ISRN AIDS

Volume 2012 (2012), Article ID 485307, 5 pages

doi:10.5402/2012/485307

Clinical Study

Prevalence and Factors Associated with Vitamin D Deficiency and Hyperparathyroidism in HIV-Infected Patients Treated in Barcelona

Elisabet Lerma, M. Ema Molas, M. Milagro Montero, Ana Guelar, Alicia González, Judith Villar, Adolf Diez, and Hernando Knobel

Department of Internal Medicine and Infectious Diseases, Hospital del Mar, 08003 Barcelona, Spain

Received 25 April 2012; Accepted 23 May 2012

Academic Editors: S. Blanche and P. Price

Copyright © 2012 Elisabet Lerma et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Vitamin D deficiency is an important problem in patients with chronic conditions including those with human immunodeficiency virus (HIV) infection. The aim of this cross-sectional study was to identify the prevalence and factors associated with vitamin D deficiency and hyperparathyroidism in HIV patients attended in Barcelona. Cholecalciferol (25OH vitamin D3) and PTH levels were measured. Vitamin D insufficiency was defined as 25(OH) D < 20 ng/mL and deficiency as <12 ng/mL. Hyperparathyroidism was defined as PTH levels >65 pg/mL. Cases with chronic kidney failure, liver disease, treatments or conditions potentially affecting bone metabolism were excluded. Among the 566 patients included, 56.4% were exposed to tenofovir. Vitamin D insufficiency was found in 71.2% and 39.6% of those had deficiency. PTH was measured in 228 subjects, and 86 of them (37.7%) showed high levels. Adjusted predictors of vitamin D deficiency were nonwhite race and psychiatric comorbidity, while lipoatrophy was a protective factor. Independent risk factors of hyperparathyroidism were vitamin D < 12 ng/mL (OR: 2.14, CI 95%: 1.19–3.82, P: 0.01) and tenofovir exposure (OR: 3.55, CI 95%: 1.62–7.7, P: 0.002). High prevalence of vitamin deficiency and hyperparathyroidism was found in an area with high annual solar exposure.

1. Introduction

Vitamin D is a steroid liposoluble hormone and can be made available to the individuals in two forms. First, vitamin D3 or cholecalciferol is synthesized in the skin in response to ultraviolet B radiation and is present in oil-rich fish (salmon, mackerel, and herring), egg yolks, and liver [1]. Second, vitamin D2 or ergocalciferol is obtained from the UV irradiation of the yeast sterol ergosterol and is found in sun-exposed mushrooms [2]. Both vitamins undergo identical metabolism, and as they are biologically inert, they require two hydroxylations, in the liver and in the kidney, to become 1,25dihydroxyvitamin D [1,25(OH)₂D] which is the biologically active form of the hormone [3, 4]. In the intestine this stimulates the absorption of calcium and phosphorus, and it helps to regulate the metabolism of both minerals within bone and kidney interaction [5]. Assessment of vitamin D is based on measurement of serum 25(OH)D [6, 7] that is the most stable and plentiful metabolite of vitamin D in serum and has a half-life about 21 days [8].

The main source of vitamin D is exposure to sunlight [9, 10]. Therefore, an insufficient exposure to sunlight is a major cause of vitamin D deficiency. Other causes of vitamin deficiency are sunscreen sun protection [2], dark skin body mass index (BMI) greater than 30 [11, 12], malabsorptive conditions, and use of a wide variety of medications including antiretroviral drugs. Vitamin D levels are also related to seasonal fluctuation, geographic latitude, time of the day for sunlight exposure, and age.

Vitamin D deficiency results in abnormalities in bone metabolism [13, 14] and in calcium and phosphorus homeostasis [15, 16] and is increasingly recognized as a key factor in many chronic diseases [17, 18].

HIV patients suffer from several risk factors of vitamin D deficiency [19–21]. Not only they have a high prevalence of disorders related to lower vitamin D levels [22], but also they are exposed to some antiretrovirals that can lead to vitamin D deficiency [23], specially efavirenz [24–26] and protease inhibitor, which seem to be associated with higher rates of vitamin D deficiency. Moreover, some studies suggest a protective effect of vitamin D against HIV disease progression, mortality, and AIDS events that might be explained by its role in immune function [27].

2. Methods

2.1. Study Design, Setting, and Participants

This is a cross-sectional study of 566 HIV-infected patients, older than 18 years old, with and without antiretroviral treatment, recruited in a consecutive manner from August 2010 to August 2011, in a tertiary referral center for HIV patients in Barcelona, Spain.

Data were collected every 3 to 6 months and included CD4 cell counts, viral loads (COBAS, AmpliPrep/TaqMan HIV-1 test, Roche Diagnostics), HIV-related parameters including time since HIV diagnosis, CDC stage, cART regimen, medical history, hepatitis coinfection, ethnicity, and patients demographics. The primary outcomes were plasma levels of 25(OH)D (competitive chemiluminescent immunoassay, CLIA, City, Country) and intact PTH (solid-phase sequential chemiluminescent immunoassay, Manufacturer, City, Country). Secondary variables included calcium, phosphorus, cholesterol, and renal and liver parameters.

Patients with kidney insufficiency (defined as GFR < 30 mL/min) and liver insufficiency (Child Pugh Stage C) and those patients whose records contained insufficient data were excluded from the analysis.

2.2. Definitions

Vitamin D insufficiency was defined as 25(OH)D <20 ng/mL and deficiency as <12 ng/mL. Hyperparathyroidism was defined as PTH levels >65 pg/mL. Undetectable viral load (HIV RNA) was defined as HIV RNA <20 copies/mL.

2.3. Statistical Analysis

Proportions were compared with and Fisher exact test.

Quantitative data were analyzed using Student’s t-test for normal variables. The Mann Whitney U test was used to compare variables with nonnormal distribution. The variables observed as risk factor of vitamin D deficiency or hyperparathyroidism in the univariate analysis were included in the logistic regression model (SPSS version 18 was used to all statistics analysis).

3. Results

3.1. Demographic and Baseline Information

A total of 592 patients were eligible for the study. From those, participants suitable for inclusion because the records had the necessary information were 566.

Among the participants, 400 (70.7%) were men and 480 (84.8%) were Caucasian (86 from different races), with a median age of 46 years (DS 9.5). The predominant mode for HIV transmission was IDU in 291 cases (51%). The median nadir CD4 cell count was 156 (49–269) cells/mm³, and current CD4 was 537 cell/mm³. A total of 492 patients (87%) had undetectable viral load. At the time of sampling, 549 patients were taking cART, with a median exposure time of 8.9 years (4.9–11.7). From those, 319 (56.4%) were exposed to tenofovir, 115 (20.3%) to abacavir, and 453 (80%) to ritonavir-boosted protease inhibitor regimen.

3.2. Vitamin D and PTH: Predictor Factors

In the series 403 (71.2%), patients showed plasma levels of vitamin D in the range of insufficiency and 244 (39.6%) within deficiency levels. Of the 566 patients enrolled in the trial, PTH was measured in 228 cases, with high levels (>65 ng/mL) in 86 of them (37.7%). The median PTH in all patients with hyperparathyroidism was 90 pg/ml (range: 66–200).

Predictors of vitamin D deficiency were (Table 1) non-Caucasian race, OR: 3.18 (CI 95%: 1.49–6.78; P: 0.003), and psychiatric concomitant disorders OR: 1.5 (CI 95%: 1.03–2.18; P: 0.003), while lipoatrophy was a protective factor OR 0.67 (CI 95%: 0.46–0.99; P: 0.05). Independent risk factors for predicting levels of PTH > 65 pg/mL were (Table 2) vitamin D plasma levels <12 ng/mL (OR: 2.14. IC95%: 1.19–3.82, P: 0.01) and tenofovir exposure (OR : 3.55, IC95% 1.62–7.7, P: 0.002). Elevated PTH levels were associated with the use of tenofovir regardless of the vitamin D level, with a significant difference compared to patients with no exposure to this drug (Table 3). Those patients with elevated PTH receiving tenofovir present with estimated glomerular filtration rates >60 ml/min as well as normal serum calcium and phosphorus levels. Age, gender, BMI, dyslipidemia, diabetes, hepatitis B or C coinfection, current CD4 cell count, current HIV-1 RNA load, time of exposure to ART, and use of protease inhibitors were not associated with abnormal vitamin D or PTH levels.

Table 1: Predictors of vitamin D deficiency (Vit D < 12 ng/mL).

Univariate analysis
Adjusted analysis

OR (IC95%)

OR (IC95%)

Non-Caucasian
3.52 (1.69–7.3)
0.0001
3.18 (1.49–6.78)
0.003

Psychiatric comorbidity
1.48 (1.02–2.15)
0.004
1.5 (1.03–2.18)
0.003

Lipoatrophy
0.68 (0.46–1)
0.05
0.67 (0.46–0.99)
0.05

Table 2: Risk factors for high levels of PTH (PTH > 65 pg/mL).

Univariate analysis
Adjusted analysis

OR (IC95%)

OR (IC95%)

Tenofovir exposure
3.35 (1.86–6.02)
0.0001
3.55 (1.62–7.7)
0.002

Vitamin D < 12 ng/mL
2.2 (1.26–3.82)
0.007
2.14 (1.19–3.82)
0.01

Abacavir exposure
0.48 (0.23–0.98)
0.04
0.87 (0.33–2.31)
0.7

Table 3: Interactions between tenofovir exposure and vitamin D deficiency with PTH levels.

Vitamin D < 12 ng/ml
Vitamin D > 12 ng/ml

Tenofovir
No tenofovir
Tenofovir
No tenofovir
A versus B
A versus C

(: 52) A
(: 33) B
(: 78) C
(: 65) D

PTH > 65 pg/mL
31 (59.6%)
11 (33.3%)
33 (42.3%)
11 (16.9%)
: 0.002
: 0.07

Median PTH pg/mL (IQR)
76 (52.5–105.5)
47 (31.5–75)
52 (35–90)
40 (26–57)
: 0.001
: 0.007

4. Discussion

4.1. Vitamin D is Recognized as a Key Factor for Many Chronic Diseases

Therefore, it seems plausible that vitamin D screening and the use of this hormone supplementation could benefit HIV patients when beginning ART.

The present study showed a high prevalence of vitamin D deficiency in HIV patients in an area with high solar exposure.

The prevalence of vitamin D suboptimal levels among HIV patients goes from 60 to 95% (including those with insufficiency and deficiency). In EUROSIDA [28] cohorts the prevalence is of 83%, while in two cohorts in Swiss [29] and Denmark [30], respectively, it represents a 42% and 95%. Regardless, there are two different studies in the United States, where despite having a 63% to 75% of vitamin D deficiency among HIV-infected people, the prevalence was lower than in uninfected HIV people [22, 31]. It is difficult to compare the prevalence of vitamin D insufficiency and deficiency because different definitions were used. Some studies define vitamin D deficiency as a 25OH D less than 20 ng/mL, and insufficiency as a 25OH D of 21–29 ng/mL [32, 33]. However, in cohorts of HIV patients, such as EuroSIDa, insufficiency is defined as 25OH D levels below 20 ng/mL and deficiency as levels less than 12 ng/mL [28]. The vitamin D deficiency was observed regardless of age, sex, BMI, dyslipidemia, diabetes, hepatitis B or C coinfection, current CD4 cell count, current HIV-1 RNA load, time of exposure to ART, and use of protease inhibitors.

Race, seasonality, overweight, nadir CD4 cell count less than 200 cell/mm³, and exposure to some ART, specially efavirenz, have been identified in other studies as risk factors for vitamin D deficiency [34, 35].

In our study, predictors of vitamin D deficiency were being non-Caucasian OR: 3.18 (CI 95%: 1.49–6.78, P: 0.003) and having psychiatric comorbidity OR: 1.5 (CI 95%: 1.03–2.18; P: 0.003), while lipoatrophy was a protective factor OR 0.67 (CI 95%: 0.46–0.99; P: 0.05).

Black ethnic was associated with vitamin D deficiency, because they require a longer exposure to produce the same amount of vitamin D than white people do [36]. Patients with psychiatric disorders are also identified as patients with a higher risk for vitamin D deficiency, as poorest diets and a lack of exposure to sunlight are common in this kind of patients [37]. Both factors play a major role in the development of vitamin D deficiency [38, 39].

We identified lipoatrophy as a protective factor. This could be explained by the fact that vitamin D is a liposoluble hormone, so lipoatrophy could avoid the accumulation of this vitamin in the body fat, increasing circulating levels. Some studies do identify lipoatrophy as a risk factor for vitamin D deficiency [40] against other studies where lipoatrophy is not related with vitamin D [41].

Independent risk factors for high levels of PTH were vitamin  ng/mL and tenofovir exposure. Elevated PTH levels were associated with the use of tenofovir regardless of the values of vitamin D, with a significant difference compared to patients with no exposure to tenofovir. Cross-sectional data have linked tenofovir with higher parathyroid hormone (PTH) concentration in patients with vitamin D deficiency [42]. Masiá et al. [43] performed a longitudinal study to evaluate sequential changes in PTH and 25-hydroxyvitamin D [25(OH) D] levels in patients starting cART with either tenofovir/emtricitabine or abacavir/lamivudine. Tenofovir/emtricitabine exposure was an independent predictor of high PTH levels (≥53 ng/liter). The underlying mechanisms of this effect are not well understood, but it is presumable that tenofovir causes a decrease in intestinal calcium absorption, leading to an increase in PTH secretion. Moreover, vitamin D supplementation has shown a decrease in PTH levels, regardless of baseline levels of vitamin D [44].

Limitations of the present study are mainly related to the observational design and cross-sectional nature of the current analyses. In this respect, the results reported herein are only associations from which no conclusions regarding causality can be drawn. The 25(OH) D measurements were done along the whole year, using the highest value as the referent value in each patient, independently of seasonallity.

Due to the wide range of biological actions of vitamin D and the fact that its deficiency has been associated with many adverse outcomes, such as infectious diseases, muscle weakness, metabolic, and cardiovascular and bone disease, we suggest the importance of screening in HIV patients in whom some of these pathologies are more prevalent and, in accordance with the results, starting a systematic supplementation given the high prevalence of the problem in HIV patients.

Conflict of Interests

The authors declare no conflict of interests.

References

T. D. Thacher and B. L. Clarke, “Vitamin D insufficiency,” Mayo Clinic Proceedings, vol. 86, no. 1, pp. 50–60, 2011.

M. F. Holick, N. C. Binkley, H. A. Bischoff-Ferrari et al., “Evaluation, treatment, and prevention of vitamin D deficiency: an endocrine society clinical practice guideline,” Journal of Clinical Endocrinology and Metabolism, vol. 96, no. 7, pp. 1911–1930, 2011.

M. F. Holick, “Medical progress: vitamin D deficiency,” New England Journal of Medicine, vol. 357, no. 3, pp. 266–281, 2007.

H. F. DeLuca, “Overview of general physiologic features and functions of vitamin D,” The American Journal of Clinical Nutrition, vol. 80, pp. 1989S–1969S, 2004.

S. Christakos, P. Dhawan, Y. Liu, X. Peng, and A. Porta, “New insights into the mechanisms of vitamin D action,” Journal of Cellular Biochemistry, vol. 88, no. 4, pp. 695–705, 2003.

M. F. Holick, “Vitamin D status: measurement, interpretation, and clinical application,” Annals of Epidemiology, vol. 19, no. 2, pp. 73–78, 2009.

H. A. Bischoff-Ferrari, A. Shao, B. Dawson-Hughes, J. Hathcock, E. Giovannucci, and W. C. Willett, “Benefit-risk assessment of vitamin D supplementation,” Osteoporosis International, vol. 21, no. 7, pp. 1121–1132, 2010.

R. J. Singh, R. L. Taylor, G. S. Reddy, and S. K. G. Grebe, “C-3 epimers can account for a significant proportion of total circulating 25-hydroxyvitamin D in infants, complicating accurate measurement and interpretation of vitamin D status,” Journal of Clinical Endocrinology and Metabolism, vol. 91, no. 8, pp. 3055–3061, 2006.

B. W. Hollis, “Circulating 25-hydroxyvitamin D levels indicative of vitamin D sufficiency: implications for establishing a new effective dietary intake recommendation for vitamin D,” Journal of Nutrition, vol. 135, no. 2, pp. 317–322, 2005.

C. Brot, P. Vestergaard, N. Kolthoff, J. Gram, A. P. Hermann, and O. H. Sørensen, “Vitamin D status and its adequacy in healthy Danish perimenopausal women: relationships to dietary intake, sun exposure and serum parathyroid hormone,” British Journal of Nutrition, vol. 86, supplement 1, pp. S97–S103, 2001.

C. M. Lenders, H. A. Feldman, E. Von Scheven et al., “Relation of body fat indexes to vitamin D status and deficiency among obese adolescents,” American Journal of Clinical Nutrition, vol. 90, no. 3, pp. 459–467, 2009.

M. Blum, G. Dolnikowski, E. Seyoum et al., “Vitamin D3 in fat tissue,” Endocrine, vol. 33, no. 1, pp. 90–94, 2008.

M. Pfeifer, B. Begerow, H. W. Minne, C. Abrams, D. Nachtigall, and C. Hansen, “Effects of a short-term vitamin D and calcium supplementation on body sway and secondary hyperparathyroidism in elderly women,” Journal of Bone and Mineral Research, vol. 15, no. 6, pp. 1113–1118, 2000.

M. Pfeifer, B. Begerow, H. W. Minne, K. Suppan, A. Fahrleitner-Pammer, and H. Dobnig, “Effects of a long-term vitamin D and calcium supplementation on falls and parameters of muscle function in community-dwelling older individuals,” Osteoporosis International, vol. 20, no. 2, pp. 315–322, 2009.

P. Lips, “Interaction between vitamin D and calcium,” Scandinavian Journal of Clinical and Laboratory Investigation, Supplementum, vol. 243, pp. 60–64, 2012.

H. A. Bischoff-Ferrari, W. C. Willett, J. B. Wong et al., “Prevention of nonvertebral fractures with oral vitamin D and dose dependency: a meta-analysis of randomized controlled trials,” Archives of Internal Medicine, vol. 169, no. 6, pp. 551–561, 2009.

S. Khosla, “What do we tell our patients about calcium and vitamin D supplementation?” Journal of Clinical Endocrinology and Metabolism, vol. 96, no. 1, pp. 69–71, 2011.

L. Milazzo, C. Mazzali, G. Bestetti et al., “Liver-related factors associated with low vitamin D levels in HIV and HIV/HCV coinfected patients and comparison to general population,” Current HIV Research, vol. 9, no. 3, pp. 186–193, 2011.

S. Mehta, E. Giovannucci, F. M. Mugusi et al., “Vitamin D status of HIV-infected women and its association with hiv disease progression, anemia, and mortality,” PLoS ONE, vol. 5, no. 1, Article ID e8770, 2010.

A. I. Choi, J. C. Lo, K. Mulligan et al., “Association of vitamin D insufficiency with carotid intima-media thickness in HIV-infected persons,” Clinical Infectious Diseases, vol. 52, no. 7, pp. 941–944, 2011.

C. N. Dao, P. Patel, E. T. Overton et al., “Low vitamin D among HIV-infected adults: prevalence of and risk factors for low vitamin D levels in a cohort of HIV-infected adults and comparison to prevalence among adults in the us general population,” Clinical Infectious Diseases, vol. 52, no. 3, pp. 396–405, 2011.

S. Kappor, “The role of vitamin D supplementation in HIV: an often forgotten perspective,” Climacteric, vol. 14, article 294, 2011.

J. Fox, B. Peters, M. Prakash, J. Arribas, A. Hill, and C. Moecklinghoff, “Improvement in vitamin D deficiency following antiretroviral regime change: results from the MONET trial,” AIDS Research and Human Retroviruses, vol. 27, no. 1, pp. 29–34, 2011.

M. Yin and E. Stein, “The effects of TAR on vitamin,” Clinical Infectious Diseases, vol. 52, pp. 406–408, 2011.

A. Pasquet, N. Viget, F. Ajana et al., “Vitamin D deficiency in HIV-infected patients: associated with non-nucleoside reverse transcriptase inhibitor or efavirenz use?” AIDS, vol. 25, no. 6, pp. 873–874, 2011.

A. Conesa-Botella, E. Florence, L. Lynen, R. Colebunders, J. Menten, and R. Moreno-Reyes, “Decrease of vitamin D concentration in patients with HIV infection on a non nucleoside reverse transcriptase inhibitor-containing regimen,” AIDS Research and Therapy, vol. 7, article 40, 2010.

S. Mehta, F. M. Mugusi, D. Spiegelman et al., “Vitamin D status and its association with morbidity including wasting and opportunistic illnesses in HIV-infected women in Tanzania,” AIDS Patient Care STDS, vol. 25, no. 10, pp. 579–585, 2011.

J. P. Viard, J. C. Souberbielle, O. Kirk et al., “Vitamin D and clinical disease progression in HIV infection: results from the EuroSIDA study,” AIDS, vol. 25, no. 10, pp. 1305–1315, 2011.

N. J. Mueller, C. A. Fux, B. Ledergerber et al., “High prevalence of severe vitamin D deficiency in combined antiretroviral therapy-naive and successfully treated Swiss HIV patients,” AIDS, vol. 24, no. 8, pp. 1127–1134, 2010.

U. C. Bang, S. A. Shakar, M. F. Hitz et al., “Deficiency of 25-hydroxyvitamin D in male HIV-positive patients: a descriptive cross-sectional study,” Scandinavian Journal of Infectious Diseases, vol. 42, no. 4, pp. 306–310, 2010.

O. M. Adeyemi, D. Agniel, A. L. French et al., “Vitamin D deficiency in HIV-infected and HIV-uninfected women in the United States,” Journal of Acquired Immune Deficiency Syndromes, vol. 57, no. 3, pp. 197–204, 2011.

M. G. Sweet, J. M. Sweet, M. P. Jeremiah, and S. S. Galazka, “Diagnosis and treatment of osteoporosis,” American Family Physician, vol. 79, no. 3, pp. 193–200, 2009.

A. C. Ross, J. E. Manson, S. A. Abrams et al., “The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know,” Journal of Clinical Endocrinology and Metabolism, vol. 96, no. 1, pp. 53–58, 2011.

T. Welz, K. Childs, F. Ibrahim et al., “Efavirenz is associated with severe vitamin D deficiency and increased alkaline phosphatase,” AIDS, vol. 24, no. 12, pp. 1923–1928, 2010.

A. M. Caro-Murillo, A. Gil Luciano, G. Navarro Rubio, M. Leal Noval, and J. R. Blanco Ramos, “HIV infection in different age groups: potential implications for prevention. CoRIS Cohort, Spain, 2004–2008,” Medicina Clinica, vol. 134, no. 12, pp. 521–527, 2010.

T. L. Clemens, J. S. Adams, S. L. Henderson, and M. F. Holick, “Increased skin pigment reduces the capacity of skin to synthesise vitamin D3,” The Lancet, vol. 1, no. 8263, pp. 74–76, 1982.

J. B. Koster and B. A. Künbauch, “Vitamin D deficiency and phsychiatrics patients,” Tijdschrift Voor Psychiatrie, vol. 53, pp. 561–565, 2011.

A. Agorastos, A. Weinas, A. D. Agorastos, and K. Wiedemann, “Psychosis-induced vitamin D deficiency with secondary hyperparathyroidism and osteoporotic fractures,” General Hospital Psychiatry, vol. 33, pp. 641.e3–641.e5, 2011.

G. Parker and H. Brotchie, “D for depression: any role for vitamin D? Food for Thought’ll,” Acta Psychiatrica Scandinavica, vol. 124, pp. 243–249, 2011.

J. R. Curtis, B. Smith, M. Weaver, et al., “Ethnic variations in the prevalence of metabolic bone disease among HIV-positive patients with lipodistrophy,” AIDS Research and Human Retroviruses, vol. 22, pp. 125–131, 2006.

E. Hammond, E. McKinnon, P. Glendenning, R. Williams, S. Mallal, and E. Phillips, “Association between 25-OH vitamin D and insulin is independent of lipoatrophy in HIV,” Clinical Endocrinology, vol. 76, pp. 201–206, 2012.

K. E. Childs, S. L. Fishman, C. Constable et al., “Short communication: inadequate vitamin D exacerbates parathyroid hormone elevations in tenofovir users,” AIDS Research and Human Retroviruses, vol. 26, no. 8, pp. 855–859, 2010.

M. Masiá, S. Padilla, C. Robledano, N. López, J. M. Ramos, and F. Gutiérrez, “Early changes in parathyroid hormone concentrations in HIV-infectes patients initiating antiretroviral therapy with tenofovir,” AIDS Research and Human Retroviruses, vol. 28, no. 3, pp. 242–246, 2012.

P. L. Havens, C. B. Stephensen, and R. Hazra, “Vitamin D3 supplementation decreases parathyroid hormone (PTH) in HIV-infected youth being treated with tenofovir-containing combination antiretroviral therapy: randomized, double-blind, placebo-controlled trial,” Clinical Infectious Diseases, vol. 54, no. 7, pp. 1013–1025, 2012.