Because whole grains are rich in phytochemicals and fibres that are metabolised by intestinal microflora, researchers investigated whether the health benefits of whole grains might be attributable to changes in the gut microbiota.
A total of 50 overweight or obese adults (aged 20–65 years) completed the study.
All participants were considered to be at risk for metabolic syndrome.
Participants were randomised to a whole-grain diet or a refined-grain diet for 8 weeks and then crossed over to the other arm after a 6-week washout period.
The whole-grain diet aimed to provide at least 75 g of whole grains per day, and the refined-grain diet aimed to provide less than 10 g of whole grains per day.
Compared with the refined-grain diet, the whole-grain diet resulted in a lower caloric intake, greater weight loss (p<0.0001), decreased interleukin-6 (p=0.009), and decreased C-reactive protein (p=0.003).
The weight loss was associated with the lower caloric intake, and the decreased interleukin-6 was associated with greater consumption of rye.
Higher plasma concentrations of butyrate were also noted in the whole-grain group (p=0.02).
The whole-grain diet did not significantly change markers of insulin sensitivity or glucose metabolism (HOMA-IR, HbA1c, fasting serum c-peptide, fasting glucose, and fasting insulin).
It did not significantly alter the richness or diversity of the intestinal microbiome (evaluated by sequencing-based metagenomics and 16S rRNA profiling), colonic fermentation (evaluated by a hydrogen breath test), intestinal permeability (evaluated by urinary metabolites), or intestinal transit time (evaluated by X-ray after intake of radiopaque markers).
The authors of the study concluded that only subtle findings pointed toward a microbiome response to the whole-grain diet and that the health benefits observed (weight loss and reduced inflammation) are independent of changes in the gut microbiome during the course of 8 weeks.
