1. Antioxidant Defense: Selenium is a crucial component of the antioxidant enzyme glutathione peroxidase, which protects cells from oxidative damage by neutralizing harmful free radicals. Selenium also works in concert with vitamin E to scavenge free radicals and reduce oxidative stress.
2. Thyroid Function: Selenium is required for the conversion of thyroxine (T4) to triiodothyronine (T3), the active form of thyroid hormone. Selenium deficiency can impair thyroid hormone metabolism and lead to hypothyroidism.
3. Immune System Function: Selenium plays a role in immune system function, helping to activate immune cells and promoting the production of cytokines, which are important signaling molecules involved in the immune response.
4. Reproductive Health: Selenium is important for male reproductive health, as it is a key component of the enzyme responsible for the synthesis of sperm cell membranes. In women, selenium has been shown to improve fertility and may also play a role in preventing pregnancy complications.

1. Brazil nuts
2. Seafood, such as fish, shellfish, and seaweed
3. Organ meats, such as liver and kidney
4. Meat, including beef, pork, and poultry
5. Whole grains, such as brown rice, oats, and wheat
6. Dairy products, including milk and cheese
7. Eggs
8. Vegetables, such as broccoli and spinach
9. Legumes, such as chickpeas and lentils

The amount of selenium in these foods can vary depending on the soil where they are grown or raised. Brazil nuts are one of the richest dietary sources of selenium and can provide more than the recommended daily intake in just one or two nuts.

Note: Excessive intake of selenium can be toxic and cause symptoms such as hair loss, nail brittleness, gastrointestinal upset, and neurological symptoms. Therefore, it is important to consume selenium in moderation and within the recommended daily intake.

Selenium deficiency can occur in areas where the soil is low in selenium, leading to crops that are also low in this mineral. Other factors that can contribute to selenium deficiency include gastrointestinal disorders that interfere with nutrient absorption, prolonged intravenous feeding without selenium supplementation, and certain genetic disorders.

Selenium deficiency can cause a range of symptoms, including:

1. Weak immune system
2. Fatigue
3. Hair loss
4. Cognitive decline and brain fog
5. Infertility
6. Muscle weakness
7. Joint pain
8. Thyroid dysfunction
9. Cardiomyopathy (enlargement of the heart)
10. Keshan disease (a form of heart disease)

It’s worth noting that selenium deficiency is rare in developed countries, as most people get enough selenium from their diet. However, in certain parts of the world where the soil is low in selenium, deficiency can be more common.