Describe Selenium under the headings of 'its dietary sources', 'its biochemical roles' and 'its deficiency diseases' with their clinical manifestations.

1. Dietary Sources of Selenium

The selenium content in plant-based foods varies significantly depending on the concentration and form of selenium in the soil where they are grown. Animal products generally have more consistent selenium levels as animals maintain predictable tissue concentrations through homeostatic mechanisms. Foods high in protein tend to be good sources of selenium due to its binding with protein.

• Brazil Nuts: These are exceptionally rich sources of selenium, with just one nut often exceeding the daily recommended intake.
• Seafood: Fish and shellfish such as tuna, halibut, sardines, and oysters are excellent sources.
• Meats: Lean meats, poultry (chicken, turkey), and organ meats (liver, kidney) provide significant amounts of selenium.
• Grains and Cereals: Whole grains like pasta, rice, and oatmeal, as well as breads and fortified cereals, contribute to selenium intake. The selenium content in these foods depends on the soil quality of the region where they are grown.
• Dairy Products: Milk, cheese (e.g., cottage cheese), and yogurt contain selenium.
• Eggs: A good source, with one large hard-boiled egg providing a moderate amount.
• Legumes and Seeds: Lentils and sunflower seeds also contain selenium.

2. Biochemical Roles of Selenium

Selenium's primary biochemical function is as an integral component of selenoproteins, which are enzymes and proteins with diverse and crucial roles in cellular metabolism and defense. Approximately 25 selenoproteins have been identified in the human proteome.

• Antioxidant Defense: Selenium is a key component of glutathione peroxidases (GPx), a family of antioxidant enzymes. GPx enzymes protect cells from oxidative damage by catalyzing the reduction of hydrogen peroxide and organic hydroperoxides, thereby neutralizing harmful reactive oxygen species (ROS). This protection is crucial for maintaining cellular integrity and preventing chronic diseases.
• Thyroid Hormone Metabolism: Deiodinase enzymes, another group of selenoproteins, are essential for the conversion of the inactive thyroid hormone thyroxine (T4) to its active form, triiodothyronine (T3). This process is vital for regulating metabolism, growth, and development.
• Immune Function: Selenoproteins play a role in modulating immune responses, including enhancing T-cell proliferation, natural killer cell activity, and antibody production. Adequate selenium status supports a robust immune system, crucial for fighting infections.
• DNA Synthesis and Repair: Some selenoproteins are involved in DNA synthesis and repair mechanisms, contributing to genomic stability and potentially reducing the risk of mutations and cancer.
• Reproduction and Fertility: Selenium is critical for male fertility, influencing sperm motility and viability. It also plays a role in female reproductive health.
• Detoxification: Selenium aids in detoxifying heavy metals like mercury by forming insoluble complexes, thus reducing their toxicity in the body.

3. Selenium Deficiency Diseases and Clinical Manifestations

Selenium deficiency, though rare in well-nourished individuals, can lead to severe health issues, particularly in regions with selenium-poor soils or in individuals with compromised intestinal function. The clinical manifestations primarily reflect the disruption of selenoprotein functions.

A. Keshan Disease

• Description: A severe form of congestive cardiomyopathy primarily affecting children and young women. It is prevalent in certain selenium-deficient areas of China, especially when combined with a coxsackievirus infection.
• Clinical Manifestations:
  • Cardiac Enlargement: The heart muscle weakens and enlarges.
  • Heart Failure: Symptoms include shortness of breath, fatigue, and fluid retention.
  • Electrocardiogram (ECG) Abnormalities: Irregular heart rhythms are common.
  • Cardiogenic Shock: In severe cases, the heart's pumping ability becomes dangerously low, leading to multi-organ failure.

B. Kashin-Beck Disease

• Description: A chronic osteoarthropathy (joint and bone disease) that primarily affects growing children. It results in the degeneration and necrosis of cartilage tissue, particularly in the joints, leading to disabling deformities. This disease is also linked to selenium deficiency in combination with other factors like iodine deficiency or fungal toxins, common in parts of Siberia and China.
• Clinical Manifestations:
  • Joint Pain and Stiffness: Especially in the knees, ankles, wrists, and elbows.
  • Limited Joint Mobility: Progressive restriction of movement due to cartilage damage.
  • Short Stature and Deformities: Affected children often experience growth retardation and develop severe joint deformities, leading to dwarfism.
  • Gait Disturbances: Difficulty walking due to joint damage.

C. Hypothyroidism

• Description: Since selenoproteins are crucial for thyroid hormone metabolism, selenium deficiency can exacerbate or contribute to symptoms of an underactive thyroid gland, especially when coupled with iodine deficiency.
• Clinical Manifestations:
  • Fatigue and Weakness: Persistent tiredness and lack of energy.
  • Weight Gain: Unexplained increase in body weight despite normal dietary intake.
  • Goiter: Enlargement of the thyroid gland.
  • Mental Slowing: Impaired cognitive function, poor concentration, and memory issues.
  • Hair Loss and Dry Skin: Common dermatological symptoms.
  • Recurrent Miscarriage: In women, thyroid dysfunction can impact reproductive health.

D. Other Manifestations of Selenium Deficiency

• Muscle Weakness and Pain (Myopathy): Generalized muscle weakness, fatigue, and tenderness. This can be observed in patients on long-term total parenteral nutrition without selenium supplementation.
• Impaired Immune Function: Increased susceptibility to infections, particularly viral infections, due to a weakened immune response.
• Infertility: Both male and female infertility can be associated with selenium deficiency, affecting sperm quality and overall reproductive health.
• Neurological Symptoms: Though less common, severe deficiency has been linked to depression, anxiety, and cognitive decline.

The severity and specific presentation of selenium deficiency symptoms can vary widely depending on the degree of deficiency, genetic factors, and the presence of co-existing nutritional deficiencies or environmental stressors.