Impaired glucose tolerance (IGT) is a form of prediabetes diagnosed when the 2-hour glucose reading on an oral glucose tolerance test (OGTT) falls between 140 and 199 mg/dL. It reflects difficulty clearing a glucose load from the bloodstream and is one of three ADA-accepted criteria for prediabetes, alongside impaired fasting glucose and A1C 5.7 to 6.4 percent.
How Impaired Glucose Tolerance Is Diagnosed
The OGTT requires an overnight fast, then ingestion of a 75-gram glucose drink, followed by a blood draw 2 hours later. The result places you in one of three categories:
| 2-Hour OGTT Glucose | Category |
|---|---|
| Under 140 mg/dL | Normal glucose tolerance |
| 140-199 mg/dL | Impaired glucose tolerance (prediabetes) |
| 200 mg/dL or higher | Diabetes (confirmed x2) |
Some labs also measure a 1-hour value. Elevated 1-hour glucose (155 mg/dL or higher) can independently predict progression to type 2 diabetes even when the 2-hour value is normal.
IGT Versus Other Forms of Prediabetes
The three ADA criteria for prediabetes capture different physiologic defects:
- Impaired fasting glucose (IFG): fasting glucose 100-125 mg/dL. Reflects hepatic insulin resistance and overnight glucose production.
- Impaired glucose tolerance (IGT): 2-hour OGTT 140-199 mg/dL. Reflects muscle insulin resistance and impaired first-phase insulin secretion.
- Elevated A1C: 5.7-6.4 percent. Reflects 3-month average glucose across both fasting and post-meal periods.
About one third of people with prediabetes have IGT alone, one third have IFG alone, and one third have both. The “both” group has the highest progression risk, roughly 10-15 percent per year. Learn more at our prediabetes hub.
What Causes Impaired Glucose Tolerance
IGT develops when muscle tissue becomes less responsive to insulin and the pancreas cannot produce enough additional insulin to compensate during and after meals. The typical sequence plays out over years:
- Excess calorie intake and sedentary lifestyle lead to visceral fat accumulation.
- Visceral fat releases free fatty acids and inflammatory signals that impair insulin signaling in muscle.
- The pancreas compensates by producing more insulin, keeping fasting glucose normal.
- First-phase insulin secretion (the rapid burst released within minutes of eating) begins to fail.
- Post-meal glucose rises, creating IGT.
- Fasting glucose eventually rises too, producing IFG and progression toward diabetes.
Risk factors include BMI over 25, family history of type 2 diabetes, age over 45, physical inactivity, gestational diabetes history, polycystic ovary syndrome, hypertension, and high-risk ethnicity (South Asian, Hispanic, African American, Pacific Islander, Native American).
Symptoms and Why Screening Matters
Most people with IGT have no noticeable symptoms. That absence is precisely why the CDC estimates that 1 in 3 US adults has prediabetes, and more than 80 percent do not know it. Occasional fatigue after carbohydrate-heavy meals, mild increased thirst, or slow-healing cuts can be clues, but they are not diagnostic. For a full symptom review, see our symptoms of prediabetes guide.
The USPSTF recommends screening all adults aged 35 to 70 with overweight or obesity. Screening methods include A1C, fasting plasma glucose, or OGTT. Among these, the OGTT is the most sensitive for detecting IGT specifically.
Complications and Progression Risk
IGT is not benign. According to a 2012 Lancet review by Tabak and colleagues, people with IGT face:
- 5-10 percent annual risk of progression to type 2 diabetes
- 20 percent higher cardiovascular event risk compared to people with normal glucose tolerance
- Higher risk of non-alcoholic fatty liver disease
- Moderately increased risk of certain cancers (liver, pancreas, colorectal)
The 2-hour post-load glucose is more strongly associated with cardiovascular mortality than fasting glucose, which is one reason some guidelines emphasize OGTT over fasting-only testing.
How to Reverse Impaired Glucose Tolerance
The Diabetes Prevention Program (DPP), published in the New England Journal of Medicine in 2002, is the landmark trial. It randomized 3,234 adults with IGT to intensive lifestyle intervention, metformin 850 mg twice daily, or placebo. At 2.8 years:
- Lifestyle arm: 58 percent reduction in progression to type 2 diabetes
- Metformin arm: 31 percent reduction
- Placebo arm: 11 percent annual incidence
The lifestyle intervention was not extreme. Targets were 7 percent weight loss and 150 minutes per week of moderate activity (brisk walking). Benefits persisted 15 years later in the DPP Outcomes Study.
Practical Steps
- Weight. Aim for 5-7 percent loss. For a 180-pound adult, that is 9-13 pounds.
- Exercise. 150 minutes weekly of brisk walking, cycling, or swimming, split across 5 sessions. Add resistance training twice weekly.
- Diet. Emphasize vegetables, legumes, whole grains, nuts, fatty fish, and olive oil. Limit sugary drinks, refined starches, and added sugars. See our diet guide.
- Sleep. 7-9 hours nightly. Screen for sleep apnea if snoring or daytime fatigue.
- Medication. Consider metformin if BMI over 35, age under 60, or prior gestational diabetes.
Monitoring After Diagnosis
The ADA recommends annual glucose testing after a prediabetes diagnosis, or more frequent if risk factors are high. Options include A1C (most common), fasting glucose, or repeat OGTT. Many clinicians rotate between A1C every 6 months and an OGTT every 2-3 years to capture both the integrated and post-load signals. For treatment options beyond lifestyle, see our treatment hub.
The Bottom Line
Impaired glucose tolerance is a 2-hour OGTT glucose of 140 to 199 mg/dL, one of three ways to diagnose prediabetes. It reflects post-meal insulin resistance, carries a 5-10 percent annual progression risk to type 2 diabetes, and increases cardiovascular risk by about 20 percent. The Diabetes Prevention Program showed that modest weight loss and exercise reduce progression by 58 percent. With focused intervention, many people return to normal glucose tolerance within 1-2 years.
