The Insulin Cycle: Understanding How Insulin Works in the Body

During digestion, foods that contain carbohydrates are converted into glucose. Most of this glucose is sent into your bloodstream, causing a rise in blood glucose levels. This increase in blood glucose signals your pancreas to produce insulin.

The insulin tells cells throughout your body to take in glucose from your bloodstream. As the glucose moves into your cells, your blood glucose levels go down. Some cells use the glucose as energy. Other cells, such as in your liver and muscles, store any excess glucose as glycogen. Since glucose in the bloodstream is toxic, if the liver and muscle tissues have reached their capacity to store glycogen, the insulin will trigger the removal and storage of the remaining glucose in the bloodstream as fat.

About four to six hours after you eat, the glucose levels in your blood decrease, triggering your pancreas to produce glucagon. This hormone signals your liver and muscle cells to change the stored glycogen back into glucose. These cells then release the glucose into your bloodstream so your other cells can use it for energy.

This whole feedback loop with insulin and glucagon is constantly in motion. It keeps your blood sugar levels from dipping too low, ensuring that your body has a steady supply of energy, while at the same time keeping blood sugar levels from getting too high.

Deeper Dive

The Pancreas
The pancreas secretes both insulin and glucagon in an attempt to balance blood sugar levels.

Beta cells of the pancreas are sensitive to blood glucose concentrations. When glucose levels are high, the beta cells secrete insulin into the blood; when glucose levels are low, secretion of insulin is inhibited. Their neighboring alpha cells, by taking their cues from the beta cells, secrete glucagon into the blood in the opposite manner: increased secretion when blood glucose is low, and decreased secretion when glucose concentrations are high. Glucagon, through stimulating the liver to release glucose by glycogenolysis and gluconeogenesis, has the opposite effect of insulin. The secretion of insulin and glucagon into the blood in response to the blood glucose concentration is the primary mechanism of glucose homeostasis.

Carbohydrates are made up of three components: fiber, starch, and sugar.

Fiber and starch are complex carbs, while sugar is a simple carb. Depending on how much of each of these is found in a food determines its nutrient quality.

Simple Carbs
The difference between glucose and fructose (sucrose = glucose + fructose). Fructose goes directly to the liver for storage (as glycogen and fat) upon absorption into the bloodstream, whereas glucose is processed via the insulin cycle for use throughout the body.

Fructolysis - Under one percent of ingested fructose is directly converted to plasma triglyceride. 29% - 54% of fructose is converted in the liver to glucose, and about a quarter of fructose is converted to lactate. 15% - 18% is converted to glycogen. Glucose and lactate are then used normally as energy to fuel cells all over the body. Unlike glucose, fructose does not trigger an insulin release, in fact it lowers circulating insulin.

Glycolysis - An oxygen independent metabolic pathway that converts glucose into pyruvate and a hydrogen ion. The free energy released in this process is used to form the high-energy molecules ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide).

DHAP formed as a by-product of glycolysis is a source of the glycerol that combines with fatty acids to form fat.

Complex Carbs (Fiber, Starch, and Glycogen)
Complex carbs break down more slowly than simple carbs and thus help avoid blood sugar spikes and limit the need and subsequent release of insulin.

Incredibly false info about carbs from SF Gate - What Do Carbohydrates Taken in as Food Break Down Into?
 - This is why so many Americans struggle with weight gain - they are given “permission” to eat carbs from the mainstream media.

Insulin (from Latin insula, island) is a peptide hormone produced by beta cells of the pancreatic islets; it is considered to be the main anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats and protein by promoting the absorption of carbohydrates, especially glucose from the blood into liver, fat and skeletal muscle cells. In these tissues the absorbed glucose is converted into either glycogen via glycogenesis or fats (triglycerides) via lipogenesis, or, in the case of the liver, into both. Glucose production and secretion by the liver is strongly inhibited by high concentrations of insulin in the blood. Circulating insulin also affects the synthesis of proteins in a wide variety of tissues. It is therefore an anabolic hormone, promoting the conversion of small molecules in the blood into large molecules inside the cells. Low insulin levels in the blood have the opposite effect by promoting widespread catabolism, especially of reserve body fat.

Ingestion of a meal rich in carbohydrates triggers the release of insulin. Insulin in turn stimulates the uptake of large neutral branched-chain amino acids (BCAA). For a deeper dive:

Glucagon is a peptide hormone, produced by alpha cells of the pancreas. It works to raise the concentration of glucose and fatty acids in the bloodstream, and is considered to be the main catabolic hormone of the body. It is also used as a medication to treat a number of health conditions. Its effect is opposite to that of insulin, which lowers extracellular glucose.

The pancreas releases glucagon when the concentration of insulin (and indirectly glucose) in the bloodstream falls too low. Glucagon causes the liver to convert stored glycogen into glucose, which is released into the bloodstream. High blood-glucose levels, on the other hand, stimulate the release of insulin. Insulin allows glucose to be taken up and used by insulin-dependent tissues. Glucagon increases energy expenditure and is elevated under conditions of stress.

Insulin Resistance
People with type 2 diabetes have insulin resistance, which means that the cells do not respond properly when insulin instructs them to absorb glucose from the bloodstream.

How to improve insulin resistance

Improving your diet:
1. Eating foods that keep blood sugar levels low (low glycemic foods) - avocado, banana, blueberry, cinnamon, garlic, honey, peanut butter, slow-cooked oatmeal, vinegar, yogurt without added sugars.

2. Avoiding foods that cause insulin spikes - high sugar foods (candies and chocolates), dried fruits, sugary drinks.

3. Following a low-carb diet - Cutting the simple carbs that are converted rapidly to glucose.

Lifestyle changes:
1. Losing weight - In obese individuals, adipose tissue releases increased amounts of non-esterified fatty acids, glycerol, hormones, pro-inflammatory cytokines and other factors that are involved in the development of insulin resistance.
85% of patients diagnosed with type 2 diabetes also suffer from being overweight or obesity.

2. Exercise regularly - Exercise seems to fine tune the insulin system through regular use within moderate parameters. Either aerobic or resistance training alone can improve insulin resistance but the improvements are greatest when combining aerobic and resistance training.

3. Resistance training - Muscle tissue acts as a reservoir for glucose as glycogen, which can help buffer any spikes in blood glucose levels.

4. Reduce stress - Cortisol is the main hormone released when humans are confronted with a fight or flight situation (i.e. stress). Cortisol triggers an increase in the availability of glucose for our bodies to use for either fighting or flighting, while suppressing insulin sensitivity (preventing the reduction of blood sugar levels). Under fight or flight, when the event is over, cortisol levels drop back to normal and insulin is free to mop up all the extra glucose.

When stress levels are chronic, cortisol tends to linger, thereby impeding insulin sensitivity. Acute stress (i.e. exercise) is good, chronic long-term stress is bad.

Understanding how the insulin cycle operates may be the single most important reason why we must maintain a proper diet and exercise regimen in order to maintain a healthy lifestyle and extend our lifespan, our healthspan, for decades.