Walnut Consumption Influences the Human Gut Microbiome
Authors: Guetterman Heather M , Swanson Kelly S , Novotny Janet A , Baer David J , Holscher Hannah D 
Published: June 2021

Background

Diet and the microbiome play an important role in human health. However, the interplay of diet, the microbiome, and health and disease is under‐investigated. Furthermore, there is a dearth of information on the impact of specific foods on the gastrointestinal microbiome. Diets rich in nuts have beneficial effects on cardiovascular disease risk factors, including reduced LDL cholesterol and inflammation. The exact mechanisms of these cardioprotective effects may be linked to the gastrointestinal microbiome.

Objective

We aimed to assess the impact of walnut consumption on the human gastrointestinal microbiome as a means for understanding the underlying mechanisms of the cardiometabolic protective effects of nut consumption.

Methods

A controlled‐feeding, randomized, crossover study was undertaken in healthy adult males (n=10) and females (n=8). Study participants received isocaloric treatment diets containing 0 grams or 42 grams/day of walnut pieces for a period of 3 weeks with a 1‐week washout between periods. Blood, urine, and fecal samples were collected at the beginning and end of each treatment period for metabolic, immunologic, and microbial analyses. Barcoded amplicon pools of bacterial, fungal, and archaeal sequences were generated using a Fluidigm Access Array system prior to high‐throughput sequencing on an Illumina MiSeq. Sequence data were analyzed with QIIME 1.8. Data were analyzed using the mixed‐model procedure of SAS with post‐hoc Tukey adjustments for multiple comparisons (SAS 9.4).

Results

Principal coordinates analysis (PCoA) of UniFrac distances between samples based on their 97% OTU composition and abundances indicated that bacterial communities were impacted by walnut consumption (p<0.05). Walnut consumption increased (p=0.05) the relative abundance of Clostridium from baseline to end. Compared to the end of the control period, walnut consumption increased the relative abundances of Roseburia (p=0.02) and Dialister (p=0.02), and tended to increase the proportion of Faecalibacterium (p=0.07). The relative abundance of Oscillospira tended (p=0.08) to decrease from the end of the control period (no walnuts) compared to the end of the treatment period when walnuts were consumed.

Conclusions

These novel results revealed that walnut consumption significantly affects the microbial composition of the human gastrointestinal microbiota. More specifically, walnut consumption increased bacterial genera associated with anti‐inflammatory properties and production of the short‐chain fatty acids butyrate and propionate. Short‐chain fatty acids are purported to mediate some of the hypocholesterolemic effects of dietary fibers. These data help fill the gap in knowledge related to the cardioprotective effects of nut consumption. Further research on the bacterial fermentation of walnuts is needed to determine if the changes in these microbial communities translate to increased production of short‐chain fatty acids.

Support or Funding Information

This study was funded by USDA and California Walnut Commission.