What is the natural source of lactulose?

January 10, 2026 •

3 min read

While often assumed to be a natural sugar, lactulose is not found naturally in significant amounts. Instead, this disaccharide is a synthetic sugar derived primarily from lactose, the sugar found in milk. The misconception likely arises because of its dairy origin and its presence in some dairy products due to manufacturing processes.

Quick Summary

Lactulose is a synthetic disaccharide created from lactose, not a sugar that occurs naturally in foods. Trace amounts can form in heat-treated milk, but commercial production uses a chemical process to isomerize lactose from milk sugar.

Key Points

Synthetic Origin: Lactulose is a man-made sugar, not one that occurs naturally in foods.
Made from Lactose: Its precursor is lactose, or milk sugar, which is chemically transformed through a process called isomerization.
Heat-Treated Milk Presence: Trace amounts of lactulose can be found in milk that has been subjected to high heat, such as UHT milk.
Not Digested by Humans: Unlike lactose, lactulose is not absorbed by the human small intestine and reaches the colon intact.
Promotes Gut Health: Once in the colon, lactulose is fermented by beneficial bacteria, acting as a prebiotic and promoting better intestinal function.
Used as a Laxative: By drawing water into the colon, it softens stool and helps relieve constipation.
Medicinal and Food Applications: It is widely used as a pharmaceutical product and as a functional ingredient in some foods, especially infant formula.

Lactulose: A Synthetic Disaccharide

Lactulose is a synthetic sugar composed of two simple sugars, fructose and galactose. Despite being derived from lactose (milk sugar), it does not exist in nature on its own. Its creation is a controlled process that alters the structure of lactose to form a different, non-digestible molecule. The final product, lactulose, is poorly absorbed by the human body and is not broken down by our intestinal enzymes. This is the key property that gives it its therapeutic and prebiotic effects.

The Creation Process: From Lactose to Lactulose

The primary source for creating lactulose is lactose. During commercial production, lactose is subjected to a chemical reaction called isomerization. This process typically involves a basic, or alkaline, catalyst and heat, which rearranges the molecular structure of the lactose molecule.

The industrial process generally follows these steps:

An aqueous solution of lactose is prepared.
An alkaline catalyst, such as sodium hydroxide or sodium aluminate, is added.
The mixture is heated, causing the lactose to isomerize into lactulose.
Purification steps follow to remove any remaining unconverted lactose, catalysts, and other byproducts.
The final lactulose product is concentrated into a syrup or crystallized into a powder.

Formation in Heat-Treated Milk

While lactulose is primarily a synthetic product, it can be detected in very small quantities in milk that has undergone heat processing. This is a byproduct of the high temperatures used during pasteurization or sterilization, specifically ultra-high temperature (UHT) milk. The level of lactulose produced correlates directly with the intensity of the heat treatment, making it a reliable indicator of how extensively milk has been heated. However, the amounts found in these dairy products are trace and not comparable to the concentrated forms used for medicinal or supplemental purposes.

Lactulose vs. Lactose

To better understand lactulose, it's helpful to compare it to its precursor, lactose. While both are disaccharides and share a similar name, their molecular structure and how the human body processes them are fundamentally different.


Feature	Lactulose (Synthetic)	Lactose (Natural)
Source	Produced industrially from lactose.	Naturally occurs in raw milk and dairy products.
Molecular Composition	Galactose + Fructose.	Galactose + Glucose.
Enzymatic Digestion	Not digested by human enzymes; passes mostly intact to the colon.	Digested by the enzyme lactase in the small intestine.
Effect on the Body	Prebiotic effect and osmotic laxative; promotes beneficial gut bacteria.	Acts as a source of energy.
Presence in Raw Milk	Absent.	Present.
Detection in Heat-Treated Milk	Found in small amounts in heat-processed milk, especially UHT milk.	Found in lower concentrations after heat treatment compared to raw milk.

Therapeutic and Prebiotic Applications

Despite its non-natural origin, lactulose has significant and beneficial applications. As a laxative, it draws water into the colon via osmosis, which softens the stool and promotes a bowel movement. As a prebiotic, it serves as a food source for beneficial bacteria in the gut, such as Bifidobacteria and Lactobacilli. The fermentation of lactulose by these microbes produces short-chain fatty acids (SCFAs), which are important for colon health.

Conclusion

In summary, the search for a natural source of lactulose reveals that this is a misconception. Lactulose is a man-made sugar, synthesized primarily through the isomerization of lactose. While traces can be found in heat-treated dairy products as a result of the manufacturing process, the commercially available product for medicinal and supplemental use is entirely synthetic. Its unique properties, which are not found in naturally occurring sugars, make it a valuable tool in medical and nutritional fields. Understanding its synthetic origin is crucial for appreciating its function and purpose as a non-digestible prebiotic and laxative.

For more information on the chemistry and applications of lactulose, see the National Institutes of Health (NIH) resource on the subject.(https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2021.672925/full)

Frequently Asked Questions

Yes, lactulose is commercially derived from lactose, which is the natural sugar found in milk. However, it is a synthetic derivative and not naturally present in its final form in raw milk.

Small amounts of lactulose are formed during the thermal processing of milk, especially ultra-high temperature (UHT) treatment. This heat-induced chemical change isomerizes some of the milk's lactose into lactulose.

Yes, lactulose is widely regarded as safe. It has been used for decades as both a laxative and prebiotic and is listed on the World Health Organization's List of Essential Medicines. Common side effects are typically mild and gastrointestinal.

Lactulose functions as an osmotic laxative. Because it is not absorbed in the small intestine, it draws water into the colon, which helps to soften the stool and make it easier to pass.

Commercial lactulose is produced by chemically isomerizing lactose in an alkaline and heated solution. The resulting lactulose is then purified from other sugars and byproducts to create a concentrated syrup or powder.

No, lactulose does not cause lactose intolerance symptoms because it is not the same molecule as lactose. It is not digested by the enzyme lactase and therefore does not produce the same effects in individuals with lactose intolerance.

You cannot obtain lactulose directly from natural, unprocessed foods. While it exists in trace amounts in heat-treated milk, this is a byproduct of processing rather than a natural source.