The findings, which appear in the journal Nature, are of importance in showing how constipation is a risk factor in conditions such as colorectal cancer and Parkinson’s disease as well as disorders where constipation often occurs such as Attention Deficit Hyperactivity Disorder (ADHD) and autism.
Evidence suggests that the waste products produced by the gut bacteria affect colonic cells associated with these conditions, although the mechanisms remain largely unknown.
Findings of the study, carried out at the Technical University of Denmark's National Food Institute, revealed that a long colonic transit time was linked with high microbial diversity and a switch from carbohydrate digestion to protein breakdown.
A shorter gut transit time was associated with gut bacteria by-products that could indicate increased renewal of the colonic mucosa.
The observations suggest that a rich gut microbial diversity does not necessarily imply a healthy gut microbial ecosystem.
“Our study shows that the longer food takes to pass through the colon, the more harmful bacterial degradation products are produced,” explained Tine Rask Licht, professor at the National Food Institute and lead author of the study.
“Conversely, when the transit time is shorter, we find a higher amount of the substances that are produced when the colon renews its inner surface, which may be a sign of a healthier intestinal wall.”
The findings were based on the urine and stool samples of 98 adults from Denmark.
Urine and faecal samples from these subjects (61 female and 37 male, aged 22–66 years,) were taken as part of two human intervention studies.
Each participant’s habitual dietary intake was recorded based on a four-day dietary registration. On the examination day, subjects consumed a standardised drink and a standardised breakfast containing approximately 3,000 kJ. (52.6% fat, 39.7% carbohydrates and 7–8 % protein).
Current research has not gone into sufficient detail as to how colonic transit time may control gut microbial metabolism. Previous studies have shown that high bacterial richness characterises a healthy gut microbial ecosystem.
Recently, it was shown in gene profiling studies that a firm stool consistency, which is a good indicator of a long colonic transit time, was linked with a high gut microbial richness.
“Our observation that bacterial metabolites of protein catabolism are positively associated with a long transit time reflects the fact that carbohydrates are the preferred substrate for bacterial fermentation,” the study concluded.
“Bacteria will not switch to protein metabolism until carbohydrates are depleted. This assumption underpins the concept of prebiotics.”
Licht emphasised that dietary habits were a major factor in influencing transit time. She recommended a diet rich in fibre and drinking plenty of water.
“It may also be worth trying to limit the intake of for example meat, which slows down the transit time and provides the gut bacteria with lots of protein to digest. Physical activity can also reduce the time it takes for food to travel through the colon.”
The study was performed in collaboration with a number of institutes including the Technical University of Denmark, University of Copenhagen and Bispebjerg Hospital.
Published online ahead of print, doi:10.1038/nmicrobiol.2016.93
“Colonic transit time is related to bacterial metabolism and mucosal turnover in the gut.”
Authors: Tine R. Licht et al.