The Role of Insulin

Insulin is a peptide hormone made in the beta cells of the pancreas that is central to regulating carbohydrate metabolism in the body (Wikipedia, 2016). After a meal, insulin is secreted into the bloodstream. When it reaches insulin-sensitive cells—liver cells, fat cells, and striated muscle—insulin stimulates them to take up and metabolize glucose. Insulin synthesis and release from beta cells is stimulated by rising concentrations of blood glucose. Insulin has a range of effects that can be categorized as anabolic, or growth-promoting.

Functions of Insulin
Turns on	Turns off
Uptake and use of glucose by insulin-sensitive cells	Breakdown of glycogen in liver cells
Storage of glucose in the form of glycogen in the liver and skeletal muscle tissue. Storage of fat.	Breakdown of fat
Uptake of amino acids and the synthesis of proteins	Breakdown of protein
DNA synthesis	Gluconeogenesis

The Problem of Insulin Resistance

In type 2 diabetes, many patients have body cells with a decreased response to insulin known as insulin resistance. This means that, for the same amount of circulating insulin, the skeletal muscles, liver, and adipose tissue take up and metabolize less glucose than normal. Being less sensitive to insulin, the liver does not react to the usual signal of insulin, so the liver manufactures and secretes more glucose than is needed.

Insulin resistance can develop in a person over many years before the appearance of type 2 diabetes. People inherit a propensity for developing insulin resistance, and other health problems can worsen the condition. For example, when skeletal muscle cells are bathed in excess free fatty acids, the cells preferentially use the fat for metabolism while taking up and using less glucose than normal, even when there is plenty of insulin available. In this way, high levels of blood lipids decrease the effectiveness of insulin; thus, high cholesterol and body fat, overweight and obesity increase insulin resistance.

Physical inactivity has a similar effect. Sedentary overweight and obese people accumulate triglycerides in their muscle cells. This causes the cells to use fat rather than glucose to produce muscular energy. Physical inactivity and obesity increase insulin resistance.
Video