Selenium used to be treated as a very toxic substance, but modern science now regards it as essential - but in small quantities. An overdose or deficiency of selenium is equally bad, and good steady amounts should be available - but in small quantities.
Selenium is toxic and too large quantities may result in hair loss, tooth decay, brittle nails, white spots, poor appetite, sour taste in the mouth, loss of feeling in the hands and feet, change in skin pigmentation and the breath may have a garlic smell.
Men need more selenium than women as it is lost in the seminal fluid, and people staying in areas where the soil is poor in selenium, should also pay attention to their selenium intake. Brazil nuts are excellent sources of selenium, but are also found in whole grains, shellfish.