Selenium in soil is a delicate situation. The amount of selenium in grains that we (and animals) consume depends on the selenium concentration in the soil where the grains have been cultivated. According to Stonehouse et al. , “deficiency and toxicity affect over a billion people worldwide.” Hemp may play an important role.
Moderate amounts of selenium are essential for DNA formation, metabolism, and thyroid function. It also helps to protect the body from oxidative stress and infection. Some research has suggested that when the body lacks selenium it has increased risk for certain types of cancer.  Other complications that may arise due to selenium deficiency include cardiovascular disease, thyroid disease, and mental disorders.
But like everything else, too much is never too good. This is a trace element and only trace amounts (a dose of 0.005 mg/kg body weight/day) are needed to support physiology. Taking too much selenium will cause what is referred to as selenosis, which is basically selenium toxicity. Selenosis may cause gastrointestinal symptoms, respiratory distress, and cardiovascular complications.
Phytoremediation is a biological process where contaminants (such as excess selenium) are removed from soil and water by plants. On the other hand, biofortification is the process through which the density of nutrients in plants is increased.
Where Does Hemp Come In?
A recent study published in the Environmental Science & Technology Journal has shown that hemp has potential in the phytoremediation and biofortification of selenium from soil.  This means that hemp can help to moderate the amount of this element to prevent potential deficiency or toxicity.
Field surveys were carried out in agricultural areas of Colorado with soil naturally rich in selenium. The researchers found that seeds had 15–25 μg/g, noting “4 g of this hemp seed provides the U.S. recommended daily allowance of 55–75 μg of [selenium].” In experimental (selenate-treated) greenhouse plants, however, much higher levels of selenium accrued—up to 150–200 μg/g in seeds and 1,300 μg/g in shoots. Selenium also accumulated in the leaf tissue, flowers, and seed embryos of hemp. This did not affect cannabinoid or terpenes concentrations in the plant.
As an interesting side note, the researchers analyzed the selenium content of commercial beer produced with hemp hearts from the tested field locations. One bottle equated to about 25% of an adult’s daily selenium needs, roughly 2.5 times higher compared to four non-hemp brands.
The researchers therefore concluded that hemp has potential in phytoremediation of selenium while also acting as a source of selenium-biofortified dietary product. 
Further research is needed to shed more light on the role of hemp in mitigating selenium toxicity and in the same breath providing a rich source of dietary selenium.
- Stonehouse GC, et al. Selenium metabolism in hemp (Cannabis sativa)—Potential for phytoremediation and biofortification. Environmental Science & Technology. 2020;54(7):4221-4230. Journal Impact Factor: 7.149; Times Cited: 2
- Brinkman M, et al. Are men with low selenium levels at increased risk of prostate cancer? European Journal of Cancer. 2006;42(15):2463–2471. Journal Impact Factor 6.68; Times Cited: 111