The oral glucose tolerance test (OGTT) is a common diagnostic procedure used to screen for diabetes, prediabetes, or gestational diabetes. This test requires consuming a standardized, highly concentrated glucose drink, typically containing 75 to 100 grams of sugar, after an overnight fast. The purpose is to measure how efficiently the body processes this large sugar load over a period of two to three hours. Many people experience digestive upset after consuming the sugary solution.
The Osmotic Effect and Bowel Changes
The concentrated glucose solution often affects the digestive system because of a process known as osmosis. When the drink reaches the small intestine, its very high sugar content creates a hyperosmotic environment, meaning the concentration of solute (sugar) is much higher than that of the surrounding body tissues.
This high concentration of unabsorbed sugar acts as a strong draw for water. To balance this concentration difference, the body pulls water from the bloodstream and surrounding tissues into the intestinal lumen. This rapid influx of fluid significantly increases the volume of liquid within the intestines.
The increased fluid volume accelerates the movement of contents through the digestive tract, resulting in loose stools or diarrhea, often referred to as osmotic diarrhea. For some individuals, the speed of this reaction can lead to an urgent need to use the restroom shortly after finishing the drink. While not everyone experiences this side effect, loose stools and an upset stomach are among the recognized, non-serious adverse effects of the OGTT.
Managing Nausea and Other Common Side Effects
Beyond bowel changes, the most frequently reported side effect of the glucose drink is nausea. The sweet, sometimes cloying taste of the solution can be difficult to tolerate, especially on an empty stomach after an 8- to 12-hour fast. Some people also report experiencing lightheadedness, dizziness, or a headache during the testing period.
Mitigating Discomfort
To mitigate discomfort, remain still and avoid strenuous movement during the two to three hours of testing. Physical activity can disrupt the test results and potentially exacerbate feelings of dizziness. Sipping the drink slowly over the allowed five-minute period may help reduce the immediate onset of nausea.
Improving Palatability
Maintaining proper hydration by drinking plain water before and during the test, as permitted by the lab, is helpful. Some laboratories may chill the glucose solution or offer different flavor options to make the taste more palatable. Resting quietly throughout the test helps keep the body calm and can minimize feelings of weakness or wooziness.
When to Seek Medical Guidance
While most side effects are mild and temporary, certain symptoms warrant immediate attention from the healthcare professionals administering the test. Severe vomiting, for example, is a concern because if the glucose solution cannot be kept down, the test must be stopped and rescheduled. This is necessary because the accurate dose of sugar was not absorbed.
Extreme dizziness, fainting, or feeling short of breath also require immediate communication with the staff. These symptoms could indicate an exaggerated drop in blood sugar, known as reactive hypoglycemia, or a severe reaction to the procedure. Though uncommon, a severe reaction needs to be evaluated quickly to ensure safety. It is important to communicate any unusual or intense discomfort to ensure the safest and most accurate testing experience.