Description of the condition
Cardiovascular disease (CVD) remains the number one cause of death globally (WHO 2011a). CVDs are the result of disorders of the heart and blood vessels and include cerebrovascular disease, coronary heart disease (CHD), and peripheral arterial disease (PAD) (WHO 2011b). In 2008, an estimated 17.3 million people died from CVDs, representing 30% of all global deaths. Of these deaths, an estimated 7.3 million were due to CHD and 6.2 million were due to stroke (WHO 2011a). Over 80% of CVD deaths occur in low- and middle-income countries, and the number of CVD deaths is expected to increase to 23.3 million by 2030 (Mathers 2006; WHO 2011a).
One of the main mechanisms thought to cause CVD is atherosclerosis, in which the arteries become clogged by plaques or atheromas (NHS 2012). Atherosclerosis can cause CVD when the arteries are completely blocked by a blood clot or when blood flow is restricted by a narrowed artery, limiting the amount of blood and oxygen that can be delivered to organs or tissue (British Heart Foundation 2012). Whilst arteries may naturally become harder and narrower with age, this process may be accelerated by factors such as smoking, high cholesterol, high blood pressure (BP), obesity, a sedentary lifestyle and ethnicity (NHS 2012). Prevention of CVD by targeting modifiable factors remains a key public health priority. Diet plays a major role in the aetiology of many chronic diseases including CVD, thereby contributing to a significant geographical variability in morbidity and mortality rates across different countries and populations worldwide (WHO 2003). A number of dietary factors have been found to be associated with CVD risk, such as a low consumption of fruit and vegetables (Begg 2007), a high intake of saturated fat (Siri-Tarino 2010) and a high consumption of salt (He 2011). Dietary factors are important since they can be modified in order to lower CVD risk, making them a prime target for interventions aimed at primary prevention and management of CVD.
Description of the intervention
The intervention to be examined in this review is vitamin C supplementation as a single ingredient. No limit will be placed on the dose or frequency at which vitamin C is taken. Vitamin C (ascorbic acid or ascorbate) is an essential micronutrient that acts as a powerful water-soluble antioxidant, reducing oxidative stress. It cannot be synthesised in the body and is acquired primarily through the consumption of fruit, vegetables, supplements, fortified beverages, and fortified breakfast or 'ready-to-eat' cereals (Frei 1989; WHO 2006).
Data on the adverse effects of vitamin C supplementation are rare. A survey of 9328 patients who had used high dose intravenous vitamin C during the preceding 12 months revealed that only 101 had side effects, mostly minor, including lethargy/fatigue in 59 patients, change in mental status in 21 patients and vein irritation/phlebitis in 6 patients (Padayatty 2010). In a recent meta-analysis of the effects of vitamin C supplementation, alone and in combination with other agents (such as vitamin E, Magnesium, Zinc, Selenium) on BP (Juraschek 2012), few trials (6 of 29) reported adverse effects. Details of adverse effects were not described in the paper, however.
How the intervention might work
Population-based observational studies have shown an inverse association between plasma vitamin C concentrations and vitamin C intake with BP (McCarron 1984; Moran 1993). They have also shown an inverse relationship between vitamin C intake and mortality due to CVD (Jacques 1995; Simon 1998). However, results from randomised controlled trials (RCTs) have been inconsistent with regard to the potential effectiveness of vitamin C supplementation in the prevention of cardiovascular events (Cook 2007; Sesso 2008) or mortality outcomes (Bjelakovic 2007).
In the early stages of atherosclerosis, monocytes adhere to the walls of the endothelium, causing the vessel walls to thicken and lose their elasticity. Research has shown that vitamin C supplementation can reduce the rate of monocyte adhesion to the endothelial cell wall. The study looked at the effects of vitamin C (250 mg per day, six weeks duration) in healthy adults with normal and below-average plasma vitamin C concentration at baseline. Before the study, subjects with below average levels of vitamin C had 30% greater monocyte adhesion than normal, putting them at higher risk for atherosclerosis. After six weeks of vitamin C supplementation, the rate of monocyte adhesion fell by 37% (Woollard 2002).
Furthermore, intercellular adhesion molecule-1 (ICAM-1) is an inducible surface glycoprotein that mediates the adhesion of monocytes to the endothelium. The researchers went on to demonstrate that the same dose and duration of vitamin C supplementation was able to reduce monocyte ICAM-1 expression by 50% in subjects with below-average plasma vitamin C concentration (Rayment 2003). Vitamin C supplementation might improve nitric oxide bioactivity (Huang 2000), as well as endothelial function of brachial and coronary arteries, as suggested by short-term interventions among high-risk individuals (Grebe 2006; McNulty 2007; Silvestro 2002; Solzbach 1997).
Why it is important to do this review
From preliminary searching of The Cochrane Library, we identified five systematic reviews which assessed the effects of vitamin C supplementation on BP (Juraschek 2012; McRae 2006a; Ness 1997), low-density lipid (LDL) cholesterol and triglycerides (McRae 2008), and total cholesterol (McRae 2006b).
Two of these systematic reviews incorporated meta-analyses of RCTs only (Juraschek 2012; McRae 2008). However, one of these looked at RCTs with vitamin C supplementation alone, and in combination with other agents (such as vitamin E, Mg, Zn, Se) in trials between 2 and 26 weeks duration. In addition, the review covered primary and secondary prevention of CVD (Juraschek 2012) and was subject to publication bias because four of the trials in the review specifically stated their decision not to report BP results because of a lack of significant results. The review concluded that vitamin C supplementation reduced systolic and diastolic BP in short-term trials, however, long-term trials on the effects of vitamin C supplementation on BP and clinical events are needed.
The other meta-analysis of RCTs only (McRae 2008) included trials of between 3 and 24 weeks duration. The authors concluded that supplementation with at least 500 mg/day of vitamin C, for a minimum of four weeks, can result in a significant decrease in serum LDL cholesterol and triglyceride concentrations. However, the lack of quality assessment and analysis of statistical heterogeneity, and the small sample sizes of the included trials, limit the reliability of the authors' conclusions.
Of the remaining 3 reviews, 1 review looked at 13 trials (including 9 RCTs with either parallel or crossover design, 1 non-randomised study and 2 baseline comparisons) in hypertensive subjects (McRae 2006a). Dose and duration of vitamin C ranged from 400 to 2000 mg/day for 4 to 12 weeks duration, and in 1 trial, Mg supplementation was used as the control. The review concluded that vitamin C supplementation in hypertensive patients appears to have modest effects on reducing systolic and diastolic BP.
Another review looked at 51 experimental studies (mostly baseline comparisons with only a small number of RCTs) (McRae 2006b). Dose and duration of vitamin C supplementation ranged from 300 to 5000 mg/day for 2 to 48 weeks. Subjects were separated into categories based on their baseline total serum cholesterol concentrations. It was observed that the category with the highest baseline cholesterol started out with the lowest plasma vitamin C levels and obtained the highest mean percent change in total serum cholesterol compared to the other categories. The review concluded that the cholesterol-lowering benefit of vitamin C supplementation may be in its ability to elevate plasma vitamin C concentrations in those patients who initially possess lower than normal vitamin C concentrations.
The final review was the first systematic review of epidemiological studies of vitamin C and BP (Ness 1997). Cross-sectional data were available from 18 populations. Ten of 14 reported an inverse association between plasma vitamin C and BP, and 3 of 4 reported an inverse association with vitamin C intake. The review also included six small trials (two non-randomised and four RCTs), of which one reported a significant decrease in BP, one a non-significant decrease and two were uninterpretable. The review found a consistent cross-sectional association between higher vitamin C intake or status and lower BP, however none of the studies controlled adequately for confounding by other dietary factors.
For this review we will aim to examine evidence from RCTs of the supplementation of vitamin C in the general population and those at high risk of CVD to determine if this intervention is effective in the primary prevention of CVD. From preliminary searching of The Cochrane Library we identified a number of RCTs involving vitamin C supplementation (as a single supplement) which cover the primary prevention of CVD. This review will build on the existing systematic reviews discussed above by assessing vitamin C supplementation (as a single supplement only), including trials with a minimum of three months duration and within healthy populations only. It will also assess a wider range of outcomes.
The primary objective is to determine the effectiveness of vitamin C supplementation as a single supplement for the primary prevention of cardiovascular disease (CVD).
Criteria for considering studies for this review
Types of studies
We will include all randomised controlled trials (RCTs) including cross-over trials. We will include studies reported as full-text, those published as abstract only, and unpublished data.
Types of participants
We will include healthy adults (18 years old or over) of all ages from the general population and those at moderate to high risk of cardiovascular disease (CVD). As the review will focus on the primary prevention of CVD, we will exclude those who have experienced a previous myocardial infarction (MI), stroke, revascularisation procedure (coronary artery bypass grafting (CABG) or percutaneous transluminal coronary angioplasty (PTCA)), and those with angina or angiographically-defined coronary heart disease (CHD).
Types of interventions
The intervention will be vitamin C supplements alone as a single ingredient. No limit will be placed on the dose or frequency of vitamin C taken. Trials will only be considered where the comparison group is placebo or no intervention. If there is a sufficient number of trials we will also stratify results by dose of vitamin C. Multi-factorial intervention studies (including other additional interventions such as dietary changes and exercise) will not be included in this review, in order to avoid confounding.
Types of outcome measures
We will include studies with follow-up periods of at least three months. Follow-up is considered to be the time elapsed since the start of the intervention.
- Cardiovascular mortality
- All-cause mortality
- Non-fatal endpoints such as MI, CABG, PTCA, angina, or angiographically-defined CHD, stroke, carotid endarterectomy, peripheral arterial disease (PAD)
- Changes in blood pressure (BP) (systolic and diastolic BP) and blood lipids (total cholesterol, high-density lipid (HDL) cholesterol, low-density lipid (LDL) cholesterol, triglycerides)
- Occurrence of type 2 diabetes as a major CVD risk factor
- Validated health-related quality of life measures
- Adverse effects
Search methods for identification of studies
We will identify trials through systematic searches of the following bibliographic databases:
- The Cochrane Library (including the Cochrane Central Register of Controlled Trials (CENTRAL), Health Technology Assessment (HTA) Database, Database of Abstracts of Reviews of Effects (DARE) and NHS Economic Evaluation Database (NEED)
- MEDLINE (Ovid)
- EMBASE (Ovid)
- Web of Science (Thomson Reuters)
We will use Medical subject headings (MeSH) or equivalent and text word terms. Searches will be designed in accordance with the Cochrane Heart Group methods and guidance.
The preliminary search strategy for MEDLINE (Ovid) (Appendix 1) will be adapted for use in the other databases. The Cochrane sensitivity-maximising RCT filter (Lefebvre 2011) will be applied to MEDLINE (Ovid) and adaptations of it to the other databases, except CENTRAL.
We will search all databases from their inception to the present, and we will impose no restriction on language of publication.
Searching other resources
We will check reference lists of reviews for additional studies. We will search the metaRegister of controlled trials (mRCT) (http://www.controlled-trials.com/mrct), ClinicalTrials.gov (http://www.clinicaltrials.gov/) and the World Health Organization (WHO) International Clinical Trials Registry platform (ICTRP) search portal (http://apps.who.int/trialsearch/) for ongoing trials. We will also search OpenGrey to identify any relevant grey literature.
On key articles we will perform citation searches by using the ‘cited by’ function in Web of Science. We will contact experts in the field for unpublished and ongoing trials and, where necessary, contact authors for any additional information.
Data collection and analysis
Selection of studies
Two authors (NF, RW) will independently screen for inclusion titles and abstracts of all the studies we identify as a result of the search, and code them as 'retrieve' (eligible or potentially eligible/unclear) or 'do not retrieve'. If there are any disagreements, we will ask a third author to arbitrate (KR/SS). We will retrieve the full-text study reports/publication and two authors (NF, RW) will independently screen the full-text and identify studies for inclusion, and identify and record reasons for exclusion of the ineligible studies. We will resolve any disagreement through discussion or, if required, we will consult a third author (KR/SS). We will identify and exclude duplicates and collate multiple reports of the same study so that each study rather than each report is the unit of interest in the review. We will record the selection process in sufficient detail to complete a PRISMA flow diagram and 'Characteristics of excluded studies' table.
Data extraction and management
Two authors (NF, RW) will independently extract study characteristics from included studies using a pre-standardised data extraction form, and contact chief investigators to request additional relevant information if necessary. We will extract details of the study design, participant characteristics, study setting, intervention (including dose and duration), and outcome data including details of outcome assessment, adverse effects, and methodological quality (randomisation, blinding, attrition) from each of the included studies. We will resolve disagreements by consensus or by involving a third author (KR/SS). One author (NF) will transfer data into the Review Manager (RevMan 2012) file. We will double-check that data is entered correctly by comparing the data presented in the systematic review with the study reports. A second author (RW) will spot-check study characteristics for accuracy against the trial report.
Assessment of risk of bias in included studies
Two authors (NF, RW) will independently assess the risk of bias for each study using the criteria outlined in the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011). We will resolve any disagreements by discussion or by involving another author (KR/SS). We will assess the risk of bias according to the following domains.
- Random sequence generation
- Allocation concealment
- Blinding of participants and personnel
- Blinding of outcome assessment
- Incomplete outcome data
- Selective outcome reporting
- Other bias. (Bias due to problems not covered elsewhere, e.g. industry funding)
We will grade each potential source of bias as having a 'low risk of bias', a 'high risk of bias' or an 'unclear risk of bias'. Studies will be regarded as at high risk of bias if any of the domains listed above are regarded at high risk of bias.
Assessment of bias in conducting the systematic review
We will conduct the review according to this published protocol and report any deviations from it in the 'Differences between protocol and review' section of the systematic review.
Measures of treatment effect
Data will be processed in accordance with the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011). We will express dichotomous outcomes as odds ratios (ORs) or relative risks (RRs), with 95% confidence intervals (CIs) calculated for each study. For continuous outcomes, net changes will be compared (i.e. intervention group minus control group differences) and a mean difference (MD) or standardised mean difference (SMD) and 95% CIs calculated for each study.
We will narratively describe skewed data reported as medians and interquartile ranges.
Unit of analysis issues
We will use data only from the first half as a parallel group design. We will only consider risk factor changes (i.e. BP, lipid levels) before patients cross over to the other therapy and where the duration is a minimum of three months before cross over occurs.
Studies with multiple intervention groups
Data for the control group will be used for each intervention group comparison. We will reduce the weight assigned to the control group by dividing the control group N by the number of intervention groups.
Cluster randomised trials
We will analyse cluster randomised trials using the unit of randomisation (cluster) as the number of observations. Where necessary, individual-level means and standard deviations (SDs) adjusted for clustering will be utilised together with the number of clusters in the denominator, in order to weight the trials appropriately.
Dealing with missing data
We will contact investigators or study sponsors in order to verify key study characteristics and obtain missing numerical outcome data where possible (e.g. when a study is identified as abstract only).
Missing data will be captured in the data extraction form and reported in the risk of bias table. If a trial collected an outcome measure at more than one time point, the longest period of follow up with 20% or fewer dropouts will be utilised.
Assessment of heterogeneity
For each outcome, we will conduct tests of heterogeneity using the Chi
Assessment of reporting biases
If there are sufficient studies (10 or more) trial effect will be plotted against standard error and presented as funnel plots (Sterne 2011). Since asymmetry could be caused by a relationship between effect size and sample size or by publication bias, we will examine any observed effect for clinical heterogeneity and additional sensitivity tests may be carried out (Sterne 2011).
Statistical analysis will be carried out using the Cochrane Collaboration’s statistical software, (RevMan 2012) Dichotomous data will be entered as events and the number of participants and continuous data will be entered as means and SDs. In the absence of moderate to substantial heterogeneity (40% to 100%) and provided that there are sufficient trials we will combine the results, using a fixed-effect model.
Subgroup analysis and investigation of heterogeneity
If there are sufficient trials (10 or more) we will stratify results by dose of vitamin C, and by high risk of CVD versus the general population.
We plan to carry out sensitivity analyses with studies of six months or more follow up, excluding studies at a high risk of bias. Studies will be regarded as at high risk of bias if any of the domains in the risk of bias tool are regarded at high risk of bias.
Appendix 1. MEDLINE search strategy
1. Ascorbic Acid/
3. (vit* adj6 c).tw.
7. exp Cardiovascular Diseases/
20. atrial fibrillat*.tw.
23. (sick adj sinus).tw.
24. exp Stroke/
25. (stroke or stokes).tw.
27. cerebral vascular.tw.
29. (brain adj2 accident*).tw.
30. ((brain* or cerebral or lacunar) adj2 infarct*).tw.
31. exp Hypertension/
33. peripheral arter* disease*.tw.
34. ((high or increased or elevated) adj2 blood pressure).tw.
35. exp Hyperlipidemias/
42. exp Arteriosclerosis/
43. exp Cholesterol/
45. "coronary risk factor* ".tw.
46. Blood Pressure/
47. blood pressure.tw.
49. randomized controlled trial.pt.
50. controlled clinical trial.pt.
53. drug therapy.fs.
57. 49 or 50 or 51 or 52 or 53 or 54 or 55 or 56
58. exp animals/ not humans.sh.
59. 57 not 58
60. 6 and 48 and 59
Contributions of authors
All authors contributed to the protocol development. NF took the lead on writing the first draft which was circulated and all authors provided feedback and comments which were then incorporated.
Declarations of interest
Sources of support
- Warwick Medical School, University of Warwick, UK.
- NIHR Cochrane Programme Grant, UK.
- National Institute for Health Research (NIHR) Collaboration for Leadership in Applied Health Research and Care, West Midlands at University Hospitals Birmingham NHS Foundation Trust, UK.Support to Karen Rees