Intestinal Absorption Tests
Chronic abdominal bloating and gas, diarrhea or unintentional weight loss (malabsorption syndrome) may result from reduced absorption of nutrients in the small intestine due to:
- Lactose intolerance.
- Fructose malabsorption.
- Food allergies and food intolerances (Tests)
- Food poisoning, parasites, Crohn’s Disease (stool analysis and blood work results)
- Glucose or galactose malabsorption
- Fat malabsorption. Fecal fat test, D-xylose test
- Small intestinal bacterial overgrowth (SIBO). Test
- Short bowel syndrome (SBS). Test
Tests for Lactose Intolerance
When you suspect dairy products cause chronic bloating and diarrhea you have, you may ask your gastroenterologist to perform hydrogen breath test. Test is described in food allergy and food intolerance tests.
Tests for Fructose Malabsorption
When you suspect fruits cause chronic bloating or diarrhea you have you may ask for hydrogen breath test with fructose. Test for reducing substances in the stool is performed in small children and when breath test is not available (see below).
Fecal Reducing Substances
When it is not clear if diarrhea in a small child is due to infection or sugar malabsorption a test for fecal reducing substances may be performed.
Reducing substances are sugars (lactose, fructose, glucose, galactose) that, when present in the stool in certain amount, reduce the added reagent copper sulphate into copper oxide and thus change its color. Reducing substances in the stool are increased in lactose intolerance, fructose malabsorption, glucose/galactose malabsorption, short bowel syndrome, toddler’s diarrhea, etc. The test helps a doctor to distinguish between:
- Infectious diarrhea (reducing substances are not increased) and diarrhea due to sugar malabsorption (reducing substances are increased).
- Genetic and acquired sugar malabsorption.
Breath Test with Lactulose in SIBO and Rapid Intestinal Transit
Both small intestinal bacterial overgrowth (SIBO) and rapid intestinal transit may cause chronic diarrhea, bloating, gas and deficiency of some vitamins (A, D, E, K, B12) and minerals (iron, magnesium, calcium, zinc, copper) due to their reduced intestinal absorption. Both conditions can be detected by hydrogen breath test with lactulose.
How to prepare? Discontinue all antibiotics 2 weeks before the test (only after doctor approval!). Do not ingest fiber-rich foods (bran, coarse breads, beans, vegetables) or alcohol 24 hours before the test and fast completely (you may drink pure water) 12 hours before the test. Do not smoke, sleep, or exercise for at least 1 hour before the test.
Procedure. You will drink an emulsion containing lactulose – a substance that cannot be absorbed from the bowel. Lactulose is radio-labeled (to make it slightly radioactive). If you have a lot of bacteria in the small intestine (SIBO), these bacteria will break down some lactulose and yield hydrogen that is radio-labeled and thus detectable in expired air by a special machine. When the remaining lactulose (not broken down by small intestinal bacteria) reaches the colon, it will be broken down the 2nd time by normal colonic bacteria and hydrogen will be detected in the expired air again. If only the later hydrogen rise is detected, it means there are bacteria only in the colon, so there is no small intestinal bacterial overgrowth. In another disorder – rapid transit of the food through the small intestine – the 2nd hydrogen rise will occur sooner as in a healthy person (1). The test is completed in three hours.
Tests for Fat Malabsorption
Fecal Fat Test
Procedure. Doctor prescribes you exact diet containing about 100 g of fats daily. During three days of such a diet you collect all your stool in a special container and then deliver it to the laboratory where amount of fat in the stool will be measured (fecal fat test) (4).
Explanation. When fats are not appropriately absorbed in the small intestine they are excreted in the stool in increased amount. Finding more than 6 grams of fat in the stool from one day speaks for fat malabsorption.
D-xylose test is made to distinguish between pancreatic and intestinal causes of fat malabsorption. D-xylose is a carbohydrate absorbed in the small intestine without the help of any enzyme. If few hours after D-xylose ingestion its concentration in the blood and urine is raised, it means small intestinal absorption is not affected, so the likely cause of fat malabsorption is a pancreatic disorder. Absent or sub-normal level of D-xylose in the blood after the test speaks for intestinal disorder.
Pancreatic Function Tests
To evaluate, if pancreatic disorder is a cause of diarrhea (usually white colored loose bowel movements), or to evaluate an extent of pancreatic damage, pancreatic function tests may be done.
Pancreatic function tests measure the activity of pancreatic enzymes thus revealing an extent of pancreatic damage.
- Fecal elastase (pancreatic enzyme appearing in the stool) is lowered in pancreatic insufficiency from any cause (2).
- Lipase in the blood serum is raised in acute pancreatitis but often normal in chronic pancreatitis and pancreatic cancer.
- Secretin/CCK stimulation test. Hormones secretin and cholecystokinin (CCK) are administered to duodenum via a tube inserted through the nose. Hormones stimulate secretion of pancreatic juice containing bicarbonate and enzymes into the duodenum. After a while, pancreatic juice is sucked out, and concentrations of pancreatic enzymes and bicarbonate are determined; their low concentrations reflect low pancreatic function.
- In the bentiromide test, a substance bentiromide is ingested. Bentiromide is broken down by pancreatic enzymes and its breakdown products are absorbed into the blood and excreted with the urine. Low concentration of bentiromide breakdown products in the urine reflects low pancreatic function.
The test is usually used in short bowel syndrome (SBS) to determine the required amount of nutrients that will meet body demands for energy (3).
With indirect calorimetry the extent of absorption in the small intestine can be determined.
Principle. For every calorie of energy produced by the body, body cells consume 208 milliliters of oxygen from the blood. This means that less amount of oxygen will be expired from the lungs when the body produces energy than when it does not produce energy. For a meal with a known calorie value, an expected reduction of amount of oxygen in the expired air can be calculated. When a person, after ingestion of such a meal, exhales more oxygen than expected, it can be concluded that the meal was not absorbed in the small intestine completely, so some oxygen was not taken up by the cells and remained free to be expired through the lungs.