Vitamin D, a fat-soluble essential vitamin, aids in the absorption of calcium and helps maintain the balance of calcium and phosphorus in the blood -- necessary functions for the growth and development of strong bones. Adequate levels of vitamin D may also play a role in protecting against osteoporosis, high blood pressure, certain cancers and autoimmune diseases. Knowing the daily recommended intake of vitamin D, which includes vitamin D-3, helps keep the incidence of vitamin D deficiency diseases in the United States low.
Types
Vitamin D exists in two main forms: vitamin D-2 and vitamin D-3. Plants synthesize vitamin D-2, while humans produce vitamin D-3 in response to sun exposure. Very few foods naturally contain vitamin D, but fatty fish, beef liver, cheese and egg yolks contain some vitamin D in the form of vitamin D-3. Dietary supplements may contain vitamin D-2 or D-3 and at nutritional doses, the two seem equivalent. The National Institutes of Health Office of Dietary Supplements suggests that vitamin D-3 is more potent than vitamin D-2 at high doses.
Synthesis
The human body has the ability to synthesize vitamin D. Skin contains a substance known as 7-dehydrocholesterol. The ultraviolet rays from the sun stimulate the conversion of this compound into previtamin D-3, which then becomes vitamin D-3. Many factors affect the production of vitamin D-3 within the body. The season, time of day, length of day, cloud cover, smog and sun screen can all reduce the ability of ultraviolet rays to penetrate the skin and begin the production of vitamin D-3. Complete cloud cover reduces the ultraviolet rays by 50 percent, while smog can reduce it by 60 percent. Sun screens with a sun protection factor of 8 or higher can completely block the rays and inhibit vitamin D3 production. Most people still produce adequate amounts of vitamin D-3 since insufficient coverage allows some sun penetration.
Adequate Intake
The National Institute of Medicine Food and Nutrition Board provides the adequate intake -- the amount believed to cover the needs of the majority of the group. The adequate intake for children and adults up to the age of 50 is at least 5 micrograms per day, while those ages 51 to 70 need at least 10 micrograms per day. Because there is no significant difference between vitamin D-2 and D-3, both forms of the vitamin count toward the recommended intake.
Recommended Daily Allowance
The Recommended Dietary Allowance describes the amount of the nutrient sufficient to meet the needs of nearly all healthy people in that specific group. The recommended allowance for vitamin D is 10 micrograms per day from 0 to 12 months, 15 micrograms per day for ages 1 to 70, and 20 micrograms per day for those over the age of 70, according to the National Institutes of Health Office of Dietary Supplements. Since 1 microgram equals to 40 IU, or International Units, this translates to 400 IU for ages 0 to 12 months, 600 IU for those up to age 70 and 800 IU for those over 70.
Fortified Sources
To help meet the daily recommended intake, food manufacturers fortify foods with vitamin D. The majority of fortified foods contain vitamin D-3. Since the 1930's the milk supply produced in the United States has been fortified with 100 IU per cup, a step that has dramatically reduced the number of cases of rickets -- the vitamin D deficiency disease in children. Other foods fortified with vitamin D-3 include breakfast cereals, orange juice, yogurt and margarine.
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