Summary of the role of vitamin D in human metabolic processes
Overview of the role of vitamin D
Populations at risk for vitamin D deficiency
Evidence used for estimating recommended vitamin D intake
Considerations in viewing recommended intakes for vitamin D
Summary of the RNIs for vitamin D by age group
Vitamin D toxicity
Future research
References
Vitamin D is required to maintain normal blood levels of calcium and phosphate, that are in turn needed for the normal mineralisation of bone, muscle contraction, nerve conduction, and general cellular function in all cells of the body. Vitamin D achieves this after its conversion to the active form 1,25-dihydroxyvitamin D [1,25-(OH)2D], or calcitriol. This active form regulates the transcription of a number of vitamin D-dependent genes coding for calcium-transporting proteins and bone matrix proteins.
Vitamin D also modulates the transcription of cell cycle proteins, that decrease cell proliferation and increase cell differentiation of a number of specialised cells of the body (e.g., osteoclastic precursors, enterocytes, keratinocytes, etc.). This property may explain the actions of vitamin D in bone resorption, intestinal calcium transport, and skin. Vitamin D also possesses immuno-modulatory properties that may alter responses to infections in vivo. The cell differentiating and immuno-modulatory properties underlie the reason why vitamin D derivatives are now used successfully in the treatment of psoriasis and other skin disorders.
Clinical assays measure 1,25-(OH)2D2 and 1,25-(OH)2D3, collectively called 1,25-(OH)2D. Similarly, calcidiol is measured as 25-OH-D but it is a mixture of 25-OH-D2 and 25-OH-D3. For the purposes of this document, 1,25-(OH)2D and 25-OH-D will be used to refer to calcitriol and calcidiol, respectively.
Vitamin D, a seco-steroid, can either be made in the skin from a cholesterol-like precursor (7-dehydrocholesterol) by exposure to sunlight or can be provided pre-formed in the diet (1). The version made in the skin is referred to as vitamin D3 whereas the dietary form can be vitamin D3 or a closely related molecule of plant origin known as vitamin D2 . Because vitamin D can be made in the skin, it should not strictly be called a vitamin, and some nutritional texts refer to the substance as a prohormone and to the two forms as cholecalciferol (D3) or ergocalciferol (D2).
From a nutritional perspective, the two forms are metabolised similarly in humans, are equal in potency, and can be considered equivalent. It is now firmly established that vitamin D3 is metabolised first in the liver to 25-hydroxyvitamin-D (25-OH-D or calcidiol) (2) and subsequently in the kidneys to 1,25-(OH)2D (3) to produce a biologically active hormone. The 1,25-(OH)2D, like all vitamin D metabolites, is present in the blood complexed to vitamin D binding protein, a specific a-globulin. The 1,25-(OH)2D is believed to act on target cells similarly to the way a steroid hormone would act. Free hormone crosses the plasma membrane and interacts with a specific nuclear receptor known as the vitamin D receptor, a DNA-binding, zinc-finger protein with a molecular weight of 55,000 (4). This ligand-receptor complex binds to a specific vitamin D-responsive element and, with associated transcription factors (e.g., retinoid X receptor), enhances transcription of mRNAs which code for calcium-transporting proteins, bone matrix proteins, or cell cycle-regulating proteins (5). As a result of these processes ,1,25-(OH)2D stimulates intestinal absorption of calcium and phosphate and mobilises calcium and phosphate by stimulating bone resorption (6). These functions serve the common purpose of restoring blood levels of calcium and phosphate to normal when concentrations of the two ions are low.
Lately, interest has focused on other cellular actions of 1,25-(OH)2D. With the discovery of 1,25-(OH)2D receptors in many classical non-target tissues such as brain, various bone marrow-derived cells, skin, thymus, etc. (7), the view has been expressed that 1,25-(OH)2D induces fusion and differentiation of macrophages (8, 9). This effect has been widely interpreted to mean that the natural role of 1,25-(OH)2D is to induce osteoclastogenesis from colony forming units-granulatory monocytes in the bone marrow. The 1,25-(OH)2D also suppresses interleukin 2 production in activated T lymphocytes (10, 11), an effect which suggests the hormone might play a role in immuno-modulation in vivo. Other tissues (e.g., skin) are directly affected by exogenous administration of vitamin D, though the physiologic significance of these effects is poorly understood. The pharmacologic effects of 1,25-(OH)2D are profound and have resulted in the development of vitamin D analogues, that are approved for use in hyper-proliferative conditions such as psoriasis (12).
In calcium homeostasis 1,25-(OH)2D works in conjunction with parathyroid hormone (PTH) to produce its beneficial effects on the plasma levels of ionised calcium and phosphate (5, 13). The physiologic loop (Figure 10) starts with calcium sensing by the calcium receptor of the parathyroid gland (14). When the level of ionised calcium in plasma falls, PTH is secreted by the parathyroid gland and stimulates the tightly regulated renal enzyme 25-OH-D-1-aa-hydroxylase to make more 1,25-(OH)2D from the large circulating pool of 25-OH-D. The resulting increase in 1,25-(OH)2D (with the rise in PTH) causes an increase in calcium transport within the intestine, bone, and kidney. All these events raise plasma calcium levels back to normal, that in turn is sensed by the calcium receptor of the parathyroid gland. The further secretion of PTH is turned off not only by the feedback action of calcium, but also by a short feedback loop involving 1,25-(OH)2D directly suppressing PTH synthesis in the parathyroid gland (not shown in figure).
Figure 10. Calcium homeostasis
Source: Adapted from Jones et al. (13).
Although this model oversimplifies the events involved in calcium homeostasis it is easy to see from it that sufficient 25-OH-D must be available to provide adequate 1,25-(OH)2D synthesis and hence an adequate level of plasma calcium and that vitamin D deficiency will result in inadequate 25-OH-D and 1,25-(OH)2D synthesis, inadequate calcium homeostasis, and a constantly elevated PTH level (termed: secondary hyperparathyroidism).
It becomes evident from this method of presentation of the role of vitamin D that the nutritionist can focus on the plasma levels of 25-OH-D and PTH to gain an insight into vitamin D status. Not shown but also important is the endpoint of the physiologic action of vitamin D, namely adequate plasma calcium and phosphate ions, that provide the raw materials for bone mineralisation.
Infants
Infants constitute a population at risk for vitamin D deficiency because of relatively large vitamin D needs brought about by their high rate of skeletal growth. At birth, infants have acquired in utero the vitamin D stores that must carry them through the first months of life. A recent survey of French neonates revealed that 64 percent had 25-OH-D values below 30 nmol/l, the lower limit of the normal range (15). Breast-fed infants are particularly at risk because of the low concentrations of vitamin D in human milk (16). This problem is further compounded in some infants fed human milk by a restriction in exposure to ultraviolet (UV) light for seasonal, latitudinal, cultural, or social reasons. Infants born in the autumn months at extremes of latitude are particularly at risk because they spend the first 6 months of their life indoors and therefore have little opportunity to synthesise vitamin D in their skin during this period. Consequently, although vitamin D deficiency is rare in developed countries, sporadic cases of rickets are still being reported in many northern cities but almost always in infants fed human milk (17-20).
Infant formulas are supplemented with vitamin D at levels ranging from 40 international units (IUs) or 1 mg /418.4 kJ to 100 IU or 2.5 mg /418.4 kJ, that provide approximately between 6 mg and 15 mg of vitamin D, respectively. These amounts of dietary vitamin D are sufficient to prevent rickets.
Adolescents
Another period of rapid growth of the skeleton occurs at puberty and increases the need not for the vitamin D itself, but for the active form 1,25-(OH)2D. This need results from the increased conversion of 25-OH-D to 1,25-(OH)2D in adolescents (21). Furthermore, unlike infants, adolescents are usually outdoors and therefore usually are exposed to UV light sufficient for synthesising vitamin D for their needs. Excess production of vitamin D in the summer and early fall months is stored mainly in the adipose tissue (22) and is available to sustain high growth rates in the winter months that follow. Insufficient vitamin D stores during these periods of increased growth can lead to vitamin D insufficiency (23).
Elderly
Over the past 20 years, clinical research studies of the basic biochemical machinery handling vitamin D have suggested an age-related decline in many key steps of vitamin D action (24) including rate of skin synthesis, rate of hydroxylation leading to activation to the hormonal form, and response of target tissues (e.g., bone) as well as reduced skin exposure (25). Not surprisingly a number of independent studies from around the world have shown that there appears to be vitamin D deficiency in a subset of the elderly population, as characterised by low blood levels of 25-OH-D coupled with elevations of plasma PTH and alkaline phosphatase (26). There is evidence that this vitamin D deficiency contributes to declining bone mass and increases the incidence of hip fractures (27). Although some of these studies may exaggerate the extent of the problem by focusing on institutionalised individuals or in-patients with decreased sun exposures, in general they have forced health professionals to re-address the intakes of this segment of society and look at potential solutions to correct the problem. Several groups have found that modest increases in vitamin D intakes (between 10 and 20 µg/day) reduce the rate of bone loss and the fracture rate (25-29).
These findings have led agencies and researchers to suggest an increase in recommended vitamin D intakes for the elderly from the suggested 2.5-5 µg /day to a value that is able to maintain normal 25-OH-D levels in the elderly, such as 10-15 µg/day. This vitamin D intake results in lower rates of bone loss and is suggested for the middle-aged (50-70 years) and old-aged (>70 years) populations (33). The increased requirements are justified mainly on the grounds of the reduction in skin synthesis of vitamin D, a linear reduction occurring in both men and women, that begins with the thinning of the skin at age 20 years (24).
Pregnancy and lactation
Elucidation of the changes in calciotropic hormones occurring during pregnancy and lactation has revealed a role for vitamin D in the former but probably not the latter. Even in pregnancy, the changes in vitamin D metabolism which occur, namely an increase in the maternal plasma levels of 1,25-(OH)2D (34) due to a putative placental synthesis of the hormone (35), do not seem to impinge greatly on the maternal vitamin D requirements. The concern that modest vitamin D supplementation might be deleterious to the foetus is not justified. Furthermore, because transfer of vitamin D from mother to foetus is important for establishing the newborn’s growth rate, the goal of ensuring adequate vitamin D status with conventional prenatal vitamin D supplements probably should not be discouraged.
In lactating women there appears to be no direct role for vitamin D because increased calcium needs are regulated by PTH-related peptide (36, 37), and recent studies have failed to show any change in vitamin D metabolites during lactation (38, 39). As stated above, the vitamin D content of human milk is low (16). Consequently, there is no great drain on maternal vitamin D reserves either to regulate calcium homeostasis or to supply the need of human milk. Because human milk is a poor source of vitamin D, rare cases of nutritional rickets are still found, but these are almost always in breast-fed babies deprived of sunlight exposure (17-20). Furthermore, there is little evidence that increasing calcium or vitamin D supplements to lactating mothers results in an increased transfer of calcium or vitamin D in milk (38). Thus, the current thinking, based on a clearer understanding of the role of vitamin D in lactation, is that there is little purpose in recommending additional vitamin D for lactating women. The goal for mothers who breast-feed their infants seems to be merely to ensure good nutrition and sunshine exposure in order to ensure normal vitamin D status during the perinatal period.
Table 19
Randomised, controlled trials with dietary vitamin D supplements
|
Reference |
Study Group |
n b |
Age (years) Mean SD |
Regimen |
Duration (years) |
Resultsa |
|
|
Dawson-Hughes et al., 1991 (28) |
Healthy, post-menopausal women living independently |
249 |
62 |
0.5 |
10 µg vitamin D + 400 mg calcium |
1.0 |
Reduced late wintertime bone loss from vertebrae; net spine
BMD; no change in whole-body BMD.. |
|
Chapuy et al., 1992 (29) |
Healthy, elderly women living in nursing homes or in
apartments for the elderly |
3270 |
84 |
6 |
20 µg vitamin D + 1200 mg calcium |
1.5 |
Hip fractures 43% ; non-vertebral fractures 32%; in subset
(n=56), BMD of proximal femur 2.7% in vitamin D group and 4.6% in placebo
group |
|
Chapuy et al., 1994 (30) c |
|
|
|
|
|
3.0 |
Hip fractures 29%; non-vertebral fractures 24%. |
|
Dawson-Hughes et al., 1995 (31) |
Healthy post-menopausal women living independently |
261 |
64 |
5 |
2.5 µg or 17.5 µg vitamin D + 500 mg
calcium |
2.0 |
Loss of BMD from femoral neck lower in 17.5µg group
(-1.06%) than in 2.5 µg group (-2.54%); no difference in BMD at
spine. |
|
Lips et al., 1996 (32) |
Healthy elderly living independently, in nursing homes, or in
apartments for the elderly |
2578 (1916-Fem.) (662-Male) |
80 |
6 |
10 µg vitamin D |
|
No difference in fracture incidence; in subset (n=248)
of women from nursing homes, BMD 2.3% after 2 years. |
Lack of accuracy in estimating dietary intake and skin synthesis
The unique problem of estimating total intake of a substance that can be provided in the diet or made in the skin by exposure to sunlight makes it difficult to estimate adequate total intakes of vitamin D for the general population. Accurate food composition data are not available for vitamin D, accentuating the difficulty for estimating dietary intakes. Whereas this has led two recent US national surveys to avoid attempting this task, the second National Health and Nutrition Examination Survey (NHANES II) estimated vitamin D intakes to be 2.9 µg/day and 2.3 µg/day for younger and older women, respectively. A recent study of elderly women by Kinyamu et al. (40) concurred with this assessment, finding an intake of 3.53 mg/day.
Skin synthesis is equally difficult to estimate, being affected by such imponderables as age, season, latitude, time of day, skin exposure, sun screen use, etc. In vitamin D - replete individuals, estimates of skin synthesis are put at around 10 µg /day (24, 41), with total intakes estimated at 15 µg/day (24).
Use of plasma 25-OH-D as a measure of vitamin D status
Numerous recent studies have used plasma 25-OH-D as a measure of vitamin D status, and there is a strong presumptive relationship of this variable with bone status. Thus, it is not surprising that several nutritional committees (e.g., the Food and Nutrition Board of the US National Academy of Sciences’ Institute of Medicine in conjunction with Health Canada) have chosen to use a biochemical basis for estimating required intakes and used these estimates to derive recommended intakes (33). The method used involved the estimation of the mean group dietary intake of vitamin D required to maintain the plasma 25-OH-D levels above 27 nmol/l, that is necessary to ensure normal bone health. Previously, many studies had established 27 nmol/l as the lower limit of the normal range (e.g., NHANES III [41]). This dietary intake of vitamin D for each population group was rounded to the nearest 50 IU (1.25 µg) and then doubled to cover the needs of all individuals within that group irrespective of sunlight exposure. They termed this amount adequate intake (AI) and used it in place of recommended dietary allowance (RDA), that had been used by US agencies since 1941. We are recommending the use of those figures here as recommended nutrient intakes (RNIs) because it is an entirely logical approach to estimating the vitamin D needs for the whole population.
Table 20
Preliminary unweighted results from NHANES III dataa
|
Percentile |
25-OH-Db ng/ml |
|
1st 5th |
7.6 |
|
10th |
10.9 |
|
|
13.2 |
|
50th |
24.4 |
|
90th |
40.1 |
|
95th |
45.9 |
|
99th |
59.0 |
Because study after study had been recommending increases in vitamin D intakes for the elderly, it might have been expected that the proposed increases in suggested intakes from 5 µg/day (RDAs in the United States [43], RNIs in Canada [44]) to 10 µg/day or 15 µg/day (AI) would be welcomed. However, a recent editorial in a prominent medical journal attacked the recommendations as being too conservative (45). This came on the heels of an article in the same journal (46) reporting the level of hypovitaminosis D to be as high as 57 percent in a population of ageing (mean 62 years) medical in-patients in the Boston area.
Of course, such in-patients are by definition sick and should not be used to calculate normal intakes. Indeed, the new NHANES III study (42) of 18 323 normal individuals from all regions of the United States suggests that approximately 5 percent had values of 25-OH-D below 27 nmol/l (47) (Table 20). Although the data are skewed by sampling biases that favour sample collection in the southern states in winter months and northern states in the summer months, even subsets of data collected in northern states in September give the incidence of low 25-OH-D in the elderly in the 6-18 percent range (47) as compared with 57 percent in the institutionalized in-patient population (43). Ideally, such measurements of the normal population should be made at the end of the winter months and before UV irradiation has reached a strength sufficient to allow skin synthesis of vitamin D. Thus, the NHANES III study may still underestimate the incidence of hypovitaminosis D in a northern elderly population in winter. Nevertheless, in lieu of additional studies of selected human populations, it would seem that the recommendations of the Food and Nutrition Board are reasonable guidelines for vitamin D intakes, at least for the near future. This considered approach allows for a period of time to monitor the potential shortfalls of the new recommendations as well as to assess whether the suggested guidelines can be attained, a point that was repeatedly stated about the RDAs.
In recommending intakes for vitamin D, it must be recognised that in most locations in the world in a broad band around the equator (between latitudes 42oN and 42oS), the most physiologically relevant and efficient way of acquiring vitamin D is to synthesise it endogenously in the skin from 7-dehydrocholesterol by sun (UV) light exposure. In most situations, approximately 30 minutes of skin exposure (without sunscreen) of the arms and face to sunlight can provide all the daily vitamin D needs of the body (24). However, skin synthesis of vitamin D is negatively influenced by factors which may reduce the ability of the skin to provide the total needs of the individual (24):
Because not all of these problems can be solved in all geographic locations, particularly during winter at latitudes higher than 42o where synthesis is virtually zero, it is recommended that individuals not synthesising vitamin D should correct their vitamin D status by consuming the amounts of vitamin D appropriate for their age group (Table 21).
Table 21
RNIs for vitamin D according to age groups
|
Age group |
RNI µg/day |
|
|
Infants |
|
|
|
|
0-6 months |
5 |
|
|
7-12 months |
5 |
|
|
1-3 years |
5 |
|
|
4-6 years |
5 |
|
|
7-9 years |
5 |
|
Adolescents, 10-18 years |
5 |
|
|
Adults |
|
|
|
|
19-50 years |
5 |
|
|
Older adults, 51-65 years |
10 |
|
|
Elderly adults, 65+ years |
15 |
|
Pregnant women |
5 |
|
|
Lactating women |
5 |
|
The adverse effects of high vitamin D intakes - hypercalciuria and hypercalcemia - do not occur at these new recommended intake levels. In fact, it is worth noting that the recommended intakes for all age groups are still well below the lowest observed adverse effect level of 50 µg/day and have not yet even reached the no observed adverse effect level of 20 µg/day (33, 48). Outbreaks of idiopathic infantile hypercalcemia in the United Kingdom in the post-World War II era led to the withdrawal of vitamin D fortification from all foods in that country because of concerns that they were due to hypervitaminosis D. There are some suggestions in the literature that these outbreaks of idiopathic infantile hypercalcemia may have been multifactorial with genetic and dietary components and were not just due to technical problems with over-fortification as was assumed (49,50). In retrospect, the termination of the vitamin D fortification may have been counter productive because it exposed segments of the UK community to vitamin deficiency and may have discouraged other nations from starting vitamin D fortification programmes (50). This is all the more cause for concern because hypovitaminosis D is still a problem worldwide, particularly in developing countries at high latitudes and in countries where skin exposure to sunlight is discouraged (51).
Further research is needed to determine:
1. Feldman, D., Glorieux, F.H. & Pike, J.W. 1997. Vitamin D, Academic Press.
2. Blunt, J.W., DeLuca, H.F. & Schnoes, H.K. 1968. 25-hydroxycholecalciferol. A biologically active metabolite of vitamin D3. Biochem., 7: 3317-3322.
3. Fraser, D.R. & Kodicek, E. 1970. Unique biosynthesis by kidney of a biologically active vitamin D metabolite. Nature, 228: 764-766.
4. Haussler, M.R. 1986. Vitamin D receptors: nature and function. Ann. Revs. Nutr., 6: 527-562.
5. Jones, G., Strugnell, S. & DeLuca, H.F. 1998. Current understanding of the molecular actions of vitamin D. Physiol. Revs., 78: 1193-1231.
6. DeLuca, H.F.1988. The vitamin D story: a collaborative effort of basic science and clinical medicine. FASEB J., 2 : 224-236.
7. Pike, J.W. 1991. Vitamin D3 receptors: Structure and function in transcription. Ann. Revs. Nutr., 11: 189-216.
8. Abe, E., Miyaura, C., Tanaka, H., Shiina, Y., Kuribayashi, T., Suda, S., Nishii, Y., DeLuca, H.F. & Suda, T. 1983. 1,25-Dihydroxyvitamin D3 promotes fusion of mouse alveolar macrophages both by a direct mechanism and by a spleen cell-mediated indirect mechanism. Proc. Natl. Acad. Sci. U S A, 80: 5583-5587.
9. Bar-Shavit, Z., Teitelbaum, S.L., Reitsma, P., Hall, A., Pegg, L.E., Trial, J. & Kahn, A.J. 1983. Induction of monocytic differentiation and bone resorption by 1á,25- dihydroxyvitamin D3. Proc. Natl. Acad. Sci. U S A, 80: 5907-5911.
10. Bhalla, A.K., Amento, E.P., Clemens, T., Holick, M.F. & Krane, S.M. 1983. Specific high affinity receptors for 1,25-dihydroxvitamin D3 in Human peripheral blood mononuclear cells: presence in monocytes and induction in T lymphocytes following activation J. Clin. Endocrinol. Metab., 57: 1308-1310.
11. Tsoukas, C.D. Provvedini, D.M. & Manolagas, S.C. 1984. 1,25-Dihydroxyvitamin D3: A novel immunoregulatory hormone. Science, 224: 1438-1440.
12. Kragballe, K. 1992. Vitamin D analogs in the treatment of psoriasis. J. Cell. Biochem., 49: 46-52.
13. Jones, G. & DeLuca, H.F. 1988. HPLC of vitamin D and its metabolites. In: "High Performance Liquid Chromatography and its Application to Endocrinology". Eds. Makin, H.L.J. and Newton, R. Monographs on Endocrinology, Vol.30. p.95-139. Berlin. Springer-Verlag,
14. Brown, E.M. Pollak, M. & Hebert, S.C. 1998. The extracellular calcium-sensing receptor: its role in health and disease. Annu. Rev. Med., 49: 15-29.
15. Zeghund, F., Vervel, C., Guillozo, H., Walrant-Debray, O., Boutignon, H. & Garabedian, M. 1997. Subclinical vitamin D deficiency in neonates: definition and response to vitamin D supplements. Am. J. Clin. Nutr., 65:771-778.
16. Specker, B.L., Tsang, R.C. & Hollis, B.W. 1985. Effect of race and diet on Human milk vitamin D and 25-hydroxyvitamin D Am. J. Dis. Child., 139: 1134-1137.
17. Pettifor, J.M. & Daniels, E.D. 1997. Vitamin D deficiency and nutritional rickets in children. In: Vitamin D, Feldman D, Glorieux FH, Pike JW. P. 663-678. Academic Press.
18. Binet, A. & Kooh, S.W. 1996. Persistence of vitamin D deficiency rickets in Toronto in the 1990s. Can. J. Public Health, 87: 227-230.
19. Brunvand, L. & Nordshus, T. 1996. Nutritional rickets-an old disease with new relevance. Nord. Med., 111: 219-221.
20. Gessner, B.D., deSchweinitz E., Petersen K.M. & Lewandowski C. 1997. Nutritional rickets among breast-fed black and Alaska Native children. Alaska Med., 39: 72-74.
21. Aksnes, L. & Aarskog, D. 1982. Plasma concentrations of vitamin D metabolites at puberty: Effect of sexual maturationand implications for growth. J. Clin. Endocrinol. Metab., 55: 94-101.
22. Mawer, E.B., Backhouse, J., Holman, C.A., Lumb, G.A. & Stanbury, D.W. 1972. The distribution and storage of vitamin D and its metabolites in Human tissues. Clin. Sci., 43: 413-431.
23. Gultekin, A., Ozalp, I., Hasanoglu, A. & Unal, A. 1987. Serum 25-hydroxycholecalciferol levels in children and adolescents. Turk. J. Pediatr., 29: 155-162.
24. Holick, M.F. 1994. McCollum award lecture, 1994: Vitamin D-new horizons for the 21st century. Am. J. Clin. Nutr., 60: 619-630.
25. Shearer, M.J. 1997.The roles of vitamins D and K in bone health and osteoporosis prevention. Proc. Nutr. Soc., 56:915-937.
26. Chapuy, M-C. & Meunier, P.J. 1997. Vitamin D insufficiency in adults and the elderly. In: Vitamin D, Feldman D, Glorieux FH, Pike JW. P. 679-693. Academic Press.
27. Dawson-Hughes, B., Harris, S.S., Krall, E.A. & Dallal, G.E. 1997. Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older. N. Engl. J. Med., 337: 670-676.
28. Dawson-Hughes B., Dallal G.E., Krall E.A., Harris S., Sokoll, L.J. & Falconer G. 1991. Effect of vitamin D supplementation on wintertime and overall bone loss in healthy postmenopausal women. Ann. Intern. Med., 115: 505-512.
29. Chapuy, M-C, Arlot, M.E. & Duboeuf, F. 1992.Vitamin D3 and calcium prevent hip fractures in elderly women. N. Engl. J. Med., 327: 1637-1642.
30. Chapuy, M-C., Arlot, M.E., Delmans, P.D. & Meunier, P.J. 1994. Effect of calcium and cholecalciferol treatment for three years on hip fractures in elderly women. BMJ, 308: 1081-1082.
31. Dawson-Hughes, Harris, S.S., Krall, E.A., Dallal, G.E., Falconer, G. & Green, C.L. 1995. Rates of bone loss in postmenopausal women randomly assigned to one of two dosages of vitamin D. Am. J. Clin. Nutr., 61: 1140-1145.
32. Lips, P., Graafmans, W.C., Ooms, M.E., Bezemer, P.D. & Bouter, L.M. 1996. Vitamin D supplementation and fracture incidence in elderly persons: a randomised, placebo-controlled clinical trial. Ann. Internal Med., 124: 400-406.
33. National Academy of Sciences. 1997. Report on Dietary Reference Intakes for Calcium, Phosphorus, Magnesium and Vitamin D, Food & Nutrition Board, Institute of Medicine, US National Academy of Sciences. P. 7.1-7.30. . National Academy Press.
34. Bouillon, R., Van Assche, F.A., Van Baelen, H., Heyns, W. & De Moor, P. 1981. Influence of the vitamin D-binding protein on the serum concentration of 1,25-dihydroxyvitamin D3. Significance of the free 1,25-dihydroxyvitamin D3 concentration. J. Clin. Invest., 67: 589-596.
35. Delvin, E.E., Arabian, A., Glorieux, F.H. & Mamer, O.A. 1985. In vitro metabolism of 25-hydroxycholecalciferol by isolated cells from Human decidua. J. Clin. Endocrinol. Metab., 60: 880-885.
36. Sowers, M.F., Hollis, B.W., Shapiro, B., Randolph, J., Janney, C.A., Zhang, D., Schork, A., Crutchfield, M., Stanczyk, F. & Russell-Aulet, M. 1996. Elevated parathyroid hormone-related peptide associated with lactation and bone density loss. JAMA, 276(7): 549-54
37. Prentice, A. 1998. Calcium requirements of breast-feeding mothers. Nutr. Revs., 56: 124-127.
38. Sowers, M., Zhang, D., Hollis, B.W., Shapiro, B., Janney, C.A., Crutchfield, M., Schork, M.A., Stanczyk, F. & Randolph, J. 1998. Role of calciotrophic hormones in calcium mobilisation of lactation. Am. J. Clin. Nutr., 67 (2): 284-91.
39. Kovacs, C.S. & Kronenberg, H.M. 1997. Maternal-Foetal calcium and bone metabolism during pregnancy, puerperium, and lactation. Endocr. Rev., 18: 832-72.
40. Kinyamu, H.K., Gallagher, J.C., Rafferty, K.A. & Balhorn, K.E. 1998. Dietary calcium and vitamin D intake in elderly women: effect on serum parathyroid hormone and vitamin D metabolites. Am. J. Clin. Nutr., 67: 342-348.
41. Fraser, D.R. 1983. The physiological economy of vitamin D. Lancet, I: 969-972.
42. Centers for Disease Control and Prevention. 1998. National Health and Nutrition Examination Survey III, 1988-1994. CD-ROM Series 11, No. 2A. Hyatsville, MD.
43. National Research Council. 1989. Recommended Dietary Allowances 10th Edition. Report of the Subcommittee on the Tenth Edition of the RDA, Food and Nutrition Board and the Commission on Life Sciences . Washington DC: National Academy Press .
44. Health and Welfare Canada. 1990. Nutrition Recommendations. P. 90-93. Ottawa, ON Canada. Published by Health and Welfare Canada.
45. Utiger, R.D. 1998. The need for more vitamin D. N. Engl. J. Med., 338: 828-829.
46. Thomas, M.K., Lloyd-Jones, D.M., Thadhani, R.I., Shaw, A.C., Deraska, D.J., Kitch, B.T., Vamvakas, E.C. Dick, I.M., Prince, R.L. & Finkelstein, J.S. 1998. Hypovitaminosis D in medical inpatients. N. Engl. J. Med., 338: 777-783.
47. Looker, A.C. & Gunter, E.W. 1998. Hypovitaminosis D in medical inpatients. A letter to N. Engl. J. Med., 339: 344-345.
48. Lachance, P.A. 1998. International Perspective: Basis, need and application of recommended dietary allowances. Nutr. Revs., 56: S2-S5.
49. Jones, K.L. 1990. Williams syndrome: an historical perspective of its evolution, natural history, and etiology. Am. J. Med. Genet. Suppl., 6: 89-96.
50. Fraser, D. 1967. The relation between infantile hypercalcemia and vitamin D - public health implications in North America. Pediatr., 40: 1050-1061.
51. Mawer, E.B. & Davies, M. 1997.Vitamin D deficiency, rickets and osteomalacia, a returning problem worldwide. In: Vitamin D. Chemistry, Biology and Clinical Applications of the Steroid Hormone. p. 899-906. Norman AW, Bouillon R, Thomasset M eds. University of California.
