This Article Abstract Full Text (PDF) Submit a related Letter to the Editor View responses Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Binkley, N. Articles by Drezner, M. K. Search for Related Content PubMed PubMed Citation Articles by Binkley, N. Articles by Drezner, M. K. Related Collections Calcium and Bone Metabolism

Low Vitamin D Status despite Abundant Sun Exposure

University of Wisconsin Osteoporosis Clinical Research Program (N.B., D.K., T.K., M.K.D.), Madison, Wisconsin 53705; Human Nutrition, Food and Animal Sciences (R.N., Y.G.D.), University of Hawaii at Manoa, Honolulu, Hawaii 96822; Laboratory Medicine (G.L.), University of Wisconsin, Madison, Wisconsin 53792; and Medical University of South Carolina (B.W.H.), Charleston, South Carolina 29425

Address all correspondence and requests for reprints to: Neil Binkley, M.D., University of Wisconsin Osteoporosis Research Program, Suite 100, 2870 University Avenue, Madison, Wisconsin 53705. E-mail: nbinkley{at}wisc.edu.

	Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Context: Lack of sun exposure is widely accepted as the primary cause of epidemic low vitamin D status worldwide. However, some individuals with seemingly adequate UV exposure have been reported to have low serum 25-hydroxyvitamin D [25(OH)D] concentration, results that might have been confounded by imprecision of the assays used.

Objective: The aim was to document the 25(OH)D status of healthy individuals with habitually high sun exposure.

Setting: This study was conducted in a convenience sample of adults in Honolulu, Hawaii (latitude 21°).

Participants: The study population consisted of 93 adults (30 women and 63 men) with a mean (SEM) age and body mass index of 24.0 yr (0.7) and 23.6 kg/m² (0.4), respectively. Their self-reported sun exposure was 28.9 (1.5) h/wk, yielding a calculated sun exposure index of 11.1 (0.7).

Main Outcome Measures: Serum 25(OH)D concentration was measured using a precise HPLC assay. Low vitamin D status was defined as a circulating 25(OH)D concentration less than 30 ng/ml.

Results: Mean serum 25(OH)D concentration was 31.6 ng/ml. Using a cutpoint of 30 ng/ml, 51% of this population had low vitamin D status. The highest 25(OH)D concentration was 62 ng/ml.

Conclusions: These data suggest that variable responsiveness to UVB radiation is evident among individuals, causing some to have low vitamin D status despite abundant sun exposure. In addition, because the maximal 25(OH)D concentration produced by natural UV exposure appears to be approximately 60 ng/ml, it seems prudent to use this value as an upper limit when prescribing vitamin D supplementation.

	Introduction

Top Abstract Introduction Subjects and Methods Results Discussion References

 
LOW VITAMIN D status ¹ is extremely common (1, 2, 3, 4), and may contribute to the development of osteoporosis and osteomalacia/rickets, as well as increase the risk for falls (5, 6). Moreover, low vitamin D status may play a role in nonmusculoskeletal diseases, including a variety of cancers, multiple sclerosis, infection, hypertension, and diabetes mellitus (7, 8). Although it is widely accepted that vitamin D status is determined by the measurement of the circulating concentration of 25-hydroxyvitamin D [25(OH)D] (9), the cutoff value to define low vitamin D status and a definition for success of vitamin D repletion therapy remain controversial (10, 11). This is partially due to the variability of vitamin D concentration by geographical location and differences in assay methodology (12, 13, 14, 15, 16). Despite this controversy, clinicians often endeavor to correct vitamin D deficiency by prescribing high-dose vitamin D (17). However, the goal for such therapy is unclear and could include achieving a serum 25(OH)D level greater than an accepted cutpoint (e.g. 30 ng/ml) or, alternatively, the upper limit of normal, a value that varies between laboratories (18).

The high prevalence of low vitamin D status is assumed to result from inadequate sun exposure. Because highly sun-exposed individuals likely possess normal vitamin D status from an evolutionary standpoint, the use of such individuals to define normal 25(OH)D status has been proposed (19). This argument is based on the view that contemporary humans are genetically adapted to the environment of our ancestors and that the profound lifestyle changes that have occurred over the past approximately 10,000 yr (importantly including reduced sun exposure) have been much too rapid for the human genome to adjust (20, 21). The current study was designed to assess whether, in fact, people living at a low latitude with high amounts of sun exposure have adequate vitamin D status, as expected, and to identify a target value of 25(OH)D for use in vitamin D therapy.

	Subjects and Methods

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Subjects and study design

Subjects older than 18 yr were recruited approximately equally from the University of Hawaii at Manoa (UH) and from patrons of the A’ala Park Board Shop, Honolulu, Hawaii (latitude 21° north), in late March 2005. The A’ala Park Board Shop is a skateboard shop frequented by young adults. Recruitment was performed by posted notice at the Board Shop and on the UH campus; volunteers were reimbursed for study participation. Volunteers were required to have self-reported sun exposure of 3 or more hours per day on 5 or more days per week for at least the preceding 3 months, and not to be currently taking phenobarbital, phenytoin, or prednisone. A total of 93 subjects (63 male and 30 female) participated.

The University of Wisconsin Health Sciences Institutional Review Board and the Committee on Human Studies at the UH reviewed and approved this research. All subjects provided written informed consent before the conduct of any study procedure.

Data acquisition

Blood was collected for serum chemistry, 25(OH)D, and PTH measurement. Participants were not required to fast for blood collection, which was performed by routine venipuncture. Samples were allowed to clot for 30–45 min at room temperature, centrifuged, and serum was promptly frozen on dry ice. All specimens were shipped and received frozen on dry ice, then stored at –80 C until thawed for analyses. The 25(OH)D analyses were performed in batches of nine to 16 samples (a total of eight HPLC runs were performed over 14 d) from 10–26 d after specimen acquisition. Three internal controls were run with each HPLC batch; the results of each control were consistently within previously established acceptable ranges. The PTH samples were performed 66–71 d after specimen acquisition in a single run. All participants completed a nonvalidated, self-administered questionnaire that included questions about ethnicity, sun exposure, sunscreen use, and dietary vitamin D intake.

To document sun exposure, skin color was measured by reflectance colorimetry (IMS SmartProbe, Millford, CT). The Commission Internationale de l’Eclairage L scale was used, which ranges from 0 (black) to 100 (white) and represents a system created by the International Commission on Illumination to represent accurately human color perception. A measurement was taken on the back of the hand and front of the distal thigh for the darkest measurement. In addition, skin color was measured under the arm and at the self-reported least sun-exposed area, often the breast or buttock, to determine the lightest or natural skin color. The lowest and highest L scale measurements were subtracted to determine the change in skin color or the delta skin color. A previously developed sun exposure index (22) was used to estimate the amount and duration of skin sun exposure. These data were obtained by asking the subjects to depict their usual amount of skin exposed on a diagram with and without sunscreen use. Subsequently, the rule of nines (23) was used to calculate skin sun exposure in which the front and back torso and each leg were counted as 18%, the arms and head as 9%, and the face only as 5%. This number was then multiplied by the reported average sun exposure per week without sunscreen to calculate the sun exposure index for each subject.

Serum analyses

Serum chemistries were measured using a Roche Integra autoanalyzer at General Medical Laboratories (Madison, WI). Serum 25(OH)D was determined by reverse-phase HPLC (24). The intraassay percent coefficient of variation (CV) for this assay ranges from 1.9% at a 25(OH)D concentration of 61.5 ng/ml to 6.3% at a 25(OH)D concentration of 14.3 ng/ml. The interassay percent CV is 3.2% at a 25(OH)D concentration of 59.8 ng/ml and 3.9% at a 25(OH)D concentration of 14.3 ng/ml. In assay proficiency evaluation, correlation with liquid chromatography mass spectroscopy performed at the Mayo Medical Laboratories (Rochester, MN) revealed essentially identical results, with r² values of 0.99 and 0.97 for 25(OH)D₂ and 25(OH)D₃, respectively. In addition, 25(OH)D was also determined by RIA (Diasorin RIA, Stillwater, MN) in the laboratory of B.W.H. who also performed serum vitamin D measurement using HPLC (25) in a subset of 19 individuals. The intraassay and interassay percent CVs for these assays are less than 10%. Scantibodies Clinical Laboratory (Santee, CA) using the Scantibodies Laboratory whole PTH or (CAP; cyclase activating PTH) assay measured specific 1–84 PTH or whole PTH ("CAP PTH") (26). For this assay, the intraassay percent CV is 5% at 30.2 pg/ml, and the interassay percent CV is 7.4% at 31.9 pg/ml.

Statistical analyses

Normal data distribution was documented using the Shapiro-Wilk test. Subsequently, relationships between 25(OH)D and sun index, hours of sun exposure, PTH, etc., were evaluated by linear regression. Differences between the lowest quartile and remainder of the cohort were evaluated by the unpaired t test. 25(OH)D assay comparison (HPLC to RIA) was evaluated by linear regression and Bland-Altman analysis. All analyses were performed using Statview software (Abacus, Cary, NC) or Analyze-it software (Leeds, UK).

	Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Subject demographics

A total of 93 subjects (63 male and 30 female) participated in this study. Overall, their mean (SEM) age was 24.0 yr (0.7), body mass index (BMI) was 23.6 kg/m² (0.4), and supplemental vitamin D intake was 107 IU (18) daily. In addition, their mean (SEM) creatinine, albumin, and calcium concentrations were 0.9 mg/dl (0.02), 4.5 g/dl (0.02), and 9.8 mg/dl (0.04), respectively. The mean (SEM) lightest skin color L scale value was 63.7 (0.5) and the darkest 50.5 (0.5), with a difference of 13.2 (0.4) (data not shown). On average, the 93 subjects reported being outside for 22.4 h/wk (1.6) with no sunscreen, and 28.9 h/wk (1.5) with and without sunscreen (Fig. 1). Of subjects, 40% (37 of 93) reported never using sunscreen. The resultant mean sun exposure index score, indicating hours per week of total body skin exposure with no sunscreen used, was 11.1 ± 0.7 (range 1.0–38.4). Only two subjects reported use of tanning booths; as such, the skin darkening noted previously reflects natural sunlight exposure.

View larger version (19K): [in this window] [in a new window]   FIG. 1. Amount of sun exposure without and with sunscreen. The mean self-reported sun exposure without sunscreen use was 22.4 h (range 2–96), with a mean total of 28.9 ± 1.5 h/wk. Of this cohort, 40% reported no sunscreen use.

Using the HPLC assay for serum 25(OH)D and applying a widely recommended cutpoint of 30 ng/ml (10), 51% (47 of 93) of these subjects had low vitamin D status (Fig. 2). The highest serum 25(OH)D concentration observed was 62 ng/ml. No correlation between serum whole PTH and 25(OH)D concentration was observed (Fig. 3). Moreover, there was no correlation between serum 25(OH)D measured by HPLC and age, lightest or darkest skin color, delta skin color, hours/week of sun exposure without sunscreen, sun index, total hours of sun exposure/week, or BMI (data not shown). Specifically, delta skin color was not correlated with either PTH (P = 0.10; r² = 0.03) or serum 25(OH)D (P = 0.18; r² = 0.02).

View larger version (16K): [in this window] [in a new window]   FIG. 2. Low vitamin D status in highly sun-exposed subjects. When an accepted cutpoint of 30 ng/ml is used to define low vitamin D status, 51% of these subjects (open bars) are low.

View larger version (20K): [in this window] [in a new window]   FIG. 3. PTH and 25(OH)D. Specific 1–84 PTH or whole PTH ("CAP PTH") was not related to serum 25(OH)D status measured by HPLC in this cohort.

Serum 25(OH)D as measured by reverse-phase HPLC and RIA was highly correlated (r² = 0.76; Fig. 4). However, a systematic bias was present with 25(OH)D values determined by RIA being approximately 6.8 ng/ml higher than by HPLC. Thus, if the Diasorin RIA had been used to determine the prevalence of low vitamin D status (using a cutpoint of 30 ng/ml), fewer individuals would have been classified as "low." However, even using the RIA, 25% of this population would be classified as having low vitamin D status. Finally, although these assays were highly correlated, greater scatter at higher 25(OH)D values was observed with the RIA, as has been previously reported (18). This greater scatter at higher values slightly increases the mean bias noted previously; however, even when limiting the analysis to the 47 individuals with a 25(OH)D less than 30 ng/ml by HPLC, a positive bias of 5.2 ng/ml persisted.

View larger version (17K): [in this window] [in a new window]   FIG. 4. Comparison of 25(OH)D as measured by HPLC and RIA. Although the correlation between these assays is good, a positive bias of 6.8 ng/ml is present using the Diasorin RIA in comparison to the HPLC assay used in this study.

Serum cholecalciferol (D₃)

In the subset of 19 subjects in whom circulating D₃ was measured, a logarithmic relationship (r² = 0.67) between serum D₃ and 25(OH)D was observed. It was not until serum D₃ exceeded approximately 15–20 ng/ml that serum 25(OH)D was definitively higher than 30 ng/ml (Fig. 5). Serum D₃ concentration was not correlated with sun index, delta skin color, or BMI (data not shown).

View larger version (19K): [in this window] [in a new window]   FIG. 5. Circulating vitamin D3 in relationship to 25(OH)D concentration. In this population of 19 individuals with a mean sun exposure without sunscreen of 20.2 h/wk and a mean sun exposure index of 10.9, low circulating vitamin D3 values exist.

	Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

Although the presence of "low" 25(OH)D concentration in this population seems counterintuitive, this might be anticipated from an evolutionary standpoint because the high calcium intake of early humans (27) may have allowed maintenance of calcium homeostasis despite low vitamin D status. Moreover, it is certainly plausible that genetic differences exist in the amount of vitamin D necessary to maintain optimal physiological function. Such differences could contribute to the lack of a direct relationship between serum PTH and 25(OH)D on an individual basis that is observed in many studies (28, 29). This absence of a direct relationship between PTH and 25(OH)D emphasizes that PTH measurement cannot be used clinically as a surrogate marker of vitamin D deficiency, as exemplified by enhanced calcium absorption at higher vitamin D levels despite normal PTH status (30). In addition, it is possible that genetic differences in the cytochrome P450 enzymes activating and degrading vitamin D exist. Finally, the data reported here are consistent with prior reports of highly sun-exposed individuals that also demonstrate substantial variability in 25(OH)D status. For example, in 18 Puerto Rican farmers with self-reported sun exposure from 32–70 h/wk, two individuals had a 25(OH)D level less than 30 ng/ml (31). Similarly, low 25(OH)D values were reported in some subjects who used a tanning bed at least once a week for 6 wk (32) and among outdoor workers with a sun index of 11.5 (22). Thus, even substantial sunlight or UV exposure does not ensure maintenance of vitamin D adequacy for all individuals, according to currently accepted standards. This implies that the common clinical recommendation to allow sun exposure to the hands and face for 15 min may not ensure vitamin D sufficiency.

A probable explanation for the "low" 25(OH)D status of some individuals is found in their failure to obtain high circulating D₃ concentrations. Possible explanations for this include inadequate cutaneous production of D₃, enhanced cutaneous destruction of previtamin D₃ or vitamin D₃, down-regulation of cutaneous synthesis by sun-induced melanin production, or abnormalities of transport from the skin to the circulation. In this regard, Holick et al. (33) documented that human skin has the intrinsic ability to limit vitamin D production. Moreover, a reduction in cutaneous concentration of 7-dehydrocholesterol and a concomitant declining capacity of the skin to make vitamin D occur with advancing age (34, 35). However, in our study the population was predominantly young, which should have obviated such reduced capability for vitamin D synthesis. Importantly, lizards with behaviorally high sun exposure have a lower capacity to produce vitamin D than closely related species with habitually less sun exposure (36). Thus, it appears likely that factors exist, which are not yet well understood, that can restrict skin production of vitamin D in response to UV radiation. In any case, it is crucial that we do not wantonly accept the concept that vitamin D deficiency is due exclusively to inadequate UV exposure. Rather, it seems self-evident from this study that low vitamin D status, as it is currently defined, may occur despite "more than adequate" sun exposure.

An alternate explanation for the "low" values in these highly sun-exposed adults and the corresponding high prevalence of low vitamin D status might reflect 25(OH)D assay variability. However, the prevalence of low vitamin D status in this population is substantial whether 25(OH)D is measured using HPLC or RIA. The systematic bias between HPLC and RIA observed in this study emphasizes the difficulty with setting a single cutpoint value, e.g. 30 ng/ml, below which individuals are classified as having low vitamin D status. The widespread availability of 25(OH)D assay calibrators currently being developed by the National Institute of Standards and Technology could be expected to reduce the magnitude of systematic bias noted here.

These results may allow for rational provision of guidance for clinicians prescribing vitamin D treatment, in that the highest 25(OH)D concentration achievable by sun exposure is approximately 60 ng/ml. An apparent physiological ceiling does not support attempts to achieve higher values by pharmacological intervention. It is of interest that the highest 25(OH)D values observed in this study are quite similar to that reported in other highly sun-exposed populations. For example, the individuals in this study with the three highest levels had serum 25(OH)D concentrations of approximately 60 ng/ml. Similarly, among Nebraska outdoor workers, the three highest reported values were between 81 and 84 ng/ml (22). However, it should be noted that these values were obtained using a competitive protein binding assay for 25(OH)D that measures other vitamin D metabolites in addition to 25(OH)D (37) and, therefore, results in a higher value than that obtained with the HPLC system used in this study.

Limitations of this report include the cross-sectional design and self-report of sun exposure. It is possible that some individuals incorrectly reported their sun exposure and/or body surface exposed. Despite this limitation, this population was clearly highly sun exposed as documented by darkening of exposed skin. In addition, because this study included only highly sun exposed individuals, these observations may not be generalizable to those with less sun exposure. Additionally, it may be argued that the use of 30 ng/ml as a cutpoint is inappropriately high. However, even if a more conservative cutpoint of 20 ng/ml, as suggested by some (10, 29), is used, a substantial minority (10%) of these individuals would still be "low." Moreover, as noted previously, it is possible that racial and/or genetic differences underlay differences in vitamin D status. However, the racial groups in this study are of insufficient size to define such potential differences. Further investigation of this possibility is appropriate. Finally, this study was conducted following the Hawaiian equivalent of winter during which time there is reduced capability for cutaneous vitamin D production. Despite this limitation, given the low latitude of Hawaii, substantial UV exposure and, therefore, vitamin D production are possible year round (38, 39).

In conclusion, high amounts of sun exposure do not ensure what is currently accepted as vitamin D adequacy. Thus, clinicians should not assume that individuals with abundant sun exposure have adequate vitamin D status. In the event of vitamin D deficiency, the goal of vitamin D replacement therapy should be no greater than the maximum that appears attainable, a serum 25(OH)D concentration of approximately 60 ng/ml.

	Footnotes

 
Disclosure Statement: N.B., R.N., D.K., T.K., Y.G.D., G.L., and M.K.D. have no conflicts of interest relevant to this work. B.W.H. is a consultant for Diasorin.

First Published Online April 3, 2007

Abbreviations: BMI, Body mass index; CV, coefficient of variation; 25(OH)D, 25-hydroxyvitamin D; UH, University of Hawaii at Manoa.

¹ We have chosen to use the terminology "low vitamin D status" for a syndrome that others have variously described as "vitamin D insufficiency," "vitamin D inadequacy," or "hypovitaminosis D."

Received October 16, 2006.

Accepted March 26, 2007.

	References

Top Abstract Introduction Subjects and Methods Results Discussion References

 

1. Rucker D, Allan JA, Fick GH, Hanley DA 2002 Vitamin D insufficiency in a population of healthy western Canadians. Can Med Assoc J 166:1517–1524[Abstract/Free Full Text]
2. Looker AC, Dawson-Hughes B, Calvo MS, Gunter EW, Sahyoun NR 2002 Serum 25-hydroxyvitamin D status of adolescents and adults in two seasonal subpopulations from NHANES III. Bone 30:771–777[Medline]
3. Nesby-O’Dell S, Scanlon KS, Cogswell ME, Gillespie C, Hollis BW, Looker AC, Allen C, Doughertly C, Gunter EW, Bowman BA 2002 Hypovitaminosis D prevalence and determinants among African American and white women of reproductive age: third National Health and Nutrition Examination Survey, 1988–1994. Am J Clin Nutr 76:187–192[Abstract/Free Full Text]
4. Holick MF, Siris ES, Binkley N, Beard MK, Khan A, Katzer J, Petruschek RA, Chen E, dePapp AE 2005 Prevalence of vitamin D inadequacy among postmenopausal North American women receiving osteoporosis therapy. J Clin Endocrinol Metab 90:3215–3224[Abstract/Free Full Text]
5. Holick MF 2003 Vitamin D: a millennium perspective. J Cell Biochem 88:296–307[CrossRef][Medline]
6. Heaney RP 2000 Vitamin D: how much do we need, and how much is too much. Osteoporos Int 11:553–555[CrossRef][Medline]
7. Grant WB, Holick MF 2005 Benefits and requirements of vitamin D for optimal health: a review. Altern Med Rev 10:94–111[Medline]
8. Liu PT, Stenger S, Li H, Wenzel L, Tan BH, Krutzik SR, Ochoa MT, Schauber J, Wu K, Meinken C, Kamen DL, Wagner M, Bals R, Steinmeyer A, Zugel U, Gallo RL, Eisnberg D, Hewison M, Hollis BW, Adams JS, Bloom JR, Modlin RL 2006 Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response. Science 311:170–173[Abstract/Free Full Text]
9. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes Food and Nutrition Board, Institute of Medicine 1997 DRI Dietary Reference Intakes for calcium phosphorus, magnesium, vitamin D and fluoride. Washington, DC: National Academy Press
10. Dawson-Hughes B, Heaney RP, Holick MF, Lips P, Meunier PJ, Vieth R 2005 Estimates of optimal vitamin D status. Osteoporos Int 16:713–716[CrossRef][Medline]
11. Lips P 2004 Which circulating level of 25-hydroxyvitamin D is appropriate? J Steroid Biochem Mol Biol 89–90:611–614
12. Lips P, Chapuy MC, Dawson-Hughes B, Pols HAP, Holick MF 1999 An international comparison of serum 25-hydroxyvitamin D measurements. Osteoporos Int 9:394–397[CrossRef][Medline]
13. Binkley N, Krueger D, Cowgill C, Hansen KE, Drezner MK 2003 Assay variation confounds hypovitaminosis D diagnosis: a call for standardization. J Clin Endocrinol Metab 89:3152–3157[CrossRef]
14. Carter GD, Carter CR, Gunter E, Jones J, Jones G, Makin HLJ, Sufi S 2004 Measurement of vitamin D metabolites: an international perspective on methodology and clinical interpretation. J Steroid Biochem Mol Biol 89–90:267–271
15. Carter GD, Carter R, Jones J, Berry J 2004 How accurate are assays for 25-hydroxyvitamin D? Date from the international vitamin D external quality assessment scheme. Clin Chem 50:2195–2197[Free Full Text]
16. Hollis BW 2000 Comparison of commercially available ¹²⁵I-based RIA methods for the determination of circulating 25-hydroxyvitamin D. Clin Chem 46:1657–1661[Abstract/Free Full Text]
17. Malabanan A, Veronikis IE, Holick MF 1998 Redefining vitamin D insufficiency. Lancet 351:805–806[Medline]
18. Lensmeyer GL, Binkley N, Drezner MK 2006 New horizons for assessment of vitamin D status in man. 2nd ed. San Diego: Academic Press
19. Hollis BW 2005 Circulating 25-hydroxyvitamin D levels indicative of vitamin D sufficiency: implications for establishing a new effective dietary intake recommendation for vitamin D. J Nutr 135:317–322[Abstract/Free Full Text]
20. Cordain L, Eaton SB, Sebastian A, Mann N, Lindeberg S, Watkins BA, O’Keefe JH, Brand-Miller J 2005 Origins and evolution of the Western diet: health implications for the 21st century. Am J Clin Nutr 81:341–354[Abstract/Free Full Text]
21. Eaton SB 2006 The ancestral human diet: what was it and should it be a paradigm for contemporary nutrition? Proc Nutr Soc 65:1–6[CrossRef][Medline]
22. Barger-Lux MJ, Heaney RP 2002 Effects of above average summer sun exposure on serum 25-hydroxyvitamin D and calcium absorption. J Clin Endocrinol Metab 87:4952–4956[Abstract/Free Full Text]
23. Knaysi GA, Crikelair GF, Cosman B 1968 The rule of nines: its history and accuracy. Plast Reconstr Surg 41:560–563[Medline]
24. Lensmeyer GL, Wiebe DA, Binkley N, Drezner MK 2006 HPLC method for 25-hydroxyvitamin D measurement: comparison with contemporary assays. Clin Chem 52:1120–1126[Abstract/Free Full Text]
25. Hollis BW 2005 Detection of vitamin D and its major metabolites. In: Feldman D, Pike JW, Glorieux FH, eds. Vitamin D. 2nd ed. Burlington, MA: Elsevier; 931–950
26. Gao P, Scheibel S, D’Amour P, John MR, Rao SD, Schmidt-Gayk H, Cantor TL 2001 Development of a novel immunoradiometric assay exclusively for biologically active whole parathyroid hormone 1–84: implications for improvement of accurate assessment of parathyroid function. J Bone Miner Res 16:605–614[CrossRef][Medline]
27. Boyd-Eaton S, Nelson DA 1991 Calcium in evolutionary perspective. Am J Clin Nutr 54:281S–287S
28. Dawson-Hughes B, Harris SS, Dallal GE 1997 Plasma calcidiol, season, and serum parathyroid hormone concentrations in healthy elderly men and women. Am J Clin Nutr 65:67–71[Abstract/Free Full Text]
29. Lips P 2001 Vitamin D deficiency and secondary hyperparathyroidism in the elderly: consequences for bone loss and fractures and therapeutic implications. Endocr Rev 22:477–501[Abstract/Free Full Text]
30. Heaney RP, Dowell MS, Hale CA, Bendich A 2003 Calcium absorption varies within the reference range for serum 25-hydroxyvitamin D. J Am Coll Nutr 22:142–146[Abstract/Free Full Text]
31. Haddock L, Corcino J, Vazques MD 1982 25(OH)D serum levels in the normal Puerto Rican population and in subjects with tropical sprue and parathyroid disease. P R Health Sci J 1:85–91
32. Tangpricha V, Turner A, Spina C, Decastro S, Chen TC, Holick MF 2004 Tanning is associated with optimal vitamin D status (serum 25-hydroxyvitamin D concentration) and higher bone mineral density. Am J Clin Nutr 80:1645–1649[Abstract/Free Full Text]
33. Holick MF, MacLaughlin JA, Doppelt SH 1981 Regulation of cutaneous previtamin D₃ photosynthesis in man: skin pigment is not an essential regulator. Science 211:590–593[Abstract/Free Full Text]
34. Holick MF 1995 Environmental factors that influence the cutaneous production of vitamin D. Am J Clin Nutr 61:638S–645S
35. MacLaughlin JA, Holick MF 1985 Aging decreases the capacity of human skin to produce vitamin D₃. J Clin Invest 76:1536–1538[Medline]
36. Ferguson GW, Gehrmann WH, Karsten KB, Landwer AJ, Carman EN, Chen TC, Holick MF 2005 Ultraviolet exposure and vitamin D synthesis in a sun-dwelling and a shade-dwelling species of Anolis: are there adaptations for lower ultraviolet B and dietary vitamin D₃ availability in the shade? Physiol Biochem Zool 78:193–200[CrossRef][Medline]
37. Heaney RP, Davies KM, Chen TC, Holick MF, Barger-Lux MJ 2003 Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol. Am J Clin Nutr 77:204–210[Abstract/Free Full Text]
38. Kimlin MG, Schallhorn KA 2004 Estimations of the human ‘vitamin D’ UV exposure in the USA. Photochem Photobiol Sci 3:1067–1070[CrossRef][Medline]
39. Engelsen O, Brustad M, Aksnes L, Lund E 2005 Daily duration of vitamin D synthesis in human skin with relation to latitude, total ozone, altitude, ground cover, aerosols and cloud thickness. Photochem Photobiol 81:1287–1290[CrossRef][Medline]

This article has been cited by other articles:

				J. L. Lapp Vitamin D: Bone Health and Beyond American Journal of Lifestyle Medicine, September 1, 2009; 3(5): 386 - 393. [Abstract] [PDF]

				J. H. O'Keefe, J. H. Lee, D. S.H. Bell, and M. F. Holick Reply J. Am. Coll. Cardiol., May 26, 2009; 53(21): 2012 - 2013. [Full Text] [PDF]

				G. Devereux, H. Macdonald, and C. Hawrylowicz Vitamin D and Asthma: Time for Intervention? Am. J. Respir. Crit. Care Med., May 1, 2009; 179(9): 739 - 740. [Full Text] [PDF]

				J. M. Brehm, J. C. Celedon, M. E. Soto-Quiros, L. Avila, G. M. Hunninghake, E. Forno, D. Laskey, J. S. Sylvia, B. W. Hollis, S. T. Weiss, et al. Serum Vitamin D Levels and Markers of Severity of Childhood Asthma in Costa Rica Am. J. Respir. Crit. Care Med., May 1, 2009; 179(9): 765 - 771. [Abstract] [Full Text] [PDF]

				S. Penckofer, J. Kouba, D. E. Wallis, and M. A. Emanuele Vitamin D and Diabetes: Let the Sunshine In The Diabetes Educator, November 1, 2008; 34(6): 939 - 954. [Abstract] [Full Text] [PDF]

				D. E. Wallis, S. Penckofer, and G. W. Sizemore The "Sunshine Deficit" and Cardiovascular Disease Circulation, September 30, 2008; 118(14): 1476 - 1485. [Full Text] [PDF]

				P. M Brannon, E. A Yetley, R. L Bailey, and M. F. Picciano Overview of the conference "Vitamin D and Health in the 21st Century: an Update" Am. J. Clinical Nutrition, August 1, 2008; 88(2): 483S - 490S. [Abstract] [Full Text] [PDF]

				B. A Gilchrest Sun exposure and vitamin D sufficiency Am. J. Clinical Nutrition, August 1, 2008; 88(2): 570S - 577S. [Abstract] [Full Text] [PDF]

				J. C. Fleet, C. Gliniak, Z. Zhang, Y. Xue, K. B. Smith, R. McCreedy, and S. A. Adedokun Serum Metabolite Profiles and Target Tissue Gene Expression Define the Effect of Cholecalciferol Intake on Calcium Metabolism in Rats and Mice J. Nutr., June 1, 2008; 138(6): 1114 - 1120. [Abstract] [Full Text] [PDF]

				N. Binkley, D. Krueger, D. Gemar, and M. K. Drezner Correlation among 25-Hydroxy-Vitamin D Assays J. Clin. Endocrinol. Metab., May 1, 2008; 93(5): 1804 - 1808. [Abstract] [Full Text] [PDF]

				S T Weiss and A A Litonjua Author's reply Thorax, March 1, 2008; 63(3): 293 - 293. [Full Text] [PDF]

eLetters:

This Article Abstract Full Text (PDF) Submit a related Letter to the Editor View responses Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Binkley, N. Articles by Drezner, M. K. Search for Related Content PubMed PubMed Citation Articles by Binkley, N. Articles by Drezner, M. K. Related Collections Calcium and Bone Metabolism