Vitamin D supplementation for sickle cell disease

  • Protocol
  • Intervention



This is the protocol for a review and there is no abstract. The objectives are as follows:

To determine the effects on general health, bone and respiratory health, and the safety of vitamin D supplementation in children and adults with SCD.

To investigate the hypothesis that vitamin D supplementation increases serum 25-hydroxyvitamin D level in children and adults with SCD.


Description of the condition

Sickle cell disease (SCD) is an inherited chronic hemolytic and pro-inflammatory disorder that affects over 100,000 people in the USA and millions around the globe (Ashley-Koch 2000; CDC 2012b; Weatherall DJ). The disease is most commonly found in sub-Saharan Africa as well as in the Mediterranean basin, Saudi Arabia, Central America and India. In some African countries, the prevalence of the sickle gene in the population ranges from less than 1% to as high as 45% (WHO 2010). The clinical manifestations of SCD result from the presence of mutations on the beta globin genes that generate an abnormal hemoglobin product (called hemoglobin S) within the red blood cell (WHO 2010). During periods of hypoxemia and deoxygenation, a conformational change occurs in the sickle hemoglobin that results in the deformation of red blood cells into a 'sickle' shape. Downstream effects of this include increased vascular adhesivity that leads to vaso-occlusion and exaggerated hemolysis, which in turn leads to a chronic anemia. There are several genotypic variants that comprise the range of disorders referred to as SCD. The most common and perhaps most clinically severe form, is sickle cell anemia (homozygous SS disease), which results from the inheritance of two abnormal sickle S genes. The co-inheritance of two different abnormal beta globin genes (one being the sickle S gene) results in the various subtypes of SCD. Common subtypes of SCD include haemoglobin SC disease (co-inheritance of one sickle S gene and an abnormal haemoglobin (C) gene) (CDC 2012a) and  haemoglobin Sβ+/0 thalassaemia (co-inheritance of the sickle S genes (S) with an abnormal beta thalassemia gene) (Rees 2010).

Pain and musculoskeletal complications are the most common reason for health care utilization among people with SCD and remains an important cause of acute and chronic morbidity (Almeida 2005). Acute bone complications such as painful vaso-occlusive crisis, osteomyelitis, stress fractures, orbital compression syndrome, vertebral collapse and bone marrow necrosis are common in patients with SCD. Chronic bone complications such as osteonecrosis, chronic degenerative arthritis, osteoporosis, compression spine deformities, dental gnathopathy and impaired growth are also common (Almeida 2005). Individuals with SCD suffer acute and chronic end-organ damage secondary to recurrent episodes of vaso-occlusion and chronic ongoing hemolysis. Acute complications not related to bones include strokes, splenic and hepatic sequestration, acute chest syndrome, priapism and aplastic crisis. Chronic end-organ damage in SCD may present as sickle nephropathy, retinopathy, leg ulcers, pulmonary hypertension and chronic endocrinopathies (Kato 2007). In addition, SCD patients are at greater risk of bacterial infection (Ramakrishnan 2010) which contributes to early mortality (Platt 1994).

Individuals with SCD suffer global deficits in energy and nutrients intake (Hyacinth 2010; Kawchak 2007), with increased catabolism that ultimately causes multiple macro- and micro-nutritional deficiencies (Hyacinth 2010). Among the micronutrient deficiencies, vitamin D deficiency has been found to be extremely common in people with SCD, regardless of age and season (Rovner 2008). The prevalence of vitamin D deficiency among individuals with SCD ranges from 33% to 100% (Adewoye 2008; Buison 2004; Goodman 2010; Gorrido 2012; Jackson 2012; Rovner 2008). Since vitamin D regulates the absorption and excretion of calcium and is essential for bone mineralization (AFMC 2009), its deficiency in SCD patients may contribute to the myriad of musculoskeletal health problems encountered. These include muscle weakness, chronic debilitating bone pain, avascular necrosis, bone fragility and compression fractures (Osunkwo 2011), which in turn can lead to functional impairment in mobility and interfere in education, employment and psychosocial development (Swanson 2011).

Description of the intervention

Sources of vitamin D include skin exposure to ultraviolet B radiation in sunlight, dietary sources such as oily fish and variably fortified food, and supplementation (Kennel 2010; Pramyothin 2012). There are two important forms of vitamin D: calciferol (ergocalciferol) (vitamin D2); and cholecalciferol (vitamin D3), which are both biologically inert form (Holick 2007; Kennel 2010; Park 2011). Vitamin D is metabolized in the liver to 25-hydroxyvitamin D (25(OH)D) which is the major form circulating in the blood stream (Chung 2009; Pramyothin 2012). Subsequently, the 25-hydroxyvitamin D (25(OH)D) is metabolized into its active form 1,25-dihydroxyvitamin D (1,25(OH)2D), mainly in the kidney and some other organs such as the colon, prostate and mammary gland, etc by the action of the 1 α-hydroxylase enzyme.

The primary functions of vitamin D are to maintain calcium homeostasis and support bone health (Gonzalez 2010; Kennel 2010). Vitamin D affects the calcium and phosphorous absorption in the intestine and indirectly affects bone mineralization by maintaining normal calcium and phosphorous concentration in the blood. Moreover, vitamin D has extra-skeletal effects on other systems such as the cardiovascular, immune and pancreatic systems, as well as on muscles, on the brain and on the control of cell cycles (Chung 2009; Gonzalez 2010; Holick 2006). The daily requirements of vitamin D recommended by the USA's Institute of Medicine (IOM) (Ross 2011) and the US Endocrine Practice Guidelines Committee (Holick 2011; Pramyothin 2012) are described as follows.

Life-stage group (both sexes)IOM

Committee recommendations for

patients at risk for vitamin D deficiency

EARRDADaily requirement
0 to 12 months  400 to 1000 IU
1 to 18 years400 IU600 IU600 to 1000 IU
19 to 70 years400 IU600 IU1500 to 2000 IU
over 70 years400 IU800 IU1500 to 2000 IU

EAR: estimated average requirement; RDA: recommended dietary allowance

Vitamin D supplementation is usually safe and its toxicity is very rare; however, prolonged periods of excessive intake of vitamin D may result in malaise, drowsiness, nausea, vomiting, abdominal pain, thirst, constipation and loss of appetite (Bennett 2003; Park 2011). It can also lead to hypercalcaemia and causes confusion, cardiac arrhythmias, renal failure and coma (Bennett 2003).

How the intervention might work

The role of vitamin D in calcium homeostasis and bone health has been recognized for many years (Holick 2007; Kennel 2010). Recent studies have also shown that vitamin D has a role in the prevention of cancer (e.g. prostate, colorectal, breast, and pancreas), heart disease, fractures and falls, periodontal disease, autoimmune disease, respiratory infections (e.g. tuberculosis, influenza), type 2 diabetes and depression (Bischoff-Ferrari 2006; Chung 2009; Gonzalez 2010). In SCD, vitamin D supplementation can normalize the serum 25-hydroxyvitamin D (25(OH)D), which not only improves bone health and bone mineral density (Adewoye 2008), but also reduces chronic pain and improves quality of life (Osunkwo 2012). Moreover, vitamin D is also important for the immune system (Hewison 1992) for fighting infectious diseases (Chung 2009; Gonzalez 2010). Supplementing vitamin D in SCD patients may reduce the respiratory complications that can contribute to early mortality (Ramakrishnan 2010).   

Why it is important to do this review

Vitamin D supplementation is relatively simple as both vitamin D2 and D3 supplements are easily available (Kennel 2010). The empirical evidence shows the importance of vitamin D on bone mineralization and musculoskeletal health. Moreover, numerous studies suggest that vitamin D plays a role in many chronic diseases (Bischoff-Ferrari 2006; Chung 2009; Gonzalez 2010; Holick 2007) and in supporting the immune system (Hewison 1992). Cochrane systematic reviews of randomised controlled trials have been carried out on the effect of vitamin D supplementation in other conditions (Avenell 2009; Bjelakovic 2011; Ferguson 2012; Jagannath 2010; Palmer 2009a; Palmer 2009b; Straube 2010; Winzenberg 2010). However, the effect of vitamin D supplementation in SCD has never been reviewed. It is therefore important to determine the effects of supplementing vitamin D in order to reduce complications that contribute to significant morbidity and mortality in people with SCD.


To determine the effects on general health, bone and respiratory health, and the safety of vitamin D supplementation in children and adults with SCD.

To investigate the hypothesis that vitamin D supplementation increases serum 25-hydroxyvitamin D level in children and adults with SCD.


Criteria for considering studies for this review

Types of studies

Randomized controlled trials and quasi-randomized controlled trials (controlled clinical trials).

Types of participants

People with SCD, of all ages, gender, and phenotypes including sickle cell anaemia (HbSS), haemoglobin SC disease (HbSC) and Sβ-thalassaemia (HbSβ+ and HbSβ0) diseases.

Types of interventions

Oral administration of any form of vitamin D supplementation at any dose and for any duration. A comparison of one type of vitamin D versus another type or versus either placebo or no supplementation.

Types of outcome measures

Primary outcomes
  1. Serum 25-hydroxyvitamin D (25(OH)D) level (reported as absolute change from baseline)

  2. Bone mineral density (BMD) of lumbar spine, hips, forearm and total body (measured by using dual energy X-ray absorptiometry (DEXA), reported as per cent change from baseline)

  3. Adverse events (e.g. nausea, drowsiness, vomiting, loss of appetite, constipation, confusion, cardiac arrhthymias, renal failure, coma).

Secondary outcomes
  1. Serum parathyroid hormone level

  2. Bone fracture (confirmed by radiology)

  3. Muscle health (assessed by handgrip)

  4. Pain

    1. Presence of pain

    2. Frequency of pain

    3. Duration of pain

    4. Severity or Intensity of pain (measured using a validated scale)

  5. Growth status

    1. Height

    2. Weight

    3. Body mass index (BMI)

  6. Respiratory outcomes     

    1. Acute chest syndrome

    2. Acute exacerbation of asthma

    3. Respiratory infections such as tuberculosis, pneumonia, etc

    4. Lung function tests (forced expiratory volume at one second (FEV1), forced vital capacity (FVC))

  7. Health-related quality of life (HRQOL) (using a validated scale)

Search methods for identification of studies

Electronic searches

We will identify relevant studies from the Cystic Fibrosis & Genetic Disorders Group's Haemoglobinopathies Trials Register using the terms: (sickle cell OR (haemoglobinopathies AND general)) AND vitamin D.

The Haemoglobinopathies Trials Register is compiled from electronic searches of the Cochrane Central Register of Controlled Trials (CENTRAL) (updated each new issue of The Cochrane Library) and quarterly searches of MEDLINE. Unpublished work is identified by searching the abstract books of five major conferences: the European Haematology Association conference; the American Society of Hematology conference; the British Society for Haematology Annual Scientific Meeting; the Caribbean Health Research Council Meetings; and the National Sickle Cell Disease Program Annual Meeting. For full details of all searching activities for the register, please see the relevant section of the Cochrane Cystic Fibrosis and Genetic Disorders Group Module.

Searching other resources

We will check the reference lists of retrieved relevant articles to identify further relevant studies. We will search the clinical trial registries such as (, Current Controlled Trials metaRegister of Controlled Trial (mRCT) ( We will also contact manufacturers of vitamin D supplements, experts and researchers who are in the field to ask for additional studies including unpublished and ongoing trials.  

Data collection and analysis

Selection of studies

Two authors (HHKS, NNT) will independently assess the titles and abstracts of all articles identified from the literature searches to identify potentially relevant reports. We will not be blinded to information about articles such as the journal of publication, author names, institution, or the study results. If we cannot ascertain from the titles or abstracts whether the studies are eligible for inclusion, we will retrieve and review the full-text articles for eligibility in accordance with the specified criteria for the inclusion of studies. If there is disagreement, we will aim to resolve this by discussion, and if necessary, consult a third review author (ALA). If it is not possible to resolve the disagreement without additional information, we will list the study as "awaiting assessment" and will contact the study author in an effort to obtain further details. We will present the excluded studies with reasons for exclusion in the 'Characteristics of excluded studies' table in RevMan (RevMan 2011).

Data extraction and management

Two authors (HHKS, JS) will independently extract the data from included studies using a standard data collection form and check for consistency. If there is any discrepancy, we aim to resolve this by discussion and if required, we will consult with a third review author to reach a consensus. If the information in the published article is not clear, we will contact the study authors for further details. One author (HHKS) will enter relevant data into RevMan, and two authors will check for errors (JS, ALA) (RevMan 2011).  

We will extract the following information. 

1. Source

  • Study identifier (ID) (created by review author)

  • Report ID (created by review author)

  • Review author ID (created by review author)

  • Citation (journal or conference, year of publication, etc)

  • Contact details

 2. Eligibility

  • Confirm eligibility for review

  • Reason for exclusion

 3. Methods

  • Study design

  • Study setting

  • Time and duration of study

  • Randomisation

  • Allocation sequence concealment

  • Blinding of participants, care providers and outcome assessors

  • Other concerns about bias

  • Intention-to-treat analysis

4. Participants

  • Total number

  • Eligibility criteria (inclusion and exclusion criteria)

  • Age and sex of participants

5. Interventions

  • Total number of intervention groups

For each intervention and comparison group of interest

  • Dose of intervention

  • Type of administration

  • Timing of administration

  • Frequency of administration

  • Duration of intervention

  • Co-interventions (if any)

6. Outcomes

For each outcome of interest

  • Outcome definition (diagnostic method, name of scale, definition of threshold)

  • Units of measurement (if relevant)

  • For scales, upper and lower limits, and whether a high or low score is favourable

7. Results

  • Number of participants allocated to each intervention group

For each outcome of interest

  • Sample size

  • Missing participants

  • Summary data for each intervention group (mean and standard deviation (SD) for continuous data, 2 x 2 table for dichotomous data, etc)

8. Miscellaneous

  • Key conclusions of the study authors

  • References to other relevant studies

  • Funding source

  • Correspondence required

We will collate information from multiple reports of the same study. We plan to group the outcome data into those measured at one month, over one month and up to three months, over three months and up to six months, over six months and up to one year and annually thereafter. We will also record which outcomes were measured at other time periods.

Assessment of risk of bias in included studies

Two authors (HHKS, JS) will independently assess the risk of bias of included studies using a method described in the Cochrane Handbook of Systematic Reviews of Interventions (Higgins 2011). We will assess each study independently for the risk of bias in seven domains (sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, selective outcome reporting, and other potential sources of bias). In each domain, we will assign a judgment of either low, high or unclear risk of bias, according to the criteria described in the Cochrane Handbook of Systematic Reviews of Interventions (Higgins 2011).

If there is any discrepancy in the assessment of the risk of bias in a study, we will aim to resolve this by discussion. If we are not able to reach a consensus by discussion, we will consult a third review author (ALA). If necessary we will contact the study authors for additional information. 

Measures of treatment effect

Dichotomous data

For dichotomous data, we will present the results using the risk ratio (RR) with 95% confidence intervals (CI).

Continuous data

For continuous data, we will present the results using the mean difference (MD) and their corresponding 95% CIs for the outcomes which are measured using same scale between studies. If different scales are used for measuring same outcome, we will use standardized mean difference (SMD) and corresponding 95% CIs.  

Count data

For counts of rare events, we will present the results using the rate ratio (RR) with 95% CIs.

For counts of common events, we will present the results using mean difference (MD) comparing the difference in the mean number of events occurred in participants of intervention group with participants in control group and their corresponding 95% CIs.

Unit of analysis issues

If we identify any cluster-randomised studies, and if there is little heterogeneity between the studies, we will combine the results from both individually randomised studies and cluster-randomised studies. If possible, we will also perform a sensitivity analysis to investigate the effects of the randomisation unit (see below).

If the authors of the cluster-randomised studies ignored the clustering in their analyses, we will calculate the studies' effective sample sizes using an estimate of the intracluster correlation coefficient (ICC). We will derive the ICC from a study (if available) or from similar studies, and we will calculate the design effect using the formula given in the Cochrane Handbook of Systematic Reviews of Interventions (Higgins 2011). If we use the ICC from other similar studies, we will perform a sensitivity analysis to investigate the effect of variations in the ICC.

If we identify cross-over studies, if we believe there is a carryover effect which will outlast any washout period included in the study or where second period data are unavailable, we will include only data from the first arm in the meta-analysis. If we identify studies with more than two intervention groups, we will combine the groups to create a single pair-wise comparison.

Dealing with missing data

If we identify studies published only in abstract form, or presented at meetings or conferences, we will contact the study authors for further details or data as needed.

We will investigate the attrition rate including dropouts, withdrawals and losses to follow up in the included studies. If the data are unclear or missing with no reported reason for dropouts, we will contact the study authors to request further information. In order to allow an intention-to-treat analysis, we will collect the data by allocated treatment groups (regardless of compliance or whether enrolled participants were later found to be ineligible or excluded from treatment group).

Assessment of heterogeneity

We will visually asses the forest plots to determine whether there is heterogeneity. The Chi2 test will be used and a P value of less than 0.10 will be considered to demonstrate statistically significant heterogeneity. We will also use the I2 statistic to quantify the inconsistency across the studies. We will categorize the I2 value of 0% to 40% as not important heterogeneity, 30% to 60% as moderate heterogeneity, 50% to 90% as substantial heterogeneity and 75% to 100% as considerable heterogeneity (Deeks 2011).

Assessment of reporting biases

We will aim to identify any selective outcome reporting in the included studies by comparing the study protocols with the final reports. If the study protocols are not available, we will compare the 'Methods' section with the 'Results' section of the study reports to assess whether the study authors reported all the outcomes which were stated to be measured. If we include 10 or more studies in a meta-analysis, we will use funnel plots to assess publication bias. If there is asymmetry, we will investigate the possible causes of this, such as publication bias, true heterogeneity and a high risk of bias.  

Data synthesis

If there is no clinical variation (e.g. variability of interventions, participants and diverse outcomes) between the studies, we will pool the results and perform meta-analyses. We will use RevMan for the data analyses (RevMan 2011). If there is no statistically significant heterogeneity between the studies, we will use a fixed-effect model. If there is substantial or considerable heterogeneity identified (as defined above), we will use a random-effects model. If there is considerable heterogeneity, we will check the data entry for accuracy and also investigate the cause of the heterogeneity according to the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011).

Subgroup analysis and investigation of heterogeneity

If we identify statistically significant heterogeneity in the results for the primary outcomes, we will perform subgroup analyses as follows:

  1. different forms of vitamin D supplementation (vitamin D2 versus D3);

  2. different durations of vitamin D supplementation (less than six months versus six months or greater);

  3. different doses of vitamin D supplementation (up to and including 400 IU versus more than 400 IU daily);

  4. different types of SCD (HbSS, HbSC, HbSβ+ and HbSβ0);

  5. demographic status (children age 18 years and below versus adults aged over 18 years; male versus female).

Sensitivity analysis

We will perform a sensitivity analysis to determine the robustness of the findings. If there are a sufficient number of studies included (10 or more) we plan to undertake an additional analysis excluding those studies with a high risk of bias for allocation concealment or incomplete outcome data, or both. We also plan to repeat the meta-analyses using both the fixed-effect and the random-effects models to assess how the findings are robust to this choice of method. We will also repeat the analyses if we include any cluster-randomised trials to investigate the effects of the randomisation unit. We will perform a further sensitivity analysis if we use the ICC from other similar studies to calculate the studies' effective sample sizes in cluster-randomised studies. We will produce summary tables reporting on any sensitivity analyses undertaken.


The authors would like to acknowledge Cystic Fibrosis & Genetic Disorders Group for their kind support and valuable guidance on this protocol. We would like to thank Dr Soe Moe (Department of Community Medicine, Melaka-Manipal Medical College), Dr Aung Win Thein (Department of Surgery, Melaka-Manipal Medical College) and Dr Simerjit Singh (Department of Orthopedics, Melaka-Manipal Medical College) for their comments and kind contributions during the preparation of this protocol.

Contributions of authors

Roles and responsibilities
Protocol stage: draft the protocol HHKS, NNT, HN, IO
Review stage: select which trials to include (2 + 1 arbiter) HHKS, NNT + ALA
Review stage: extract data from trials (2 people) HHKS, JS
Review stage: enter data into RevMan HHKS, JS
Review stage: carry out the analysis HHKS, ALA, ARMS
Review stage: interpret the analysis HHKS, ALA, ARMS
Review stage: draft the final review HHKS, NNT, HN, IO
Update stage: update the review HHKS, NNT, ALA

Declarations of interest

None known.