Impaired glucose tolerance represents a critical stage where blood sugar levels sit higher than normal but not yet high enough to be diagnosed as diabetes—a window of opportunity where the progression to type 2 diabetes can often be prevented or delayed through changes in daily habits and lifestyle.

Epidemiology

Impaired glucose tolerance affects a substantial portion of the population across the world. In the United States, between 10 and 15 percent of adults live with this condition or its related state, impaired fasting glucose (IFG), which refers to elevated blood sugar levels after not eating for several hours.^[1] More specifically, among U.S. adults aged 40 to 74 years, approximately 15.6 percent—roughly 14.9 million people—have impaired glucose tolerance, while 9.7 percent (9.6 million people) have impaired fasting glucose.^[1]

The prevalence of impaired glucose tolerance has been rising steadily worldwide. From 2009 through 2012, the prevalence of what is often called 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, approximately 84.1 million Americans age 18 and older had prediabetes.^[2] Globally, in 2010, the worldwide prevalence of glucose intolerance was estimated at about 8 percent.^[2]

An important characteristic of impaired glucose tolerance is that it often goes unnoticed. People with this condition are generally without symptoms and have only elevated glucose levels, which means a large proportion of these individuals remain undiagnosed unless they undergo screening tests.^[2] This silent nature of the condition makes awareness and screening particularly important for those at higher risk.

Impaired fasting glucose and impaired glucose tolerance are metabolically distinct disorders with limited overlap. Among those who have one or both of these conditions, only 16 percent have both, 23 percent have impaired fasting glucose alone, and 60 percent have impaired glucose tolerance alone.^[1]

Causes

The precise causes of impaired glucose tolerance remain not fully understood, but researchers recognize that multiple factors come together to create this condition. There appears to be an interaction between genetic factors—traits passed down through families—and what are called epigenetic factors, which involve changes in how genes work without altering the DNA itself. These genetic elements interact with lifestyle factors such as a sedentary lifestyle and poor dietary habits to produce the condition.^[2]

At the core of impaired glucose tolerance are defects in two key processes: insulin resistance and insulin secretion. Insulin resistance occurs when the body’s cells—particularly in fat, liver, and muscle tissues—do not respond properly to insulin, a hormone that helps move sugar from the blood into cells for energy. When cells become resistant to insulin, the body needs more of it to maintain normal blood sugar levels.^[2]

In addition to insulin resistance, problems with insulin secretion also play a role. The pancreas, the organ that produces insulin, may not release enough insulin to meet the body’s needs, or the insulin it produces may not work as efficiently as it should. Both of these defects are important in declaring the clinical syndrome of impaired glucose tolerance.^[2]

The condition develops gradually over time. Initially, when insulin resistance begins, the pancreas responds by producing more insulin to compensate. This can maintain normal blood sugar levels for a while. However, eventually, the pancreas cannot keep up with the increased demand, and blood glucose levels begin to rise above normal, though not yet to diabetic levels. This intermediate stage is what defines impaired glucose tolerance.^[6]

Risk Factors

Multiple factors increase the likelihood of developing impaired glucose tolerance, and being aware of these risk factors is essential because the condition often has no symptoms. A family history of type 2 diabetes is a significant risk factor—having a parent or sibling with diabetes substantially increases the chances of developing impaired glucose tolerance.^[1]

Body weight plays a major role. Being overweight or obese, particularly with a body mass index (BMI) greater than 25 kg per m², significantly raises the risk. Obesity is a major underlying cause of insulin resistance. A study found that participants who had obesity were about six times more likely to develop type 2 diabetes than those at a healthy weight, regardless of genetic predisposition; people who were overweight had 2.4 times the normal risk.^[23] Waist measurement is also important—in Caucasian men, a waist circumference greater than 94 cm, in Asian men greater than 90 cm, and in women of any background greater than 80 cm indicates increased risk.^[8]

A sedentary lifestyle—exercising fewer than three times per week—is another important risk factor. Physical inactivity contributes to weight gain and worsens insulin resistance.^[1] Smoking also increases the risk of developing impaired glucose tolerance.^[8]

Other medical conditions can elevate risk. High blood pressure (hypertension) and abnormal cholesterol levels (dyslipidemia), either alone or together, increase the likelihood of impaired glucose tolerance.^[1] Women who have experienced gestational diabetes—diabetes that develops during pregnancy—or who have given birth to a baby weighing more than 9 pounds are at higher risk. Women with polycystic ovary syndrome (PCOS), a hormonal disorder, also face increased risk.^[1] Some antipsychotic medications can raise the risk as well.^[8]

Age is another factor. The American Diabetes Association recommends that diabetes screening for most adults begin at age 35, though screening before this age is advised for those who are overweight and have additional risk factors.^[13]

Symptoms

One of the most challenging aspects of impaired glucose tolerance is that it typically produces no noticeable symptoms. This condition usually develops silently, without obvious signs that would prompt someone to seek medical attention. Pre-diabetes usually has no symptoms, which is why it is so important for people to be aware of the risk factors and have regular checks by their doctor.^[8]

The absence of symptoms means that a person can have impaired glucose tolerance for years without knowing it. Symptoms that are associated with type 2 diabetes, such as increased urination, excessive thirst, or unusual hunger, won’t necessarily surface with impaired glucose tolerance.^[23] These more obvious symptoms typically only appear once blood sugar levels rise high enough to be diagnosed as diabetes.

In rare cases, some people may develop a visible sign on their skin. Some individuals with impaired glucose tolerance develop darkening of the skin around the armpits, neck, and elbows—a condition called acanthosis nigricans. However, this sign doesn’t appear in everyone with the condition.^[4]

Because symptoms are absent or subtle, the only reliable way to identify impaired glucose tolerance is through blood testing. Without screening, early signs of insulin resistance can be very difficult to identify. This is why doctors recommend regular screening for people who have risk factors, even if they feel perfectly healthy.^[23]

Prevention

The progression from impaired glucose tolerance to type 2 diabetes can be prevented or delayed, and this represents a critical opportunity for health intervention. Research has demonstrated that modifying lifestyle can significantly reduce the risk of developing diabetes in people with impaired glucose tolerance.^[1]

When the diagnosis of impaired glucose tolerance is made, physicians 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] Even a modest weight loss can make a substantial difference in reducing diabetes risk. 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 placebo in individuals with impaired glucose tolerance.^[10]

Dietary changes form a cornerstone of prevention. Consuming a healthy diet that is high in polyunsaturated fatty acids, monounsaturated fatty acids, fiber, and whole grains can improve glucose tolerance and reduce the risk of type 2 diabetes.^[16] Eating a “clean” diet, which consists of healthier choices, can help restore normal blood sugar levels. This includes incorporating fruits with complex carbohydrates, vegetables, lean meats, whole grains, and healthy fats like those found in avocado and fish.^[17]

Physical activity is not only great for energy and mental health, but it can also lower blood sugar by increasing insulin sensitivity. This allows the cells in the 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] Activities can include walking, cycling, swimming, or any form of moderate exercise that raises the heart rate.

Other lifestyle modifications also contribute to prevention. Quitting smoking is important, as smoking increases the risk of impaired glucose tolerance and its progression to diabetes.^[16] Consuming alcohol in moderation, rather than in excess, also helps manage glucose tolerance.^[16] Managing stress and ensuring adequate sleep are additional factors that support healthy blood sugar levels.

For some individuals, drug therapy may be considered. Medications such as metformin or acarbose have been shown to delay or prevent the onset of diabetes. Metformin therapy reduced the risk of developing diabetes by 31 percent in the Diabetes Prevention Program.^[10] However, medications are not as effective as lifestyle changes, and it is not known if treatment with these drugs is cost effective in the management of impaired glucose tolerance.^[1]

Regular screening is also a form of prevention. Patients at higher risk should be screened with a fasting plasma glucose level. If impaired glucose tolerance is diagnosed, early intervention can begin immediately.^[1] For those who have had gestational diabetes, healthcare providers will likely check blood sugar levels at least once every three years.^[13]

Pathophysiology

Understanding how impaired glucose tolerance develops requires looking at what happens inside the body when normal glucose regulation goes awry. In a healthy person, blood glucose levels are tightly regulated by insulin, a hormone produced by specialized cells called beta cells in the pancreas. Insulin acts like a key that unlocks cells, allowing glucose from the bloodstream to enter cells where it can be used for energy.^[8]

In impaired glucose tolerance, two main problems disrupt this normal process. The first is insulin resistance, which means that the body’s cells—particularly those in muscle, fat, and the liver—stop responding properly to insulin. When insulin resistance develops, these tissues require more insulin than usual to move the same amount of glucose from the blood into the cells. It’s as if the locks on the cells have become sticky, and the insulin key doesn’t work as smoothly as it should.^[2]

Initially, when insulin resistance begins to develop, the pancreas tries to compensate by producing more insulin. This increased insulin production can maintain normal blood glucose levels for a period of time. The body is essentially working harder behind the scenes to maintain balance. However, this compensatory mechanism has limits.^[6]

The second problem is that the pancreas eventually cannot keep up with the increased demand for insulin. Over time, the beta cells in the pancreas that produce insulin may become exhausted or damaged. They either cannot produce enough insulin to overcome the insulin resistance, or the insulin they produce doesn’t work as efficiently as it should. This leads to a progressive loss of beta cell insulin secretion, frequently occurring on the background of continuing insulin resistance.^[6]

As these two processes—insulin resistance and inadequate insulin secretion—progress, glucose begins to accumulate in the bloodstream instead of entering cells. Blood glucose levels rise above normal but remain below the threshold that defines diabetes. This intermediate state is impaired glucose tolerance.^[2]

The metabolic disturbances in impaired glucose tolerance extend beyond just glucose handling. The condition frequently occurs alongside what is called metabolic syndrome, a cluster of conditions that includes high blood pressure, abnormal cholesterol levels, increased waist circumference, and elevated blood sugar. These conditions often occur together and compound each other’s effects, creating a complex metabolic disturbance that affects multiple body systems.^[1]

Compared with people who have normal glucose levels, patients with impaired glucose tolerance are at substantially greater risk of developing cardiovascular disease. The elevated glucose levels, even though not yet in the diabetic range, can begin to damage blood vessels throughout the body. Some of the long-term damage to blood vessels, heart, and kidneys may already be starting in people with impaired glucose tolerance, even before diabetes is diagnosed.^[1]

In an analysis of six prospective studies, the risk of developing diabetes was found to be approximately 3.6 to 8.7 percent per year in patients with impaired glucose tolerance. Elevated fasting glucose levels, elevated two-hour post-challenge glucose values, and body mass index greater than 27 kg per m² were associated with the development of diabetes in these patients.^[1]