What is insulin resistance?
Insulin resistance is a state where the tissues of the body do not respond normally to the hormone insulin. The pancreas, a gland in the abdomen, secretes insulin in response to certain signals mainly a rise in blood glucose levels. Insulin then acts on various sites in the body to initiate different mechanisms that ultimately removes glucose from the bloodstream. One of these mechanisms is to increase uptake of glucose by cells when insulin acts upon it. In insulin resistance the cells fail to respond to insulin and do not take up as much glucose from the bloodstream. The blood glucose levels therefore remain elevated. The body attempts to overcome this defect by then secreting more insulin, leading to high levels of insulin in the blood known as hyperinsulinemia.
Insulin resistance has gained widespread attention in the past 20 years due to its association with obesity and subsequent development of diabetes mellitus (sugar diabetes). It is also a common syndrome in gynecological disorders such as polycystic ovarian syndrome (PCOS). However, insulin resistance is more commonly observed in middle aged men and can also occur in children. It is a significant, yet often missed, syndrome since it is may not be specifically identified in terms of its clinical presentation. Insulin resistance may lead to elevated blood glucose levels but the higher than normal levels of insulin in the blood may compensate and it can therefore be asymptomatic. The association of insulin resistance with other disorders such as high blood pressure, high cholesterol levels and abdominal obesity is collectively termed metabolic syndrome and indicates a greater risk of a heart attack, stroke and developing diabetes mellitus.
Meaning of insulin resistance
In order to understand insulin resistance, it is important to first have a working knowledge of normal glucose metabolism. Glucose in the bloodstream is mainly due to the breakdown of foods with digestion and subsequent absorption of nutrients into the bloodstream. It is also produced in the liver by processing these nutrients or converting storage forms of nutrients such as glycogen or even fat into glucose. These processes are essential to maintain a glucose supply since glucose is used by cells to produce energy. However, too much of glucose can be dangerous because high levels damages the cells of the body.
Two hormones help to control the metabolism of glucose – insulin and glucagon. Other hormones may also play a role in glucose metabolism but is not relevant for an overview of insulin resistance. Whereas insulin helps to lower blood glucose levels, glucagon has the opposite effect. Insulin, however, has various complex actions on the body to ultimately reduce blood glucose levels. It is produced by the beta cells in the islets of Langerhans of the pancreas. There are several triggers for the secretion of insulin into the bloodstream but this is essentially to prevent or reverse a rise of blood glucose above the norm.
The circulating insulin then has the following effects on the body :
- increased uptake of glucose by cells particularly the muscle cells.
- increased utilization of glucose by the cells (glycolysis).
- decreased production of glucose by the liver (gluconeogenesis).
- conversion of glucose to glycogen for storage in the liver.
- conversion of excess glucose into fatty acids for storage in adipose (fat) tissue.
- prevents breakdown and release of fatty acids from the adipose tissue.
With insulin resistance these processes are disrupted. Glucose remains in the blood and the liver continues with glucose production thereby leading to elevated blood glucose levels. However, it is important to note that insulin is not totally ineffective in insulin resistance. It continue to act on the body and has the desired effect although not as efficiently as is normally the case. The body attempts to compensate by increasing insulin production and secretion. The insulin levels in the blood are therefore higher than normal.
To adapt for greater insulin production, the beta cells hypertrophy (enlarged). In this state it is able to maintain higher levels of insulin secretion. The regulation of glucose can remain normal or be slightly defective in that the glucose levels may rise but not as high as in diabetes mellitus. This transient state of defective glucose metabolism is known as impaired glucose tolerance or pre-diabetes. Eventually the cells become exhausted and can no longer maintain this adaptation phase. The cells shrink and some are even destroyed. This is usually irreversible. The insulin production and secretion drops and diabetes mellitus sets in.
Causes of Insulin Resistance
Insulin resistance may be due to both inherited or acquired causes. The most common cause, however, is obesity which is due to lifestyle factors.
Genetic factors may contribute to abnormal insulin, antibodies against insulin, reduced number of insulin receptors, defects in insulin receptors, antibodies that block insulin receptors or impairment of mechanisms that are initiated when insulin binds to receptors. There are several other mechanisms by which insulin resistance may occur due to genetic causes that have not as yet been clearly identified.
Acquired causes includes a sedentary lifestyle, advancing age, diet, medication and obesity. Although diet and a sedentary lifestyle contribute to obesity, it does not mean that only the obese will develop insulin resistance. A person with a normal BMI may also have insulin resistance and the obese, although at greater risk, will not definitely suffer with insulin resistance.
Although insulin resistance may be commonly seen with conditions such as polycystic ovarian syndrome, this does not mean that PCOS causes insulin resistance. In fact it is thought that insulin resistance may be a contributing factor to PCOS. Sometimes insulin resistance may occur temporarily with certain infections, in pregnancy and with the short term use of some medication.
Insulin Resistance vs Diabetes Mellitus
Diabetes mellitus is a deficiency of insulin or unresponsiveness of the body’s cells to insulin. Although insulin resistance may precede the development of diabetes mellitus, it is known that insulin resistance could be reversed. Diabetes mellitus is irreversible. In insulin resistance, the glucose metabolism may be normal or there may be impaired glucose tolerance (IGT). The latter presents as a higher than normal blood glucose level within 1 to 2 hours after eating. Fasting glucose levels may sometimes be normal. This means that the body is unable to efficiently cope with the intake of glucose. With diabetes mellitus, the body’s ability to cope with glucose intake is completely compromised.
Symptoms of Insulin Resistance
There are no definitive symptoms of insulin resistance. It is commonly associated with a group of conditions that are collectively known as metabolic syndrome. The symptoms of these other conditions like high blood pressure (hypertension), high blood cholesterol (hypercholesterolemia) and high glucose levels (hyperglycemia) often in the presence of abdominal obesity may be indicative of insulin resistance. Sometimes a person with severe insulin resistance may develop darkening of the skin at the folds of the body, a condition known as acanthosis nigricans.
Other conditions which may be seen with insulin resistance includes :
- Fatty liver disease
- Polycystic ovarian syndrome
- Growth abnormalities
Reverse Insulin Resistance
Insulin resistance is reversible. However, if it is left untreated for long periods of time and when it progresses to diabetes mellitus, then it is irreversible. The period of time that it takes for insulin resistance to progress to diabetes mellitus can vary significantly and may be altered by a number of factors. It could take months or even years. Furthermore it is unclear in many case as to how long a person already had insulin resistance. Therefore a person who is diagnosed with insulin resistance needs to take immediate steps to reverse this state. Measures to reverse insulin resistance primarily revolves around weight loss, diet and physical activity.
Insulin Resistance Weight Loss
The key to weight loss, irrespective of insulin resistance, is balancing the number of calories consumed in food with calories utilized in physical activity. An additional factor in insulin resistance, however, is that the body’s glucose regulation is already impaired. Therefore a calorie-restricted diet comprising of low glycemic index (GI) foods is essential as discussed under insulin resistance diet. Physical activity should ideally be about 30 minutes per day for at least 5 days a week. An exercise regimen should only be commenced after approval by a medical doctor. The goal of any weight loss program is to restore a normal body mass index (BMI) which is under 25. Ideally a person should maintain a BMI of 18 to 22.
Insulin Resistance Tests
Insulin resistance is not a specific diagnosis but a disordered process that has been associated with several conditions. Therefore tests for these other conditions should also be conducted when identifying insulin resistance.
- Glucose tolerance test (GTT) which measures the body’s ability to handle a measured dose of glucose.
- Insulin levels in the blood which may be elevated (hyperinsulinemia) can be monitored fasting insulin test although sometimes an insulin tolerance test (ITT) may be conducted.
- Lipid profile to measure the level of fats (cholesterol and triglycerides) in the blood.
- High sensitivity C-reactive protein may be used to determine low grade inflammation of the cardiac (heart) tissue which may be seen with insulin resistance.
Insulin Resistance Treatment
There is no specific treatment for insulin resistance. The focus is largely on diet and exercise to manage insulin resistance with the goal of reversing it. As discussed above, weight control is a key factor in reversing insulin resistance. However, certain diabetes medication (non-insulin antidiabetic drugs) and metformin in particular may be used in the management of insulin resistance. These are short term approaches to delay the onset of diabetes mellitus and will not reverse or ‘cure’ insulin resistance.
Insulin Resistance Diet
Dietary modification for insulin resistance should involve avoiding high glycemic index (GI) carbohydrates. These foods cause the blood glucose level to become significantly elevated after a meal thereby requiring insulin. Low glycemic index (GI) foods are broken down slowly and do not cause spikes in the blood glucose level as is the case with high GI foods. In addition, the calories should be carefully be monitored for weight loss goals. Therefore the ideal insulin resistance diet would be a low GI calorie-restricted diet. A person with insulin resistance should consult with a registered dietitian to develop an eating plan that is calorie-restricted for specific weight loss goals, comprising low GI foods, be palpable for individual tastes yet be convenient for one’s lifestyle.
Some of the main considerations when developing a low GI diet is :
- Refined carbohydrates like white sugar and white flour are high GI foods.
- Starches like white bread and potato are high GI foods compared to rye bread and brown rice.
- Protein helps to ‘lower’ the GI of certain foods consumed in the same meal.
- Fiber also helps to slow down the absorption from the gut thereby ‘lowering’ the GI of certain foods.