It was previously believed that fructose, which is the sugar found in fruit and fruit juice, is processed by the liver. However, a new study suggests that fructose is mainly processed in the small intestine.

The study, which is published in the journal Cell Metabolism, reveals that processed high-sugar food and drink only spills over into the liver for processing when the small intestine becomes overwhelmed.

The recent findings add to the body of scientific knowledge on the effects of too much fructose on the body.

We know from previous research that excessive consumption of sugar is harmful to the liver, and that chronic overconsumption causes obesity, increases resistance to insulin, and creates conditions for the onset of diabetes.

Last year, a study that found that fructose-containing products such as sweetened drinks can increase the risk of non-alcoholic steatohepatitis, a form of non-alcoholic fatty liver disease, "which can lead to cirrhosis or liver cancer."

Study observed fructose digestion in mice

The researchers, from Princeton University in New Jersey, used mice to study how fructose travels through the digestive system. Their findings suggest that there is a physiological difference in how the body processes different amounts of sugar.

Rather than the liver processing all the sugar in the body, the team observed that more than 90% of fructose was processed in the small intestines of the mice in the study.

The team found that fructose not absorbed into the small intestine is passed through to the colon, where it comes into contact with the microbiome, which is the microbiotic flora that inhabits the large intestine and colon.

The researchers explain that the microbiome is not designed to process sugar. So, while a person could eat a large amount of carbohydrates without exposing their microbiome to any sugar, this changes significantly when high-sugar products—such as soda and juice—are consumed.

While the findings do not prove that fructose influences the microbiome, the team believes that "an effect is likely." They suggest that this link should be further investigated in future studies, as it may provide new insights into the adverse effects of high sugar intake.

'Eat sugar only after meals'

In the study, the small intestine was found to clear fructose more efficiently after a meal.

The team theorizes that during periods of fasting, such as in the morning or mid-afternoon, individuals may be more vulnerable to fructose as the small intestine has reduced ability to process it during these times.

As study author Joshua D. Rabinowitz, of the Lewis-Sigler Institute for Integrative Genomics at Princeton University, explains, "We can offer some reassurance—at least from these animal studies—that fructose from moderate amounts of fruits will not reach the liver."

"We saw that feeding of the mice prior to the sugar exposure enhanced the small intestine's ability to process fructose," Rabinowitz continues. "And that protected the liver and the microbiome from sugar exposure."

Rabinowitz says that the results support "the most old-fashioned advice in the world," which is to "limit sweets to moderate quantities after meals" and avoid sugary drinks outside of meal times.