Selenium accumulation, speciation and localization in Brazil nuts (Bertholletia excelsa H.B.K.)

Authors: LIMA, LEONARDO - Colorado State University; STONEHOUSE, GAVIN - Colorado State University; Walters, Christina; EL MEHDAWI, ALI - Colorado State University; FAKRA, SIRINE - Lawrence Berkeley National Laboratory; PILON-SMITS, ELIZABETH - Colorado State University

Submitted to: Plants
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2019
Publication Date: 8/16/2019
Citation: Lima, L.W., Stonehouse, G.C., Walters, C.T., El Mehdawi, A.F., Fakra, S.C., Pilon-Smits, E.A. 2019. Selenium accumulation, speciation and localization in Brazil nuts (Bertholletia excelsa H.B.K.). Plants. 8(8):289. https://doi.org/10.3390/plants8080289.
DOI: https://doi.org/10.3390/plants8080289

Interpretive Summary: This paper characterizes selenium in Brazil nuts (Bertholletia excelsa H.B.K.). B. exelsa produces oil-rich seeds (nuts) that tend to also accumulate selenium. Hence, Brazil nut is an excellent source of selenium. Usually only one nut is needed to meet recommended daily requirements and eating more than one nut per day could lead to toxicity. The anatomy of B. excelsa seeds is unique in that it consists of mostly undifferentiated cells with very rudimentary cotyledons and radicle. Only a ring of meristematic cells near the periphery of the seed can be distinguished using vital staining methods. Micro X-ray fluorescence was used to localize selenium within the seed and demonstrated that selenium was most concentrated in the proximity of the meristematic ring. The authors speculate that accumulation of selenium in the seed may be an evolved strategy to defend against herbivory.

Technical Abstract: More than a billion people worldwide may be selenium (Se) deficient, and supplementation with Se-rich Brazil nuts may be a good strategy to prevent deficiency. Since different forms of Se have different nutritional value, and Se is toxic at elevated levels, careful seed characterization is important. Variation in Se concentration and correlations of this element with other nutrients were found in two batches of commercially available nuts. Selenium tissue localization and speciation were further determined. Mean Se levels were between 28 and 49 mg kg-1, with up to 8-fold seed-to-seed variation (n = 13) within batches. Brazil nut Se was mainly in organic form. While present throughout the seed, Se was most concentrated in a ring 1 to 2 mm below the surface. While healthy, Brazil nuts should be consumed in moderation. Consumption of one seed (5 g) from a high-Se area meets its recommended daily allowance; the recommended serving size of 30 g may exceed the allowable daily intake (400 µg) or even its toxicity threshold (1200 µg). Based on these findings, the recommended serving size may be re-evaluated, consumers should be warned not to exceed the serving size and the seed may be sold as part of mixed nuts, to avoid excess Se intake.