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C My BP? Vitamin C for Hypertension
Abstract & Commentary
By Russell H. Greenfield, MD
Synopsis: A systematic review of 29 studies using vitamin C to help lower blood pressure showed a slightly positive effect, though of debatable clinical value. The study highlights the variable nature of existing data in this realm with regards to quality and methodology.
Source: Juraschek SP, et al. Effects of vitamin C supplementation on blood pressure: A meta-analysis of randomized controlled trials. Am J Clin Nutr 2012;95:1079-1088.
The researchers conducted a systematic review and meta-analysis of randomized, controlled trials to determine the effects of vitamin C supplementation on blood pressure (BP) in adults. Trials were identified by searching Medline, EMBASE, and the Cochrane Central Register of Controlled Trials databases for papers published from January 1966 through December 2010, without language restrictions. In addition, bibliographies of original research were reviewed. Pre-specified inclusion criteria were: 1) use of a randomized controlled trial design; 2) trial reported effects on systolic (SBP), diastolic (DBP), or both; 3) trial used oral vitamin C supplementation and concurrent control groups; and 4) trial had a minimum duration of 2 weeks. Two investigators independently abstracted the articles (which included crossover trials, factorial studies, and parallel trials) and achieved consensus where initial disagreement occurred. The authors determined between-group differences in BP change for vitamin C and placebo groups, and then pooled the results. Mean BP was used for trials reporting multiple BP measurements. Subgroup analyses were performed that focused on specific participant and trial characteristics including vitamin C dosage, sample size, and study duration. Heterogeneity and individual study quality (degree of blinding, adverse events reporting) also was assessed.
A number of studies were excluded for reasons such as coadministration of additional vitamin-C containing supplements or beverages, incomplete reporting of BP data, and lack of concurrent control group. A total of 29 studies were included in the primary analysis (sample sizes ranged from n = 10-120, with mean participant age ranging from 22-74 years; duration ranged from 2-26 weeks). Vitamin C dosage ranged from 60-4000 mg/d. Fifteen trials permitted concurrent use of antihypertensive agents.
Vitamin C supplementation reduced SBP by an average of -3.84 mm Hg (95% confidence interval [CI]: -5.29, -2.38 mm Hg; P < 0.001) and DBP by -1.48 mm Hg (95% CI: -2.86, -0.10 mm Hg; P = 0.036) in pooled analysis. BP effects did not differ by hypertensive status for either SBP (P = 0.28) or DBP (P = 0.85); however, more significant SBP reductions were observed in trials with younger participants, and greater impact on both SBP and DBP was noted in studies with fewer total measurements.
Based on the results of their analyses, the researchers conclude that supplemental vitamin C may have a useful role in lowering BP, but make clear that the existing data are largely flawed.
Systematic reviews and meta-analyses often provide valuable information, but it almost always comes at a steep price, usually in the form of caution in interpreting the results. The best researchers, such as the authors of this study, demonstrate the difficulties inherent in performing the task of meta-analysis by pointing out the variable nature of the studies at hand that use different dosages over disparate timeframes in far-ranging patient populations where a variety of measurements were obtained. And then a clinically relevant conclusion is expected.
An accompanying editorial in the journal comments that the problems associated with interpreting systematic reviews are compounded by the difficulties in performing nutrition research.1 It is an excellent read.
Most practitioners would agree that the best way to encourage patients to get vitamin C into their systems is by eating a healthy diet that emphasizes fresh or frozen vegetables and fruits. Most practitioners also understand that the majority of people do not follow widely recognized nutrition guidelines of eating 5-9 servings of vegetables and fruit per day, and that dietary supplement use is commonplace. Vitamin C supplementation is widely considered safe (save for an increased risk of urolithiasis), but questions about pro-oxidant actions and other potential adverse effects have lingered, and recommendations about dosage, while not consistent, are trending lower.
Hypertension as a public health issue remains inadequately addressed for multiple reasons that include issues of access, funding, and compliance with medical recommendations. A simple and inexpensive supplement that might help lower blood pressure generally would certainly be attractive. As relates specifically to vitamin C, however, the jury is still out.
The authors of this review did a terrific job of parsing the available data and worked hard to create the cogent conclusion that vitamin C may indeed be a useful adjunct in the care of people with high blood pressure. They also painstakingly describe the lack of uniformity among the trials, making firm conclusions nearly impossible. Heterogeneity and methodological shortcomings aside, the clinical import of the reported effects from vitamin C supplementation on BP is not compelling, and the ultimate impact of vitamin C on heart disease and stroke, while promising, is still being investigated.
Vitamin C has many proponents, especially among those promoting a healthy diet as a centerpiece of a healthy lifestyle. Supplemental vitamin C also has its proponents, and while fewer in number they are often passionate in their support for the agent. That's all well and good provided the dosage employed is relatively low and that patients understand, at least in the setting of hypertension, that a clear scientific rationale for using the agent has yet to be firmly established.
1. Newberry SJ. What is the evidence that vitamin C supplements lower blood pressure? Am J Clin Nutr 2012;95:997-998.