Recommended Dietary Intake of Vitamin D

Vitamin D is very rare in foods naturally, with the exception of fatty fish and some fish liver oils. Although milk in the US is fortified with 400 IU of vitamin D/quart, several surveys during the past decade have demonstrated that approximately 80% of milk in the US contained less than 300 IU/quart. Fifty pecent of the milk samples contained less than 200IU/quart and 15% had no detectable vitamin D. Some orange juice and other juice products are fortified with calcium and 100 IU of vitamin D3/8oz. Multivitamin preparations that contain vitamin D are a good source of vitamin D as are pharmaceutical preparations.

In 1997, the Institute of Medicine and the National Academy of Sciences reviewed the recommended dietary intake for several nutrients and vitamins including vitamin D. The recommended dietary allowance (RDA) was defined as the daily intake level that is sufficient to meet nutrient requirements for nearly all (97-98%) individuals in life-stage and gender group. The RDA was meant to apply to individuals and not groups. When sufficient scientific evidence was not available to calculate an estimated average requirement (EAR), i.e., a nutrient value that was estimated to meet the requirement defined by a specified indicator of adequacy in 50% of individuals in a life-stage and gender group, the Committee recommended using an adequate intake (AI). The AI is based on the observation of experimentally determined approximations of average nutrient intake by a defined population or subgroup that appears to sustain a defined nutritional state

Milk Orange juice

Prostate gland, breast, colon, lung, immune cells

1,25(OH)2D

Regulation of cell growth (cancer prevention)

Regulation of immune function (diabetes type 1, MS, RA, autoimmune disease prevention)

25(OH)D

Milk Orange juice

Prostate gland, breast, colon, lung, immune cells

1,25(OH)2D

Regulation of cell growth (cancer prevention)

Regulation of immune function (diabetes type 1, MS, RA, autoimmune disease prevention)

Kidney 1

Calcium, muscle bone health & regulation of blood pressure insulin production (heart disease and diabetes prevention)

Figure 7 Photoproduction and sources of vitamin D. Vitamin D is metabolized in the liver to 25-hydroxyvitamin D [25(OH)D], which is responsible for maintaining calcium homeostasis. 25(OH)D is also converted to 1,25(OH)2 D in a variety of other cells and tissues for the purpose of regulating cell growth, immune function, as well as a variety of other physiologic processes that are important for the prevention of many chronic diseases. MS, multiple sclerosis; RA, rheumatoid arthritis. (Copyright Michael F Holick (2004) Vitamin D: Importance in the prevention of cancers, type 1 diabetes, heart disease, and osteoporosis. American Journal of Clinical Nutrition 79: 362-371, used with permission.)

such as normal circulation nutrient values or growth. Because sunlight played such an important role in providing humans with their vitamin D requirement and, therefore, was a variable that was difficult to quantify in most studies that were reviewed by the Committee, it was concluded that an AI rather than an RDA should be used for vitamin D (Table 1).

Adequate Intake for Ages 0-6 Months

It is well documented that human and cows' milk has very little vitamin D naturally. Human milk contains on average between 10 and 50IUl-1 (0.25-1.25 mg). This is dependent on the mother's exposure to sunlight and her vitamin D intake. Several studies have suggested that infant intakes of vitamin D of between 8.5

Table 1 Adequate Intake (AI) and Tolerable Upper Limit (UL) for Vitamin D

Table 1 Adequate Intake (AI) and Tolerable Upper Limit (UL) for Vitamin D

0-6 m

5 (200 IU)

25 (1000 IU)

6m-12yr

5 (200 IU)

25 (1000 IU)

1 yr-18yr

5 (200 IU)

50 (2000 IU)

19 yr-50 yr

5 (200 IU)

50 (2000 IU)

51 yr-70yr

10 (400IU)

50 (2000 IU)

71 + yr

15 (600IU)

50 (2000 IU)

and 15 mgday-1 would provide the maximum effect on their linear growth. A study in infants from Northern China (40-47 °N) found that vitamin D supplements of 2.5, 5, or 10 mg day-1 resulted in 36, 29, and 2% of the infants being vitamin D deficient with 25(OH)D levels of less than 25nmoll-1 (10ngml-1). None of the infants, however, had manifestations of rickets. Chinese infants from two southern cities (22 °N and 30 °N) maintained normal vitamin D status on as little as 2.5 mgday-1 of vitamin D.

There was a seasonal variation of vitamin D status of infants when they were fed human milk only and did not receive vitamin D supplements; their 25(OH)D levels decreased in the winter due to less exposure to sunlight. However, this decrease did not occur in infants receiving a vitamin D supplement of 10 mg/day beginning at 3 weeks of age.

Therefore, based on the available literature, it was concluded that a minimum intake of 2.5 mg day-1 of vitamin D was adequate to prevent rickets. However, at this intake and in the absence of sunlight, infants are at risk for developing vitamin D insufficiency; therefore, it was recommended that an AI of 5 mgday-1 (200IU) was prudent. 10 mgday-1 (400IU), the current amount in 1 l of standard infant formula or one quart of commercial cows' milk, was not considered to be excessive.

Adequate Intake for Ages 6-12 Months

Infants between 6 and 12 months of age who were fed human milk and exposed to an average of 35minday-1 of sunshine had similar 25(OH)D concentrations at 1 year of age whether the infants received 400IU of vitamin D or no vitamin D supplementation. However, in Norway, in the winter, older infants who received an average of 5 mg day-1 of vitamin D had normal 25(OH)D levels that were intermediate between those of infants studied at the end of the summer and formula-fed infants.

Therefore, in the absence of any sunlight exposure, an AI of 5 mgday-1 was recommended. However, an intake of 10 mgday-1 was not considered to be excessive.

Adequate Intake for Ages 1-18Years

There are no studies in the scientific literature that systematically evaluated the influence of different amounts of vitamin D on either serum 25(OH)D or bone mineral content in this age group. Sunlight exposure is very important for this age group to obtain its required vitamin D. In South Africa, children aged 1-8 years of mixed race showed no evidence of vitamin D deficiency. A longitudinal study in Norway, where sun exposure was presumed to vary widely over a year, an intake of vitamin D of about 2.5 mgday-1 from fortified margarine in children aged 8-18 years was adequate to prevent vitamin D deficiency.

During puberty, there is a need to increase the efficiency of dietary calcium absorption in order to satisfy the rapid growth of the skeleton. As a result, there is an increase in the metabolism of 25(OH)D to 1,25(OH)2D. Because the blood levels of 1,25(OH)2D are approximately 1000 times less than 25(OH)D, this increase in metabolism does not appear to increase the requirement of vitamin D for either boys or girls between the ages of 8 and 18 years. An average daily intake of 2.5 mgday-1 prevented any evidence of vitamin D deficiency in Scandinavian children in this age group. However, intakes less than 2.5 mg day-1 in Turkish children aged 12-17 years resulted in a decrease in 25(OH)D levels consistent with vitamin D deficiency.

Therefore, based on the available literature, it appears that children between 1 and 18 years obtain most of their vitamin D from exposure to sunlight and do not normally need to ingest vitamin D. However, for children who live in far northern and southern latitudes, vitamin D supplementation may be necessary. An AI of 5 mg day-1 (200 IU) was recommended to maintain vitamin D sufficiency in this age group regardless of exposure to sunlight.

Adequate Intake for Ages 19-50 Years

There is only sparse literature regarding the role that sunlight and diet play in maintaining an adequate vitamin D status for men and women in this age group. This age group depends on sunlight for most of its vitamin D requirement. Regardless of exposure to sunlight, it was estimated that an AI of 5 mg day-1 is sufficient for preventing vitamin D deficiency in this age group.

Adequate Intake for Ages 51-70 Years

The Committee recommended a doubling of the dietary intake of vitamin D for this age group. This was based on several studies that demonstrated the importance of increasing dietary intakes of vitamin D to maximize bone health. An evaluation of 333 ambulatory Caucasian women (mean age 58 ± 6 years) found that serum PTH concentrations were elevated in the winter (between March and May) in women consuming less than 5.5 mg (220 IU) day-1 of vitamin D. There was no seasonal variation in serum PTH concentrations when vitamin D intakes were greater than 5.5 mg (220 IU) day-1. When bone loss was evaluated between seasons in women (62 ± 0.5 years) who had a usual vitamin D intake of 2.5 mg day-1, a dietary supplement of 10 mgday-1 decreased spinal and hip-bone density loss.

Thus, since this age group does not obtain as much of its vitamin D from exposure to sunlight, it is at more risk for developing vitamin D deficiency. Therefore, in the absence of exposure to sunlight, there appears to be an increased requirement for vitamin D in this age group and an AI of 10 mg (400 IU) day-1 was recommended. This is twice the previous RDA for this age group.

Adequate Intake for Ages Greater Than 70 Years

There was strong evidence-based literature that demonstrated a decrease in the circulating concentration of 25(OH)D, and an increase in the PTH level correlated with an increased risk of skeletal fractures in both the hip and spine in this age group. Studies in both men and women supplemented with 10-25 mg day-1 of vitamin D demonstrated reduced bone resorption, increased bone mineral content, and a decrease in vertebral and nonverte-bral fractures. Therefore, because this age group is even less likely to receive an adequate amount of exposure to sunlight than adults aged 50-70 years and because they have a reduced capacity to produce vitamin D in their skin, it was recommended that men and women in this age group, regardless of exposure to sunlight, have an AI of 15 mg (600 IU) day-1, which is three times the previous RDA for vitamin D for this age group.

Adequate Intake for Pregnancy and Lactation

Although there is an increase in the metabolism of 25(OH)D to 1,25(OH)2D during the last trimester of pregnancy and during lactation there is nothing in the evidence-based literature to suggest that there is an increased vitamin D requirement for pregnant and lactating women. Therefore, it was recommended that the AI of vitamin D for pregnancy and lactation follow that recommended for their age group, i.e., 5 mg (200IU)/day-1. However, the 400 IU of vitamin D found in prenatal supplements was not considered to be excessive.

Healthy Vitamin D Intakes

Since the publication of these recommendations there have been a multitude of studies that suggest that the AIs for vitamin D are inadequate if there is no exposure to sunlight. In the absence of sunlight, children above 1year and all adults need 1000 IU of vitamin D to maintain a healthy level of 25(OH)D (above 20ngml-1) in their circulation.

Tolerable Upper Intake Levels and Vitamin D Intoxication

An excessive intake of vitamin D can lead to vitamin D intoxication. This is characterized by a marked increase in serum 25(OH)D that is usually greater than 375nmoll-1 (150ngml-1), and is associated with hypercalciuria and hypercalcemia. This can lead to soft tissue calcification and increased risk of kidney stones. The safe upper limit for vitamin D, as recommended by the Committee, is found in Table 1.

Vitamin D intoxication usually occurs when a person ingests more than 5000 IU of vitamin D daily for several months. A person does not need to be concerned about becoming vitamin D intoxicated if they take a multivitamin that contains 400 IU of vitamin D, drink a quart of milk that contains 400 IU of vitamin D, and are exposed to sunlight.

See also: Calcium. Lactation: Dietary Requirements. Pregnancy: Nutrient Requirements. Vitamin D: Rickets and Osteomalacia.

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