Oral Glucose Tolerance Test in Horses

The absorptive capacity of the small intestine can be assessed by the D-glucose or D-xylose absorption tests. The method of carrying out these tests for either carbohydrat source is similar. D-xylose is preferable to use as it is not metabolized by the small intestine mucose and its absorption is not affected by insulin. However, D-xylose is not readily available. D-glucose is more commonly used and the test is known as an oral glucose tolerance test.

The horse should be starved for 18 - 24 hours prior to the test and then administered 1g / kg of D-glucose via a nasogastric tube. Blood samples to measure the glucose concentraions should be collected at 0,30,60,90,120,150,180,210 and 240 minutes after the carbohydrate has been given. More samples after 6 hours should be taken if the results are of questionable significance. The blood samples should be put into sodium flouride tubes to measure the glucose. Sedation and prolonged fasting prior to the test can alter the rate of absorption of glucose and the gastrointestinal transit times and so should be avoided to obtain reliable results. A glucose curve should be created to assist the interpretation of the results.

A normal result should have a peak absorbtion between 90 and 120 minutes that is greater than 85% above the resting glucose level. Partial malabsorption is defined as a peak between 15 - 85% above the resting glucose level. Complete malabsorption is defined as a peak less than 15% above the resting glucose level.

Whilst this test is very sensitve, it is not very specific. There are many variables that can affect the absorption of the carboydrate. These include age, diet, endocrine function, length of fasting, medications, gastric emptying and gastrointestinal transit time.