SIBO Malabsorption and Testing

Use and abuse of hydrogen breath tests

This post is a review of a research article from 2006 by M Simren and P-O Stotzer.

Basics of testing

The idea is that you eat easy-to-digest foods for 24 hours prior to your test, then you eat nothing except a lactulose solution for the duration of the test. This allows most of the food to be cleared from your gastrointestinal tract for testing time.

The lactulose is used because it is only digestible by bacteria in the gut, not by our entercytes. The bacteria get this food, then produce hydrogen or methane gas, which is measured in your exhalation at regular intervals.

Smoking raises hydrogen production. Exercise lowers hydrogen production. Therefore, neither are allowed during testing.

Problems

There may be some bacteria in there that you don't want, but they might not produce hydrogen or methane. They could be doing you harm, but not measured on the test. This situation could lead to false negative results.

False positives may result from carbohydrate malabsorption in chronic pancreatitis and celiac disease. The gas is produced from fermentation in the colon.

Delayed transit could produce false negatives.

Rapid transit could produce false positives.

Oral bacteria or not adhering to low fiber diet the day prior to testing could also lead to false positives.

Carbohydrate malabsorption

With poor secretion or activity of pancreatic amylase or intestinal epithelial lactase, maltase, and sucrase, carbohydrates may not be able to break down into simple sugars for absorption. If they make it to the colon still as complex carbohydrates, then they will be fermented, which can lead to the gas problems. The three most studied carbohydrates that may be culprits are lactose, fructose, and sorbitol. More complex carbohydrates could be at least as problematic, though.

Lactose malabsorption

Lactose is a dairy disaccharide cleaved into galactose and glucose by the enzyme lactase, found in the brush border of the small intestinal epithelium.

Lactose would stick around too long if there were a lactase deficiency or secondary to disorders of the epithelium, like celiac disease. Lactase deficiency is more likely for certain races:

• 3-8% in Scandinavia and Northwest Europe
• 70% in southern Italy and Turkey (southen and eastern Europe)
• ~100% in southeast Asia

The most common test (as of 2006) for lactase deficiency is the lactose hydrogen breath test. It works just like the aforementioned lactulose breath test, but instead of ingesting lactulose, the patient ingests lactose. Positive test results mean an increase of 10-20 ppm above baseline hydrogen values. It's worth paying attention to gastrointestinal distress during the test, as well.

Lactose maldigesters can usually tolerate small amounts of lactose, up to 12.1 grams, daily with minimal or no symptoms.

It seems lactase deficiency is no more common in irritable bowel syndrome (IBS) than it is in normal populations. Therefore, lactase could be a red herring.

Fructose malabsorption

Fructose occurs as a ketohexose or as sucrose, which is cleaved by sucrase into glucose and fructose. It's commonly used as an added sweetener and in fruits. It is absorbed by carrier-mediated facilitated diffusion.

The capacity for fructose absorption is apparently small. One study observed 10% absorption in a 50 gram solution of fructose. Plus, simultaneous ingestion of glucose increases fructose absorption. The opposite is observed when fructose is co-ingested with sorbitol.

The fructose test is similar to the lactose test, both in methodology and in usefulness.

Prevalence is also similar in patients with IBS to normal populations.

Small Intestinal Bacterial Overgrowth

SIBO is characterized by > 10^5 colonic-type bacteria present in the small intestine. See my other post to learn more about SIBO.

Tests for SIBO

1. Small intestinal culturing of jejunal aspirate
2. Breath tests

Small intestinal culturing of jejunal aspirate

The jejunum is about as far away as you could get. Grabbing some intestinal fluid might not be representative of the jejunum as a whole, plus it's easily contaminated by things like the oral flora.

Breath tests

The original test was a bile acid breath test, attempting to quantify the number of bacteria present that could deconjugate bile acids. This test is no longer used.

The 1g ^14C-D-xylose breath test uses one gram of ^14C labelled xylose. After absorption, minimal xylose is metabolized and most of it gets secreted through the kidneys. If bacteria are in the small bowel, more of this xylose gets metabolized into 14CO2, which is absorbed and exhaled.

Hydrogen breath tests, using a solution of glucose (50-75 grams) or lactulose (10 grams), are the most commonly used tests for SIBO today. Hydrogen gas expired is measured every 15-30 minutes for three hours.

This figure is from Simren & Stotzer, 2006.

Glucose hardly ever makes it to the large intestine barring any severe transit abnormalities. Results are positive with any clearly recognizable hydrogen peak exceeding 10-20 ppm. High fasting hydrogen concentrations may mean SIBO, but they most likely mean the patient did not follow instructions of low fiber diet prior to testing.

Lactulose is not absorbed until reaching the colon or colonic bacteria. Since unaffected by the small intestine, it can also be used to measure orocecal transit time. Ideal positive results show two hydrogen gas peaks above 20 ppm: the first representing colonic bacteria in the small intestine, and the second representing passage through the large intestine. It has been shown, however, that two peaks can represent the same ingestion of testing solution. This explains the low sensitivity and specificity for the lactulose test.