Brazil nuts - a variable source of selenium
Selenium is a chemical element that has received substantial attention as an antioxidant which plays a role in preventing cell damage and may help prevent certain cancers. Selenium is necessary in trace amounts in the diet because it is a component of certain enzymes such as glutathione peroxidase, but excess selenium can be toxic. The Food and Drug Administration (FDA) has set the Daily Value for selenium at 70 micrograms (mcg). The tolerable upper level of selenium is 400 mcg/day for adults based on the prevention of hair and nail brittleness and early signs of chronic selenium toxicity. Toxic effects have occurred when blood selenium concentrations reach a level corresponding to an intake of 850 mcg/day.
Brazil nuts are high in selenium content relative to other human foods. Since the limits between meeting human selenium dietary requirements and toxicity are narrow, it is important to know the variation in selenium content of individual nuts. Analysis of Brazil nuts has shown that the average selenium content is approximately 14.7 ppm with a range of 0.2 to 253. The amount of selenium in the nuts depends on the selenium content of the soil where the trees grow.
It is significant that some Brazil nuts have almost no selenium, whereas others have very high values. This means that it may be very difficult to regulate the amount of selenium in the diet by eating Brazil nuts. Two Brazil nuts weigh about 10 grams, so two Brazil nuts with the average concentration of 14.7 ppm contain 147 mcg of selenium. However, two Brazil nuts containing 253 ppm of selenium will contain a whopping 2,530 mcg or 2.5 mg of selenium. This is substantially in excess of the tolerable upper level and already in the toxic range.
It is important not to overeat Brazil nuts.
 Carol L. Secor, Donald J. Lisk (1989), Variation in the selenium content of individual brazil nuts, Journal of Food Safety, 9(4), 279-281 (1989).
Shirley Crawley said,
2008-02-17 @ 10:03:04
1. How can I tell the difference in the selenium content when I buy brazil nuts? 2. Why do brazil nuts have less selenium if we buy them already shelled (25 mcg as opposed to 100 mcg).
2008-02-17 @ 14:58:22
Unless a batch of Brazil nuts is sampled and tested for selenium content, it is not possible to know how much selenium is present. This is the reason why it is important to eat Brazil nuts in moderation.
I don't know where you got your information about shelled vs. unshelled Brazil nuts, but it does not make sense. Brazil nuts are always eaten unshelled. If the shells contain selenium, the nut with the shell would have a greater quantity of selenium, but this is irrelevant since the shells are always discarded.
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