Improving bowel movements through diet is one of the most common strategies for treating constipation. For instance, dietary fibre has shown to enhance the intestinal environment and increase stool frequency.
Although kale is known to be a good source of dietary fibre and minerals, its effect on the intestinal environment remains to be evaluated.
To examine how the gut microbiota and stool frequency are affected by the consumption of kale, researchers from Japan conducted a randomised, double-blind, placebo-controlled, crossover study.
As women have been reported to suffer from constipation more than men, 24 female participants aged between 20 and 59 with constipation tendencies were selected for the trial.
The participants underwent a four-week intake of kale (collard-type kale and powdered whole kale leaf), or control food (corn starch and maltodextrin powder) processed with colouring and flavouring to mask differences in taste and odour.
The test or control food was mixed with 100 to 150ml of water and consumed twice daily.
During the trial, faecal samples were collected at baseline and at two weeks, and four weeks after the dietary intervention and frozen at -20°C until gut microbiome and metabolome analysis was performed.
It was found that kale consumption for four weeks significantly increased the frequency of stool defecation in the control group, compared to that in the placebo group (p = 0.037).
Notably, participants with improved bowel movements had greater stool amount following intake of kale (p = 0.090).
A total of 215 genera of gut microbes and 352 metabolites were obtained from 120 samples to evaluate the effect of kale consumption on gut metabolite abundance.
The results indicated an increase in Eubacterium eligens group abundance and a decrease in Ruminococcus gnavus (R. gnavus) group abundance for the kale-treated participants.
In addition, pimelic acid content was consistently higher upon intake of kale than the control. Pimelic acid is a precursor of biotin (vitamin B7), a gut microbiota-derived compound that is essential for energy metabolism in humans.
There was also a positive correlation between increased lactic acid levels and increased defecation frequency after four weeks of kale intake.
Lactic acid is an intermediate in propionic acid production. Propionic acid and other short-chain fatty acids are associated with peristalsis (involuntary muscle movement that moves food through the gastrointestinal tract) via the production of serotonin and calcitonin gene-related peptides. Thus, elevated lactic acid levels could lead to higher stool frequency.
Conversely, morpholine content, reportedly harmful to the respiratory tract, kidneys and liver, was consistently lower at both second and fourth week of kale intake.
“Previous studies found that individuals’ responses to foods or drugs are partly attributed to the differences in their gut microbiota. This study provided novel insights into the effects of kale on the intestinal environment and defecation events in a Japanese population.
“Our findings indicate that kale consumption improves stool frequency, particularly in those with low stool amounts at baseline. The correlation analysis also showed that the intervention modifies certain gut microbes and faecal metabolites,” the authors wrote.
Lower intestinal inflammation
Cruciferous vegetables have been widely studied for the prevention of dietary-associated diseases, with broccoli and cabbage being the two most popular crops.
Often consumed as a green juice, kale not only contains flavonoid glycosides, such as quercetin and kaempferol, but also high levels of insoluble dietary fibre.
Insoluble fibre has been shown in studies to possess a high water-holding capacity that increases faecal bulk, and some of the fibre is metabolised by the gut microbiota to form butyric acid, which further raises the frequency of defecation.
Eubacterium eligens is said to utilise dietary fibre, especially pectin, as an energy source, and was observed in in-vitro cell-based assays to “strongly promote” production of the anti-inflammatory cytokine interleukin-10.
Therefore, the results of this study suggest that kale consumption may reduce intestinal inflammation through increased Eubacterium] eligens.
On the other hand, R. gnavus is a mucolytic bacteria that is linked to inflammatory bowel disease. However, it has also been reported to have both beneficial and harmful effects on the host, depending on the subspecies.
It should be noted that this study has some limitations, including the small sample size and simultaneous analysis of numerous metabolites, which made drawing conclusions a challenge.
“Further validations in another cohort model are necessary, especially considering that the gut microbiota has complex parameters consisting of many different bacterial microbes and metabolites,” said the authors.
Source: Frontiers
https://doi.org/10.3389/fnut.2023.1247683
“Kale improves bowel movements in constipated women and affects some intestinal microbes and metabolites: a pilot study”
Authors: Yuichiro Nishimoto, et al
