Alterations in digestion and absorption

"The main treatment for food intolerance is the elimination of the food in question from the regular diet".


The gastrointestinal tract is the muscular tube that runs from the mouth to the anus. Its structure is different in the different sections according to the requirements of food digestion and absorption.

The tube at the intestinal level is covered by microvilli containing enzymatic systems responsible for digestion and allows the development of very efficient absorption mechanisms. In addition, the tract is regulated by a complex neurohormonal control system.

Alterations in the processes of digestion and absorption can occur at different levels and affect various components of the food.

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When is the study of absorption and digestion indicated?

These tests are routinely performed in clinical practice.

They require the ingestion of a specific amount of the element to be studied, waiting for a period of time for it to be absorbed, and then taking a sample of blood, urine or feces to analyze its composition.

Most frequent indications of this test:

  • Fructose, galactose, glucose and lactose intolerance.
  • Intolerance to gluten.
  • Bacterial overgrowth.

Do you have any of these diseases?

A study of intestinal digestion and absorption may be necessary

Types of analysis of food digestion and absorption

It allows to rule out/detect absorption problems due to alteration in the intestinal surface. 

The patient ingests 25 grams of D-xylose dissolved in half a liter of water. Afterwards, the urine emitted during the following 5 hours is obtained.

If there is no renal alteration, it is normal for at least 25% of the administered dose to be detected in urine.

Reduced levels of xylose in urine suggest the presence of an absorption defect at the level of the small intestine, although low levels can also be observed in situations of ascites, vomiting, delayed gastric emptying, high consumption of aspirin, in some pharmacological treatments and in altered renal function.

In cases where the patient presents a diarrheal process, in which some alteration in the digestion or absorption process is suspected, it is recommended to obtain a stool sample in which the color, appearance and pH are determined and also observed under the microscope to detect the presence of undigested fat, starch or muscle fibers.

If fat is detected in the stool, steatorrhea, a syndrome characterized by poor absorption of fat from the diet, may be suspected. It is advisable to carry out a balance of fats in the stool, for which the faeces emitted during 3 days are collected while the patient ingests a known amount of fat. In children, no more than 2 g of fat should be detected in 24 hours, and in adults, no more than 7 g.

If the amount of fat in the stool exceeds normal values, it may be due to a Zollinger-Ellison syndrome, an increase in duodenal acid, abnormal bile excretion, pancreatic insufficiency leading to a deficit in digestive enzymes, disorders of the intestinal mucosa or a disease of the large intestine leading to an interruption in enterohepatic circulation.

Carbohydrates ingested in the diet must be converted into simple molecules, monosaccharides, which can be absorbed at the intestinal level. This conversion involves various enzymes such as salivary amylase and intestinal amylase and disaccharidases.

One of the most frequent digestive problems, traditionally in children and increasingly in adults, is lactose intolerance. It is due to the deficit of one of these disaccharidases, lactase. If people with this deficit ingest milk or milk products, colic, gas and diarrhea will occur.

The diagnosis of this disorder is made with the lactose tolerance test. The patient ingests 50 gr. of lactose dissolved in water and basal blood samples are collected at 5, 10, 30, 60, 90 and 120 minutes to determine glucose. For glucose to appear in the blood, the lactose must have been adequately degraded and absorbed. It is normal for blood glucose to rise more than 200 mg/dl above the baseline.

Recently it has been observed that the best way to determine the digestion and absorption of glucose is by measuring the amount of hydrogen in the exhaled air after oral administration of lactose. Typically, less than 10 parts per million of hydrogen is detected in the breath, while in patients with intolerance it may be more than 50.

Stable isotopes are found in nature. They are harmless to the environment, with no adverse effects, and allow repeated studies to be performed on the same patient. Recently, tests have been developed that use stable isotopes to assess pancreatic function, gastric emptying, digestion processes, orocecal transit, bactarian overgrowth and the presence of Helicobacter pylori.

In the test to assess pancreatic function, the patient ingests a mixed triglyceride in which one of the fatty acids that form it is marked with 13C. If pancreatic function is normal, the lipase secreted by the pancreas into the intestine will degrade the triglyceride and the marked fatty acid will be oxidized to form 13 CO2 which will be eliminated and quantified in the breath.

With a similar test using another marked compound, 13C-octanoic, the speed of gastric emptying can be assessed, detecting cases where gastric emptying is accelerated or delayed.

In order to detect the presence of Helicobacter pylori in the stomach, associated with gastritis and gastric tumors, another breath test is performed. The patient ingests urea marked with 13C. By the action of the uresase that the Helicobacter possesses, the urea is degraded forming 13CO2 that will be detected in the breath. If the result is positive, after the antibiotic treatment, the test is repeated to confirm its elimination.

Where do we do it?


The Clinical Biochemistry Service
of the Clínica Universidad de Navarra

The Clinical Biochemistry Service of the Clinica Universidad de Navarra is responsible for performing the biochemical analyses requested by the medical specialists of our center.

We carry out technical quality controls to guarantee the proper functioning of the equipment and the highest precision in the results obtained from the samples.

In order to guarantee excellence in patient service, we offer the response with the results of the analyses in the shortest possible time, responding in only 46 minutes in some cases of general analysis.

Organized in care units

  • General biochemistry.
  • Electrolytes.
  • Hormones, urine and proteins.
  • Markers.
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Why at the Clinica?

  • Maximum speed in offering the analytical results.
  • We carry out quality controls to guarantee the correct and most precise results obtained.
  • We work in a multidisciplinary way with all the departments of the Clinic.