From the Guidelines

The optimal fasting time before a glucose tolerance test is at least 8 hours, as defined by the most recent and highest quality studies 1.

For most standard oral glucose tolerance tests (OGTTs), patients should fast overnight for at least 8 hours but not more than 14 hours before the test.
During this fasting period, patients should consume only water and avoid all food, beverages containing calories, alcohol, caffeine, and tobacco.
Medications that can affect glucose metabolism should be discussed with the healthcare provider before the test, as some may need to be temporarily discontinued.
This fasting period is crucial because it establishes a baseline glucose level that isn't influenced by recent food intake.
A shorter fasting time could result in elevated baseline glucose levels from recent meals, while fasting too long might trigger stress hormones that can raise blood glucose.

Patients should schedule their test for early morning to make the fasting period more manageable.
Patients should inform the laboratory staff if they've eaten anything during the fasting period, as this would invalidate the test results.
The definition of fasting as no caloric intake for at least 8 hours is consistent across multiple studies, including those from 2004 1 and 2010 1, but the most recent and highest quality study from 2014 1 is used to inform this recommendation.

From the Research

The optimal fasting time before a glucose tolerance test is a crucial factor in obtaining accurate results, as it can affect the body's glucose and insulin responses.
A study published in 2023 2 found that fasting for 6 or more hours before the 1-hour oral glucose tolerance test (OGTT) resulted in a significantly higher screen-positive rate and mean glucose value compared to eating within 2 hours of the test.
The study suggests that fasting for 6 or more hours may be the optimal fasting time before the OGTT, but it is essential to consider individual factors and consult with a healthcare professional for personalized guidance.
Other studies have investigated the effects of fasting and glucose intake on glucose metabolism and insulin sensitivity, but they do not provide direct evidence for the optimal fasting time before a glucose tolerance test 3, 4, 5, 6.
These studies highlight the importance of considering individual factors, such as glucose metabolism and insulin sensitivity, when determining the optimal fasting time before a glucose tolerance test.

Fasting for 6 or more hours before the OGTT resulted in a higher screen-positive rate and mean glucose value compared to eating within 2 hours of the test 2.
The optimal fasting time before a glucose tolerance test may vary depending on individual factors, such as glucose metabolism and insulin sensitivity.
Further research is needed to determine the optimal fasting time before a glucose tolerance test and to develop personalized guidelines for different populations.

A study published in 2025 3 investigated the associations between fluctuations in blood glucose and insulin with hypoglycemic symptoms, but it did not provide direct evidence for the optimal fasting time before a glucose tolerance test.
Other studies have examined the effects of metformin and other medications on glucose metabolism and insulin sensitivity in patients with impaired glucose tolerance or type 2 diabetes 4, 5, 6, but they do not provide direct evidence for the optimal fasting time before a glucose tolerance test.