Creedon, A.C., E.S. Hung, S.E. Berry, K. Whelan, 2020. Nuts and their effect on gut microbiota, gut function and symptoms in adults: A systematic review and meta-analysis of randomised controlled trials. Nutrients. 12, 2347; doi:10.3390/nu12082347
Nuts contain fibre, unsaturated fatty acids and polyphenols that may impact the composition of the gut microbiota and overall gut health. This study aimed to assess the impact of nuts on gut microbiota, gut function and gut symptoms via a systematic review and meta-analysis of randomised controlled trials (RCTs) in healthy adults. Eligible RCTs were identified by systematic searches of five electronic databases, hand searching of conference abstracts, clinical trials databases, back-searching reference lists and contact with key stakeholders. Eligible studies were RCTs administering tree nuts or peanuts in comparison to control, measuring any outcome related to faecal microbiota, function or symptoms. Two reviewers independently screened papers, performed data extraction and risk of bias assessment. Outcome data were synthesised as weighted mean difference (WMD) or standardised mean difference (SMD) using a random effects model. This review was registered on PROSPERO (CRD42019138169). Eight studies reporting nine RCTs were included, investigating almonds (n = 5), walnuts (n = 3) and pistachios (n = 1). Nut consumption significantly increased Clostridium (SMD: 0.40; 95% CI, 0.10, 0.71; p = 0.01), Dialister (SMD: 0.44; 95% CI, 0.13, 0.75; p = 0.005), Lachnospira (SMD: 0.33; 95% CI, 0.02, 0.64; p = 0.03) and Roseburia (SMD: 0.36; 95% CI, 0.10, 0.62; p = 0.006), and significantly decreased Parabacteroides (SMD: −0.31; 95% CI, −0.62, −0.00; p = 0.05). There was no effect of nuts on bacterial phyla, diversity or stool output. Further parallel design RCTs, powered to detect changes in faecal microbiota and incorporating functional and clinical outcomes, are needed.
Tindall, A.M., C.J. McLimans, K.S. Petersen, P.M. Kris-Etherton, R. Lamendella, 2019. Walnuts and vegetable oils containing oleic acid differentially affect the gut microbiota and associations with cardiovascular risk factors: Follow-up of a randomized, controlled, feeding trial in adults at risk for cardiovascular disease. J Nutr. 150(4):806-817.
Background: It is unclear whether the favorable effects of walnuts on the gut microbiota are attributable to the fatty acids, including α-linolenic acid (ALA), and/or the bioactive compounds and fiber. Objective: This study examined between-diet gut bacterial differences in individuals at increased cardiovascular risk following diets that replace SFAs with walnuts or vegetable oils.Methods: Forty-two adults at cardiovascular risk were included in a randomized, crossover, controlled-feeding trial that provided a 2-wk standard Western diet (SWD) run-in and three 6-wk isocaloric study diets: a diet containing whole walnuts (WD; 57-99 g/d walnuts; 2.7% ALA), a fatty acid-matched diet devoid of walnuts (walnut fatty acid-matched diet; WFMD; 2.6% ALA), and a diet replacing ALA with oleic acid without walnuts (oleic acid replaces ALA diet; ORAD; 0.4% ALA). Fecal samples were collected following the run-in and study diets to assess gut microbiota with 16S rRNA sequencing and Qiime2 for amplicon sequence variant picking. Results: Subjects had elevated BMI (30 ± 1 kg/m2), blood pressure (121 ± 2/77 ± 1 mmHg), and LDL cholesterol (120 ± 5 mg/dL). Following the WD, Roseburia [relative abundance (RA) = 4.2%, linear discriminant analysis (LDA) = 4], Eubacterium eligensgroup (RA = 1.4%, LDA = 4), LachnospiraceaeUCG001 (RA = 1.2%, LDA = 3.2), Lachnospiraceae UCG004 (RA = 1.0%, LDA = 3), and Leuconostocaceae (RA = 0.03%, LDA = 2.8) were most abundant relative to taxa in the SWD (P ≤ 0.05 for all). The WD was also enriched in Gordonibacter relative to the WFMD. Roseburia (3.6%, LDA = 4) and Eubacterium eligensgroup (RA = 1.5%, LDA = 3.4) were abundant following the WFMD, and Clostridialesvadin BB60group (RA = 0.3%, LDA = 2) and gutmetagenome (RA = 0.2%, LDA = 2) were most abundant following the ORAD relative to the SWD (P ≤ 0.05 for all). Lachnospiraceae were inversely correlated with blood pressure and lipid/lipoprotein measurements following the WD. Conclusions: The results indicate similar enrichment of Roseburia following the WD and WFMD, which could be explained by the fatty acid composition. Gordonibacter enrichment and the inverse association between Lachnospiraceae and cardiovascular risk factors following the WD suggest that the gut microbiota may contribute to the health benefits of walnut consumption in adults at cardiovascular risk. This trial was registered at clinicaltrials.gov as NCT02210767.
McArthur, B.M., R.D. Mattes, R.V. Considine, 2018. Mastication of nuts under realistic eating conditions: implications for energy balance. Nutrients. 10(6), 710; https://doi.org/10.3390/nu10060710.
The low digestibility and high satiety effects of nuts have been partly attributed to mastication. This work examines chewing forces and the bolus particle size of nuts (walnuts, almonds, pistachios) varying in physical properties under different conditions (with and without water, juice, sweetened yogurt and plain yogurt) along with satiety sensations and gut hormone concentrations following walnut consumption (whole or butter). In a randomized, cross-over design with 50 adults (25 males, 25 females; Body Mass Index (BMI) 24.7 _ 3.4 kg/m2; age: 18–52 years old (y/o), the chewing forces and particle size distribution of chewed nuts were measured under different chewing conditions. Appetite sensations were measured at regular intervals for 3 h after nut intake, and plasma samples were collected for the measurement of glucose, insulin and Glucagon-like peptide-1 (GLP-1). The three nuts displayed different particle sizes at swallowing though no differences in chewing forces were observed. Walnuts with yogurt yielded larger particle sizes than the other treatments. Particle size was not correlated with either food palatability or flavor. Fullness sensations were higher after whole nut than nut butter consumption though there were no significant changes in glucose, insulin, or GLP-1 concentrations under any condition. Changing the conditions at swallowing might influence the release of energy from nuts.
Holscher, H.D., H.M. Guetterman, K.S. Swanson, R. An,N.R. Matthan, A.H. Lichtenstein, J.A. Novotny, D.J. Baer, 2018. Walnut consumption alters the gastrointestinal microbiota, microbially derived secondary bile acids, and health markers in healthy adults: a randomized controlled trial. J Nutr. doi: 10.1093/jn/nxy004. [Epub ahead of print]
Background: Epidemiologic data suggest that diets rich in nuts have beneficial health effects, including reducing total and cause-specific mortality from cancer and heart disease. Although there is accumulating preclinical evidence that walnuts beneficially affect the gastrointestinal microbiota and gut and metabolic health, these relations have not been investigated in humans. Objective: We aimed to assess the impact of walnut consumption on the human gastrointestinal microbiota and metabolic markers of health. Methods: A controlled-feeding, randomized crossover study was undertaken in healthy men and women [n = 18; mean age = 53.1 y; body mass index (kg/m2): 28.8]. Study participants received isocaloric diets containing 0 or 42 g walnuts/d for two 3-wk periods, with a 1-wk washout between diet periods. Fecal and blood samples were collected at baseline and at the end of each period to assess secondary outcomes of the study, including effects of walnut consumption on fecal microbiota and bile acids and metabolic markers of health. Results:
Compared with after the control period, walnut consumption resulted in a 49-160% higher relative abundance of Faecalibacterium, Clostridium, Dialister, and Roseburia and 16-38% lower relative abundances of Ruminococcus, Dorea, Oscillospira, and Bifidobacterium (P < 0.05). Fecal secondary bile acids, deoxycholic acid and lithocholic acid, were 25% and 45% lower, respectively, after the walnut treatment compared with the control treatment (P < 0.05). Serum LDL cholesterol and the non-cholesterol sterol campesterol concentrations were 7% and 6% lower, respectively, after walnut consumption compared with after the control treatment (P < 0.01). Conclusions: Walnut consumption affected the composition and function of the human gastrointestinal microbiota, increasing the relative abundances of Firmicutes species in butyrate-producing Clostridium clusters XIVa and IV, including Faecalibacterium and Roseburia, and reducing microbially derived, proinflammatory secondary bile acids and LDL cholesterol. These results suggest that the gastrointestinal microbiota may contribute to the underlying mechanisms of the beneficial health effects of walnut consumption.
