The body of evidence about nuts and gut health continues to grow, with new local and international research papers regularly published.

Body of evidence

Effect of 1-year lifestyle intervention with energy-reduced Mediterranean diet and physical activity promotion on the gut metabolome and microbiota: A randomized clinical trial. (Garcia- Gavilan et al, 2024).
This intervention study, involving 400 participants, found that an energy-reduced MedDiet and physical activity promotion, compared with an ad libitum MedDiet, produced significant changes in gut metabolomics and microbiota in a Mediterranean population of older adults (aged 55-75 years) with overweight/obesity and metabolic syndrome. And these changes, in faecal metabolites and faecal microbiota, were associated with changes in several cardiometabolic risk factors, including a greater reduction in adiposity and improvements in lipid profile and markers of glucose metabolism.

Peanuts supplementation affects compositions and functions of gut microbiome in Ugandan children. (Wang et al, 2024).
This trial involved 120 Ugandan children, aged 6-9 years. Half received daily roasted peanut snacks (57g peanuts), while the other half did not (control group). Faecal samples were collected at baseline, day 60, and day 90. Significant differences were seen between the control and peanut-supplemented groups after 90 days, indicating a role for peanuts in modulating the gut metabolome, enriching beneficial bacteria, and inhibiting pathogens. In turn, the authors suggest this may offer a novel approach to mitigating child malnutrition and improving health status.

Daily cashew and Brazil nut consumption modifies intestinal health in overweight women on energy-restricted intervention: A randomized controlled trial (Brazilian Nuts Study). (Kelly Souza Silveira et al, 2024).
This 8-week randomised controlled trial found consuming a combined 30g of cashews and 15g of Brazil nuts per day, within an energy-restricted context (−500kcal/day), positively impacted the gut microbiota profile of women with overweight or obesity. Compared with control, cashew and Brazil nut consumption increased faecal propionic acid and potentially-beneficial gut bacteria, such as Ruminococcus and Roseburia, and mitigated increased intestinal permeability.

Effect of nuts on gastrointestinal health. (Mandalari et al, 2023)
This narrative review summarises the most important findings and new research perspectives on nut consumption and gastrointestinal health. It outlines the physiological processes that contribute to the digestion of nuts, and how this may impact gut microbiota composition. Some scientific evidence suggests an increase in faecal butyrate after almond consumption, and a role for walnuts in the prevention of ulcerative colitis and protection against gastric mucosal lesions. However, research on the effect of nuts on gastrointestinal health is still at an early stage.

Dietary fibers of tree nuts differ in composition and distinctly impact the fecal microbiota and metabolic outcomes in vitro. (Sahin et al, 2023).
This study analysed and compared the effects of dietary fibres within tree nuts (almond, cashew, hazelnut, pistachio, and walnut) on gut microbiota in vitro. It found that nut fibers are generally capable of promoting beneficial microbes in the colon, although the degree of promotion depends on the nut type. The researchers suggest this is a contributing factor to the health-promoting effects of regular nut consumption.

Nuts and their effect on gut microbiota, gut function and symptoms in adults: A systematic review and meta-analysis of randomised controlled trials (Creedon et al, 2020)
This study assessed 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. Eight studies reporting nine RCTs were included, investigating almonds (n = 5), walnuts (n = 3) and pistachios (n = 1). Nut consumption significantly increased Clostridium, Dialister, Lachnospira and Roseburia, and significantly decreased Parabacteroides. There was no effect of nuts on bacterial phyla, diversity or stool output.

Peanuts as a nighttime snack enrich butyrate-producing bacteria compared to an isocaloric lower-fat higher-carbohydrate snack in adults with elevated fasting glucose: A randomized crossover trial. (Sapp et al, 2022).
This randomised crossover trial, involving 50 adults, examined the effect on gut microbiota composition of 28g/day of peanuts for 6-weeks, compared to an isocaloric lower-fat, higher-carbohydrate (LFHC) snack. Following peanut intake, Ruminococcaceae (a known butyrate producer, with a role in role in the maintenance of gut health) were significantly more abundant, compared to the LFHC snack. The researchers say this adds support for peanut-induced gut microbiome modulation.

Microbiome connections with host metabolism and habitual diet from 1,098 deeply phenotyped individuals. (Asnicar et al, 2021).
Researchers performed metagenomic sequencing of 1,203 gut microbiomes from 1,098 people enrolled in the Personalised Responses to Dietary Composition Trial (PREDICT 1) study, where dietary information and cardiometabolic blood marker measurements were also available. They found that people who ate a diet rich in healthy, plant-based foods were more likely to have high levels of ‘good’ gut microbes. Among the findings, Prevotella copri, a species of bacteria, was associated with favourable post-prandial blood sugar control. These bacteria were more common when people ate a diet high in polyunsaturated fat, found in fish, nuts, and seeds.

Does ‘activating’ nuts affect nutrient bioavailability? (Kumari et al, 2020)
This study assessed the effects of different soaking regimes on phytate and mineral concentrations of whole and chopped almonds, hazelnuts, peanuts, and walnuts. The treatments were: 1. Raw; 2. soaked for 12 h in salt solution; 3. soaked for 4 h in salt solution; 4. soaked for 12 h in water. Although there were some statistically significant differences in phytate concentrations between treatments, no soaking treatment reduced phytate concentrations to a level that would result in clinically meaningful improvements in the bioavailability of minerals. In summary, the authors found no evidence that soaking is an effective strategy to reduce phytate concentrations and improve the nutrient bioavailability of almonds, hazelnuts, peanuts and walnuts.

Mediterranean diet intervention alters the gut microbiome in older people reducing frailty and improving health status: the NU-AGE 1-year dietary intervention across five European countries. (Shankar Ghosh et al, 2020).
The Mediterranean diet is positively associated with health. The results of this study show that a greater adherence to the MedDiet modulates specific components of the gut microbiota that were associated with reductions in risk of frailty, improved cognitive function and reduced inflammatory status.

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. (Tindall et al, 2019).
Similarities between enrichment of SCFA-producing bacteria, including Roseburia and Eubacterium, following the walnut diet (WD) and walnut fatty acid matched diet (WFMD) illustrate the effect that the high unsaturated fat content, including ALA, may have on gut bacteria. The unique enrichment of Gordonibacter following the WD suggests that walnut-derived bioactive compounds and fibre modulate gut microbiota. The associations between Lachnospiraceae and improved cardiovascular risk factors suggest that the gastrointestinal microbiota may contribute to the underlying mechanisms of the beneficial health effects of walnut consumption.

Urolithin metabotypes can determine the modulation of gut microbiota in healthy individuals by tracking walnuts consumption over three days. (Garcia Mantrana et al, 2019).
Walnuts are rich in polyphenols ellagitannins, modulate gut microbiota (GM), and exert health benefits after long-term consumption. This study aimed to assess whether urolithin metabotypes (reported to predict host responsiveness to a polyphenol-rick intervention) were associated with differential GM modulation after short-term walnut consumption. In this study, 27 healthy individuals consumed 33 g of peeled raw walnuts over three days. The results show that walnuts consumption after only three days modulates GM in a urolithin metabotype-depending manner and increases the production of short-chain fatty acids (SCFA).

Walnut consumption alters the gastrointestinal microbiota, microbially derived secondary bile acids, and health markers in healthy adults: A randomized controlled trial. (Holscher et al, 2018).
Walnut consumption affected the gastrointestinal microbiota and microbially derived secondary bile acids and reduced serum total and LDL cholesterol in healthy adults. These results suggest that the gastrointestinal microbiota may contribute to the underlying mechanisms of the beneficial health effects of walnut consumption, including cardiometabolic and gastrointestinal health.

The effects of ‘activating’ almonds on consumer acceptance and gastrointestinal tolerance. (Taylor et al, 2018).
According to a study published in the European Journal of Nutrition, soaking (or activating) almonds did not reduce phytates, nor did it improve GI tolerance when compared to unsoaked nuts.

Prebiotic nut compounds and human microbiota. (Lamuel-Raventos et al, 2017).
Nuts contain non-bio-accessible material (mainly polysaccharides and polymerised polyphenols) and emerging evidence suggests that they appear to have a prebiotic effect, which may help to explain their many health benefits. However, data are very limited and more research is required, particularly from human intervention studies, as well as research on different types of nuts, doses, and over a sufficiently long period of time.

Health benefits of walnut polyphenols: An exploration beyond their lipid profile. (Sanchez-Gonzalez et al, 2017).
The main polyphenol in walnuts is pedunculagin, an ellagitannin. After consumption, ellagitannins are hydrolysed to release ellagic acid, which is converted by gut microflora to urolithin A and other derivatives such as urolithins B, C, and D. Ellagitannins possess well known antioxidant and anti-inflammatory bioactivity, and several studies have assessed the potential role of ellagitannins against disease initiation and progression, including cancer, cardiovascular, and neurodegenerative diseases.

Dietary pattern and colonic diverticulosis. (Tursi, 2017).
High-fibre diet does not prevent diverticulosis occurrence, and results about prevention/treatment of diverticular disease and acute diverticulitis are still conflicting. No association was seen between nut, corn or popcorn consumption and occurrence of diverticulosis, diverticular disease and acute diverticulitis.

Diverticular disease: Reconsidering conventional wisdom. (Peery et al, 2013).
This paper suggests that the theories of prior generations, with respect to diverticular disease, have been proven to be questionable and in some cases unsupportable. Among these, is the advice to avoid foods that may leave coarse particulate in the stool, including nuts and seeds. The researchers provide evidence that nuts and seeds do not increase the risk of diverticulitis or diverticular bleeding. And they state that it is time to reconsider conventional wisdom about diverticular disease.

Lifestyle factors and the course of diverticular disease. (Strate, 2012).
Contrary to a long-standing belief, a large prospective study found that nuts and corn did not increase the risk of diverticulitis or diverticular bleeding.

Low-residue diet in diverticular disease: putting an end to a myth. (Tarleton et al, 2011)
Historically, low-residue diets have been recommended for diverticulosis because of a concern that indigestible nuts, seeds, corn, and popcorn could enter, block, or irritate a diverticulum and result in diverticulitis and possibly increase the risk of perforation. To date, there is no evidence supporting such a practice. In contrast, dietary fibre supplementation has been advocated to prevent diverticula formation and recurrence of symptomatic diverticulosis, although this is based mostly on low-quality observational studies.