Xylose – Wood Sugar, Absorption Test and Metabolism

What is Xylose?

Xylose (also known as D-Xylose or wood sugar) is a naturally occurring monosaccharide belonging to the group of pentoses – sugar molecules containing five carbon atoms. It is found predominantly in plant cell walls, particularly in hemicellulose-rich structures such as wood, straw, and various vegetables and cereals. While xylose plays a minor role in human metabolism compared to glucose, it holds significant relevance in medicine and nutritional science.

Sources and Occurrence

Xylose is found in a wide variety of plant-based foods:

• Fruits and vegetables (e.g., carrots, peas, spinach)
• Cereals and dietary fiber (e.g., wheat bran, oats)
• Wood and straw (primary sources for industrial extraction)
• Legumes

In the food industry, xylose serves as a precursor for the production of xylitol – a sugar alcohol widely used as a sweetener.

Biological Functions and Metabolism

Xylose is absorbed in the small intestine through both active and passive transport mechanisms. Unlike glucose, it is not subject to hormonal regulation, which makes it a useful marker for assessing the absorption capacity of the small intestinal mucosa. After absorption, a portion of xylose is excreted unchanged via the kidneys into the urine – a property that forms the basis of the D-xylose absorption test.

Medical Relevance: The D-Xylose Absorption Test

The D-xylose absorption test is a diagnostic procedure used to evaluate the absorptive function of the small intestine. It is commonly applied when the following conditions are suspected:

• Celiac disease (gluten-related intestinal disorder)
• Short bowel syndrome
• Tropical sprue
• General malabsorption syndromes

How the Test is Performed

Following an overnight fast, the patient ingests a defined amount of D-xylose (typically 25 g dissolved in water). Blood and/or urine samples are then collected over a set time period to measure the amount of xylose absorbed. A reduced xylose level in blood or urine suggests impaired absorption in the small intestine.

Interpreting the Results

Reference values vary depending on the protocol used, but general guidelines include:

• Urinary excretion of more than 4–5 g of xylose within 5 hours is considered normal.
• Lower values may indicate mucosal damage in the small intestine.
• Impaired kidney function can affect results and must be considered during interpretation.

Xylose in Nutrition and Health Promotion

Xylose has a low glycemic index and is absorbed slowly by the body, making it a topic of interest in functional foods and dietary supplements as an alternative to table sugar. However, clinical evidence supporting specific health benefits remains limited compared to other sugar substitutes. The sugar alcohol xylitol, derived from xylose, has well-established evidence for its anti-cariogenic properties in dental health.

Safety and Tolerability

In amounts typically found in food, xylose is generally well tolerated. In the higher doses used during diagnostic testing, some individuals may experience:

• Bloating
• Abdominal discomfort
• Diarrhea

Individuals with impaired kidney function should only undergo the D-xylose test under medical supervision, as renal clearance directly affects test results.

References

1. Kasper, H. & Burghardt, W. – Ernaehrungsmedizin und Diaetetik (2014), Elsevier Verlag, 12th Edition.
2. Lomer, M. C. E. – Advanced Nutrition and Dietetics in Gastroenterology (2014), Wiley-Blackwell.
3. World Health Organization (WHO) – Carbohydrates in Human Nutrition, FAO/WHO Expert Consultation Report, FAO Food and Nutrition Paper No. 66 (1998). Available at: https://www.fao.org/3/W8079E/W8079E00.htm