Fat-soluble vitamins are stored in the liver to be used when needed. Small amounts of these vitamins are needed for healthy body functioning, but taking supplements containing large doses of these vitamins can be toxic. Eating a well-balanced diet should not lead to toxicity, but some people have been found to have low-grade deficiencies of fat-soluble vitamins. International units are used to express recommended daily intake since the body makes some fat-soluble vitamins.
The beta-carotene found in leafy green vegetables and orange fruit and vegetables, such as carrots, pumpkins, cantaloupe, apricots and sweet potatoes, is converted to vitamin A in the body. The active form of vitamin A, retinol, is in liver, egg yolks, cheese and fortified dairy products. The recommended intake of vitamin A is 2,333 IU, and the upper level is around 3,000 IU. Vitamin A is best known for its vision function. It also plays a key role in keeping skin, eyes and mucosal membranes in the nose, mouth and lungs moist. Night blindness, diarrhea and dry skin are signs of true vitamin A deficiency, which is unusual in the U.S. Decreased resistance to infection and slow bone growth can indicate low levels of vitamin A. High doses of vitamin A have been associated with severe birth defects and an increased risk of hip fractures. Oregon State University's Linus Pauling Institute cites studies showing that toxicity is associated with extended consumption of 25,000 IU of vitamin A daily. Signs of vitamin A toxicity include dry skin, headache, nausea and lack of appetite.
Vitamin D is made in the body in response to sunlight. It is also found in fish, fish oils, egg yolk and fortified dairy products. Recommended intake is 200 to 400 IU, while the upper limit is set at 2,000 IU. Colorado State University states that 800 to 1,000 IU vitamin D may be needed for normal function in the absence of sun exposure. The Linus Pauling Institute recommends adults take 2,000 IU daily and indicates that the safe upper limit may be as high as 10,000 IU. Vitamin D is important for bone development and maintenance, immunity, insulin secretion and blood pressure regulation. Low levels of vitamin D have been associated with increased risk of cancers, infectious disease and autoimmune disease. Excess calcium in the blood, slowed mental and physical growth, decreased appetite, nausea and vomiting are associated with too much vitamin D.
Instead of using 22.5 IU to express the recommended daily allowance of vitamin E, 15mcg of the most active form, alpha-tocopherol, is used. The upper limit of intake is 1,000mcg alpha-tocopherol. Vitamin E is an antioxidant that protects vitamin A and C and essential fatty acids from destruction. Low levels of vitamin E in fat malabsorption diseases can produce neuropathies. Oxidation of the cholesterol-transporting low-density lipoproteins has been associated with heart disease. Findings indicate a diet high in antioxidants may be protective against heat disease and cancer, but no benefit is seen from supplement use. According to the Linus Pauling Institute, about 30 percent of U.S. adults have a low blood level of vitamin E, which is associated with increased cardiovascular risk. High levels of vitamin E interfere with statins and blood-thinning medications. Vegetable oils, grains, nuts seeds and fortified cereals provide vitamin E.
Vitamin K is produced by bacteria in the intestine. Other sources of vitamin K are green vegetables, cruciferous vegetables such as cabbage, cauliflower and broccoli, and vegetable oils. The adequate intake for vitamin K is 90mcg for adult females and 120mcg for males. Because it is part of the blood coagulation system, a deficiency of vitamin K manifests as a bleeding disorder. Due to the risks of bleeding, the Linus Pauling Institute states that American Academy of Pediatrics recommends that all newborns be given an injection of a form of vitamin K. In children and adults, vitamin K is associated with bone formation and remodeling and cell growth. Excessive intake of vitamin K is associated with liver damage.