Sugar intolerance is a digestive condition, more accurately described as carbohydrate malabsorption, where the small intestine struggles to properly break down and absorb specific sugars. This difficulty usually stems from a deficiency in necessary digestive enzymes or a problem with transport proteins moving sugar molecules across the intestinal lining. When sugars are not processed correctly, they continue to the large intestine, leading to uncomfortable gastrointestinal symptoms. Since these symptoms are common to many digestive issues, medical testing is employed to pinpoint the exact sugar causing the problem and confirm the diagnosis.
Types of Intolerance and Common Symptoms
The most frequently encountered forms of sugar malabsorption involve the disaccharide lactose and the monosaccharide fructose. Lactose intolerance occurs due to an insufficient amount of the enzyme lactase, which is required to split lactose into its absorbable components, glucose and galactose. Fructose malabsorption involves a saturation or deficiency of the specific transporter proteins that carry fructose across the intestinal wall.
When these sugars remain undigested, they draw excess water into the bowel through osmosis, contributing to loose stool and diarrhea. The unabsorbed sugar then passes into the colon, where gut bacteria rapidly ferment it. This bacterial process creates gaseous byproducts, primarily hydrogen and methane, which manifest as bloating, excessive gas, and abdominal cramping or pain. These reactions typically begin a few minutes to several hours after consuming the offending sugar.
The Hydrogen Breath Test Procedure
The most common non-invasive method used to diagnose sugar malabsorption is the Hydrogen Breath Test (HBT), which directly measures the gaseous byproducts of bacterial fermentation. This test relies on the fact that human cells do not produce hydrogen or methane gas; therefore, any significant amount detected in the breath must originate from gut bacteria. Specialized versions of this test exist for different sugars, such as the Lactose Breath Test or the Fructose Breath Test.
Preparation for the HBT is a highly regulated process to ensure accurate results. Patients must typically fast for 8 to 12 hours before the test to clear the digestive tract of fermentable food residue. Certain medications, including antibiotics, laxatives, and proton pump inhibitors, must often be avoided for up to four weeks beforehand, as they can alter gut bacteria or transit time. Smoking, vigorous exercise, and sleeping are also restricted immediately before and during the procedure, as they can impact the results.
The test begins with the patient providing a baseline breath sample by blowing into a specialized collection device. Following this initial sample, the patient drinks a measured solution containing the specific sugar being tested, such as 25 grams of lactose or a similar dose of fructose. Over the next two to three hours, breath samples are collected at regular intervals, often every 15 to 30 minutes, to track the concentration of exhaled gases.
If the small intestine fails to absorb the ingested sugar, the solution travels to the colon where bacteria ferment it, releasing hydrogen and/or methane gas. These gases are absorbed into the bloodstream and carried to the lungs, where they are exhaled. A positive result is generally defined by a rise of at least 20 parts per million (ppm) in hydrogen or a significant rise in methane above the baseline measurement. The timing of this gas spike helps distinguish between sugar malabsorption and small intestinal bacterial overgrowth (SIBO), which produces an earlier gas spike.
Secondary Diagnostic Methods
While the HBT is the primary diagnostic tool, other methods are used when the breath test is inconclusive, unavailable, or for complex cases. One alternative is measuring blood glucose levels following the ingestion of the sugar solution, often performed simultaneously with the HBT for lactose intolerance. If the sugar is properly broken down and absorbed, a noticeable rise in blood glucose concentration occurs as the monosaccharides enter the bloodstream.
A failure of the blood glucose level to rise by a set amount, often 20 to 30 milligrams per deciliter, suggests the sugar was not absorbed and passed into the colon. This blood test is an older method that serves as a supplement to the HBT, especially for patients considered non-hydrogen producers who only produce methane or no gas.
An elimination and challenge diet is another practical, non-invasive approach, often implemented before or after formal testing. This involves removing all sources of the suspected sugar from the diet for several weeks to see if symptoms improve. The sugar is then systematically reintroduced to determine if symptoms return, providing a strong clinical indication of the intolerance.
For the rarest forms of sugar intolerance, such as congenital sucrase-isomaltase deficiency, a small intestinal biopsy remains the definitive diagnostic test. During an endoscopy, a tissue sample is collected from the intestinal lining and analyzed to measure the activity level of specific digestive enzymes. Although invasive, this method provides direct confirmation of an enzyme deficiency that is otherwise difficult to assess.