Why is Brazil nut not the best source of selenium?

Brazil nuts are one of the most concentrated natural sources of selenium, but they also carry certain risks of toxicity due to the presence of heavy metals and radionuclides.

The Brazil nut tree (Bertholletia excelsa) grows in soils rich not only in selenium but also in a number of other elements. These nuts are not mass-cultivated in agriculture but are mainly harvested from wild trees in the Amazon. The tree's root system is very deep (up to 50 m), allowing it to extract trace elements and heavy metals from the lower layers of soil, where their concentration is higher.

What toxic elements are present:

• Barium and strontium — can replace calcium in bone tissue, disrupting bone mineralization and muscle function.
• Radium-226 and Radium-228 — natural radioactive isotopes that the nuts can accumulate from the soil. Their levels are not dangerous with occasional consumption, but excessive intake creates additional radiation exposure to the body.
• Cadmium and lead — found in trace amounts, but in combination with other dietary sources, they can create chronic toxic effects.

Why it is important not to overconsume:

• Selenium itself has a narrow range between beneficial and toxic doses. 1–2 nuts a day cover the daily requirement, but already 4–6 can lead to signs of selenosis (metallic taste, brittle nails, hair loss, irritability).
• With frequent consumption, not only selenium accumulates but also the aforementioned toxic elements.