|
 
 |
| ORIGINAL ARTICLE |
|
| Year : 2023 | Volume
: 14
| Issue : 1 | Page : 27 |
|
The prevalence and determinants of vitamin D status among older adults: Data from a longitudinal aging study
Omolbanin Motamed Rezaei1, Farshad Sharifi2, Mitra Moodi3, Asghar Zarban4, Reza Tahergorabi5, Zoya Tahergorabi6
1 Instructor of Environmental Toxicology, Social Determinants of Health Research Center, Department of Occupational Health, School of Health, Birjand University of Medical Sciences, Birjand, Iran
2 Department of Geronoto-Epidemiology, Elderly Health Research Center, Endocrinology and Metabolism Research Institute, Tehran University of Medical Sciences, Tehran, Iran
3 Social Determinants of Health Research Center, Birjand University of Medical Sciences, Birjand, Iran
4 Department of Clinical Biochemistry, School of Medicine, Birjand University of Medical Sciences, Birjand, Iran
5 Medical Toxicology and Drug Abuse Research Center, Birjand University of Medical Sciences, Birjand, Iran
6 North Carolina Agricultural and Technical State University, Greensboro, NC, USA
| Date of Submission | 15-Aug-2021 |
| Date of Acceptance | 27-Oct-2022 |
| Date of Web Publication | 25-Feb-2023 |
Correspondence Address:
Zoya Tahergorabi
Department of Physiology, School of Medicine, Birjand University of Medical Sciences, Ghafari Street, Birjand
USA
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/ijpvm.ijpvm_366_21
Background: Aging is identified as a risk factor for vitamin D deficiency (VDD) therefore this investigation was designed to determine the prevalence of VDD and its determinants in a sample of older adults. Methods: The data of this study were obtained from the baseline wave of the Longitudinal Aging Study (LAS). Demographic, past medical history, medication history, and smoking behavior were collected using an interview approach. The physical activity and nutritional status of the participants were assessed using a standard questionnaire. Anthropometric indices were measured according to a standard protocol then body mass index (BMI) was calculated. Serum vitamin D and calcium levels were measured by autoanalyzers. Univariate and multiple logistic regression models were applied to detect the associated factors with VDD. Results: Mean age of the participants was 71.82 (SD = 7.63) years. A total of 1319 people participated in our study, and 51.16% (n = 688) were female. A total of 8.42% (n = 111) of the participants had VDD and 17.06% of them (n = 225) had insufficient vitamin D levels. In the multivariable logistic regression model, the age group of 70–79 years in comparison with the age group of 60–69 years had a 43% less chance of VDD. Furthermore, being overweight (OR = 0.36, P = 0.01) and obese (OR = 0.35, P = 0.02), and taking vitamin D supplements (OR = 0.31, P = 0.04) were significantly associated with VDD. Conclusions: Our results showed that 25% of older adults had vitamin D deficiency or insufficiency. In addition, some modifiable lifestyle factors were associated with VDD. Given that, old age is considered a risk factor for VDD. Therefore, detection and improvement of VDD may be a preventive measure in at-risk subjects.
Keywords: Older adults, prevalence, prevention, vitamin D deficiency
How to cite this article:
Rezaei OM, Sharifi F, Moodi M, Zarban A, Tahergorabi R, Tahergorabi Z. The prevalence and determinants of vitamin D status among older adults: Data from a longitudinal aging study. Int J Prev Med 2023;14:27 |
How to cite this URL:
Rezaei OM, Sharifi F, Moodi M, Zarban A, Tahergorabi R, Tahergorabi Z. The prevalence and determinants of vitamin D status among older adults: Data from a longitudinal aging study. Int J Prev Med [serial online] 2023 [cited 2023 Mar 26];14:27. Available from: https://www.ijpvmjournal.net/text.asp?2023/14/1/27/370608 |
| Introduction | |  |
As a result of socioeconomic development, life expectancy steadily increased in the past few decades and this increment is expected to continue in industrialized countries as well as in developing countries. According to a United Nations report, it is estimated that the number of people aged 65 years and older rise from 0.7 billion (9%) in 2019 to 1.5 billion (16%) in 2050.[1] According to the United Nations definition, if a population over 60 years old is beyond 7% in a country that country is considered to be aged herein, it is estimated that Iran experienced a sudden increase in the elderly population between 2030 and 2040.[2]
The aging process is associated with lifestyle, socioeconomic, and biological changes, which is related to growing diseases and injuries, which can impose a growing disease burden on the healthcare system worldwide.[3]
Therefore, in this context, the World Health Organization (WHO) and other global organizations emphasize the promotion of a concept known as “successful” or “healthy” aging.[2] Most studies investigated the role of modifiable lifestyle factors such as moderate physical activity, a high-quality diet including dietary components such as vitamin D, and maintaining a normal body weight for achieving “successful” or “healthy” aging.[2]
Vitamin D is a steroid hormone, which is mostly synthesized in the skin following ultraviolet B (UVB) sunlight exposure and to a lesser extent from the diet.[4] Also, calcium absorption from the intestine is mediated by the active transport vitamin D-dependent and passive diffusion.[5] Geographical location related to the intensity of the solar ultraviolet (UV), darker skin pigmentation, obesity, malabsorption, and advanced age were identified as risk factors for vitamin D deficiency.[6]
Older adults are particularly at higher risk for vitamin D deficiency because of the debilitated ability of dermal synthesis following exposure to UVB sunlight, activation of vitamin D in the kidney, and age-related behavioral changes such as clothing, less outdoor exercise, and activity and dietary patterns such as decreased food quality, quantity, and variety.[7]
There is no consensus on the optimal level of vitamin D; therefore, there are various designations of 25(OH) D concentration cutoffs for vitamin D deficiency (VDD). The North American Institute of Medicine (IOM) and the UK Scientific Advisory Committee on Nutrition (SACN) consider a threshold of 30 nmol/L while European Food Safety Authority (EFSA) and the Endocrine Society (ES) established a threshold of 50 nmol/L.[8]
A study performed by Aspell et al.[9] on 6004 adults, aged >50 years from the English Longitudinal Study of Ageing (ELSA), showed that the overall prevalence of VDD was 26.4% and 58.7% based on IOM criteria and ES definitions, respectively.
Since recent studies show a high prevalence of VDD even in areas of the world with high sunlight exposure including Iran, and association of low vitamin D with many diseases, especially in older adults, therefore, the present study was designated for the prevalence of calcium deficiency and VDD and investigated the demographical, seasonal, and lifestyle factors among older adults of Birjand (a sunny city in eastern Iran, capital of South Khorasan province,) in 2019.
Also, we found a recent study by Mansouri et al.[10] in 2021 on 600 Iranian men and women 60 years of age and older who attended health centers in Tehran; however, the present study has population-based multistage stratified cluster random designation on more than twice the population (1319 subjects) than mentioned study furthermore, demographical, nutritional status, physical activity level, and lifestyle factors were considered seasonal variation in two season of Fall and winter due to dependency of vitamin D on sunlight.
| Methods | | |
Study design, population, and sampling
This study is a cross-sectional approach to data of the baseline of the Brijand Longitudinal Aging Study (BLAS). The sampling of this study was a multistage stratified cluster random method and the sample was representative of older adults ≥60 years, who live in Birjand. Inclusion criteria of the study were age ≥60 years and willingness to participate in the study and exclusion criteria were very short life expectancy and illiterate. The sampling models and data-gathering methods of this study were discussed in more detail elsewhere.[11]
Data acquisition
Demographic data such as age, education levels, employment, and smoking status (cigarette and hookah) were collected using a validated questionnaire. History of chronic diseases such as diabetes mellitus, hypertension, chronic kidney disease, (CKD), and chronic obstructive pulmonary disease (COPD) and medication history were collected. This information was obtained by asking the participants and their accompanied related informants as well as by checking the medical documents of the participants by the researchers. Levels of physical activity were evaluated by LASA Physical Activity Questionnaire (LAPAQ). The metabolic equivalents (METs) of each physical activity were extracted from the international table of METs.[12] The height of the participants was measured in an upright position along with the vertical axis and weights were measured with the least clothes without any headcover and shoes to the nearest 0.5 cm and 0.1 kg by a calibrated stadiometer (Seca, Hamburg, Germany,). All anthropometric measurements were performed according to the National Health and Nutrition Examination Survey (NHANES) anthropometric protocol.[13] Body mass index (BMI) was calculated by dividing weight by the square of height in meters.
A mini nutrition assessment (MNA) questionnaire was utilized for screening malnutrition in the participants. The application of this tool was popular in clinical and research settings for a rapid national evaluation of older adults. The tool has 18 items and the lowest score of the tool is zero and the highest score is 30. The validation of the Persian version of MNA was approved in Iran[14] Blood samples were collected after at least 12-h overnight fasting. The serum was separated from the clot at 6000 rpm centrifuging as soon as possible after samples were collected. Then the serums were stored at less than −70°C temperature. Calcium and vitamin D were measured with the calorimetry method (Pars Azmoon Kit with Authoanalyzer (Prestige 241, Tokyo, Japan) and intra-assay coefficient = 1.07) and Vitamin D was measured by ELISA method [kit of 25-OH vitamin D, Padtan Gostar Isar (PGI Co.) using Ap22 Speedy Auto-analyzer (Rome, Italy)] with an intra-assay coefficient equal to 5.97.
Protocol of analysis
The serum levels of 25(OH) vitamin D <12 ng/mL were considered as VDD, and 12–20 ng/mL were considered as insufficient serum vitamin D. Survey analysis was standardized according to WHO population 2000–2025 to estimate the prevalence of insufficient and VDD. The weight of the age group in Birjand was also considered in the survey. Associated factors of VDD were evaluated in univariate and multiple logistic regression models. The backward approach was considered for multiple regression models. Sex remained in the model despite it being removed in a backward approach. All analyses were performed using STATA12 (StataCorp Texas, USA).
Ethical consideration
This study was approved by the Research Ethical Committee of the Endocrine and Metabolism Research Institute (EMRI) of Tehran (Ethical code: IR.TUMS.EMRI.REC.1396.00158) and Birjand University of Medical Sciences (BUMS) Research Committee (Ethical code: IR.bums.REC.1395.215). All the participants signed the informed consent. In cases of cognitive impairment (AMTS ≤6), informed consent was also signed by the official guardian of the participants.
Patient consent
Informed consent was obtained from all participants.
| Results | | |
A total of 1319 subjects were monitored for VDD. The mean age of the participants was 71.82 (SD = 7.63) years. The mean vitamin D level was 31.16 (SD = 16.00) ng/mL and the mean serum calcium level was equal to 9.26 (SD = 0.46) mg/dL. Of the total participants, 688 (52.16%) were women. Only 111 (8.425%) of the participants had a deficient serum level of vitamin D, (60; 8.72% of women and 51; 8.08% of men), 225 (17.06%) of the participant had insufficient levels of vitamin D (96; 13.95% of women and 129; 20.44% of men), and 983 (74.53%) had a sufficient serum level of vitamin D (532; 77.33% of women and 451; 71.47% of men). Only 101 (7.66%) of the participants used vitamin D supplements. There was a statistical difference between the mean serum vitamin D in those whose samples were collected in fall than in those whose serum samples were collected in winter (31.33, SD = 16.01 ng/mL in group collected in fall and 30.31, SD = 15.97 mg/dL in winter, P = 0.04). The serum level of vitamin D was lower in subjects who were smokers than those who were not a smoker (26.52; SD = 13.89 vs. 31.52; SD = 16.10 ng/mL), respectively. Normally, a history of vitamin D supplement intake was more common in those with normal than lower serum levels of vitamin D (11.47% vs. 4.61% and P < 0.001) [Table 1]. | Table 1: Comparison of demographic and anthropometric characteristics between low and normal vitamin D groups
Click here to view |
The WHO population age-standardized prevalence of VDD was higher in males than females (7.98%; 5.09%–12.30% in men and 7.60%; 2.26%–12.66% in women). The highest age-standardized prevalence of VDD was observed in women in the age group of 60–69 years old (10.77%; 95% CI 7.9–14.38) [Table 2]. | Table 2: The crude and WHO population age-standardized prevalence of levels of vitamin D in the older population of Birjand city
Click here to view |
In univariate logistic regression models, there was a negative association between VDD and being overweight (odds ratio for being overweight in comparison to low body weight was 0.421, P value = 0.031, use vitamin D supplements). In the multivariable logistic regression model, there was a negative association between VDD and age groups as well as being overweight or obese and vitamin D supplementation intake [Table 3].
| Discussion | | |
Our result showed that 8.42% of the participants had a deficient serum level of vitamin D <12 ng/mL, which is similar to the study conducted by Pellicane et al. (2011)[15] on inpatients (n = 101) admitted to the Rehabilitation Institute of Chicago that 8.1% had vitamin D deficiency. In contrast, Kader et al. (2019)[16] showed that out of 6774 adult participants, 83.8% of women and 18.2% of men population (10–91 years old) who were admitted to Karapınar city, in Turkey had VDD. These discrepancies between our study and other studies can be attributed to the effect of latitude because people who live at lower latitudes may have more sun exposure and a lower prevalence of VDD. Also, the measurement methods used in these studies (tandem-mass spectrometry, radioimmunoassay, and automated immunoassay systems)[17] could be other reasons for these discrepancies
There was a statistical difference between the mean of serum's vitamin D in those whose samples were collected in fall than in those whose serum samples were collected in winter.
Our result is almost similar to a previous study conducted by Hoteit et al. (2014).[18] In their study, 25(OH) D concentrations were lower in the winter (20.1 ± 9.2 ng/mL) compared with the fall (24.8 ± 10.4 ng/mL) 9147 Lebanese outpatients. However, this is in contrast with Boyages and Bilinski (2012)[19] that reported VDD was prevalent in summer and increased in spring but not in winter.
These differences could be due to the dependency of vitamin D production on sunlight. The seasonal variation previously has been reported in vitamin D concentrations and the seasonal variation itself depends on latitude.[20]
Smoking was more common in subjects who had low levels of vitamin D than those who had a normal level of serum vitamin D.
Our result is in line with Jiang et al. (2016).[21] In this study, there were 612 Guangzhou residents aged 50 years or older. However, it is in disagreement with the study conducted by Scragg et al. (1992)[22] that smoking had no correlation with 25(OH) D levels. One possible explanation is the direct effect of chemicals in tobacco which may influence the metabolism and function of vitamins as there is some evidence for the change in some genes in the metabolic pathway of vitamin D.[23] Also, one more reason is that smokers usually have bad lifestyles including low physical activity, bad dietary habits, and reduced sun exposure, which can influence the synthesis of vitamin D.[23]
Our study indicated that the prevalence of VDD was higher in males than females. This is in line with the longitudinal study done by Elizondo-Montemayor et al. (2017)[20] on healthy adults 55–86 years old in the northeastern part of Mexico who found women had higher 25(OH) D levels than men. However, our results are in contrast with the study done by Huang et al. (2017).[24] In this study, 30.6% of men and 57.7% of women of 170 healthy volunteers aged 65 or older in rural areas of southern Taiwan had a low vitamin D status. Since Abboud et al.[25] indicated >30 min/day of physical exercise has a significant protective effect against VDD in men but not in women, therefore, a lack of regular physical activity (<30 min/day) in men can be a risk factor. Also, older male adults refused to take a sufficient dosage of a vitamin D supplement, which may have led to continuous deficiency.
In this study, there was a negative association between VDD and being overweight and obesity.
Our findings are similar to those of cross-sectional observational study conducted by Sousa-Santos et al. (2018)[26] in Portugal that was done on 1447 individuals aged 65 years or older. They showed that there was an inverse association between obesity with serum 25(OH) D concentrations. However, our study is in contrast with the study done by Al Hayek et al. (2018)[27] in Lebanon on 344 subjects (50% men and 50% women) aged between 20 and 74 years that indicated BMI was not associated with vitamin D status. This is because vitamin D is a fat-soluble vitamin and adipose tissues act as a reservoir for vitamin D therefore leading to the sequestration of vitamin D by fat tissue and inadequate serum 25(OH) D concentrations in obese individuals.[28] Another mechanism that can affect serum vitamin D levels can be less outdoor activity in obese individuals and herein less skin exposure to sunlight.[29]
Our results indicated that the highest age-standardized prevalence of VDD was observed in women in the age group of 60–69 years old and the multivariable logistic regression model showed a negative association between VDD and age groups.
Our result is in agreement with the study by Kader et al. (2019),[16] which was done on 6774 adult (10–91 years old) admitted to Karapınar public hospital, Turkey. Our results, however, are in disagreement with a retrospective study conducted in Turkey, which found that the lowest VDD was in the age group of 0–10 years and the highest VDD was in the age group of 10–40 years old.[30] A possible explanation is that skin production of vitamin D due to UV radiation exposure, decreases with aging.[31] In addition, since the main source of vitamin D is the sun exposure and the diet, Muslim women who wear veils and clothes gain inadequate UVB radiation from the sun also; women have higher body fat compared with men and are at higher risk of hypovitaminosis in a sunny city such as Birjand.[32]
In conclusion, 25% of elderly individuals had 25(OH) D insufficiency or deficiency, and some modifiable lifestyle factors were associated with VDD or insufficiency. Vitamin D can be acquired from the diet or skin exposure to solar UVB radiation. On the other hand, there is growing evidence that living in areas with abundant sunlight may not be possible to attain adequate amounts of vitamin D. Therefore, education about the importance of consuming food sources high in vitamin D and the adoption of better nutrition seems essential.
Study limitations and strength
Since this study had a cross-sectional design, seasonal variation of 25(OH) D levels has not been considered during data analysis. A second limitation was that the precise geographic status of Birjand has not been determined consequently the degree of exposure to the sun is unknown. A third limitation was that only subjects in urban areas were studied and the vitamin D status of individuals in rural areas was unavailable therefore, the study may not be representative of the general population of South Khorasan.
The strength of this study is monitoring a large sample size of the elderly people, a population category that has not been particularly well-studied previously in South Khorasan to assess vitamin D status and related factors. Another strength of this study is investigating a large number of confounders and associated diseases.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Acknowledgment
The authors would like to thank the Vice Chancellor for the Research and Technology of the Birjand University of Medical Sciences (Research No: 4330) and Elderly Health Research Center of Endocrinology and Metabolism Research Institute in Tehran University of Medical Sciences.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
| References | | |
| 1. | United Nations Population Division. World population prospects 2019. New York: United Nations Population Division; 2019 [cited 2019 Aug 16]. Available from: https://population.un.org/wpp/Download/Standard/Population/.  |
| 2. | Estebsari F, Dastoorpoor M, Khalifehkandi ZR, Nouri A, Mostafaei D, Hosseini M, et al. The concept of successful aging: a review article. Current aging science 2020;13:4-10. |
| 3. | Prince MJ, Wu F, Guo Y, Robledo LMG, O'Donnell M, Sullivan R, et al. The burden of disease in older people and implications for health policy and practice. The Lancet 2015;385:549-62. |
| 4. | Laird E, Ward M, McSorley E, Strain J, Wallace J. Vitamin D and bone health; Potential mechanisms. Nutrients 2010;2:693-724. |
| 5. | Hill TR, Aspray TJ. The role of vitamin D in maintaining bone health in older people. Therapeutic advances in musculoskeletal disease 2017;9:89-95. |
| 6. | Lips P. Vitamin D status and nutrition in Europe and Asia. The Journal of steroid biochemistry and molecular biology 2007;103:620-5. |
| 7. | Ross AC, Manson JE, Abrams SA, Aloia JF, Brannon PM, Clinton SK, et al. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know. The Journal of Clinical Endocrinology & Metabolism 2011;96:53-8. |
| 8. | Spiro A, Buttriss J. Vitamin D: an overview of vitamin D status and intake in E urope. Nutrition bulletin 2014;39:322-50. |
| 9. | Aspell N, Laird E, Healy M, Shannon T, Lawlor B, O'Sullivan M. The prevalence and determinants of vitamin D status in community-dwelling older adults: results from the English Longitudinal Study of Ageing (ELSA). Nutrients 2019;11:1253. |
| 10. | Mansouri S, Safabakhsh M, Forooshani AR, Shab-Bidar S. The prevalence of vitamin D and calcium supplement use and association with serum 25-hydroxy-vitamin D (25 (OH) D) and demographic and socioeconomic variables in Iranian elderly. International Journal of Preventive Medicine 2021;12:36. |
| 11. | Moodi M, Firoozabadi MD, Kazemi T, Payab M, Ghaemi K, Miri MR, et al. Birjand longitudinal aging study (BLAS): the objectives, study protocol and design (wave I: baseline data gathering). Journal of Diabetes & Metabolic Disorders 2020;19:551-9. |
| 12. | Stel VS, Smit JH, Pluijm SM, Visser M, Deeg DJ, Lips P. Comparison of the LASA Physical Activity Questionnaire with a 7-day diary and pedometer. Journal of clinical epidemiology 2004;57:252-8. |
| 13. | Control CfD, Prevention. National health and nutrition examination survey (nhanes): Anthropometry procedures manual. Atlanta: Centers for Disease Control and Prevention. 2007. |
| 14. | Doroudi T, Alizadeh-Khoei M, Kazemi H, Hormozi S, Taati F, Ebrahimi M, et al. Comparison of two validation nutrition tools in hospitalized elderly: Full mini nutritional assessment and short-form mini nutritional assessment. International journal of preventive medicine 2019;10. |
| 15. | Pellicane AJ, Wysocki NM, Mallinson TR, Schnitzer TJ. Prevalence of 25-hydroxyvitamin D deficiency in the acute inpatient rehabilitation population and its effect on function. Archives of physical medicine and rehabilitation 2011;92:705-11. |
| 16. | Kader S, Comaklı H, Tekindal MA. Evaluation of serum vitamin D levels according to gender and age at Karapınar City: a follow-up study from Turkey. Dubai Medical Journal 2019;2:141-5. |
| 17. | Leary PF, Zamfirova I, Au J, McCracken WH. Effect of latitude on vitamin D levels. J Am Osteopath Assoc 2017;117:433-9. |
| 18. | Hoteit M, Al-Shaar L, Yazbeck C, Sleiman MB, Ghalayini T, Fuleihan GE-H. Hypovitaminosis D in a sunny country: time trends, predictors, and implications for practice guidelines. Metabolism 2014;63:968-78. |
| 19. | Boyages S, Bilinski K. Seasonal reduction in vitamin D level persists into spring in NSW Australia: implications for monitoring and replacement therapy. Clinical endocrinology 2012;77:515-23. |
| 20. | Elizondo-Montemayor L, Castillo EC, Rodríguez-López C, Villarreal-Calderón JR, Gómez-Carmona M, Tenorio-Martínez S, et al. Seasonal variation in vitamin D in association with age, inflammatory cytokines, anthropometric parameters, and lifestyle factors in older adults. Mediators of Inflammation 2017;2017. |
| 21. | Jiang C, Chan Y, Xu L, Jin Y, Zhu T, Zhang W, et al. Smoking and serum vitamin D in older Chinese people: cross-sectional analysis based on the Guangzhou Biobank Cohort Study. BMJ open 2016;6. |
| 22. | Scragg R, Holdaway I, Jackson R, Lim T. Plasma 25-hydroxyvitamin D3 and its relation to physical activity and other heart disease risk factors in the general population. Annals of epidemiology 1992;2:697-703. |
| 23. | O'Shaughnessy PJ, Monteiro A, Bhattacharya S, Fowler PA. Maternal smoking and fetal sex significantly affect metabolic enzyme expression in the human fetal liver. The Journal of Clinical Endocrinology & Metabolism 2011;96:2851-60. |
| 24. | Huang C-H, Huang Y-TA, Lai Y-C, Sun C-K. Prevalence and predictors of hypovitaminosis D among the elderly in subtropical region. PLoS One 2017;12:e0181063. |
| 25. | Abboud M, Puglisi D, Davies B, Rybchyn M, Whitehead N, Brock K, et al. Evidence for a specific uptake and retention mechanism for 25-hydroxyvitamin D (25OHD) in skeletal muscle cells. Endocrinology 2013;154:3022-30. |
| 26. | Sousa-Santos AR, Afonso C, Santos A, Borges N, Moreira P, Padrão P, et al. The association between 25 (OH) D levels, frailty status and obesity indices in older adults. PLoS One 2018;13:e0198650. |
| 27. | Al Hayek S, Mosleh JMB, Ghadieh R, Fares JEH. Vitamin D status and body composition: a cross-sectional study among employees at a private university in Lebanon. BMC nutrition 2018;4:1-11. |
| 28. | Larose TL, Chen Y, Camargo CA, Langhammer A, Romundstad P, Mai X-M. Factors associated with vitamin D deficiency in a Norwegian population: the HUNT Study. J Epidemiol Community Health 2014;68:165-70. |
| 29. | Kull M, Kallikorm R, Lember M. Body mass index determines sunbathing habits: implications on vitamin D levels. Internal medicine journal 2009;39:256-8. |
| 30. | Serdar MA, Can BB, Kilercik M, Durer ZA, Aksungar FB, Serteser M, et al. Analysis of changes in parathyroid hormone and 25 (OH) vitamin D levels with respect to age, gender and season: a data mining study. Journal of Medical Biochemistry 2017;36:73. |
| 31. | Gallagher JC. Vitamin D and aging. Endocrinology and Metabolism Clinics 2013;42:319-32. |
| 32. | Ojah RC, Welch JM. Vitamin D and musculoskeletal status in Nova Scotian women who wear concealing clothing. Nutrients 2012;4:399-412. |
[Table 1], [Table 2], [Table 3]
|
Search Pubmed for