Insulin is a protein hormone secreted by the beta cells of the pancreas, and its main job is to move glucose (sugar) from the bloodstream into the cells. You can think of it as a “key” that opens the door to muscle, liver and fat cells. Without it, sugar stays in the blood and never reaches its destination — a state that, in the long run, damages blood vessels, nerves and kidneys.

How does it work?

After a meal, especially one rich in carbohydrates, blood glucose levels rise. The pancreas detects the rise and secretes insulin, which causes:

  1. Glucose uptake into cells: insulin signals GLUT4 transporters to move to the membrane of muscle and fat cells, and glucose enters through them.
  2. Energy storage: excess glucose is stored in the liver and muscles as glycogen, and beyond that capacity it is converted to fat.
  3. Halting fat breakdown: as long as insulin levels are high, the body is in “storage” mode and suppresses fat breakdown (lipolysis).

Insulin Resistance

When cells are exposed to high insulin levels again and again — mainly due to chronic intake of sugar and refined carbohydrates, excess belly fat and physical inactivity — they become “deaf” to the signal. The pancreas compensates by secreting more insulin, creating a cycle: more insulin → less sensitivity → even more insulin. This is the stage that precedes prediabetes and type 2 diabetes, and it is also linked to fatty liver (NAFLD) and metabolic syndrome.

What improves insulin sensitivity?

  • Physical activity: muscles take up glucose even without insulin during exercise — this is the most effective tool.
  • Cinnamon: slows stomach emptying and inhibits carbohydrate-digesting enzymes.
  • EGCG and green tea: inhibit enzymes that break down carbohydrates and increase GLUT4 translocation.
  • Berberine: activates the enzyme AMPK, and has been studied as having an effect similar to metformin.
  • Magnesium: deficiency has been linked to reduced insulin sensitivity.
  • Lentils and low-glycemic-index foods: produce a gentler glycemic response.

What harms insulin sensitivity?

  • High-fructose corn syrup and sweetened drinks: directly feed fat synthesis in the liver.
  • Refined carbohydrates and industrial sweets: cause sharp, repeated spikes.
  • Lack of sleep and chronic stress: cortisol raises blood glucose and weakens insulin’s action.

Frequently asked questions about insulin

Question: Do only people with diabetes need to care about insulin? Answer: No. Insulin resistance develops many years before diabetes is diagnosed, usually without symptoms. Measures such as fasting insulin and HOMA-IR can reveal the problem early, while it is still easy to reverse the trend.

Question: Do artificial sweeteners raise insulin? Answer: This is a contested question. Some studies point to a “cephalic insulin response” — an initial release triggered by the sweet taste itself, even without sugar — but the effect varies greatly between people. More on the artificial sweeteners page.

Question: If insulin stores fat, should you avoid it entirely? Answer: No, and you couldn’t anyway. Insulin is an essential hormone: it is what brings glucose and amino acids into muscle and enables building and recovery. The goal is not “zero insulin” but good sensitivity — meaning the body does the same work with as little of the hormone as possible.

Question: Does protein raise insulin? Answer: Yes, protein raises insulin too (whey in particular), but together with another hormone — glucagon — that balances the response. So the rise after protein does not translate into fat storage the way the rise from simple sugar does.