Pistachios: Gut Microbiome

Table of Contents

1. What Reaches the Colon From a Pistachio
2. What Gut Bacteria Do With It
3. How Gut Microbes Are Measured
4. The Pistachio Stool-Sequencing Feeding Study
5. The Pooled Nut Trials
6. Prediabetes: Microbe-Linked Chemicals in Urine
7. Animal and Laboratory Work
8. How Pistachios Compare With Almonds
9. Fiber in Large Population Studies
10. What Is Not Known
11. Who the Research Raises Concerns For
12. Connections
13. Key Research Papers
14. Featured Videos

What Reaches the Colon From a Pistachio

Most of a meal is absorbed in the small intestine. What is left — the parts human enzymes cannot digest, or cannot reach — passes into the large intestine (the colon). For pistachios, three kinds of material are the main candidates.

• Fiber. USDA FoodData Central lists 10.6 grams of total dietary fiber per 100 grams of raw pistachio kernels, which works out to about 3 grams in a one-ounce (28 g) handful. Fiber is plant material, largely cell-wall carbohydrate, that human digestive enzymes cannot break down. The USDA entry gives total fiber only; it does not split pistachio fiber into soluble and insoluble types. The full numbers are on the Protein and Fiber page.
• Fat and protein locked inside cell walls. A nut kernel is made of small plant cells, each wrapped in a tough wall. Chewing breaks some cells open and leaves others intact, and the fat and protein in intact cells can pass through the small intestine without being absorbed. Dreher’s 2012 review in Nutrition Reviews describes pistachios as having a “reduced net metabolizable energy content” — fewer calories are absorbed than the label calculation predicts. Material that escapes absorption this way continues into the colon.
• Polyphenols and other plant compounds. Reviews of pistachio composition list carotenoids (lutein and zeaxanthin), phenolic acids, flavonoids and anthocyanins (Mandalari and colleagues, 2021; Mateos and colleagues, 2022). Polyphenols are a large family of plant chemicals. Many are poorly absorbed in the small intestine, and the share that reaches the colon is available to bacteria there.

Creedon and colleagues’ 2020 meta-analysis frames the question in the same terms: nuts “contain fibre, unsaturated fatty acids and polyphenols that may impact the composition of the gut microbiota.” How much of each reaches the colon from a pistachio, in grams, has not been measured in any study in the research pack for this page.

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What Gut Bacteria Do With It

The “gut microbiota” is the community of microbes living in the intestine, mostly bacteria and mostly in the colon. The word “microbiome” is often used for the same community together with its genes. These bacteria live on whatever the small intestine did not absorb.

• Fermentation of fiber. Bacteria break down fiber without oxygen, a process called fermentation. Its main end-products are short-chain fatty acids — small acids with two to four carbon atoms, chiefly acetate, propionate and butyrate — along with gases.
• Butyrate, in plain words. Butyrate is the short-chain fatty acid that the cells lining the colon use as a main fuel. Only certain groups of bacteria make much of it; researchers call them “butyrate producers.” Because butyrate feeds the colon lining, a rise in butyrate producers is often described in the research as a potentially favourable change. That is an interpretation, not a measured health outcome. The gut barrier and microbiome animation shows the process step by step.
• Fermentation of protein. Protein that reaches the colon is also broken down by bacteria, giving a different set of products. One example measured in a pistachio trial below is p-cresol, made by bacteria from the amino acid tyrosine and then converted by the liver into p-cresol sulfate, which leaves the body in urine.
• Polyphenol breakdown. Bacteria split many polyphenols into smaller molecules that can be absorbed. One end-product that appears in urine is hippurate, formed when the liver joins a bacterial breakdown product (benzoic acid) to the amino acid glycine.
• Choline breakdown. Some bacteria turn choline and related nutrients into trimethylamine, which the liver converts to TMAO (trimethylamine N-oxide); dimethylamine is a related compound. TMAO has been studied as a marker in heart and metabolic disease — see Choline: Cardiovascular and TMAO.

These chemicals matter for the pistachio research because one of the human trials below did not look at bacteria directly. It measured these bacterial end-products in urine.

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How Gut Microbes Are Measured

A few methods turn up in the studies on this page, and each one answers a different question.

• 16S rRNA sequencing of stool. Researchers read a short, standard stretch of bacterial DNA from a stool sample and sort the reads into groups. The result is relative abundance: what share of the reads belongs to each group. It does not count the total number of bacteria. A rise in one group’s share can happen because that group grew or because others shrank.
• Diversity. A summary number for how many different groups are present and how evenly they are spread.
• Phylum and genus. Levels of classification, from broad (phylum, a handful of very large groups) down to narrow (genus, for example Roseburia or Bifidobacterium). A change at genus level can be invisible at phylum level.
• Metabolites. Chemicals made by bacteria, measured in stool, blood or urine. Urine metabolites show what the body absorbed and excreted, which reflects both the bacteria and the person’s own liver and kidneys.
• Culture counts. Growing bacteria from a sample on lab plates and counting colonies — an older method, used in the rat study below.

Relative-abundance shifts are hard to interpret. The same genus can contain species with different effects, and a short study in a few dozen people can find shifts that do not repeat in the next study. That is why the sections below report which groups changed, and what the authors said about them, rather than labelling bacteria as “good” or “bad.”

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The Pistachio Stool-Sequencing Feeding Study

The study most often cited for pistachios and gut bacteria was published by Ukhanova and colleagues in 2014 in the British Journal of Nutrition, using stool samples collected during controlled feeding studies run with USDA researchers in Beltsville, Maryland. It is not in the citation pack for this page; the details below come from its PubMed abstract, linked under PubMed Topic Searches at the bottom of the page.

• Design. Two separate randomised, controlled, crossover feeding studies — one of almonds (18 volunteers) and one of pistachios (16 volunteers). In a crossover, each person eats every diet in turn, so each person serves as their own comparison.
• Dose and duration. 0, 1.5 or 3 servings a day of the nut, each diet period lasting 18 days.
• Method. Bacteria were analysed by 16S rRNA sequencing, and fungi by a separate DNA method (internal transcribed spacer sequencing).
• What changed with pistachios. The authors reported that the effect of pistachio consumption on gut microbiota composition “was much stronger than that of almond consumption” and “included an increase in the number of potentially beneficial butyrate-producing bacteria.” The words “potentially beneficial” are the authors’.
• What did not change. Bifidobacteria were not affected by either nut.
• What went down. Pistachio consumption appeared to decrease lactic acid bacteria (P < 0.05).

The study did not measure short-chain fatty acids, symptoms, bowel habit or any health outcome in its abstract. It shows that adding pistachios to a controlled diet for under three weeks shifted the bacterial mix in 16 people; it does not show what that shift does.

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The Pooled Nut Trials

Creedon and colleagues at King’s College London published a systematic review and meta-analysis in Nutrients in 2020 of randomised controlled trials in healthy adults that gave tree nuts or peanuts and measured stool bacteria, gut function or gut symptoms.

• What was included. Eight studies reporting nine trials: five of almonds, three of walnuts and only one of pistachios. The pooled result is therefore mostly an almond-and-walnut result.
• Genera that rose with nuts. Clostridium (standardised mean difference 0.40), Dialister (0.44), Lachnospira (0.33) and Roseburia (0.36), all statistically significant.
• Genus that fell. Parabacteroides (−0.31, at the edge of significance, p = 0.05).
• Null results. No effect of nuts on bacterial phyla, on diversity, or on stool output.
• The authors’ conclusion. Further parallel-design trials, “powered to detect changes in faecal microbiota and incorporating functional and clinical outcomes, are needed.”

A “standardised mean difference” is a way of putting results measured in different units on one scale; values around 0.3 to 0.4 are usually read as small-to-moderate. Roseburia and Lachnospira are groups that the research literature generally counts among the butyrate producers, but the meta-analysis did not measure butyrate itself. The paper discloses that the review was part of a doctoral fellowship funded by an almond industry body, which the authors state had no role in the design, analysis or writing.

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Prediabetes: Microbe-Linked Chemicals in Urine

The longest pistachio trial in the pack with a gut-microbe angle comes from Hernández-Alonso, Bulló and colleagues in Reus, Spain, published in 2017 in The Journal of Nutritional Biochemistry. Prediabetes means blood sugar above normal but below the threshold for diabetes.

• Design. A randomised crossover trial in 39 adults with prediabetes.
• Diets. A pistachio-supplemented diet (50% of energy from carbohydrate, 33% from fat, including 57 grams of pistachios a day — about two ounces) and a control diet (55% carbohydrate, 30% fat), each for 4 months, with a 2-week break between them.
• What was measured. 24-hour urine samples analysed by nuclear magnetic resonance (NMR), a method that detects many chemicals at once. Stool bacteria were not sequenced in this paper.
• Results. Three metabolites that the authors relate to gut-microbe metabolism — hippurate, p-cresol sulfate and dimethylamine — were lower after the pistachio diet than after the control diet (P < .05). Cis-aconitate, a step in the body’s own energy cycle (the TCA cycle), was also lower. Within the pistachio period, TMAO fell (P = .034) and creatinine rose (P = .023).
• The authors’ reading. Chronic pistachio consumption “may modulate some urinary metabolites related to gut microbiota metabolism and the TCA cycle,” which they describe as associated with the metabolic changes of insulin resistance and type 2 diabetes.

Two points help in reading this. First, the TMAO result is a change within the pistachio period, not a difference between the two diets, which is a weaker comparison. Second, the direction of some changes is not simple to interpret: hippurate, for example, is partly a product of polyphenol breakdown, and a fall in it on a nut diet does not translate in any obvious way into a benefit or a harm. The trial shows a different urine chemistry on a pistachio diet; it does not identify which bacteria were responsible.

The same research group reported a crossover trial of the same design — 57 grams a day for 4 months against an isocaloric control diet — in 49 people with prediabetes, in the European Journal of Nutrition. That paper measured blood microRNAs (small molecules that regulate genes) linked to glucose and insulin, and found lower levels of miR-192 and miR-375 after the pistachio diet. It did not measure gut bacteria. The blood-sugar findings from these trials are covered on Blood Sugar and Diabetes.

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Animal and Laboratory Work

The studies in this section were done in animals or in the laboratory. None of them is evidence about what pistachios do in people.

• Diabetic rats fed a probiotic carried on pistachios (Prapa and colleagues, 2024, Nutrients). Twenty-four male rats, healthy or made diabetic with the drug streptozotocin (a model of type 1 diabetes), were fed for 4 weeks with a lactic acid bacterium, Pediococcus acidilactici SK, either as free cells or immobilised on pistachio nuts. In diabetic rats, the immobilised cells were followed by lower fecal staphylococci and enterococci, and Lactobacillus and Bifidobacterium were higher after the immobilised-cell diet; the inflammatory marker IL-1β was lower with immobilised than with free cells. The pistachio served here as a carrier for the probiotic, and the abstract does not describe a pistachio-only group, so the study cannot separate the effect of the nut from the effect of the bacterium.
• Pistachio compounds against bacteria in the lab (Mandalari and colleagues, 2024, Frontiers in Microbiology). A review of laboratory studies reports that pistachio extracts and individual compounds show antimicrobial activity, with Gram-positive bacteria such as Staphylococcus aureus the most susceptible. These are extracts tested on bacteria in dishes, and the authors discuss topical (skin and eye) uses. The review does not report what eating pistachios does to bacteria in the gut.
• A fermented pistachio drink (Reale and colleagues, 2024, Foods). A food-development study ground pistachios finely and fermented them with lactic acid bacteria to make a yogurt- or kefir-like drink. The starter bacteria grew to above 10⁸ colony-forming units per millilitre and the drink stayed microbiologically stable for 30 days at 4 °C. This shows pistachio material can feed lactic acid bacteria in a vat; no one drank it in a study, and it says nothing about the gut.

Laboratory “fermentation” models, in which ground nuts are digested with enzymes and then incubated with stool bacteria, exist for several nuts. No such study for pistachios is in the research pack for this page, so none is described here; a PubMed search link is given below.

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How Pistachios Compare With Almonds

Almonds have the larger gut-microbe evidence base among nuts, and the two nuts are close on fiber: the USDA figures on the Protein and Fiber page give about 3.5 grams of fiber per ounce for almonds and about 3 grams for pistachios.

• Number of trials. The Creedon meta-analysis included five almond trials and one pistachio trial. Any general statement about “nuts and the microbiome” leans on almonds and walnuts far more than on pistachios.
• Head-to-head. The only direct comparison is the Ukhanova study, where the almond and pistachio arms were run as two separate crossover studies with the same design. Its authors reported that the pistachio effect on microbial composition was “much stronger” than the almond effect. With 16 and 18 volunteers and 18-day diet periods, that is a single small comparison, not a settled ranking.
• Bifidobacteria. Neither nut changed bifidobacteria in that study.

The almond studies are covered in detail on Almonds: Gut Microbiome and Prebiotic, and the walnut work, including the urolithin compounds gut bacteria make from walnut polyphenols, on Walnuts: Gut Microbiome and Urolithins.

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Fiber in Large Population Studies

The reason gut researchers care about fiber at all comes from very large studies of fiber intake. These studies are not about pistachios, and most are observational: they find associations, not proof of cause and effect.

• Reynolds and colleagues, 2019, The Lancet. A series of systematic reviews covering 185 prospective studies and 58 clinical trials. Comparing the highest with the lowest fiber consumers, observational data showed a 15–30% lower rate of all-cause and cardiovascular death, coronary heart disease, stroke, type 2 diabetes and colorectal cancer. Risk reductions were greatest at 25 to 29 grams of fiber a day. The authors graded the certainty of the fiber evidence as moderate.
• Ramezani and colleagues, 2024, Clinical Nutrition. A meta-analysis of 64 cohort studies with more than 3.5 million participants. Higher total fiber intake was associated with 23% lower all-cause mortality. Fiber from nuts and seeds was associated with a 43% lower risk of cardiovascular death (HR 0.57).
• Arayici and colleagues, 2022, Nutrition and Cancer. Comparing highest with lowest intake, both soluble and insoluble fiber were associated with lower colorectal cancer risk to about the same degree (effect sizes 0.78 and 0.77).
• Reynolds and colleagues, 2020, PLOS Medicine. In 42 trials of adults with prediabetes or diabetes, higher-fiber diets lowered HbA1c, fasting glucose, LDL cholesterol, body weight and C-reactive protein compared with lower-fiber diets, with high variation between trials.

Against these figures, one ounce of pistachios supplies about 3 grams of fiber, roughly a tenth of the 25 to 29 grams associated with the largest risk reductions in the Lancet analysis. The population studies measured whole diets; they do not show that the gut-microbe route is how fiber is linked to these outcomes, and none isolated pistachios. More on fiber in general is on the Dietary Fiber page.

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What Is Not Known

• Whether the microbe changes do anything for health. No pistachio study in the pack linked a change in stool bacteria to an improvement in symptoms, bowel function, infection, inflammation in the gut or any disease. A microbe change is a measurement, not a health outcome.
• Butyrate itself. The pistachio stool study reported more butyrate-producing bacteria; no study in the pack measured butyrate or other short-chain fatty acids in stool or blood after people ate pistachios.
• Whether the findings repeat. The stool evidence for pistachios rests on one small study of 16 people over 18-day periods. Creedon and colleagues called for larger parallel-design trials.
• Which part of the pistachio does it. Fiber, cell-wall-trapped fat and protein, and polyphenols all reach the colon. No study in the pack separated their effects.
• Raw versus roasted, salted versus unsalted, whole versus ground. How finely a nut is broken down changes how much material escapes to the colon. No pistachio study in the pack compared forms for gut outcomes.
• Long-term effects. The longest trial (4 months, 57 g a day) measured urine metabolites, not bacteria.
• Who was studied. Healthy volunteers in the stool study and adults with prediabetes in the urine study. People with inflammatory bowel disease, irritable bowel syndrome or other digestive conditions were not the subject of any pistachio gut trial in the pack.
• The soluble-to-insoluble fiber split. USDA reports total fiber only for pistachios.

Industry-linked reviews also play a part in this literature: the 2021 review by Mandalari and colleagues, which describes pistachio consumption as “associated with a positive modulation of the human gut microbiota,” discloses consultancy ties of some authors to a pistachio growers’ organisation. That wording summarises the same small evidence base set out above.

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Who the Research Raises Concerns For

• People with tree nut allergy. Costa and colleagues’ 2019 review describes pistachio as a cause of moderate to severe IgE-mediated allergic reactions, with five officially listed pistachio allergens (Pis v 1 to Pis v 5), and notes the close link between pistachio and cashew allergy. In an Israeli oral immunotherapy study, all 35 patients co-allergic to cashew and pistachio who were desensitised to cashew were also cross-desensitised to pistachio (Elizur and colleagues, 2022), which illustrates how closely the two allergies are related.
• People whose digestion reacts to added fiber. The Creedon meta-analysis found no effect of nuts on stool output in healthy adults, but the pack contains no pistachio study in people with irritable bowel syndrome or other gut conditions. Information on fermentable carbohydrates and IBS is on the Low-FODMAP Diet for IBS page.
• Salt. Raw pistachios contain almost no sodium; salted pistachios carry what was added in processing.
• Calories. The prediabetes trial gave 57 grams a day within a diet designed to match calories with the control diet; it does not describe pistachios added on top of an unchanged diet.

Decisions about allergy testing, or about diet in a digestive condition, belong with a clinician.

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Connections

• Nuts — the nuts landing page
• Pistachios — the main topic page
• Pistachios — Benefits Deep Dive — the hub for this series
• Pistachios: Nutrient Profile — the full composition table
• Protein and Fiber
• Blood Sugar and Diabetes
• Weight and Mindful Eating
• Cholesterol and Lipids
• Blood Pressure and Blood Vessels
• Eye Health: Lutein and Zeaxanthin
• Almonds: Gut Microbiome and Prebiotic — the larger nut evidence base
• Walnuts: Gut Microbiome and Urolithins
• Gut Health — the gut health hub
• Microbiome — the gut microbiome in general
• Dietary Fiber — fiber in general
• Prebiotics — foods and fibers that feed gut bacteria
• Probiotics — live bacteria in foods and supplements
• Resistant Starches: Gut Microbiome — another butyrate-linked fiber
• Gut Barrier and Microbiome (animation)
• Choline: Cardiovascular and TMAO
• Prediabetes — the group in the urine-metabolite trial
• Insulin Resistance
• Polyphenols

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Key Research Papers

1. Creedon AC, Hung ES, Berry SE, et al. Nuts and their Effect on Gut Microbiota, Gut Function and Symptoms in Adults: A Systematic Review and Meta-Analysis of Randomised Controlled Trials. Nutrients. 2020;12(8). PubMed PMID: 32781516
2. Hernández-Alonso P, Cañueto D, Giardina S, et al. Effect of pistachio consumption on the modulation of urinary gut microbiota-related metabolites in prediabetic subjects. The Journal of Nutritional Biochemistry. 2017;45:48-53. PubMed PMID: 28432876
3. Hernández-Alonso P, Giardina S, Salas-Salvadó J, et al. Chronic pistachio intake modulates circulating microRNAs related to glucose metabolism and insulin resistance in prediabetic subjects. European Journal of Nutrition. 2017;56(6):2181-2191. PubMed PMID: 27383196
4. Prapa I, Yanni AE, Kompoura V, et al. Functional Modulation of Gut Microbiota and Blood Parameters in Diabetic Rats Following Dietary Intervention with Free or Immobilized Pediococcus acidilactici SK Cells on Pistachio Nuts. Nutrients. 2024;16(23). PubMed PMID: 39683613
5. Mandalari G, Pennisi R, Gervasi T, et al. Pistacia vera L. as natural source against antimicrobial and antiviral resistance. Frontiers in Microbiology. 2024;15:1396514. PubMed PMID: 39011148
6. Reale A, Puppo MC, Boscaino F, et al. Development and Evaluation of a Fermented Pistachio-Based Beverage Obtained by Colloidal Mill. Foods. 2024;13(15). PubMed PMID: 39123534
7. Dreher ML. Pistachio nuts: composition and potential health benefits. Nutrition Reviews. 2012;70(4):234-40. PubMed PMID: 22458696
8. Mandalari G, Barreca D, Gervasi T, et al. Pistachio Nuts (Pistacia vera L.): Production, Nutrients, Bioactives and Novel Health Effects. Plants. 2021;11(1). PubMed PMID: 35009022
9. Mateos R, Salvador MD, Fregapane G, et al. Why Should Pistachio Be a Regular Food in Our Diet? Nutrients. 2022;14(15). PubMed PMID: 35956383
10. Reynolds A, Mann J, Cummings J, et al. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet. 2019;393(10170):434-445. PubMed PMID: 30638909
11. Ramezani F, Pourghazi F, Eslami M, et al. Dietary fiber intake and all-cause and cause-specific mortality: An updated systematic review and meta-analysis of prospective cohort studies. Clinical Nutrition. 2024;43(1):65-83. PubMed PMID: 38011755
12. Arayici ME, Mert-Ozupek N, Yalcin F, et al. Soluble and Insoluble Dietary Fiber Consumption and Colorectal Cancer Risk: A Systematic Review and Meta-Analysis. Nutrition and Cancer. 2022;74(7):2412-2425. PubMed PMID: 34854791
13. Reynolds AN, Akerman AP, Mann J. Dietary fibre and whole grains in diabetes management: Systematic review and meta-analyses. PLoS Medicine. 2020;17(3):e1003053. PubMed PMID: 32142510
14. Costa J, Silva I, Vicente AA, et al. Pistachio nut allergy: An updated overview. Critical Reviews in Food Science and Nutrition. 2019;59(4):546-562. PubMed PMID: 28925724
15. Elizur A, Appel MY, Nachshon L, et al. Cashew oral immunotherapy for desensitizing cashew-pistachio allergy (NUT CRACKER study). Allergy. 2022;77(6):1863-1872. PubMed PMID: 35000223

PubMed Topic Searches

1. PubMed: Ukhanova et al. 2014, almond and pistachio consumption and gut microbiota composition
2. PubMed: pistachios and gut microbiota
3. PubMed: pistachio in-vitro digestion and fermentation
4. PubMed: nut cell walls and metabolizable energy
5. PubMed: nuts, butyrate and short-chain fatty acids
6. PubMed: pistachio polyphenol bioaccessibility

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