Why do I eat 2 Brazil nuts per day?

Brazil nuts have several of the same nutrients of other nuts, including healthy fats, antioxidants, vitamins, minerals and fiber. However, the reason why I eat 2 Brazil nuts almost every day is because they are the richest food source of a very important trace element and essential mineral called selenium. Given that many Europeans have a suboptimal selenium status [1], optimizing it is a very important issue to take into consideration.

And why should we make sure to eat enough selenium? Why is it so important?

Selenium is crucial to human health. Science has recognized it has potent anti-inflammatory, anti-viral, antioxidant and even anti-cancer activity [2]. It is also crucial for fertility and reproduction, and for brain health, as you can read further below.

Selenium is an essential component of many enzymes (so called selenoproteins) involved in metabolism and antioxidant defenses. For example, it is a component of glutathione peroxidase, which is an enzyme that converts hydrogen peroxide to water, protecting therefore our cells from the adverse effects of hydrogen peroxide, and having thus a potent antioxidant activity. This is thought to be the main reason why selenium is so crucial for so many body systems.

Below I will give you further details on the importance of selenium for thyroid function, cancer prevention, immune activity, fertility and reproduction, and brain health.

Thyroid function

You probably know that in order for our thyroid to function properly, it needs enough iodine. But what many people don’t know is that selenium is also crucial for our thyroid.  In fact, the thyroid is the organ with the highest selenium content per gram of tissue [3]. As thyroid cells produce hydrogen peroxide continuously throughout life, an effective cell defense system against this compound (and reactive oxygen intermediates derived thereof) is important. This happens through the proper functioning of the enzyme glutathione peroxidase (selenium-dependent), which is thus essential for the maintenance of normal thyroid function and protection of the gland [4]. In addition, selenium is a constituent of other thyroid enzymes, such as deiodinases, which produce the active thyroid hormone, the triiodothyronine (T3), from its inactive precursor thyroxine (T4) [3].

Hashimoto’s thyroiditis is an autoimmune disease affecting the thyroid gland that often results in hypothyroidism with bouts of hyperthyroidism. Grave’s disease is another thyroid autoimmune disease characterized by hyperthyroidism. Studies have shown that selenium supplementation with about 200 mcg per day (equivalent to consuming about 3 Brazil nuts per day) may improve the symptoms and disease markers (thyroid antibodies) in patients with both Hashimoto’s thyroiditis [5, 6] and Grave’s disease [7].

Cancer prevention

Selenium has important antioxidant properties. As referred above, selenium is an essential component of the enzyme glutathione peroxidase, which acts as an antioxidant, protecting the cells from the build-up of free radicals (oxidative stress), which as you have probably heard, are associated with cancer development.

Indeed, various studies showed that higher selenium levels are linked to a decreased risk of death from several types of cancer and decreased all cause-mortality [8-10]. Also, selenium can trigger apoptosis (cell death) of leukemia cancer stem cells [11].

However, it is not clear whether supplementation is helpful for cancer prevention [12], and this is part of the reason why I recommend to rather take selenium in food sources rather than in supplement form (more on this below).

Immune activity

Its key role for our immune system has now been recognized [13, 14]. Adequate levels of selenium are important for initiating immunity and also in regulating excessive immune responses and chronic inflammation [15].

Selenium is found in immune tissues such as liver, spleen and lymph nodes and it has been shown that low selenium impairs the immune system, while selenium supplementation was shown to enhance the activity of immune cells [2]. Selenium deficiency is linked to the occurrence, virulence or disease progression of some viral infections [2] and selenium supplementation could help fighting viral and bacterial infections [16]. For example, it appears to be a key nutrient in counteracting the development of virulence and inhibiting HIV progression to AIDS [2, 16, 17]. Selenium seems to be also important to fight the influenza virus [16].

In addition, selenium may also be helpful as a treatment for chronic asthma[18].

Fertility and Reproduction

Selenium is also crucial for both human conception and healthy pregnancy [19].

It is required for male fertility as it is needed for testosterone synthesis, for the formation and development of spermatozoa and for sperm motility [2, 19].

Studies have also shown that it may reduce the risk of miscarriage. Scientists found that women who had had miscarriages in the first trimester had lower selenium levels [20, 21]. They suggest that this may be linked to reduced antioxidant protection due to low concentration of the selenium-dependent enzyme glutathione peroxidase. In addition, selenium deficiency may lead to gestational complications and damaging of the nervous and immune systems of the fetus [19].

Consuming adequate amounts of selenium during pregnancy is also important to prevent postpartum thyroiditis and hypothyroidism [22].

Brain Health

Selenium is also vital to our brain.

Low selenium status is associated with a greater incidence of depression, anxiety, confusion and other negative mood states, while selenium supplementation seems to decrease these disorders [23-25].

Also, low selenium might be a risk factor for cognitive decline [26]. Indeed, low blood selenium concentrations in the elderly are associated with accelerated cognitive decline [27, 28] and brain selenium concentration is much lower in Alzheimer’s patients than in people without this disease [28]. Scientists also showed that the consumption of Brazil nuts can restore selenium deficiency, increase glutathione peroxidase activity, and have positive effects on cognitive function on older adults with mild cognitive impairment [27]. The brain tends to accumulate peroxidation products such as hydrogen peroxide and thus, the enzymes that remove it, like gutatione peroxidase (dependent on selenium, as referred above) are absolutely vital [2].

In addition, studies showed that selenium supplementation could reduce intractable epileptic seizures in children [29, 30].

Selenium supplements vs food sources

A study investigating the bioavailability (meaning, absorption) of selenium in Brazil nuts showed that eating only 2 Brazil nuts per day for 12 weeks was as effective as 100mcg of a selenomethionine supplement for increasing blood selenium status [31]. So there is really no need to supplement this important trace element, when we can obtain it directly from a food source.

Also, we don’t know yet the interactions that might exist between nutrients in food. It is possible that selenium works best when provided together with other nutrients – the truth is that we don’t know. And honestly, if there is a food source with such high amounts of selenium why would we take a supplement? I do believe that first of all we should eat healthy foods that will already provide us with a wide arrange of nutrients, and only take supplements when really needed.

However, if for any reason you prefer to take a supplement, opt rather for organic forms of selenium (selenomethionine and selenocysteine), which are more bioavailable (better absorbed) than inorganic forms (selenite and selenate) [32].

Too much selenium

Selenium is a trace mineral, meaning you only need tiny amounts of it (in the mcg range).

Some studies have shown that the effect of selenium to human health has a U-shape. This means that too little might be a risk factor for all the conditions mentioned above and more, however too much selenium is also counterproductive and may also raise the risk of cancer, for example [33]. Researchers think the ideal blood levels of selenium are between 100 and 130 mcg/L [8, 33].

The RDA of selenium is 55 mcg/day, but the upper limit of intake is 400 mcg/day for adults. The majority of the studies referred in this article used a dose of 200 mcg/day, which is equivalent to about 3 Brazil nuts. This is why for general health purposes, I do not recommend eating more than 3 Brazil nuts per day. Aim to eat at least 1-2 Brazil nuts per day (organic whenever possible!) to make sure you keep your selenium levels in check. This amount will of course depend on whether you have a health condition or on whether you did a selenium blood test and you know you are deficient, in which cases you would probably benefit from an increased intake for a period of time (in the studies I have mentioned in this article, supplementation with 200 mcg of selenium varied between 3 months and 1 year). But in these cases, please get informed with your health professional!

Hope you found this article helpful! And that you start eating Brazil nuts (in moderation) 🙂

Let me know your thoughts in the comments below 🙂

Ana Coito, PhD

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References

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