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BY-NC-ND 4.0 license Open Access Published by De Gruyter July 1, 2017

Effect of Latitude on Vitamin D Levels

Patrick F. Leary , Ina Zamfirova , Johnathan Au and Ward H. McCracken

Abstract

Context

Vitamin D levels have been linked to bone health and to numerous diseases; however, an element that lacks substantial direct data and limits the evidence basis regarding whom to screen for vitamin D deficiency is the effect of latitude on vitamin D levels.

Objectives

To determine whether latitude influences vitamin D levels and to investigate the influence of other factors that may affect vitamin D levels, including sex, race, skin type, and body mass index.

Methods

Osteopathic medical students were recruited from campuses in Bradenton, Florida, and Erie, Pennsylvania. Surveys were administered to obtain demographic information, and blood samples were drawn to measure total vitamin D levels. Two-sample t tests, Fisher exact test, and logistic regression was used to assess differences in total vitamin D levels between the 2 locations.

Results

A total of 359 medical students (aged 22-57 years) were included in the study, 194 at the Bradenton campus and 214 at the Erie campus. The mean (SD) vitamin D level was 34.5 (11.8) ng/mL among participants in Bradenton and 28.1 (12.4) ng/mL among participants in Erie. Logistic regression models revealed an adjusted OR of 3.3 (95% CI, 1.73-6.4) for deficient total vitamin D among Erie students. Non-white race, male sex, and high body mass index were also statistically significant risk factors for vitamin D deficiency in regression models (P<.05).

Conclusion

Latitude was found to be a statistically significant risk factor for vitamin D deficiency. Additionally, the findings suggest that persons with darker skin tone and, to a lesser degree, men and persons who are overweight or obese are also at increased risk for vitamin D deficiency. Physicians should be cognizant of these risk factors when deciding whom to screen.

Keywords: latitude; vitamin D

Numerous studies have implicated vitamin D deficiency in maladies such as cancer, poor bone health, poor orthopedic surgical outcomes, fractures in elderly persons, diabetes, heart disease, osteoporosis, autoimmune disorders, multiple sclerosis, schizophrenia, and depression.1-8 Studies have even linked vitamin D intake to general and musculoskeletal health in special populations, such as athletes.9 One study found that deficient and insufficient vitamin D levels are highly prevalent among participants in the NBA draft.10 Systematic reviews and meta-analyses did not show clear evidence that correction of low serum vitamin D level improves outcomes.11 Despite having several supplement guidelines to safely and effectively manage vitamin D deficiency, there is no established consensus regarding whom to screen. Part of the difficulty is that many variables have been demonstrated in the literature to play a role in affecting levels of vitamin D. For example, obese persons were found to have significantly lower vitamin D levels, likely resulting from a reduced bioavailability due to vitamin D's fat-soluble nature.12 A longitudinal study13 found that a greater amount of melanin is correlated with reduced capacity of the skin to synthesize vitamin D. Yet, one variable that has been recognized to affect vitamin D levels but for which current literature lacks consensus is latitude. Holick14 suggested that sunlight in the winter months north of 37° is insufficient to produce adequate vitamin D synthesis. An Australian study15 concluded that although latitude and season had an effect on vitamin D levels, other factors, such as behavior, had a much larger effect. Another study by Holick5 found no connection between latitude and serum vitamin D levels,5 whereas Jelinek et al16 found a significant correlation between latitude and vitamin D levels in patients with multiple sclerosis.

The present study was conducted to compare vitamin D levels in medical students at 2 campuses of the Lake Erie College of Osteopathic Medicine: Bradenton, Florida (27°N), which has an average of 101 clear days of sun per year, and Erie, Pennsylvania (42°N), which has an average of 63 clear days of sun per year.18 These locations had closely matching populations, allowing for a more focused comparison of the effects of latitude on serum vitamin D levels. As a secondary outcome, other factors that were known to correlate with vitamin D levels, specifically sex, race, skin type, and body mass index (BMI), were compared.

Methods

Data were collected from second-year medical students at the Lake Erie College of Osteopathic Medicine (LECOM) on campuses in Bradenton, Florida, and Erie, Pennsylvania, during their annual phlebotomy laboratory training from February to March 2011 and again from February to March 2012. All second-year medical students attending either campus were eligible to participate. Participation was voluntary, and no compensation was provided. The study was made known to all students via direct presentation during their required phlebotomy training in the second year. Once consent was received, participants were given surveys to elicit further demographic information before having blood collected. All blood draws were overseen by trained study coordinators and sent to a laboratory for 25-hydroxy vitamin D analysis (D2, D3, and total 25(OH)D levels with ng/mL units).

To limit potential bias and confounding factors, participants were excluded from the study if any of the following were met: history of renal disease, bone disease, dermatologic conditions interfering with vitamin D synthesis, or cancer; use of exogenous vitamin D supplementation (≥800 IU daily); or excessive tanning (≥1 time per month).

The primary outcome of interest was the average levels of 25(OH)D in students between the campuses and their relationship to latitude. We acknowledge that literature has recognized deficient, insufficient, and sufficient ranges of vitamin D10-12; however, for the purposes of this study's data analysis, levels were dichotomized to be either deficient or nondeficient, with deficient being recognized as less than 20 ng/mL. Other covariates analyzed included BMI, medical history, current medications, use of tanning beds, use of sunscreen and strength applied, and skin type. Participants classified as underweight (BMI <18.5) were excluded because of low numbers. Participants classified as overweight (BMI 25-30) or obese (BMI ≥30) were collapsed into a single category, also owing to low numbers. Skin type was graded on the Fitzpatrick scale and was self-determined by participants. Types IV through VI were grouped together in data analysis because of low numbers.

Descriptive statistics for all continuous (mean [SD]) and categorical (No. [%]) data were calculated on all participant characteristics. A 2-sample t test for continuous and the Pearson χ2 or Fisher exact tests for categorical data were performed to assess the significance between group means and proportions, respectively. Multiple logistic regression was used to calculate ORs and 95% CIs. Statistical significance was set at α = .05. Analysis was conducted using STATA statistical software (version 11.2; Stata Corp).

Results

Among the pool of 413 possible participants, 408 (98.8%) elected to participate in the study. However, 49 (13%) were excluded: 5 with medical conditions that affected vitamin D levels, 36 for exogenous vitamin D supplementation, and 8 for high tanning frequency. The final study population included 359 participants (aged 22-57 years), 194 at the Bradenton campus and 214 at the Erie campus. In the Bradenton cohort, 18 participants (5%) were found to be deficient in vitamin D, and in the Erie cohort, 49 participants (13%) were found to be deficient.

Participant demographics are presented in Table 1. The majority of participants were white, and the proportion of white participants was similar between locations. A greater proportion of women participated in Bradenton (105 [54.1%]) compared with Erie (96 [44.9%]). Most participants were of normal weight, with the mean (SD) BMI being 23.7 (3.2) and 24.7 (4.2) for Bradenton and Erie, respectively. Most participants' skin type was categorized as either II (147 [36%]) or III (182 [45%]). The mean (SD) total vitamin D levels were 34.5 ng/mL (11.8) and 28.1 ng/mL (12.4) for Bradenton and Erie, respectively.

Table 1.

Student Demographics at Bradenton, Florida, and Erie, Pennsylvania, Campuses of Lake Erie College of Osteopathic Medicinea

Demographic Bradenton Erie P Value
Race n=194 n=214 .066
 White 149 (76.8) 169 (79.0)
 Asian 14 (7.2) 16 (7.5)
 Indian 4 (2.1) 10 (4.7)
 Other 25 (12.9) 13 (6.1)
 Unknown 2 (1.0) 6 (2.8)
Sex n=194 n=214 .062
 Female 105 (54.1) 96 (44.9)
BMI n=192 n=211 .004
 Underweight 1 (0.5) 5 (2.4)
 Normal 139 (72.4) 121 (57.4)
 Overweight 44 (22.9) 63 (29.9)
 Obese 8 (4.2) 22 (10.4)
Skin Type Gradeb n=193 n=213 .461
 I 8 (4.2) 17 (8.0)
 II 71 (36.8) 76 (35.7)
 III 88 (45.6) 94 (44.1)
 IV, V, or VI 26 (13.5) 26 (12.2)
Tanning Bed Use n=194 n=211 .021
 Yes 188 (96.9) 192 (91.0)
 No 6 (3.1) 19 (9.0)
Excessive Tanning n=194 n=214 .727
 Yes 3 (1.6) 5 (2.3)
 No 191 (98.5) 209 (97.7)
Sun Block Use Frequency n=176 n=131 .001
 Never 22 (11.3) 0
 Rarely 33 (17.0) 27 (20.6)
 Occasionaly 40 (20.6) 33 (25.2)
 Usually 55 (28.4) 50 (38.2)
 Always 26 (13.4) 21 (16.0)
 Not Applicable 18 (9.3) 0
Vitamin D Supplements n=194 n=214 .014
 Yes 10 (5.2) 26 (12.2)
 No 184 (94.9) 188 (87.9)
Previous Related Disease n=194 n=214 .734
 Yes 2 (1.0) 3 (1.4)
 No 192 (99.0) 211 (98.6)
BMI, mean (SD) 23.7 (3.2) 24.7 (4.2) .005
SPF, mean (SD) 20.1 (18.9) 15.0 (16.9) .005
Total Vitamin D, mean (SD) 34.5 (11.8) 28.1 (12.4) <.001

a Tables represent data for all people surveyed who gave complete answers to the participation questionnaire (including those who eventually met study exclusion criteria and were not part of the final data analysis). The n provided for each item represents the number of respondents from each school for that item.

b Fitzpatrick scale.

Abbreviations: BMI, body mass index; SPF, sun protection factor.

Mean total vitamin D levels are reported in Table 2 for all study participants and by location. Participants who were female, white, or normal weight had higher mean total vitamin D levels. These trends were consistent for both locations. With regard to skin type classification, among participants in Bradenton, participants with darker skin types had lower vitamin D levels on average. Among participants in Erie, the highest vitamin D levels were observed for participants with type II skin. Lowest vitamin D levels were seen among those with type IV skin for either location.

Table 2.

Mean Total Vitamin D Levels for All Students and by Lake Erie College of Osteopathic Medicine Campus (Bradenton, Florida, and Erie, Pennsylvania)a

Both Campuses Bradenton Erie
Variable n Total Vitamin D, Mean (SD) P Value n Total Vitamin D, Mean (SD) P Value n Total Vitamin D, Mean (SD) P Value
Sex <.001 <.001 <.001
 Female 192 35.5 (13.5) 105 38.1 (12.4) 87 32.4 (14.3)
 Male 202 27.2 (9.9) 89 30.3 (9.4) 113 24.8 (9.7)
Race <.001 <.001 .036
 White 306 33.0 (12.1) 149 36.9 (10.7) 157 29.4 (12.3)
 Asian 30 23.8 (13.1) 14 22.9 (10.5) 16 24.7 (15.4)
 Indian 14 24.0 (10.8) 4 16.0 (7.2) 10 27.2 (10.6)
 Other 36 26.7 (12.1) 25 29.8 (11.9) 11 19.5 (9.5)
 Unknown 8 24.4 (10.2) 2 35 (14.1) 6 20.8 (6.8)
BMI <.001 .036 .097
 Underweight 5 38.2 (23.0) 1 40 (0.0) 4 37.8 (26.5)
 Normal 254 33.0 (12.5) 139 36.0 (11.1) 115 29.3 (13.2)
 Overweight 102 28.7 (11.6) 44 31.7 (13.1) 58 26.5 (9.8)
 Obese 28 25.0 (10.9) 8 26.9 (10.9) 20 24.2 (11.1)
Skin Type Gradeb <.001 <.001 .166
 I 24 31.6 (13.6) 8 39.4 (10.6) 16 27.7 (13.5)
 II 142 34.7 (11.4) 71 39.3 (10.1) 71 30.0 (10.8)
 III 175 30.7 (12.9) 88 33.5 (11.1) 87 28.0 (14.0)
 IV, V, or VI 51 23.5 (10.1) 26 23.5 (10.8) 25 23.5 (9.5)
Tanning Bed Use .001 .140 <.001
 Yes 25 39.0 (13.2) 6 41.5 (7.1) 19 38.3 (14.7)
 No 366 30.8 (12.3) 188 34.3 (11.8) 178 27.0 (11.8)
Excessive Tanning <.001 .088 <.001
 Yes 8 46.4 (13.9) 3 46.0 (4.6) 5 46.6 (18.1)
 No 386 30.9 (12.3) 191 34.3 (11.8) 195 27.6 (12.0)
Sunscreen Use .009 <.001 .743
 Never 22 30.2 (10.9) 22 30.2 (10.9) 0
 Rarely 60 27.7 (12.0) 33 28.3 (11.1) 27 27.0 (13.1)
 Occasionally 69 35.3 (13.9) 40 39.3 (9.9) 29 29.8 (16.7)
 Usually 102 33.9 (12.3) 55 36.9 (12.2) 47 30.5 (11.5)
 Always 46 33.5 (12.1) 26 36.7 (11.6) 20 29.4 (11.6)
 Not applicable 18 30.1 (9.7) 18 30.1 (9.7) 0
Vitamin D Supplements .015 .009 .032
 Yes 35 36.2 (12.4) 10 43.9 (14.9) 25 33.1 (10.1)
 No 359 30.8 (12.4) 184 34.0 (11.4) 175 27.4 (12.6)
Previous Related Disease .648 .432 .180
 Yes 5 33.8 (9.2) 2 28.0 (1.4) 3 37.7 (10.6)
 No 389 31.2 (12.6) 192 34.6 (11.8) 197 27.9 (12.4)
BMI Correlation Coefficient (r) 389 −0.230 <.001 192 −0.256 .412 197 −0.168 .018
SPF Correlation Coefficient (r) 384 0.135 .008 192 0.153 .034 192 0.042 .561

a Tables represent data for all people surveyed who gave complete answers to the participation questionnaire (including those who eventually met study exclusion criteria and were not part of the final data analysis).

b Fitzpatrick scale.

Abbreviations: BMI, body mass index; SPF, sun protection factor.

Multivariate logistic regression results are presented in Table 3. Results demonstrate higher odds of deficient vitamin D levels among participants who were male, nonwhite, overweight/obese, and living in Erie. Statistical significance was only observed with regard to race and location. Skin type II and III showed decreased odds of vitamin D deficiency, whereas type IV showed increased odds. Only type II was found to be statistically significant.

Table 3.

Multivariate Logistic Regression for Students With Deficient Vitamin D Levels in Bradenton, Florida, Compared With Erie, Pennsylvania

Variable OR (95% CI) P Value
Campus
 Bradenton 1
 Erie 5.23 (2.27-12.04) <.001
Race
 White 1
 Other 2.92 (1.08-7.83) .034
Sex
 Female 1
 Male 2.18 (0.93-5.12) .073
BMI
 Normal 1
 Overweight 1.06 (0.97-1.17) .203
Skin Type Gradea
 I 1
 II 0.15 (0.03-0.65) .011
 III 0.49 (0.13-1.86) .295
 IV, V, or VI 1.23 (0.22-6.90) .813
Tanning Bed Use
 No 1
 Yes 0.31 (0.03-2.77) .292

a Fitzpatrick scale.

Abbreviation: BMI, body mass index.

Discussion

When controlled for other confounding variables, students at the Bradenton campus had statistically significant higher vitamin D levels. Given how closely matched the sample populations were in demographics, this finding lends substantial evidence to the hypothesis that latitude is a primary risk factor for vitamin D deficiency. It should be noted that mean levels in participants from both campuses were both in the nondeficient range.

The secondary outcomes were generally consistent with existing literature. Although it did not achieve statistical significance, the breakdown by sex consistently showed higher average vitamin D levels in women at both campuses. Results of the race and skin type subcategories also showed similar results to those in the literature.19 As skin complexion darkens, vitamin D levels decrease; people with lighter skin have overall higher levels of vitamin D.13 Regarding BMI, it has been shown in other studies that BMI has an inverse relationship to vitamin D levels, and this study is in general agreement with that model.12

Several limiting factors that were difficult to plan and control for should be noted. Our sample population was one of convenience and, as such, represents a much younger segment of the overall population. Our data were likely skewed by the large proportion of the sample population being white, allowing few data points for Asian, Indian, and those categorized as “other.” Additionally, the limited number of data points available for participants with darker skin types limits the power of the study to make any conclusive findings with regards to those participants. Further research could also help substratify risk in nonwhite races.

Dietary intake differences were not controlled for, and while sunscreen use was included in the preenrollment survey, it proved difficult to meaningfully quantify and control for in our analysis and so was omitted from the final analysis.

Given the warmer temperatures in Bradenton during the winter months, residents are more likely to be outside more often and have their skin exposed to the sun than residents of Erie during the same months.

Conclusion

Although all confounding factors were difficult to control for, this study found good preliminary evidence that living at a higher latitude and self-identifying as a race other than white are risk factors for vitamin D deficiency in the studied population. Male sex was found to be a higher-risk group; further research could help clarify this risk relationship. We suggest that physicians consider testing patients for vitamin D deficiency if they have appropriate symptoms and if they are a nonwhite race, if they live at higher latitudes, and, to a lesser degree, if they are overweight or obese.


From the Lake Erie College of Osteopathic Medicine in Erie, Pennsylvania (Dr Leary); the Russell Research Institute at Lutheran General Hospital in Downers Grove, Illinois (Ms Zamfirova and Mr Au); and the University of Minnesota in Minneapolis (Dr McCracken).
Financial Disclosures: None reported.
Support: Funded by the Lake Erie Consortium for Osteopathic Medical Training (LT51511).

*Address correspondence to Patrick F. Leary, DO, MS, 5401 Peach St, Suite 3400, Erie, PA 16509-2601. E-mail:


Acknowledgments

We thank the following individuals for their contributions to making this study possible: Thurman Alvey, DO; Thomas Quinn, DO; Joseph Leary, DO; Louis C. Cavadini, DO; and Mark Kauffman, DO.

Author Contributions

All authors provided substantial contributions to conception and design, acquisition of data, or analysis and interpretation of data; all authors drafted the article or revised it critically for important intellectual content; all authors gave final approval of the version of the article to be published; and all authors agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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Received: 2017-02-19
Accepted: 2017-03-31
Published Online: 2017-07-01
Published in Print: 2017-07-01

© 2017 American Osteopathic Association

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

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