Glucose Tolerance Impaired

Impaired glucose tolerance is a serious condition that affects millions of people worldwide, placing them at significant risk of developing type 2 diabetes. While this condition often develops without symptoms, early detection and lifestyle changes can reverse the process and prevent progression to full diabetes.

Table of contents

• What Is Impaired Glucose Tolerance?
• Medical Definitions and Classifications
• How Common Is This Condition?
• Causes and Risk Factors
• How Is Impaired Glucose Tolerance Diagnosed?
• What Happens If Left Untreated?
• Lifestyle Changes for Managing the Condition
• Medical Treatment Options
• Follow-Up and Monitoring

impaired glucose tolerance, IGT, prediabetes, borderline diabetes, intermediate hyperglycemia

What Is Impaired Glucose Tolerance?

Impaired glucose tolerance is a condition where blood sugar levels are higher than normal but not yet high enough to be diagnosed as diabetes. This condition represents an intermediate stage between normal glucose control and type 2 diabetes^[1].

In people with impaired glucose tolerance, the body does not handle sugar properly after eating. When you eat, your body breaks down food into sugar, which enters the blood. The body then uses a hormone (a chemical messenger in the body) called insulin to move the sugar from blood into cells for energy. With impaired glucose tolerance, this process doesn’t work as well as it should^[2].

The condition is also commonly known as prediabetes or borderline diabetes, and these terms are often used interchangeably^[4]. The World Health Organization recommends using the term “intermediate hyperglycemia” instead of prediabetes to avoid the stigma associated with diabetes, and because not all cases progress to full diabetes^[2].

Medical Definitions and Classifications

Impaired glucose tolerance is diagnosed using specific blood sugar measurements. According to medical guidelines, the condition is defined as having blood glucose levels of 140 to 199 mg per dL (7.8 to 11.0 mmol per L) two hours after drinking a sweet liquid containing 75 grams of glucose during an oral glucose tolerance test (a diagnostic test where blood sugar is measured before and after drinking a glucose solution)^[1].

For comparison, normal blood sugar levels on this test are below 140 mg per dL. Levels of 200 mg per dL or higher indicate diabetes^[9].

Impaired glucose tolerance is different from another prediabetic condition called impaired fasting glucose (elevated blood sugar measured after not eating for at least 8 hours). Impaired fasting glucose is defined as fasting blood glucose levels of 100 to 125 mg per dL (5.6 to 6.9 mmol per L)^[1].

These two conditions are metabolically distinct disorders with limited overlap. Among people who have either or both conditions, only 16 percent have both impaired fasting glucose and impaired glucose tolerance, 23 percent have impaired fasting glucose alone, and 60 percent have impaired glucose tolerance alone^[1].

How Common Is This Condition?

Impaired glucose tolerance affects a large portion of the adult population. Among U.S. adults aged 40 to 74 years, approximately 15.6 percent (14.9 million people) have impaired glucose tolerance^[1]. From 2009 through 2012, the prevalence of prediabetes in the United States was 37 percent in adults older than 20 years and 51 percent in those older than 65 years^[2].

According to the American Diabetes Association, in 2015, 84.1 million Americans age 18 and older had prediabetes^[2]. From 10 to 15 percent of adults in the United States have either impaired glucose tolerance or impaired fasting glucose^[1].

The worldwide prevalence is also significant. In 2010, the worldwide prevalence of glucose intolerance was about 8 percent^[2]. People with impaired glucose tolerance are generally symptom-free and have only increased glucose levels; therefore, a large proportion of these patients remains undiagnosed^[2].

Causes and Risk Factors

The precise cause of impaired glucose tolerance remains unclear. There appears to be an interaction between genetic factors and lifestyle factors, particularly a sedentary lifestyle and poor dietary habits. The condition involves problems with both insulin resistance (when the body’s cells don’t respond properly to insulin) and insulin production^[2].

Several risk factors increase the likelihood of developing impaired glucose tolerance. Having a family history of type 2 diabetes significantly raises the risk. Other important risk factors include having a body mass index (a measure of body fat based on height and weight) greater than 25 kg per m2, living a sedentary lifestyle with little physical activity, and smoking^[1].

Medical conditions that increase risk include high blood pressure, high cholesterol levels, a history of gestational diabetes (diabetes during pregnancy) or giving birth to a large baby, and polycystic ovary syndrome (a hormonal disorder in women)^[1].

Certain ethnic and racial groups are at increased risk for impaired glucose tolerance and diabetes. These include Black Americans, Latin Americans, Native Americans, Asian-Pacific Islanders, Middle Eastern people, South Asian people, Pacific Islander people, North African people, and Aboriginal and Torres Strait Islander people^[1][8].

How Is Impaired Glucose Tolerance Diagnosed?

Impaired glucose tolerance is diagnosed through blood tests. The main diagnostic test is the oral glucose tolerance test. Before the test begins, you must not eat or drink anything (except water) for at least 8 hours. A blood sample is taken first to measure your starting blood sugar level. Then you drink a liquid containing a specific amount of glucose (usually 75 grams). Blood samples are taken again at regular intervals, typically every 30 to 60 minutes after drinking the solution. The test may take up to 3 hours^[9].

If your blood sugar level falls between 140 and 199 mg per dL at the 2-hour mark, you most likely have impaired glucose tolerance^[4]. Most doctors consider blood sugar levels at 200 mg per dL or above at the 2-hour mark to indicate diabetes, requiring treatment^[4].

Other tests can also help identify problems with blood sugar control. The A1C test (also called glycated hemoglobin test) measures your average blood sugar level for the past 2 to 3 months. Results between 5.7 percent and 6.4 percent indicate prediabetes, while 6.5 percent or higher indicates diabetes^[5].

A fasting plasma glucose test checks blood sugar levels after fasting. Results of 100 to 125 mg per dL indicate impaired fasting glucose, while 126 mg per dL or higher on two separate tests indicates diabetes^[5].

Impaired glucose tolerance usually has no symptoms, so it is important to be aware of the risk factors and have regular checks by your doctor^[8].

What Happens If Left Untreated?

People with impaired glucose tolerance face a significant risk of developing type 2 diabetes. In studies analyzing six prospective research projects, the risk of developing diabetes was found to be approximately 3.6 to 8.7 percent per year in patients with impaired glucose tolerance^[1].

Without lifestyle changes, approximately one in 3 people with prediabetes will develop type 2 diabetes^[8]. It is estimated that 20 percent to 70 percent of individuals with prediabetes who do not lose weight, change their dietary habits, or engage in moderate physical activity will progress to type 2 diabetes within 3 to 6 years^[16]. The yearly conversion rate appears to range between 5 percent to 10 percent, with the higher rate for impaired fasting glucose^[2].

Beyond the risk of diabetes, people with impaired glucose tolerance are at substantially greater risk of developing cardiovascular disease compared to people with normal blood sugar levels^[1]. People with these conditions have an increased risk of developing both diabetes and cardiovascular disease^[2].

In people with prediabetes, some of the long-term damage to blood vessels, heart, and kidneys may already be starting. The condition has also been linked to “silent” heart attacks, with symptoms so mild people may not realize they occurred^[23].

Lifestyle Changes for Managing the Condition

The good news is that impaired glucose tolerance can often be reversed or prevented from progressing through lifestyle changes. The progression from impaired glucose tolerance to type 2 diabetes can be prevented or delayed by modifying lifestyles in high-risk individuals, and these health benefits are well documented in various ethnicities with prediabetes across the world^[16].

When a diagnosis of impaired glucose tolerance is made, doctors should counsel patients to lose 5 to 7 percent of their body weight and engage in moderate physical activity for at least 150 minutes per week^[1]. Results from the Diabetes Prevention Program showed that intensive lifestyle intervention reduced the risk of developing type 1 and type 2 diabetes by 58 percent compared with no intervention^[10].

Eating a healthy diet is crucial. This includes consuming foods high in healthy fats like polyunsaturated fatty acids and monounsaturated fatty acids (healthy fats found in fish, nuts, and olive oil), fiber, and whole grains. A “clean” diet consisting of healthier choices can help restore normal blood sugar levels. This includes fruits with complex carbohydrates, vegetables, lean meats, whole grains, and healthy fats like avocado and fish^[16][17].

Regular physical activity is essential. Exercise lowers blood sugar by increasing insulin sensitivity, which allows the cells in your body to use insulin more efficiently. According to the American Diabetes Association, exercise can reduce blood sugar for up to 24 hours after a workout^[17].

Other important lifestyle changes include quitting smoking, consuming alcohol in moderation, and managing stress. These modifications can improve glucose tolerance and reduce the risk of type 2 diabetes^[16].

Medical Treatment Options

In addition to lifestyle changes, medication can be used to help manage impaired glucose tolerance and prevent progression to diabetes. Drug therapy with metformin (a medication that helps control blood sugar) or acarbose (a medication that slows the breakdown of carbohydrates in the intestine) has been shown to delay or prevent the onset of diabetes^[1].

The Diabetes Prevention Program showed that metformin therapy reduced the risk of developing type 1 and type 2 diabetes by 31 percent compared with placebo^[10]. However, medications are not as effective as lifestyle changes. Intensive lifestyle intervention was nearly twice as effective as metformin in preventing diabetes progression^[10].

It is not yet known if treatment with these drugs is cost effective in the management of impaired glucose tolerance^[1]. Pharmacological treatments have been successful in managing glucose intolerance; however, they have adverse effects. The implementation of lifestyle changes is preferred as the first approach^[16].

Some botanical supplements have been studied for managing glucose intolerance, but more research is warranted before a definitive recommendation can be made for their use^[16].

Follow-Up and Monitoring

If you have prediabetes, your healthcare provider will likely check your blood sugar levels at least once every three years to monitor whether the condition is progressing^[13]. Regular follow-up is important to track your progress and adjust treatment if needed.

Patients at higher risk should be screened with a fasting plasma glucose level. The American Diabetes Association recommends that diabetes screening for most adults begin at age 35. Screening before age 35 is advised if you’re overweight and have additional risk factors for prediabetes or type 2 diabetes^[13].

To make progress on this worldwide problem, efforts are needed to improve the awareness of prediabetes, increase promotion of healthy behaviors, and improve the availability of evidence-based lifestyle intervention programs to the community^[16].