October 2007

Should ‘D’ Stand for ‘Deficiency’?
By Dale Ames Kline, MS, RD, CNSD, LD
Today’s Dietitian
Vol. 8 No. 10 P. 12

CDR Learning Codes: 2060, 2090, 2000, 3020, 4000, 4030, 4040, 4130, 5000; Level 2

Many people are in the dark about the “sunshine vitamin.” You’ve probably heard that
10 to 20 minutes of exposure to sunlight each day is all you need to meet your vitamin D requirement. Unfortunately, new research is proving this statement wrong; many people may be chronically deficient in this vital nutrient, with dire consequences. This article will review some recent research and make recommendations for improving vitamin D intake and serum levels.

Vitamin D was identified in 1919 but improperly named: It is not really a vitamin, since it is not essential in the diet, but is properly called a prohormone. Structurally, it is similar to steroid hormones such as cortisol, estradiol, and aldeosterone.

It took until 1969 to identify the active metabolite of vitamin D: 1,25(OH)2D3 (1,25(OH)2D). It was not until the late 1970s that a reliable measure of serum vitamin D was available.1 To measure vitamin D concentrations, the metabolically inactive metabolite 25-hydroxyvitamin D3 (25(OH)D), which does indicate body stores, is used rather than the metabolically active 1,25(OH)2D, which does not.

Vitamin D deficiency was initially linked to the childhood disease rickets, which causes the softening and weakening of bones, often leading to fractures. (In adults, rickets is called osteomalacia.) Vitamin D fortification of milk became a common and inexpensive preventative, and it seemed people knew all they needed to know about vitamin D.

In the 1970s, however, researchers linked hip fractures to low serum vitamin D levels, finding in one study that serum levels in those with hip fractures were 39 nanomoles per liter (nmol/L) vs. 72 nmol/L in age- and sex-matched controls.1 That led to a better understanding of vitamin D’s relationship with calcium and bone metabolism.

We know vitamin D increases calcium absorption to maintain a constant serum calcium level. If vitamin D is deficient, the body maintains a constant calcium level by taking calcium from bones. Increases in parathyroid hormone occur as serum calcium falls, triggering the release of calcium from the bones. The bones become thin and brittle if they continue to lose calcium over a period of time, leading to osteomalacia or osteoporosis.

But vitamin D researchers have discovered other important functions as they began questioning the inverse relationship between sunlight exposure and increased incidence of many chronic diseases, such as cancer, autoimmune diseases, diabetes, and cardiovascular disease. Although causation has been difficult to pin down, it is recognized that people living in northern latitudes have more chronic diseases, with that tendency being linked to their inability to produce adequate amounts of vitamin D, especially during winter months, as discussed later.2

Many different body cells have vitamin D receptors—keratinocytes of the skin, islet cells of the pancreas, lymphocytes, promyleocytes, colon enterocytes, osteoblasts, distal renal cells, the parathyroid gland, the pituitary gland, and ovarian cells.3 This discovery has led researchers to identify many noncalcemic roles for vitamin D. One, its regulation of cellular growth and differentiation and T-cell immunity, may explain why vitamin D deficiency is linked to cancer, autoimmune disease, and other chronic diseases.3,4

Sunlight vs. Diet
Most foods contain little vitamin D. The best sources are oily fish, egg yolk, cod liver oil, and liver. Milk, orange juice and other fruit juices, and some breads and cereals are fortified with vitamin D; most of the dietary vitamin D consumed comes from fortified milk. Approximately 10% of our vitamin D comes from food.5 Table 1 shows the vitamin D content of some foods.

We get most of our vitamin D from exposure to sunlight, particularly ultraviolet B (UVB) radiation. The skin contains 7-dehyrocholesterol (7DHC), which is converted to previtamin D3 by UVB light and then to vitamin D3 by heat. The liver then converts the vitamin D3 to serum 25(OH)D, the major circulating form of vitamin D in the body, which is metabolically inactive. It is in the proximal convoluted tubule of the kidney that metabolically active vitamin D is formed—1,25(OH)2D. New research has shown that other tissues of the body besides the kidney can convert 25(OH)D to 1,25(OH)2D, such as the colon, prostate, breast, and immune cells.4

Humans have a love-hate relationship with sunshine. Skin fashions come and go—from the lily white skin standards of the 19th century to the bronze age of the 1960s to the skin cancer-obsessed, sunscreen-laden era of today. On the surface, it seems it should be easy to obtain adequate vitamin D from the sun. We are an increasingly active, outdoorsy society of joggers, hikers, and golfers, and more of us live in the Sun Belt states each year.

However, it is becoming clear that vitamin D deficiencies exist in more people than doctors and researchers had previously thought. Older adults, those living in extreme northern or southern latitudes, African Americans, pregnant women, infants (particularly those solely breast-fed), and children are at risk.5-7 Estimates have indicated that more than one half of the U.S. population may have suboptimal or deficient vitamin D levels.8

Several factors determine the amount of vitamin D produced by the sun in an individual. Primary factors are seasonality, latitude, 7DHC concentration in the skin, and concentration of melanin in the skin (skin color). Other factors include the amount of sun exposure, sunscreen use, and the amount of cloud cover and smog. An age-related variation has also been noted.

In the winter months, those individuals living above 35° North latitude (north of Atlanta) or below 35° South latitude (south of Sydney, Australia, or Buenos Aires, Argentina) will have difficulty producing enough vitamin D, even if they are well exposed to the sun. The sun’s rays are not strong enough to produce previtamin D. Even during the summer months, the further away from the equator you go, the weaker the sun’s rays and the more exposure you need to make adequate vitamin D. For example, sunlight exposure from November through February in Boston is insufficient to produce significant vitamin D synthesis in the skin. Complete cloud cover halves the energy of UV rays, and shade reduces it by 60%.

Older people typically get less exposure to sunlight, and their ability to make vitamin D from the sun decreases by 75% by the age of 70, which may be related to a decrease in the amount of 7DHC in the skin. Since the skin has a large capacity to produce adequate vitamin D, it is still possible for older people to maintain normal vitamin D levels, although it is more difficult. In Boston, at the end of August, Holick and associates found that 30% of older white adults, 42% of older Hispanic adults, and 84% of older African American adults were vitamin D deficient, with serum levels of 25(OH)D below 50 nmol/L.9

The darker your skin, the more difficult it is to convert 7DHC to previtamin D. Young Caucasian and African American women in Boston were studied to see if there were seasonal variations in their vitamin D concentrations and differences based on skin color.10 Both groups had seasonal variations; however, the vitamin D concentration in the African American women was at least 50% less than Caucasian women, regardless of the season. From February to March, values for vitamin D were lowest at 30.2 nmol/L in African American women and 60 nmol/L in Caucasian women. From June to July, the values were highest, with the concentration for African American women at 41 nmol/L and 85.4 nmol/L for Caucasian women. When vitamin D levels did rise, the increase was smaller in African American women than Caucasian women.

It can take 5 to 10 times the amount of sun exposure for an individual with very dark skin to produce the same amount of previtamin D compared with an individual with very light skin, depending on the latitude and season.6

Staying indoors or covering up while in the sun impedes vitamin D production. In Muslim countries, where women cover up before they go outside, most women are likely vitamin D deficient, as a study based in the United Arab Emirates showed.11 Serum concentrations of these women were usually below 30 nmol/L, low enough to cause osteomalacia and the associated nonspecific bone pain, muscle aches, and weakness, symptoms that were common in the women studied.11

Sunscreen decreases vitamin D production dramatically. If applied as directed, a sunscreen with an SPF of 15 or more decreases vitamin D production by 99%. Even sunscreen with an SPF of 8 or less decreases vitamin D production by 95%.5 The less sunscreen used, the more vitamin D produced.

Preventing skin cancer without causing a vitamin D deficiency is difficult. It is important to have adequate sun exposure to make sufficient vitamin D to prevent chronic diseases but not enough to cause skin cancer. In most latitudes, sun exposure for 15 to 20 minutes per day in the summer months before applying sunscreen is recommended for Caucasians. Hispanics and African Americans, with darker skin, need a considerably longer time in the sun to produce the same amount of vitamin D. In winter months, a vitamin D supplement is needed.

Vitamin D is stored in the body’s fat cells. In the obese, vitamin D is stored in fat tissue and is unavailable for use. So even if adequate vitamin D is produced in the sun, it may not be physiologically available.12

So, many people are probably not meeting today’s standards for vitamin D, for whatever reason. But even these standards are in question; we no longer know for sure what “normal” or “optimal” serum levels are.

In the past, the normal range for vitamin D was approximately 30 nmol/L. That level would be classified as deficient by the most prominent researchers in vitamin D today. The reason? They measured individuals most likely deficient in vitamin D and called them normal.

A consensus is emerging that levels above 50 nmol/L are necessary for proper function of the body’s cells and organs, with a range of 78 to 100 nmol/L necessary for bone health, fracture prevention, and chronic disease prevention.6,8 In determining optimal serum concentrations of 25(OH)D, Bischoff-Ferrari and colleagues found optimal fracture prevention when serum 25(OH)D levels were approximately 100 nmol/L.8

Higher levels can be tolerated without any adverse effects, as lifeguards and sunbathers can have serum vitamin D levels of up to 250 nmol/L without any signs of toxicity.6 According to Holick, exposing the body to sunlight that causes minimum redness to the skin is equivalent to ingesting 10,000 to 25,000 international units (IU) of vitamin D.9 If only 6% of the skin is exposed to sunlight, that is the equivalent of 600 to 1,000 IU of vitamin D.

Some levels will produce toxicity symptoms, however. Vitamin D toxicity appears first as hypercalcemia, with vitamin D levels above 600 to 750 nmol/L, although it can be seen in lower serum levels of 500 nmol/L.1,13

Hathcock recently applied the risk assessment methodology used by the Food and Nutrition Board to arrive at a safe tolerable upper limit for vitamin D.13 By his estimates, daily intake of 10,000 IU is safe vs. the 2,000 IU currently recommended by the Food and Nutrition Board. At 10,000 IU per day, serum vitamin D would remain well below 500 nmol/L, the concentration associated with hypercalcemia, and in most people would stay well within the new consensus range.13

Deficiency and Disease Risk
Vitamin D functions in the body in conjunction with vitamin D receptors (VDRs), which are present in many cells. The active form of vitamin D—1,25(OH)2D—binds to the VDR and is transported in the cell nucleus. Once inside the cell, it binds with a retinoic acid receptor and finally interacts with a vitamin D-responsive element, which causes an increase or inhibition of vitamin D genes that control the cell functions.6 We now know that many normal cells, as well as cancerous cells, can produce the active form of vitamin D when needed.5

For example, in breast, colon, and lung cells, vitamin D interacts with VDRs to regulate cell growth and decrease the proliferation and hyperproliferation of cells.5 Without adequate vitamin D, normal cells proliferate, which can lead to cancer. In cancerous cells, unchecked hyperproliferation leads to increased tumor size. In other organs and cells, vitamin D regulates blood pressure, insulin production, and immune function.

In a double-blind, randomized, placebo-controlled trial, Lappe and colleagues demonstrated an inverse relationship of vitamin D levels and risk for all types of non-skin cancers.14 As the serum 25(OH)D concentration rose above 80 nmol/L, there was a 50% reduction in the colorectal cancer risk. The authors also concluded that baseline and treatment-induced vitamin D levels were strong predictors of cancer risk.14

The risk of prostate cancer is reduced by 50% when serum vitamin D levels are greater than 50 nmol/L.15 However, it may not be vitamin D levels alone that are responsible. Small variations in the genes, called polymorphisms, alter how the cell functions.16 This may explain why some individuals are more prone to cancer, osteoporosis, or other diseases.

Immune Functions
Vitamin D is integral to the normal function of the immune response. In terms of autoimmune disease, vitamin D inhibits the proliferation of T cells and decreases the production of T helper 1 (Th1) cells and proinflammatory cytokines produced by Th1 cells: interleukin-2, tumor necrosis factor-alpha, and interferon-gamma.4 It is the balance of Th1 and Th2 cells that keeps the immune system functioning optimally.

If vitamin D is deficient, the production of Th1 cells increases, as does the quantity of proinflammatory cytokines, leading to the development or exacerbation of autoimmune disease and inflammation. Remember: In autoimmune disease, the immune system attacks “self” cells and does not know when to shut off, subsequently damaging tissue.

In type 1 diabetes, the immune system attacks the B-islet cells of the pancreas. The risk for type 1 diabetes increases dramatically in children who are vitamin D deficient. Hyppönen and colleagues found that by giving children (aged 1 and older) 2,000 IU of vitamin D, their risk of type 1 diabetes decreased by 80%.17 In another study, children and adolescents with type 1 diabetes were three times more likely to be vitamin D deficient with mean serum levels of 54.7 nmol/L compared with 64.6 nmol/L for the control group.18 Seasonal variations in vitamin D were found in this study.

Researchers have found a link between multiple sclerosis (MS) and vitamin D. Higher circulating levels of vitamin D are linked to a significantly lower MS risk. Individuals with the highest vitamin D levels—approximately 100 nmol/L—had a 62% lower risk of MS than those with the lowest vitamin D levels of 63 nmol/L.19

Researchers have long observed an increase in the winter-time symptoms associated with MS and rheumatoid arthritis and an increase in the diagnosis of cancer, diabetes, and other autoimmune diseases, particularly in those living above 35° North latitude. During the summer, symptoms improved and some individuals went into remission.

The following summarizes the noncalcemic functions of vitamin D:

• regulates cell growth, differentiation, and signaling;

• induces apoptosis (cell death);

• decreases malignancies, especially of the colon, breast, and prostate;

• modulates T- and B-cells, cell-mediated immunity, and cytokines
• promotes T-cells that do not react to “self” tissue;

• decreases production of proinflammatory Th1 immune cells; and

• stimulates insulin production.

Pregnancy
Along with the general population, pregnant women have been found to have a high incidence of vitamin D deficiency.7 Vitamin D status of the infant at birth is related to the vitamin D status of the mother, as the cord blood will contain 50% to 60% of the maternal circulating concentrations of vitamin D.20

A vitamin D deficiency in pregnant women can cause problems for the mother and the fetus. Maternal effects of vitamin D deficiency include decreased serum calcium concentrations and possible decreased weight gain in the third trimester.21 Fetal vitamin D deficiency can delay growth and bone ossification and cause enamel hypoplasia and problems with calcium regulation (ie, hypocalcemia and tetany), decrease bone mineral content and skeletal mineralization, and cause congenital rickets and craniotabes (softening of the skull).21,22

At issue is whether a vitamin D deficiency decreases maternal weight gain, fetal growth, and birth weight and whether the serum vitamin D level for pregnant women should be higher.

In a study of pregnant women in the Netherlands, 8% of light-skinned women and more than 50% of darker pigmented women were vitamin D deficient, with levels below 25 nmol/L (a level that causes osteomalacia in adults).22 Had the criteria for vitamin D deficiency been set higher, an even greater number of pregnant women would have been identified as vitamin D deficient—up to 100% of the women in the study.

A 2006 study found a correlation of maternal milk intake to infant birth weight in pregnant women living in Calgary, Canada (51º North latitude), independent of other risk factors. As milk intake increased, so did birth weight.23 For each microgram (40 IU) increase in vitamin D intake, birth weight increased by 11 grams. No differences in infant head circumference and length were found between women with higher and lower milk consumption. No serum vitamin D levels were taken.

A study of lactating women given either 2,000 or 4,000 IU per day for three months increased serum vitamin D from 69 to 90.25 nmol/L and 81.5 to 97.9 nmol/L, respectively.20 All these values were within the normal reference range. In addition, the infants of the lactating women had significantly better vitamin D status at the end of the study, since their breast milk was richer in vitamin D. It appears supplementation does improve vitamin D status in pregnant women and their infants, and a vitamin D supplement up to 4,000 IU per day is not harmful.20

New Recommended Daily Intakes?
With all this information, we still have this unanswered question: How much vitamin D does the body need to produce a serum level that protects against disease?

In 1997, the Dietary Reference Intake for vitamin D was published by the Institute of Medicine.24 The adequate intake recommended for infants, children, and adults aged 19 to 50 is 200 IU per day; for adults aged 50 to 70, it is 400 IU per day; and for adults aged 70 or older, it is 600 IU per day.24 The adequate intake recommended for pregnant women is 200 IU. At present, the upper limit recommended for vitamin D is 2,000 IU per day.

Many researchers now believe these recommendations are too low. While the recommended intakes may prevent bone disease, they are not high enough to promote cellular health and prevent other diseases.

In the study of hip fractures previously cited, the authors found that a vitamin D level of 100 nmol/L was optimal and could prevent up to 25% of fractures.8 Reaching that level took a minimum of 700 to 800 IU per day and more if the baseline was below 44 nmol/L.

In a study of healthy young men in Nebraska (41° North latitude), it took 500 IU of vitamin D per day to prevent a seasonal fall in vitamin D and 1,000 IU per day to raise serum vitamin D by 12.5 nmol/L.

Calcium absorption, a measure of vitamin D status, was not maximized until a serum level of vitamin D reached 75 to 80 nmol/L.25,26 To achieve a serum vitamin D level of 80 nmol/L could require a daily intake of up to 2,200 IU, depending on baseline vitamin D levels.26 Individuals exposed to adequate sunlight have serum vitamin D concentrations averaging 150 nmol/L.27

At this point, there is a consensus among researchers and policy makers that the recommended daily allowance for vitamin D does need to be reassessed. Until that happens, the best recommendation is to have the serum vitamin D level checked. Make sure the doctor checks 25(OH)D. Next, encourage clients to go in the sun at least three times per week with maximum skin exposure and use sunscreen after 15 to 20 minutes. If their skin is darker, they will need to be in the sun for a longer period of time.

If they live above 35° North latitude (Atlanta, San Francisco), supplement vitamin D in the wintertime in doses of up to 2,200 IU per day to prevent seasonal declines. Infants need either sun exposure or supplemental vitamin D in a lower dose.

The recommendation to increase serum vitamin D levels in individuals who are deficient is to take 50,000 IU once per week for eight weeks. If the level remains low, repeat the 50,000 IU once per week for another eight weeks. Once a value of 50 nmol/L or greater is achieved, a supplement of 1,000 to 2,000 IU per day will maintain vitamin D in the normal range.5

Toxicity is not seen even at serum vitamin D levels of 250 nmol/L, requiring a vitamin D intake of 10,000 IU per day.26 Individuals exposed to adequate sunlight have serum vitamin D concentrations averaging 150 nmol/L, and lifeguards have closer to 250 nmol/L.27 Hathcock’s risk assessment estimates that 10,000 IU is safe ± five times the 2,000 IU currently recommended by the Food and Nutrition Board.13

In the era of food fortification, we have become complacent and have not recognized that factors such as sunscreen can affect vitamin D levels. The official recommendations sometimes lag behind the research, and dietitians should become more aware of potential risk for inadequacies of vitamin D in their patients, particularly in northern areas and among groups identified here as most at risk. In any case, it seems prudent, absent specific testing to establish adequacy, to recommend a daily supplement of at least 1,000 IU of vitamin D. For vitamin D concent of foods click here.

— Dale Ames Kline, MS, RD, CNSD, LD, is president of Nutrition Dimension, Inc. A former hospital chief clinical dietitian and nutrition educator in the Women, Infants and Children program, she has written and edited continuing education home study courses since 1984.

Examination

1. Which of the following forms of vitamin D is used to measure vitamin D concentrations and the nutritional status of vitamin D?
a. 7-dehydrocholesterol
b. Vitamin D3
c. 25-hydroxyvitamin D
d. 1,25-dihydroxyvitamin D

2. The “classic” role for vitamin D is maintaining bone health by which of the following mechanisms?
a. Decreasing the amount of calcium deposited in the bones
b. Increasing the production of parathyroid hormone
c. Preventing the loss of calcium from the kidneys
d. Increasing the absorption of calcium from the intestines

3. One cup of milk is generally fortified with how much vitamin D?
a. 50 international units (IU)
b. 100 IU
c. 150 IU
d. 200 IU

4. In the winter months, how much vitamin D is produced by someone living in Seattle?
a. None
b. One fourth as much as in the summer
c. One half as much as in the summer
d. The same amount

5. If an individual puts on the recommended amount of sunscreen with an SPF of 15 or more, how much does vitamin D production decrease?
a. 25%
b. 58%
c. 76%
d. 99%

6. Which of the following serum vitamin D levels do most researchers believe is necessary for fracture and chronic disease prevention?
a. 25 to 40 nanomoles per liter (nmol/L)
b. 50 nmol/L
c. 78 to 100 nmol/L
d. 110 to 125 nmol/L

7. Which of the following functions may explain why vitamin D is an important cancer-preventing nutrient?
a. It prevents genetic material in the cell from mutating and becoming cancerous.
b. It regulates growth, differentiation, and proliferation of cells.
c. It destroys cells that have mutated and are traveling in the blood.
d. It decreases nutrients available to cancerous cells.

8. How does vitamin D improve the symptoms of autoimmune diseases?
a. It decreases T helper 2 (Th2) cell production.
b. It decreases production of Th1 helper cells and inflammatory cytokines.
c. It prevents autoantibodies from attacking “self” cells.
d. It blocks the action of the proinflammatory cytokines.

9. Supplementation of pregnant women with vitamin D up to 4,000 IU per day is safe and effective in raising their serum vitamin D levels.
a. True
b. False

10. Robert lives in Chicago and has a vitamin D level of 55 nmol/L in March. How much daily supplemental vitamin D would you recommend to raise his serum vitamin D level to 80 nmol/L?
a. 500 IU
b. 1,000 IU
c. 1,500 IU
d. 2,000 IU

References

1. Morris HA. Vitamin D: A hormone for all seasons—How much is enough? Clin Biochem Rev. 2005;26(1):21-32.

2. Heaney RP. Long-latency deficiency disease: insights from calcium and vitamin D. Am J Clin Nutr. 2003;78(5):912-919.

3. DeLuca HF. Overview of general physiologic features and functions of vitamin D. Am J Clin Nutr. 2004;80(6 Suppl):1689S-1696S.

4. Mullin GE, Dobs A. Vitamin D and its role in cancer and immunity: A prescription for sunlight. Nutr Clin Pract. 2007;22(3):305-322.

5. Holick MF. Vitamin D: Importance in the prevention of cancers, type 1 diabetes, heart disease, and osteoporosis. Am J Clin Nutr. 2004;79(3):362-371.

6. Holick MF. Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. Am J Clin Nutr. 2004;80(6 Suppl):1678S-1688S.

7. Hollis BW, Wagner CL. Nutritional vitamin D status during pregnancy: reasons for concern. CMAJ. 2006;174(9):1287-1290.

8. Bischoff-Ferrari HA, Giovannucci E, Willett WC, et al. Estimation of optimal serum concentrations of 25-hydroxyvitamin D for multiple health outcomes. Am J Clin Nutr. 2006;84(1):18-28.

9. Holick MF. Vitamin D: The under appreciated D-lightful hormone that is important for skeletal and cellular health. Curr Opin Endocrinol Diabetes. 2002;9:87-98.

10. Harris SS, Dawson–Hughes B. Seasonal changes in plasma 25-hydroxyvitamin D concentrations of young American black and white women.Am J Clin Nutr. 1998;67:1232-1236.

11. Saadi HF, Dawodu A, Afandi BO, et al. Efficacy of daily and monthly high-dose calciferol in vitamin D-deficient nulliparous and lactating women.Am J Clin Nutr. 2007;85(6):1565-1571.

12. Norman AW. Sunlight, season, skin pigmentation, vitamin D, and 25-hydroxyvitamin D: Integral components of the vitamin D endocrine system. Am J Clin Nutr. 1998;67(6):1108-1110.

13. Hathcock JN, Shao A, Vieth R, et al. Risk assessment for vitamin D. Am J Clin Nutr. 2007;85(1):6-18.

14. Lappe JM, Travers-Gustafson D, Davies KM, et al. Vitamin D and calcium supplementation reduces cancer risk: results of a randomized trial. Am J Clin Nutr. 2007;85(6):1586-1591.

15. Tuohimaa P, Tenkanen L, Ahonen M, et al. Both high and low levels of blood vitamin D are associated with a higher prostate cancer risk: A longitudinal, nested case-control study in the Nordic countries. Int J Cancer. 2004;108(1):104-108

16. Li H, Stampfer MJ, Hollis BW, et al. A prospective study of plasma vitamin D metabolites, vitamin D receptor polymorphisms, and prostate cancer. PLoS Med. 2007;4(3):e103.

17. Hyppönen E, Läärä E, Reunanen A, et al. Intake of vitamin D and risk of type 1 diabetes: a birth-cohort study. Lancet. 2001;358(9292):1500-1503.

18. Greer RM, Rogers MA, Bowling FG, et al. Australian children and adolescents with type 1 diabetes have low vitamin D levels. Med J Aust. 2007;187(1):59-60.

19. Munger KL, Levin LI, Hollis JB, et al. Serum 25-hydroxyvitamin D levels and risk of multiple sclerosis. JAMA. 2006;296(23):2832-2838.

20. Hollis BW, Wagner CL. Vitamin D requirements during lactation: High-dose maternal supplementation as therapy to prevent hypovitaminosis D for both the mother and the nursing infant. Am J Clin Nutr. 2004;80(6 Suppl):1752S-1758S.

21. Specker B. Vitamin D requirements during pregnancy. Am J Clin Nutr. 2004;80(6 Suppl):1740S-1747S.

22. van der Meer IM, Karamali NS, Boeke AJ, et al. High prevalence of vitamin D deficency in pregnant non-Western women in The Hague, Netherlands. Am J Clin Nutr. 2006;84(2):350-353.

23. Mannion CA, Gray-Donald K, Koski KG. Association of low intake of milk and vitamin D during pregnancy with decreased birth weight. CMAJ. 2006;174(9):1273-1277.

24. Institute of Medicine. Dietary reference intakes for calcium, phosphorus, magnesium, vitamin D, and fluoride. Washington, D.C.: National Academies Press; 1997.

25. Heaney RP. Functional indices of vitamin D status and ramifications of vitamin D deficiency. Am J Clin Nutr. 2004;80(6 Suppl):1706S-1709S.

26. Heaney RP. The vitamin D requirement in health and disease. J Steroid Biochem Mol Biol. 2005;97(1-2):13-9.

27. Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Am J Clin Nutr. 1999;69(5):824-856.