The new research, published in Food Chemistry, analysed the make-up of a variety of apple cultivars for their content of non-digestible compounds and to assess how these compounds can modulate bacterial populations.
Led by Giuliana Noratto at Washington State University, the team found ‘significant’ differences in the levels of extractable phenolics, non-extractable proanthocyanidins and dietary fibre among the seven cultivars analysed - with Granny Smith having the highest contents of these bioactive compounds.
"We know that, in general, apples are a good source of these nondigestible compounds but there are differences in varieties," said Noratto. "Results from this study will help consumers to discriminate between apple varieties that can aid in the fight against obesity."
After the initial analysis, the team tested whether the bioactivess from the Granny Smith cultivar were able to influence gut bacteria make-up using an in vitro digestion model.
"The nondigestible compounds in the Granny Smith apples actually changed the proportions of fecal bacteria from obese mice to be similar to that of lean mice," revealed Noratto.
According to the team, the tart green Granny Smith apples could benefit the growth of friendly bacteria in the colon due to their high content of non-digestible compounds, including dietary fibre and polyphenols, and low content of available carbohydrates.
The team characterised the levels of compounds in Braeburn, Fuji, Gala, Golden Delicious, Granny Smith, McIntosh and Red Delicious cultivars – finding significant differences for extractable phenolics, non-extractable proanthocyanidins, and dietary fibre among the seven cultivars.
In a further experiment the team then tested how the compounds in Granny Smith affected the fermentation of faeces from diet-induced obese mice.
“After faecal fermentation of Granny Smith non-digestible compounds, we demonstrated that relative abundances of bacterial populations in faeces from obese mice tended to be similar to the lean controls,” said the team.
Indeed, the results showed a relative abundances of Firmicutes, Bacteroidetes, Enterococcus, Enterobacteriaceae, Escherichia coli, and Bifidobacterium with an increased trend in the production of butyric acid.
Noratto and colleagues added that the results suggest that apple non-digestible compounds might help to re-establish a disturbed microbiota balance in obesity.
“These results suggest that dietary fibre and phenolic compounds remaining in apple after IVD [in vitro digestion] might help to prevent metabolic disorders driven by an altered microbiota in obesity, and potentially protect from an obesity-disturbed balance of microbiota,” concluded the team.