Background: Dietary supplements are frequently used for the treatment of several medical conditions, both prescribed by physicians or self administered. However, evidence of benefit and safety of these supplements is usually limited or absent.
Objectives: To assess the efficacy and safety of dietary supplementation for people with chronic gout.
Search methods: We performed a search in the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE and CINAHL on 6 June 2013. We applied no date or language restrictions. In addition, we performed a handsearch of the abstracts from the 2010 to 2013 American College of Rheumatology (ACR) and European League against Rheumatism (EULAR) conferences, checked the references of all included studies and trial registries.
Selection criteria: We considered all published randomised controlled trials (RCTs) or quasi-RCTs that compared dietary supplements with no supplements, placebo, another supplement or pharmacological agents for adults with chronic gout for inclusion. Dietary supplements included, but were not limited to, amino acids, antioxidants, essential minerals, polyunsaturated fatty acids, prebiotic agents, probiotic agents and vitamins. The main outcomes were reduction in frequency of gouty attacks and trial participant withdrawal due to adverse events. We also considered pain reduction, health-related quality of life, serum uric acid (sUA) normalisation, function (i.e. activity limitation), tophus regression and the rate of serious adverse events.
Data collection and analysis: We used standard methodological procedures expected by The Cochrane Collaboration.
Main results: We identified two RCTs (160 participants) that fulfilled our inclusion criteria. As these two trials evaluated different diet supplements (enriched skim milk powder (SMP) and vitamin C) with different outcomes (gout flare prevention for enriched SMP and sUA reduction for vitamin C), we reported the results separately.One trial including 120 participants, at moderate risk of bias, compared SMP enriched with glycomacropeptides (GMP) with unenriched SMP and with lactose over three months. Participants were predominantly men aged in their 50's who had severe gout. The frequency of acute gout attacks, measured as the number of flares per month, decreased in all three groups over the study period.The effects of enriched SMP (SMP/GMP/G600) compared with the combined control groups (SMP and lactose powder) at three months in terms of mean number of gout flares per month were uncertain (mean ± standard deviation (SD) flares per month: 0.49 ± 1.52 in SMP/GMP/G60 group versus 0.70 ± 1.28 in control groups; mean difference (MD) -0.21, 95% confidence interval (CI) -0.76 to 0.34; low-quality evidence). The number of withdrawals due to adverse effects was similar in both groups although again the results were imprecise (7/40 in SMP/GMP/G600 group versus 11/80 in control groups; risk ratio (RR) 1.27, 95% CI 0.53 to 3.03; low-quality evidence). The findings for adverse events were also uncertain (2/40 in SMP/GMP/G600 group versus 3/80 in control groups; RR 1.33, 95% CI 0.23 to 7.66; low-quality evidence). Gastrointestinal events were the most commonly reported adverse effects. Pain from self reported gout flares (measured on a 10-point Likert scale) improved slightly more in the SMP/GMP/G600 group compared with controls (mean ± SD reduction -1.97 ± 2.28 points in SMP/GMP/G600 group versus -0.94 ± 2.25 in control groups; MD -1.03, 95% CI -1.96 to -0.10; low-quality evidence). This was an absolute reduction of 10% (95% CI 20% to 1% reduction), which may not be of clinical relevance. Results were imprecise for the outcome improvement in physical function (mean ± SD Health Assessment Questionnaire (HAQ)-II (scale 0 to 3, 0 = no disability): 0.08 ± 0.23 in SMP/GMP/G60 group versus 0.11 ± 0.31 in control groups; MD -0.03, 95% CI -0.14 to 0.08; low-quality evidence). Similarly, results for sUA reduction were imprecise (mean ± SD reduction: -0.025 ± 0.067 mmol/L in SMP/GMP/G60 group versus -0.010 ± 0.069 in control groups; MD -0.01, 95% CI -0.04 to 0.01; low-quality evidence). The study did not report tophus regression and health-related quality of life impact.One trial including 40 participants, at moderate to high risk of bias, compared vitamin C alone with allopurinol and with allopurinol plus vitamin C in a three-arm trial. We only compared vitamin C with allopurinol in this review. Participants were predominantly middle-aged men, and their severity of gout was representative of gout in general. The effect of vitamin C on the rate of gout attacks was not assessed. Vitamin C did not lower sUA as much as allopurinol (-0.014 mmol/L in vitamin C group versus -0.118 mmol/L in allopurinol group; MD 0.10, 95% CI 0.06 to 0.15; low-quality evidence). The study did not assess tophus regression, pain reduction or disability or health-related quality of life impact. The study reported no adverse events and no participant withdrawal due to adverse events.
Authors' conclusions: While dietary supplements may be widely used for gout, this review has shown a paucity of high-quality evidence assessing dietary supplementation.