Fat-soluble vitamins
Introduction
Fat-soluble vitamins are a group of vitamins that are soluble in fats and fat solvents. They include vitamins A, D, E, and K. Unlike water-soluble vitamins, fat-soluble vitamins are stored in the body's fatty tissue and liver, and are not excreted as easily as water-soluble vitamins. This means they can accumulate in the body and potentially cause harm if consumed in excessive amounts.
Vitamin A
Vitamin A, also known as retinol, is essential for vision, growth, cell division, reproduction, and immunity. It is found in foods such as liver, fish oils, milk, eggs, and green leafy vegetables. Vitamin A deficiency can lead to night blindness, dry skin, poor growth, and decreased resistance to infections.
Vitamin D
Vitamin D is unique among vitamins because it can be synthesized in the skin in response to exposure to ultraviolet B (UVB) radiation from the sun. It is also found in foods such as fatty fish, beef liver, and egg yolks. Vitamin D is essential for the absorption of calcium and phosphate in the gut, which are necessary for the normal development and maintenance of healthy bones and teeth. Vitamin D deficiency can lead to rickets in children and osteomalacia in adults.
Vitamin E
Vitamin E, also known as tocopherol, is a powerful antioxidant that protects cell membranes from damage by free radicals. It is found in foods such as vegetable oils, nuts, seeds, and green leafy vegetables. Vitamin E deficiency is rare but can lead to neurological problems due to poor nerve conduction.
Vitamin K
Vitamin K is essential for the synthesis of proteins involved in blood coagulation. It is found in foods such as green leafy vegetables, vegetable oils, and cereal grains. Vitamin K deficiency can lead to bleeding disorders.
Absorption and Storage
Fat-soluble vitamins are absorbed in the small intestine along with dietary fat. They require bile acids for their absorption. Once absorbed, they are transported to the liver, where they are stored until needed by the body. Because of their storage in the body, fat-soluble vitamins do not need to be consumed as frequently as water-soluble vitamins.
Toxicity
Because fat-soluble vitamins are stored in the body, they can accumulate to toxic levels if consumed in excessive amounts. This is most likely to occur with vitamins A and D, which can be toxic if taken in high doses. Symptoms of vitamin A toxicity include dry skin, hair loss, vomiting, and bone pain. Symptoms of vitamin D toxicity include nausea, vomiting, poor appetite, constipation, weakness, and weight loss.
Deficiency
Deficiencies of fat-soluble vitamins can occur in individuals with malabsorption disorders, such as celiac disease, Crohn's disease, and cystic fibrosis, as well as in individuals with severe liver or pancreatic disease. Symptoms of deficiency depend on the specific vitamin but can include vision problems, poor growth, decreased resistance to infections, bleeding disorders, and bone abnormalities.