin efficiently, the pancreas compensates by producing more, which keeps insulin levels high even between meals. Elevated fasting insulin is a direct driver of fat storage, inflammation, and insulin resistance in a self-reinforcing cycle. Tracking fasting insulin alongside glucose gives a decade of warning before a diabetes diagnosis.

Common Misconception

Many people believe insulin spikes are inherently harmful and obsess over keeping insulin as low as possible at all times. Post-meal insulin is the correct response to eating. The problem is chronically elevated fasting insulin between meals, which signals that cells have become resistant. A moderate insulin spike after a protein-rich meal is not a metabolic problem.

What a Healthy Range Looks Like

Optimal

2–6 uIU/mL

Fasting insulin in a metabolically healthy adult; pancreas not working overtime

Normal

6–10 uIU/mL

Reference range lower bound; common in moderately active adults

Elevated

10–20 uIU/mL

Suggests developing insulin resistance; often years before glucose rises

High

>20 uIU/mL

Significant insulin resistance or hyperinsulinemia; requires investigation

These are fasting values only, drawn after an 8-hour fast. Post-meal insulin spikes are normal and expected. Compare your fasting insulin trends over time, not against population averages. Even within the optimal range, an upward trend year-over-year warrants attention.

Signs It Is Disrupted

Energy crashes 1 to 2 hours after eating, especially after carbohydrate-heavy meals
Hunger returning quickly after meals despite adequate calories
Difficulty losing fat even in a caloric deficit
Increasing waist circumference with stable body weight
Fasting glucose creeping upward year-over-year on annual labs

How to Improve It

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Zone 2 cardio. Aerobic exercise activates the AMPK pathway in muscle cells, driving glucose uptake independent of insulin and improving insulin sensitivity over 6 to 12 weeks of regular training.

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Resistance training. Building muscle mass increases the bodys primary glucose disposal sink; more muscle means more GLUT4 transporters and greater insulin sensitivity at rest and after meals.

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Post-meal walking. A 10 to 15 minute walk after eating reduces post-meal glucose spikes by 20 to 30% (Buffey et al., 2022) by utilizing muscle glucose uptake without requiring significant insulin signaling.

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Reduce refined carbohydrates. High-glycemic carbohydrates drive large, rapid insulin responses; replacing refined carbs with fiber-rich whole foods flattens the glucose and insulin curves at every meal.

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Improve sleep. Just two nights of restricted sleep increases insulin resistance by 25% (Spiegel et al., 1999), primarily by impairing glucose uptake in muscle and fat cells.

3 Things to Remember

Insulin is a storage and signaling hormone that is essential after every meal; chronically elevated fasting insulin between meals is the problem, not post-meal spikes.

Fasting insulin is a leading indicator of metabolic health, rising years before fasting glucose does, which is why requesting it on your annual labs is worth the ask.

Zone 2 cardio, resistance training, and sleep are the three highest-leverage inputs for improving insulin sensitivity and reducing the pancreatic workload over time.

Appears In

→The Fat Loss Protocol →The Fasting & Time-Restricted Eating Protocol →The Lab Work & Biomarkers Protocol

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