Brazil Nuts — Benefits Deep Dive

Brazil nuts have been studied for the thyroid, for the body’s antioxidant enzymes, for cholesterol and inflammation, and for memory in older age. That looks like a long list of separate claims, but nearly all of it comes back to one mineral. A Brazil nut is the richest everyday food source of selenium, and most Brazil nut research is really research on selenium delivered inside a nut. The five articles below take each subject in turn: what the biology is, what every human trial in our verified set found (including the null results), what was only seen in animals, and what is still unknown.

Two cautions run through all five pages. Most of the trials were small, short and run in Brazil, often without a control group, and they measured laboratory markers rather than illness. And much of what is written about Brazil nuts is borrowed from selenium-tablet trials, which gave a fixed dose; a nut does not. The same selenium that explains the research interest also sets a ceiling, which is why the last article is about overload and safety.

Deep-Dive Articles

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Selenium and Thyroid Health

Why the thyroid holds more selenium per gram than any other organ, and what the deiodinase and glutathione peroxidase enzymes do there. The Hashimoto’s, pregnancy and Graves’ trials used selenium tablets, mostly lowering an antibody marker without clear effects on thyroid hormones or quality of life, and no randomised trial in the set gave Brazil nuts to people with Hashimoto’s or Graves’ disease.

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Selenium Status and Antioxidant Enzymes

The most consistent finding in Brazil nut research: blood selenium and glutathione peroxidase activity rise, and in a New Zealand trial two nuts a day matched a 100 mcg selenium tablet. The page explains how status is measured, why one nut is not one dose, how genes change the response, and why a rising enzyme is a biomarker rather than a health outcome.

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Cholesterol and Inflammation

Every trial in the set that measured blood fats or inflammatory markers after Brazil nuts, Brazil nut flour or Brazil nut oil, set against the nut’s comparatively high saturated fat. The placebo-controlled flour trial and the largest mixed-nut trial found no difference in blood fats, and two inflammation trials pointed in opposite directions.

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Brain Aging and Cognition

How selenoprotein P keeps the brain supplied with selenium, and what one pilot trial in older adults with mild cognitive impairment found: 20 people finished, two sub-tests improved, and the overall cognitive score and memory tests did not change. Mouse work with purified selenomethionine and the wider nut-and-cognition research are labelled for what they are.

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Selenium Overload and Safety

The official selenium limits by age, the measured selenium per nut from about 26 to about 1,261 mcg, and the arithmetic when the two are set side by side. Also what selenosis looks like, the supplement trials on skin cancer and diabetes, natural radium and barium, aflatoxin rules and tree-nut allergy.

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Why One Nut Reaches So Many Systems

One mineral, many proteins. Selenium is a trace mineral, needed only in micrograms (millionths of a gram). It does almost nothing on its own. The body builds it into an unusual amino acid, selenocysteine, and places that inside about 25 proteins called selenoproteins (Lu 2009). Each family works in a different part of the body. The glutathione peroxidases break down peroxides, reactive molecules that damage fats, proteins and DNA. The deiodinases switch thyroid hormone from its storage form, T4, to its active form, T3. Selenoprotein P, made in the liver, carries selenium out to the tissues, and a 2022 review describes how it gives the endocrine glands and the brain a “privileged” supply that holds up when selenium is short (Schomburg 2022). So when trials of one nut measure thyroid hormones, blood enzymes, oxidised cholesterol and memory tests, they are mostly looking at the same set of proteins working in different organs.

Why the thyroid and the brain come up first. In adults the thyroid holds more selenium per gram of tissue than any other organ. It makes its hormone with the help of hydrogen peroxide, and its glutathione peroxidases protect the gland from that chemistry. The brain is one of the organs selenoprotein P supplies first, and brain cells are rich in the fats most open to oxidation. These are the reasons researchers give for testing Brazil nuts on thyroid and memory. They explain why the question was asked; they do not answer it.

The fat profile is the second thread, and it is weaker. About two thirds of a Brazil nut is fat: by USDA figures, 67.1 g per 100 g, split roughly evenly between monounsaturated oleic acid and polyunsaturated linoleic acid (an omega-6 fat), with only trace amounts of the plant omega-3. Its 16.1 g of saturated fat per 100 g is the highest of nine common nuts by USDA figures, about a quarter of its fat, and its plant-sterol content is the lowest of those listed. The nut also carries vitamin E forms, squalene and phenolic compounds such as catechin and ellagic acid. But most trials gave about one nut a day, roughly 5 g, so the doses of fat, fiber and polyphenols were tiny. The one nutrient that was not tiny was selenium. In the one trial of Brazil nut oil alone, which carries the fat but little of the selenium, HDL cholesterol fell and triglycerides rose over 30 days. For these reasons the cholesterol article reads the Brazil nut studies mainly as selenium studies.

What the evidence actually shows. The best-established result is also the simplest: eating Brazil nuts raises blood selenium and glutathione peroxidase activity. A 2022 systematic review of 24 trials, 15 of them non-randomised, found this in every study that measured it (da Silva 2022). Beyond that, the picture is thinner than popular articles suggest. The thyroid evidence comes almost entirely from selenium tablets in Hashimoto’s thyroiditis, where the most replicated effect is a fall in an antibody level, a laboratory marker; no randomised trial in the set gave Brazil nuts to people with Hashimoto’s or Graves’ disease. The placebo-controlled cholesterol trial of Brazil nut flour found no difference between groups, and the largest trial that included Brazil nuts, as one third of a mixed-nut portion, found no effect on LDL cholesterol. Two inflammation trials disagreed. The brain evidence is one pilot trial with 20 people who finished. None of these trials counted heart attacks, cancers, cases of dementia or deaths.

The same nutrient sets the ceiling. Selenium has a narrow gap between enough and too much, and the selenium in a Brazil nut is not a fixed amount. Across the trials, the measured or estimated selenium ran from about 26 mcg to about 1,261 mcg per nut, and one study found up to an eight-fold difference between seeds in the same batch. The U.S. adult upper limit is 400 mcg a day; the European Food Safety Authority lowered its adult limit to 255 mcg a day in 2023. By the USDA average of about 96 mcg per nut, one ounce (about six nuts) is above the U.S. limit, and in one trial a single nut held about three times it. In women who were not selenium-deficient, that very high-selenium nut raised the activity of genes for inflammatory signals. The large selenium-tablet trials, which gave 200 mcg a day to mostly well-nourished people, did not reduce prostate cancer or bladder-cancer recurrence, linked selenium to more squamous cell skin cancer in one trial, and, pooled by EFSA, showed more new cases of type 2 diabetes. Read together, the articles find one pattern: the measured benefits appear mostly in people who started low in selenium, while in people who already have enough the enzymes level off and extra selenium is not automatically better.

Tablet trials are not nut trials. A tablet gives a known dose of one chemical form every day. A nut gives an unknown dose, mostly in organic form, chiefly selenomethionine, together with fat, fiber, magnesium, copper and other compounds. That is why every article states, result by result, whether the finding came from nuts, from tablets or from animals. Tablet results, good or bad, cannot be transferred to the nut directly in either direction. The safety article also covers the hazards that have nothing to do with selenium: natural radium and barium taken up from the soil, mould toxins, and tree-nut allergy. Decisions about selenium intake, particularly for children, in pregnancy, alongside supplements or with thyroid disease, kidney disease or diabetes, belong with a clinician.

Research Papers: Selenium and the Thyroid

  1. Huwiler VV, Maissen-Abgottspon S, Stanga Z, et al. Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. Thyroid. 2024;34(3):295-313. PubMed PMID: 38243784
  2. Wichman J, Winther KH, Bonnema SJ, et al. Selenium Supplementation Significantly Reduces Thyroid Autoantibody Levels in Patients with Chronic Autoimmune Thyroiditis: A Systematic Review and Meta-Analysis. Thyroid. 2016;26(12):1681-1692. PubMed PMID: 27702392
  3. Winther KH, Wichman JE, Bonnema SJ, et al. Insufficient documentation for clinical efficacy of selenium supplementation in chronic autoimmune thyroiditis, based on a systematic review and meta-analysis. Endocrine. 2017;55(2):376-385. PubMed PMID: 27683225
  4. van Zuuren EJ, Albusta AY, Fedorowicz Z, et al. Selenium supplementation for Hashimoto's thyroiditis. The Cochrane database of systematic reviews. 2013;2013(6):CD010223. PubMed PMID: 23744563
  5. Gärtner R, Gasnier BC, Dietrich JW, et al. Selenium supplementation in patients with autoimmune thyroiditis decreases thyroid peroxidase antibodies concentrations. The Journal of clinical endocrinology and metabolism. 2002;87(4):1687-91. PubMed PMID: 11932302
  6. Esposito D, Rotondi M, Accardo G, et al. Influence of short-term selenium supplementation on the natural course of Hashimoto's thyroiditis: clinical results of a blinded placebo-controlled randomized prospective trial. Journal of endocrinological investigation. 2017;40(1):83-89. PubMed PMID: 27572248
  7. Negro R, Greco G, Mangieri T, et al. The influence of selenium supplementation on postpartum thyroid status in pregnant women with thyroid peroxidase autoantibodies. The Journal of clinical endocrinology and metabolism. 2007;92(4):1263-8. PubMed PMID: 17284630
  8. Kahaly GJ, Riedl M, König J, et al. Double-Blind, Placebo-Controlled, Randomized Trial of Selenium in Graves Hyperthyroidism. The Journal of clinical endocrinology and metabolism. 2017;102(11):4333-4341. PubMed PMID: 29092078

Research Papers: Selenium Status and Antioxidant Enzymes

  1. Thomson CD, Chisholm A, McLachlan SK, et al. Brazil nuts: an effective way to improve selenium status. The American journal of clinical nutrition. 2008;87(2):379-84. PubMed PMID: 18258628
  2. da Silva A, Silveira BKS, de Freitas BVM, et al. Effects of Regular Brazil Nut (Bertholletia excelsa H.B.K.) Consumption on Health: A Systematic Review of Clinical Trials. Foods (Basel, Switzerland). 2022;11(18). PubMed PMID: 36141050
  3. Huguenin GV, Oliveira GM, Moreira AS, et al. Improvement of antioxidant status after Brazil nut intake in hypertensive and dyslipidemic subjects. Nutrition journal. 2015;14:54. PubMed PMID: 26022214
  4. Stockler-Pinto MB, Mafra D, Moraes C, et al. Brazil nut (Bertholletia excelsa, H.B.K.) improves oxidative stress and inflammation biomarkers in hemodialysis patients. Biological trace element research. 2014;158(1):105-12. PubMed PMID: 24504745
  5. Coutinho-Wolino KS, da Cruz BO, Cardozo LFMF, et al. Brazil nut supplementation does not affect trimethylamine-n-oxide plasma levels in patients with coronary artery disease. Journal of food biochemistry. 2022;46(8):e14201. PubMed PMID: 35467017
  6. Duarte GBS, Reis BZ, Rogero MM, et al. Consumption of Brazil nuts with high selenium levels increased inflammation biomarkers in obese women: A randomized controlled trial. Nutrition (Burbank, Los Angeles County, Calif.). 2019;63-64:162-168. PubMed PMID: 31026738
  7. Donadio JLS, Rogero MM, Guerra-Shinohara EM, et al. SEPP1 polymorphisms modulate serum glucose and lipid response to Brazil nut supplementation. European journal of nutrition. 2018;57(5):1873-1882. PubMed PMID: 28501922
  8. Schomburg L. Selenoprotein P - Selenium transport protein, enzyme and biomarker of selenium status. Free radical biology & medicine. 2022;191:150-163. PubMed PMID: 36067902

Research Papers: Cholesterol and Inflammation

  1. Carvalho RF, Huguenin GV, Luiz RR, et al. Intake of partially defatted Brazil nut flour reduces serum cholesterol in hypercholesterolemic patients--a randomized controlled trial. Nutrition journal. 2015;14:59. PubMed PMID: 26077768
  2. Huguenin GV, Oliveira GM, Moreira AS, et al. Improvement of antioxidant status after Brazil nut intake in hypertensive and dyslipidemic subjects. Nutrition journal. 2015;14:54. PubMed PMID: 26022214
  3. Costa E Silva LM, Pereira de Melo ML, Faro Reis FV, et al. Comparison of the Effects of Brazil Nut Oil and Soybean Oil on the Cardiometabolic Parameters of Patients with Metabolic Syndrome: A Randomized Trial. Nutrients. 2019;12(1). PubMed PMID: 31877968
  4. Silveira BKS, Silva AD, Rocha DMUP, et al. Brazil Nut (Bertholletia excelsa H.B.K.) Consumption in Energy-Restricted Intervention Decreases Proinflammatory Markers and Intestinal Permeability of Women with Overweight/Obesity: A Controlled Trial (Brazilian Nuts Study). The Journal of nutrition. 2024;154(9):2670-2679. PubMed PMID: 39025334
  5. Duarte GBS, Reis BZ, Rogero MM, et al. Consumption of Brazil nuts with high selenium levels increased inflammation biomarkers in obese women: A randomized controlled trial. Nutrition (Burbank, Los Angeles County, Calif.). 2019;63-64:162-168. PubMed PMID: 31026738
  6. Bersch-Ferreira ÂC, Weschenfelder C, Vieira Machado RH, et al. Effects of mixed nuts as part of a Brazilian Cardioprotective diet on LDL-cholesterol in adult patients after myocardial infarction: a multicenter randomized controlled clinical trial. Nutrition journal. 2024;23(1):118. PubMed PMID: 39354558
  7. Nora CL, Zhang L, Castro RJ, et al. Effects of mixed nut consumption on LDL cholesterol, lipoprotein(a), and other cardiometabolic risk factors in overweight and obese adults. Nutrition, metabolism, and cardiovascular diseases : NMCD. 2023;33(8):1529-1538. PubMed PMID: 37263914
  8. Macan TP, de Amorim TA, Damiani AP, et al. Brazil nut prevents oxidative DNA damage in type 2 diabetes patients. Drug and chemical toxicology. 2022;45(3):1066-1072. PubMed PMID: 32811197

Research Papers: Brain Aging and Cognition

  1. Rita Cardoso B, Apolinário D, da Silva Bandeira V, et al. Effects of Brazil nut consumption on selenium status and cognitive performance in older adults with mild cognitive impairment: a randomized controlled pilot trial. European journal of nutrition. 2016;55(1):107-16. PubMed PMID: 25567069
  2. da Silva A, Silveira BKS, de Freitas BVM, et al. Effects of Regular Brazil Nut (Bertholletia excelsa H.B.K.) Consumption on Health: A Systematic Review of Clinical Trials. Foods (Basel, Switzerland). 2022;11(18):2925. PubMed PMID: 36141050
  3. Schomburg L. Selenoprotein P - Selenium transport protein, enzyme and biomarker of selenium status. Free radical biology & medicine. 2022;191:150-163. PubMed PMID: 36067902
  4. Gao Y, Xu Y, Yin J. Selenomethionine Ameliorates Cognitive Impairment, Decreases Hippocampal Oxidative Stress and Attenuates Dysbiosis in D-Galactose-Treated Mice. Antioxidants (Basel, Switzerland). 2022;11(1):111. PubMed PMID: 35052615
  5. Arslan J, Gilani AU, Jamshed H, et al. Edible Nuts for Memory. Current pharmaceutical design. 2020;26(37):4712-4720. PubMed PMID: 32767923
  6. Huang L, Tao Y, Chen H, et al. Mediterranean-Dietary Approaches to Stop Hypertension Intervention for Neurodegenerative Delay (MIND) Diet and Cognitive Function and its Decline: A Prospective Study and Meta-analysis of Cohort Studies. The American journal of clinical nutrition. 2023;118(1):174-182. PubMed PMID: 37105521
  7. Grosso G, Estruch R. Nut consumption and age-related disease. Maturitas. 2016;84:11-6. PubMed PMID: 26586104

Research Papers: Selenium Overload and Safety

  1. Institute of Medicine (US) Panel on Dietary Antioxidants and Related Compounds. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington (DC): National Academies Press (US). 2000. PubMed PMID: 25077263
  2. Lima LW, Stonehouse GC, Walters C, et al. Selenium Accumulation, Speciation and Localization in Brazil Nuts (Bertholletia excelsa H.B.K.). Plants (Basel, Switzerland). 2019;8(8):289. PubMed PMID: 31426292
  3. Lemire M, Philibert A, Fillion M, et al. No evidence of selenosis from a selenium-rich diet in the Brazilian Amazon. Environment international. 2012;40:128-136. PubMed PMID: 21856002
  4. Klein EA, Thompson IM Jr, Tangen CM, et al. Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2011;306(14):1549-56. PubMed PMID: 21990298
  5. Duffield-Lillico AJ, Slate EH, Reid ME, et al. Selenium supplementation and secondary prevention of nonmelanoma skin cancer in a randomized trial. Journal of the National Cancer Institute. 2003;95(19):1477-81. PubMed PMID: 14519754
  6. Baquião AC, De Oliveira MM, Reis TA, et al. Monitoring and determination of fungi and mycotoxins in stored Brazil nuts. Journal of food protection. 2013;76(8):1414-20. PubMed PMID: 23905798
  7. Kluczkovski A, Bezerra L, Januário B, et al. Nuclear Magnetic Resonance Approach in Brazil Nut Oil and the Occurrence of Aflatoxins. Journal of oleo science. 2022;71(10):1439-1444. PubMed PMID: 36089397
  8. van Boxtel EL, Koppelman SJ, van den Broek LA, et al. Heat denaturation of Brazil nut allergen Ber e 1 in relation to food processing. Food chemistry. 2008;110(4):904-8. PubMed PMID: 26047277

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