Insulin resistance is a decreased ability to respond to the effects of insulin, especially by muscle and fat (adipose) tissues. Insulin is a hormone produced by the beta cells in the pancreas. Small amounts of it are normally released after each meal to help transport glucose into the body's cells, where the glucose is needed for energy production. Since cells must have glucose to survive, the body of an insulin-resistant individual compensates by producing additional amounts of the hormone. This results in a high level of insulin in the blood (hyperinsulinemia) and over-stimulation of some tissues that have remained insulin-sensitive. Over time, this process causes an imbalance in the relationship between glucose and insulin and, without treatment, may eventually cause health complications affecting various parts of the body.
Hyperinsulinemia and insulin resistance is associated with alterations in the concentration and relative amounts of the body's lipids, significantly increasing the amount of triglycerides and LDL (low-density lipoprotein, the "bad cholesterol") in the blood and decreasing the amount of HDL (high-density lipoprotein, the "good cholesterol"). It may also increase a person's risk of developing a blood clot, cause inflammatory changes, and increase a person's sodium retention, which can lead to increased blood pressure.
The cause of insulin resistance is not fully understood. It is thought to be due partly to genetic factors, including ethnicity, and due partly to lifestyle. Most people with insulin resistance do not have any symptoms. In most cases, the body is able to keep pace with the need for extra insulin production for many years. If the body's insulin production fails to keep up with demand, then hyperglycemia will occur. Once glucose reaches a high enough level, type 2 diabetes is present. The high glucose level can damage blood vessels in many organs, including the kidneys. Insulin resistance is therefore a risk factor for developing type 2 diabetes. Changes in lipids can cause fatty plaque deposits in the arteries and lead to cardiovascular disease (CVD) and strokes.
Metabolic syndrome and insulin resistance are two terms that have often been used interchangeably to characterize some of the abnormalities associated with increased resistance to insulin and increased production of insulin, and to recognize these changes as risk factors for future disease. Metabolic syndrome is essentially a subset of insulin resistance; although several different sets of diagnostic criteria for metabolic syndrome exist, all include obesity, alterations in lipid levels, and impaired glucose processing in the definition of this disorder. The goal of screening for metabolic syndrome is to identify and work with obese, sedentary people to decrease their health risks through lifestyle changes.
Insulin resistance is not a disease or specific diagnosis, but it has been associated with conditions such as, as mentioned, CVD, type 2 diabetes, obesity, and hypertension as well as polycystic ovarian syndrome (PCOS) and nonalcoholic fatty liver disease. Some researchers also believe that there may be a link between insulin resistance and certain forms of cancer. The mechanisms of these associations, however, are not well understood. It is important to remember that many people who have these conditions do not have insulin resistance and, likewise, many people who have insulin resistance will never develop these conditions. These are just patterns of association that have emerged. They are frequently seen together and it is thought that insulin resistance may contribute to their development and exacerbate them when it is present.