Important nutrient digestion and absorption takes place in the small intestine, which is the first segment of intestine downstream of the stomach. In some disease processes, the ability to absorb nutrients is decreased.

The goal of this test is to determine whether nutrient absorption is normal or there is some barrier to absorption. It does not provide information on what is causing the barrier, only that a barrier exists. This test is often used when a horse is experiencing weight loss, despite seemingly adequate feeding.

In the glucose tolerance test, the horse is fasted for a period of time. The horse's blood sugar (glucose) is measured at time "0". A measured amount of sugar is then put into the horse's stomach through a stomach tube. The blood is then sampled for blood sugar at specific times following that. The blood sugar levels indicate the degree of absorption of the sugar. The curve for absorption in the patient is compared to normal absorption curves. A similar test is the xylose absorption test.