Glucose tolerance is your body’s ability to move sugar out of your bloodstream and into your cells after you eat. When glucose tolerance is normal, blood sugar rises briefly after a meal and then returns to baseline within a couple of hours. When it’s impaired, blood sugar stays elevated longer than it should, a warning sign that the body’s insulin system isn’t working efficiently. Glucose tolerance is measured with a specific blood test, and the results place you into one of three categories: normal, prediabetes, or diabetes.
How Your Body Handles a Sugar Load
When you eat carbohydrates, they break down into glucose and enter your bloodstream. Your pancreas detects the rise in blood sugar and releases insulin, a hormone that acts like a key, unlocking cells so they can absorb glucose and use it for energy or storage. Three tissues do most of the heavy lifting: skeletal muscle, the liver, and fat tissue.
Skeletal muscle is the biggest consumer. When insulin arrives, muscle cells ramp up glucose transport and either burn the sugar immediately for energy or pack it away as glycogen, a stored form of fuel. The liver, which sits downstream from the pancreas and receives insulin at two to three times the concentration found elsewhere in the body, responds by building glycogen stores and dialing down its own glucose production. Fat tissue plays a supporting role by pulling in glucose and, critically, by stopping the release of fatty acids. That suppression of fat breakdown sends a chemical signal to the liver to further reduce glucose output.
Good glucose tolerance means all three tissues respond quickly and in coordination. The sugar spike after eating is modest and short-lived. When any part of this system becomes sluggish, particularly if muscle and liver cells stop responding well to insulin, glucose lingers in the blood. That’s the beginning of impaired glucose tolerance.
What the Glucose Tolerance Test Measures
The oral glucose tolerance test (OGTT) is a standardized way to see how efficiently your body clears sugar. It’s a simple concept: drink a known amount of glucose, then measure how much is still in your blood two hours later.
For non-pregnant adults, the test uses 75 grams of glucose dissolved in about 250 to 300 mL of water, which you drink over five minutes. A fasting blood sample is drawn before you drink the solution, and another is drawn at the two-hour mark. You sit quietly during the wait. The test is done in the morning, typically between 7 and 9 a.m., after an overnight fast of 10 to 16 hours.
Preparation matters. For the three days leading up to the test, you should eat your normal diet with at least 150 grams of carbohydrates per day and maintain your usual activity level. Restricting carbs beforehand can artificially worsen your results because your body adapts to lower sugar intake and temporarily becomes less efficient at processing a sudden load. Illness, bed rest, and certain medications can also skew results, so let your provider know about any of these before the test.
Reading Your Results
The two-hour blood sugar reading after the 75-gram glucose drink is what determines your category:
- Normal: below 140 mg/dL (7.8 mmol/L)
- Prediabetes (impaired glucose tolerance): 140 to 199 mg/dL (7.8 to 11.0 mmol/L)
- Diabetes: 200 mg/dL (11.1 mmol/L) or higher
These thresholds have remained consistent across both World Health Organization and American Diabetes Association guidelines. A result in the prediabetes range is formally called impaired glucose tolerance, or IGT. It’s distinct from another prediabetes marker called impaired fasting glucose (IFG), which is based on a separate test: a simple fasting blood draw without the sugar drink. Fasting glucose of 100 to 125 mg/dL signals IFG. You can have one, the other, or both.
A third test, the A1C, measures your average blood sugar over the past two to three months. An A1C of 5.7% to 6.4% also falls in the prediabetes range, while 6.5% or higher indicates diabetes. Each test captures a slightly different aspect of blood sugar regulation, which is why your provider may use more than one.
Why Impaired Glucose Tolerance Matters
A result in the 140 to 199 mg/dL range often gets dismissed as “just prediabetes,” but it carries real health consequences even if it never progresses to full diabetes. A large follow-up study published in Diabetes Care found that people with impaired glucose tolerance who did not go on to develop diabetes still had a 65% higher risk of dying from any cause and more than double the risk of dying from cardiovascular disease compared to people with normal glucose tolerance. The elevated sugar levels themselves appear to damage blood vessels over time, independent of whether diabetes eventually develops.
Roughly 1 in 3 people with impaired glucose tolerance will progress to type 2 diabetes within several years if nothing changes. The good news is that this progression is not inevitable. Moderate weight loss (5% to 7% of body weight) and regular physical activity have been shown to cut the risk of progressing to diabetes nearly in half.
Glucose Tolerance Testing in Pregnancy
Pregnant women are routinely screened for gestational diabetes, a form of glucose intolerance that develops during pregnancy. Two approaches are common.
The one-step method uses a 75-gram glucose drink, similar to the standard OGTT. Gestational diabetes is diagnosed if fasting glucose reaches 92 mg/dL or higher, or if the two-hour value hits 153 mg/dL or higher. Only one elevated value is needed for a diagnosis.
The two-step method starts with a smaller screening dose of 50 grams. If your one-hour blood sugar after that drink is above 140 mg/dL, you return for a longer test using a 100-gram glucose load with blood draws at fasting, one hour, two hours, and three hours. Gestational diabetes is diagnosed if two or more values exceed specific thresholds: fasting above 95, one-hour above 180, two-hour above 155, or three-hour above 140 mg/dL. Which approach your provider uses depends on the practice and region, but both are considered valid.
What Affects Your Glucose Tolerance
Glucose tolerance isn’t fixed. It fluctuates based on several factors, some within your control and some not. Physical inactivity worsens it, sometimes significantly. Even a few days of bed rest can measurably reduce the body’s ability to clear glucose, which is why the OGTT is not performed on hospitalized or bedridden patients. Stress and acute illness raise blood sugar through the release of counter-regulatory hormones like cortisol and adrenaline.
Age gradually reduces glucose tolerance, largely because muscle tissue becomes less responsive to insulin over time. Excess body fat, particularly around the midsection, compounds this effect by promoting chronic low-grade inflammation that further blunts insulin signaling. Sleep deprivation, even a few nights of poor sleep, has been linked to temporarily impaired glucose handling.
On the positive side, regular aerobic exercise and resistance training both improve glucose tolerance by making muscle cells more sensitive to insulin. These effects can be seen within days of starting an exercise routine, well before any weight loss occurs. Dietary patterns that avoid large refined-carbohydrate loads and include fiber, protein, and healthy fats also help smooth out the blood sugar response after meals.