Vitamins come in two varieties: water soluble and fat soluble. Water soluble vitamins, such as B complex vitamins and vitamin C, dissolve in water and are excreted through the kidneys if you have excess amounts in your body. Fat-soluble vitamins--namely vitamins A, D, E and K--dissolve in fat and are stored in fat throughout the body. It is difficult for your body to excrete excess fat-soluble vitamins, so toxic levels can accumulate if you consume too many.
How Fat Soluble Vitamins Are Absorbed
When you eat foods with fat-soluble vitamins, it goes through the digestion process. When it gets to the small intestine, the fat-soluble vitamins are released. Unlike water-soluble vitamins that go directly into the bloodstream, fat-soluble vitamins are absorbed through the intestinal wall and into the lymph system, according to cardiologist Dariush Mozaffarian on Sharecare.com. Once in the lymph system, the vitamins are sent to the bloodstream where they are put out for use by the body or sent for storage in the liver and fat cells. When your body is in need of one of these vitamins, it is released from storage.
Vitamin A is necessary for vision, bone growth, reproduction, resistance to infections and several other functions in the cells of your body. Some forms of vitamin A are considered antioxidants, which may protect your body from some chronic diseases. Vitamin A is found in both plants and animals. The type of vitamin A in animals is called preformed vitamin A and is absorbed in the form of retinol. The vitamin A in plants is called provitamin A, or carotenoids. The most common carotenoid is beta carotene. Food sources of vitamin A include milk, eggs, fortified breakfast cereals and darkly colored fruits and vegetables, such as carrots, cantaloupe, sweet potatoes and spinach. The recommended dietary allowance, or RDA, is 3,000 international units per day for men and 2,310 international units for women.
Vitamin D works in conjunction with calcium to build bones and strong teeth. Very few foods contain vitamin D. Milk is fortified with vitamin D, and fatty fish, such as salmon and tuna, contain some. The best source of vitamin D is the sun. A few minutes in the sun each day provide adequate amounts of vitamin D. The RDA is 200 international units per day for teens and adults ages 14 to 50, 400 international units for adults ages 51 to 70 and 600 international units for adults over 70.
Current, ongoing studies are being done on vitamin E and its ability to protect against cancer, heart disease, stroke, dementia and liver disease. Vitamin E is considered an antioxidant, which has protective qualities to fight free radicals in your body. Free radicals damage cells throughout the body and may be associated with several chronic diseases. Vitamin E is in vegetable oils, wheat germ, corn, nuts and seeds, olives, spinach and asparagus. The RDA for adults is 15 mg per day.
Vitamin K is necessary for blood clotting. The USDA is currently studying the role of vitamin K in bone strength and integrity and how much vitamin K is needed for this purpose. Vitamin K is found in cabbage, cauliflower, fortified cereals, fortified oils and green leafy vegetables, such as spinach. The RDA is 65 micrograms per day for women and 80 micrograms for men.
Are Supplements Needed?
If you eat a well-balanced, nutritious diet, you most likely consume adequate amounts of the fat-soluble vitamins. Supplements are available if you feel your diet is lacking in these vitamins. Because they are fat-soluble and stored in your body, use caution in taking supplements. Consuming more than the RDA can be harmful. Check with your health care provider before taking high levels of supplements.