Brazil nuts come from one of the largest trees in the Amazon rainforest, Bertholletia excelsa. It can reach 50 m and live 500 years or more. The nuts are seeds, arranged like orange segments inside a woody capsule. The capsule can weigh up to 2 kg and holds 8 to 24 of them.
Trees fruit almost only in undisturbed forest, because only large bees can pollinate the flowers. So most Brazil nuts are still gathered from the wild.
Selenium is the whole story. Gram for gram, no other food among the 113 compared on this site has more. A 1 oz serving (28 g, about 6 to 8 nuts) holds 543 µg. That is ten times the Daily Value, the daily target on US food labels.
The amount in a single nut is anything but fixed. In one analysis of individual nuts, average selenium was 3.06 ppm (milligrams per kilogram) in one region and 36.0 ppm in another. Nuts within a single batch differed up to 8-fold.
That spread is why a Brazil nut is both a useful selenium source and a food to ration. Two nuts a day raised blood selenium as much as a supplement in one trial. Yet a single nut in another trial held about 1,261 µg.
The nuts also carry a little natural radium. And in a shaken mix of nuts the big ones rise to the top, a pattern physicists call the Brazil nut effect.
Health benefits of Brazil nuts
The top findings from 30 studies, strongest evidence first
What the stars mean
- Strong. Several randomized trials, pooled in a meta-analysis, point the same way, or regulators (FDA or EFSA) have approved the claim.
- Moderate. Good randomized trials support it, but there are fewer of them, or their results vary.
- Limited. Small, short or single trials, or mixed results. Promising, but it needs confirmation.
Population and lab studies get no stars: they show a link or a mechanism, not proof.
How we rate evidence →- ★★★Lower “bad” LDL cholesterol with nuts in generalAcross 61 trials, one daily serving of tree nuts lowered it (by about 4.8 mg/dL). Brazil nuts were one of eight nuts tested and were not analyzed on their own.
- ★★★No lower cancer risk from selenium supplementsIn the best-run trials of selenium supplements (not nuts), people had 1.01 times the risk of any cancer. A separate review found they did not clearly lower the risk of heart disease or death.
- ★★Raised blood seleniumTwo Brazil nuts a day raised blood selenium by 64% over 12 weeks, about as much as a supplement. Brazil nut butter did the same in 2 weeks, but selenium in the nuts varies a lot.
- ★★Higher inflammation gene activity with one nut dailyIn 55 women with obesity, one nut a day for 2 months (about 1,261 µg of selenium) did not change inflammation proteins in the blood. Activity of inflammation-related genes rose, which the authors read as a possible signal from very high selenium.
- ★★No clear cholesterol benefit over placeboIn 77 people with high cholesterol, 13 g a day of Brazil nut flour for 90 days did not clearly beat a placebo flour. Total cholesterol fell within the nut group (by 20.5 mg/dL), but blood pressure and blood fats did not differ between groups.
All 30 studies, strongest evidence first
Daily tree nuts lowered LDL cholesterol by about 4.8 mg/dL
This meta-analysis (a study that pools earlier trials) combined 61 controlled trials in 2,582 people, testing eight tree nuts. Results were scaled to one 28.4 g serving a day. Total cholesterol fell 4.7 mg/dL and triglycerides 2.2 mg/dL, and the dose mattered more than the type of nut. Brazil nuts were one of the eight and were not analyzed on their own.
Selenium supplements did not lower the risk of cancer in well-run trials
This Cochrane review (Cochrane is known for careful reviews) covered 83 studies, including 10 randomized trials with 27,232 people. In trials at low risk of bias, people on selenium had 1.01 times the risk of any cancer. Trials also suggested a slightly higher risk of type 2 diabetes. These were supplements, not nuts, and the authors found no evidence that more selenium from food or supplements prevents cancer.
Two Brazil nuts daily raised plasma selenium 64%, matching a supplement
In this randomized trial, 59 New Zealand adults got 2 Brazil nuts, 100 µg of selenium as selenomethionine, or a placebo for 12 weeks. Plasma selenium rose 64.2%, 61.0% and 7.6%. Whole blood glutathione peroxidase (GPx) activity, a selenium-dependent enzyme, rose more with nuts than with the supplement. The nuts gave an average of 53 µg of selenium a day, with a possible range of 20 to 84 µg.
Brazil nut butter raised blood selenium as much as a supplement
In this randomized trial, 44 vegans and 42 omnivores got a placebo, nut butter or a supplement for 2 weeks. The nut butter gave 55 µg of selenium a day. Both sources raised serum selenium and selenoprotein P, and the rise in selenoprotein P was larger in vegans. The authors note that Brazil nuts vary a lot in selenium and suggest eating them once in a while.
One Brazil nut daily raised selenium markers but also inflammatory genes
In this randomized trial, 55 women with obesity ate 1 nut a day for 2 months, or nothing. That nut held about 1,261 µg of selenium, a much higher dose than in other trials here. Blood levels of inflammatory proteins did not change. But gene activity for interleukin-6, tumor necrosis factor-alpha and two Toll-like receptors, all immune signals, rose. The authors read this as a possible proinflammatory signal from very high selenium.
One Brazil nut daily raised plasma cadmium and lowered manganese
This randomized trial in 55 women with obesity (29 on nuts, 26 controls) measured essential and toxic elements in the nuts and in blood. Barium was the trace element present in the highest amount, followed by copper and manganese. The authors judge regular intake of nuts from the Brazilian Amazon safe for heavy metals, though the trial lasted only 2 months.
Nut flour lowered cholesterol within its group but did not beat placebo
In this double-blind trial, 77 people with high cholesterol and high blood pressure, already on lipid-lowering drugs, took part. For 90 days they ate 13 g a day of partially defatted Brazil nut flour (227.5 µg of selenium) or a cassava flour placebo. Total cholesterol fell 20.5 mg/dL within the nut group, but blood pressure and blood lipids did not clearly differ between the two groups. Free T3, a thyroid hormone, fell in the placebo group and not in the nut group.
Brazil nut flour raised plasma selenium from 87.0 to 180.6 µg/L
In this double-blind crossover trial, 91 people had high blood pressure and abnormal blood fats. They took 13 g a day of partially defatted Brazil nut, about 227.5 µg of selenium, for 12 weeks. A 4-week washout (break) separated the treatments. GPx3 activity rose 24.8% and oxidized LDL fell 3.25%, a small change.
Selenium supplements did not lower the risk of heart disease or death
This Cochrane review covered selenium-only supplements in 12 trials with 19,715 people. Deaths from any cause (0.97 times the risk) and all cardiovascular events (1.03 times) did not clearly change. Hair loss and skin inflammation were more common in the largest trial. These were supplements, not nuts, and the authors say the limited evidence does not support them for preventing heart disease.
Selenium did not prevent type 2 diabetes and may have raised risk
This secondary analysis of a randomized trial followed 1,202 people in low-selenium areas of the eastern US. They took 200 µg of selenium a day or a placebo for about 8 years, 7.7 years on average. Diabetes developed in 58 people on selenium and 39 on placebo, 1.55 times the risk. The risk was highest in those with the highest starting blood selenium (2.70 times). Diabetes was a secondary outcome, and this was a supplement, not nuts.
Selenium supplements raised high-grade prostate cancer risk 91% in high-selenium men
This case-cohort analysis of the SELECT trial had 1,739 prostate cancer cases, 489 of them high-grade, and 3,117 men in a random comparison group. The rise was in men who already had higher selenium status. Supplements had no effect in men with low selenium status (below the 60th percentile of toenail selenium). The authors advise against selenium or vitamin E supplements above recommended dietary intakes.
Brazil nuts lowered total cholesterol, triglycerides and oxidized LDL in obese teens
In this randomized trial, 17 girls with obesity (average age 15.4) took part for 16 weeks. They got 15 to 25 g of nuts a day, about 3 to 5 nuts, or a placebo lactose capsule. Red blood cell speed in small skin vessels also rose in the nut group. It was very small, with only 8 girls on nuts.
One Brazil nut a day raised HDL and corrected selenium deficiency
This uncontrolled study had 37 severely obese women eat 1 nut a day, about 290 µg of selenium, for 8 weeks. All were selenium deficient at the start. HDL cholesterol rose, and the Castelli indexes, cholesterol ratios linked to heart risk, improved. With no control group, the changes cannot be pinned on the nuts alone.
A daily Brazil nut improved verbal fluency and constructional praxis
This randomized pilot trial had 31 adults with mild cognitive impairment (average age 77.7), and 20 finished: 11 on nuts and 9 controls. Over 6 months, the nuts gave about 289 µg of selenium a day, and selenium and GPx activity rose. Constructional praxis is the skill of copying shapes. The authors call the cognitive result preliminary evidence.
A single serving lowered LDL and raised HDL cholesterol 9 hours later
In this randomized crossover trial, 10 healthy people ate 0, 5, 20 or 50 g of Brazil nuts in a single serving. The cholesterol changes came with the 20 and 50 g servings. Blood was drawn from 1 hour up to 30 days later, and plasma selenium rose by 6 hours. Liver and kidney tests did not change. It was 10 people and a one-time serving, so it says nothing about eating them every day.
A single serving of Brazil nuts lowered inflammatory proteins in healthy volunteers
This used the same 10-person crossover design (0, 5, 20 or 50 g, blood drawn up to 30 days later). The 20 and 50 g servings lowered blood levels of interleukin-1, interleukin-6, tumor necrosis factor-alpha and interferon-gamma, and raised interleukin-10. Liver and kidney tests did not change, and the authors say regular eating needs its own test.
Brazil nuts for 15 days did not change LDL, HDL or triglycerides
In this uncontrolled study, 15 people with normal blood fats (average age 27.3) ate 45 g of Brazil nuts a day, with no comparison group. Plasma selenium rose, and HDL particles took up more cholesteryl esters. The authors say this may help move cholesterol out of the arteries. The test was short.
One nut daily lifted plasma selenium from 18.8 to 104.0 µg/L
This uncontrolled study followed 81 patients on hemodialysis (blood cleaning for kidney failure) in Rio de Janeiro for 3 months. All were selenium deficient at the start, and all were in the normal range afterward. GPx activity also rose, from 46.6 to 55.9 U/g of hemoglobin. Each nut weighed about 5 g and held an average of 58.1 µg of selenium per gram.
One nut daily raised four selenium markers; gene variants changed the response
This single-group trial (the SU.BRA.NUT study) had 130 healthy volunteers. They ate nuts for 8 weeks, then had 8 weeks of washout (a break). Responses differed by sex and by variants in the GPX1 and SELENOP genes. There was no control group.
One nut a day did not change TMAO in coronary artery disease
TMAO is a gut-made compound linked to heart disease. In this randomized trial, 37 patients with coronary artery disease ate a nut a day for 3 months (23 on nuts, 14 controls). GPx activity rose 41% in the nut group, but this could have been chance. The trial was small.
Brazil nuts raised selenium status in every study that measured it
This systematic review covered 24 studies, 15 of them non-randomized. Healthy people also improved lipid markers and fasting glucose. People on hemodialysis improved lipids, oxidative stress, inflammation and thyroid function. Results were mixed in coronary artery disease, and many studies were small.
Nuts went with about a fifth lower risk of dying
This meta-analysis pooled 20 prospective cohort studies (which follow people over time), counting tree nuts and peanuts together. Per 28 g a day, the risk of death from any cause was 0.78 times as high. For coronary heart disease it was 0.71, and for total cancer 0.85. Stroke was not clearly different (0.93). Brazil nuts were not analyzed on their own, and a link is not proof of cause.
Amazon communities with very high selenium showed no signs of selenium toxicity
Researchers examined 448 people aged 15 to 87 in 12 communities along the Tapajós River, whose selenium comes partly from Brazil nuts. Median blood selenium was 228.4 µg/L, with a top value of 1500.2 µg/L. Some values passed the level considered toxic (1000 µg/L). The diet also includes chicken, game and fish, and the authors say the chemical form of selenium may matter.
Brazil nut days gave preschoolers selenium intakes judged potentially toxic
In this study, 41 preschool children in Macapá got 15 to 30 g of Brazil nuts 3 days a week. Another 88 children in Belém served as the comparison group. Average selenium intake was 155.30 µg a day in Macapá and 44.40 µg in Belém. Blood markers in the nut group were above reference levels. No signs of selenium overload (selenosis) were seen, and both groups already had high intakes from local food.
A supplement with 200 times its labeled selenium made 201 people sick
This outbreak investigation covered a liquid supplement that made people sick in 10 US states. The median estimated dose was 41 749 µg a day, against a recommended 55 µg. Common symptoms were diarrhea (78%), fatigue (75%) and hair loss (72%), and one person was hospitalized. This was a manufacturing error in a supplement, not a food.
Brazil nuts from two regions differed more than tenfold in average selenium
This chemical analysis tested 162 individual nuts from each of two regions, by fresh weight. Nuts from Acre-Rondonia averaged 3.06 ppm of selenium (range 0.03 to 31.7). Nuts from Manaus-Belem averaged 36.0 ppm (range 1.25 to 512.0). This is food analysis, not a study in people.
Nuts from the same batch differed up to 8-fold in selenium
This lab analysis tested single nuts from two commercial batches, with mean selenium of 28 to 49 mg/kg. The selenium was mostly in organic form, concentrated in a ring 1 to 2 mm under the surface. The authors say one 5 g nut from a high-selenium area meets the daily recommended amount. A 30 g serving may pass the 400 µg limit, or even a 1200 µg toxicity threshold.
Raw eastern Amazon nuts had more selenium and aflatoxin than western ones
This analysis covered 80 raw nut samples, in shell and shelled, collected in forest sites. Aflatoxins, mold toxins that are highly carcinogenic (cancer-causing), were measured next to selenium. Moisture of the raw nuts averaged 24.5% in one region and 22.1% in the other. The authors call for more work on how selenium relates to mold growth.
Daily Brazil nut radium dose is neither clearly harmful nor clearly harmless
This review of the literature on radium in Brazil nuts also used unpublished German data. Brazil nuts stand out for radium, and 1 or 2 a day add about 88 to 220 µSv (microsieverts, a radiation dose unit) a year. Average radium-226 was 49 mBq/g (range 17 to 205) and radium-228 was 67 mBq/g (range 12 to 235). This is a measurement of the food, not a study in people, and the authors call for trials on cancer risk markers.
Brazil nut protein 2S albumin is probably a major allergen
This study used blood and skin tests in people allergic to Brazil nuts. The protein had been put into transgenic (genetically modified) soybeans to add methionine, and IgE antibodies from eight of nine people bound it. Three people reacted on skin-prick tests to the nut and the transgenic soybean, but not to ordinary soybean. So an allergen moved between foods through genetic engineering.
Who should be careful with Brazil nuts
Most people can eat Brazil nuts in small amounts. Take a little care if one of these applies:
- Too much selenium. This is the main risk. The NIH, the US National Institutes of Health, puts the upper limit (the most to get each day) at 400 µg. EFSA, the EU’s food safety agency, puts it at 255 µg, and both count food and supplements together. The NIH lists hair loss, brittle nails, garlic breath and a metallic taste as signs of too much. At the NIH figure of 68 to 91 µg a nut, about 4 to 6 nuts reach 400 µg. And 3 to 4 nuts reach 255 µg. But single nuts can hold far more, so keep to 1 or 2 a day at most.
- Tree nut allergy. Brazil nuts are a known allergenic food, and a protein in them called 2S albumin is probably a major allergen. If you are allergic to tree nuts, skip them unless your allergist says otherwise.
- Radium. Brazil nuts hold more radium than other nuts. A 2025 review put the dose from 1 or 2 a day at about 88 to 220 µSv a year. It called that neither clearly harmful nor clearly harmless. For most people, that is a reason for variety rather than worry.
How to choose, store and eat Brazil nuts
- Whole nuts: 2 a day raised plasma selenium by 64% in 12 weeks. The authors of a 2025 trial suggest eating them once in a while.
- Nut butter: 55 µg of selenium a day from Brazil nut butter raised blood selenium as much as a supplement in vegans and omnivores.
- Partly defatted flour: 13 g a day gave 227.5 µg of selenium. It lowered total cholesterol by 20.5 mg/dL within the group, but no more than placebo.
Buy from a seller you trust, because aflatoxins, which are mold toxins, have been found in raw Brazil nuts from the Amazon. University of California extension advises airtight containers in a refrigerator at 40°F or below for tree nuts such as walnuts, almonds, pecans and pistachios. Stored that way, they keep up to a year. Brazil nuts are just as rich in fat, at 67.1 g per 100 g, so the same habit makes sense.
Sources
- Selenium: fact sheet for health professionals. NIH Office of Dietary Supplements.
- Turck D, Bohn T, Castenmiller J, et al. Scientific opinion on the tolerable upper intake level for selenium. EFSA J. 2023.
- Koeder C, Keller M. Radium levels in Brazil nuts: a review of the literature. Nutr Bull. 2025.
- Nordlee JA, Taylor SL, Townsend JA, Thomas LA, Bush RK. Identification of a Brazil-nut allergen in transgenic soybeans. N Engl J Med. 1996.
- Alcocer M, Rundqvist L, Larsson G, et al. Ber e 1 protein: the versatile major allergen from Brazil nut seeds. Biotechnol Lett. 2012.
- Thomson CD, Chisholm A, McLachlan SK, Campbell JM. Brazil nuts: an effective way to improve selenium status. Am J Clin Nutr. 2008.
- Simon R, Lossow K, Pellowski D, et al. Improving the selenium supply of vegans and omnivores with Brazil nut butter compared to a dietary supplement in a randomized controlled trial. Eur J Nutr. 2025.
- Carvalho RF, Huguenin GV, Luiz RR, et al. Intake of partially defatted Brazil nut flour reduces serum cholesterol in hypercholesterolemic patients. Nutr J. 2015.
- Pacheco AM, Scussel VM. Selenium and aflatoxin levels in raw Brazil nuts from the Amazon basin. J Agric Food Chem. 2007.
- Storing and preserving fresh nuts. University of California Agriculture and Natural Resources.
- Nuts, brazilnuts, dried, unblanched (FDC 170569). USDA FoodData Central.
History of Brazil nuts
Brazil nut trees are native to the Guianas, Venezuela, Brazil, eastern Colombia, eastern Peru and eastern Bolivia. They grow as scattered trees in large forests along the Amazon, the Rio Negro, the Tapajós and the Orinoco.
A 2011 paper in Economic Botany looked at how people spread the tree across ancient Amazonia. Today the planting is done by a rodent. Agoutis gnaw open the capsules and bury seeds for later, and the ones they forget can sprout.
The English name is a little misleading. In Portuguese the nut is castanha-do-brasil, meaning chestnut from Brazil, or castanha-do-pará, chestnut from Pará. In Spanish-speaking South America it is also called nuez amazónica, the Amazon nut. The tree’s genus name honors the French chemist Claude Louis Berthollet.
Growing them on plantations has not paid off. The fruit takes 14 months to ripen after pollination. The trees need undisturbed forest and its large bees, and plantation yields are low.
So the trade still depends on gatherers, called castañeros in Spanish and castanheiros in Portuguese, who collect the fallen capsules in the forest.
That wild harvest is both a conservation story and a worry. It has been held up as a way to earn money from a tropical forest without cutting it down. But studies of harvested stands found that heavy gathering leaves too few seeds to replace old trees. A rodent study in the forest warned that intensive harvest could hurt natural regeneration.
In 2024 the world produced 79,736 tonnes of Brazil nuts in shell, and Brazil and Bolivia together made up 87% of it. Brazil is the main exporter of nuts in the shell. Bolivia and Peru ship most of the shelled kernels, and Bolivia took 54% of the export value.
Sources
- Brazil nut. Wikipedia.
- Shepard GH, Ramirez H. “Made in Brazil”: human dispersal of the Brazil nut (Bertholletia excelsa, Lecythidaceae) in ancient Amazonia. Econ Bot. 2011.
- Haugaasen JMT, Haugaasen T, Peres CA, Gribel R, Wegge P. Seed dispersal of the Brazil nut tree (Bertholletia excelsa) by scatter-hoarding rodents in a central Amazonian forest. J Trop Ecol. 2010.
- Scoles R, Gribel R. The regeneration of Brazil nut trees in relation to nut harvest intensity in the Trombetas River valley of Northern Amazonia, Brazil. For Ecol Manage. 2012.
- Rosato A, Strandburg KJ, Prinz F, Swendsen RH. Why the Brazil nuts are on top: size segregation of particulate matter by shaking. Phys Rev Lett. 1987.
- Granular convection. Wikipedia.
Frequently asked questions about Brazil nuts
Are Brazil nuts good for you?
In small amounts, yes. A 1 oz (28 g) serving has 187 calories, 4.1 g of protein and 2.1 g of fiber. It gives 25% of the Daily Value for magnesium and 55% for copper, plus about 10 times the Daily Value for selenium.
In a trial, 2 nuts a day raised plasma selenium by 64%. But selenium is also the reason to limit them, and the studies on cholesterol and inflammation are small and mixed.
How many Brazil nuts should you eat a day?
One or two is what most trials used, and a few a week also works. In one trial 2 nuts gave an average of 53 µg of selenium a day, against 290 µg from 1 nut in another.
At the NIH figure of 68 to 91 µg a nut, about 4 to 6 nuts reach the 400 µg upper limit. And 3 to 4 reach EFSA’s 255 µg, so a handful a day is too many.
Can you eat too many Brazil nuts?
Yes, because of selenium. Long-term excess shows up mainly as hair loss and brittle nails, with garlic breath and a metallic taste as early signs.
One trial gave women a nut with about 1,261 µg of selenium a day and saw inflammatory genes switch on. Yet river communities in the Amazon with a median blood selenium of 228.4 µg/L showed no signs of toxicity. So the picture is not simple.
Are Brazil nuts radioactive?
Slightly, yes. A 2025 review found average radium-226 of 49 mBq/g in Brazil nuts. It put the dose from 1 or 2 nuts a day at about 88 to 220 µSv a year. Its authors call that neither clearly harmful nor clearly harmless, and no trial has tested cancer outcomes.
Do Brazil nuts help your thyroid?
Selenium is part of how the body activates thyroid hormone, but no trial here tested Brazil nuts in people with thyroid disease. In 77 people on lipid-lowering drugs, 13 g a day of nut flour kept free T3 from falling, while it fell on placebo. A review of 24 studies reported better thyroid function in people on hemodialysis.
If you have a thyroid condition, ask your clinician before eating them daily.
Do Brazil nuts lower cholesterol?
Tree nuts as a group do, by about 4.8 mg/dL of LDL per serving across 61 trials. But Brazil nuts are only one of eight nuts in that analysis.
In Brazil nut trials, a single 20 or 50 g serving lowered LDL 9 hours later in 10 people. Meanwhile, 45 g a day for 15 days changed nothing in 15 people. The results disagree, and none is large.
How many calories are in a Brazil nut?
A single nut weighs about 5 g and has roughly 33 calories. A 1 oz (28 g) serving has 187 calories and 19 g of fat. And 100 g has 659 calories and 67.1 g of fat, of which 16.1 g is saturated.
No study here tested weight loss, so there is no evidence for or against it.
Why does the selenium in Brazil nuts vary so much?
Selenium in plants depends on the soil they grow in, according to the NIH. Nuts from one Brazilian region averaged 3.06 ppm and from another 36.0 ppm. Single nuts in one batch differed up to 8-fold. That is why the same 2 nuts that gave about 53 µg in one trial can give far more in another.