Dietary supplements for chronic gout.

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 updated the original search by searching CENTRAL, MEDLINE, Embase, CINAHL, and four trials registers (August 2020). We applied no date or language restrictions. We also handsearched the abstracts from the 2010 to 2019 American College of Rheumatology and European League against Rheumatism conferences, and checked the references of all included studies.

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 major outcomes were acute gout flares, study withdrawal due to adverse events (AEs), serum uric acid (sUA) reduction, joint pain reduction, participant global assessment, total number of AEs, and tophus regression.

Data Collection And Analysis: We used standard methodological procedures expected by Cochrane.

Main Results: Two previously included RCTs (160 participants) met our inclusion criteria; we did not identify any new trials for this update. 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 (120 participants), at unclear risk of selection and detection bias, compared SMP enriched with glycomacropeptides (GMP) with un-enriched SMP, and with lactose, over three months. Participants were predominantly men, aged in their 50s, who had severe gout. The results for all major outcomes were imprecise, except for pain. None of the results were clinically significant. The frequency of acute gout attacks, measured as the number of flares per month, decreased in all three groups over the three-month 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 not clinically significant (mean (standard deviation (SD)) flares per month: 0.49 (1.52) in SMP/GMP/G60 group versus 0.70 (1.28) in the control groups; absolute risk difference: mean difference (MD) -0.21 flares per month, 95% confidence interval (CI) -0.76 to 0.34; low-quality evidence). The number of withdrawals due to adverse effects was similar between groups (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); there were 4% more withdrawals in the SMP/lactose groups (10% fewer to 18% more; low-quality evidence). Serum uric acid reduction was similar across groups (mean (SD) -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). Pain from self-reported gout flares (measured on a 10-point Likert scale) improved slightly more in the GMP/G600 SMP group compared with controls (mean (SD) -1.97 (2.28) in SMP/GMP/G600 group versus -0.94 (2.25) in control groups; MD -1.03, 95% CI -1.89 to -0.17). This was an absolute reduction of 10% (95% CI 20% to 1% reduction; low-quality evidence), which may not be of clinical relevance. The risk of adverse events was similar between groups (19/40 in SMP/GMP/G600 group versus 39/80 in control groups; RR 0.97, 95% CI 0.66 to 1.45); the absolute risk difference was 1% fewer adverse events (1% fewer to 2% more), low-quality evidence). Gastrointestinal events such as nausea, flatulence and diarrhoea were the most commonly reported adverse effects. Data for participant global assessment were not available for analysis; the study did not report tophus regression. One trial (40 participants), at high risk of selection, performance, and detection bias, compared vitamin C alone with allopurinol, and with allopurinol plus vitamin C, in a three-arm study. We only included data from the vitamin C versus allopurinol comparison in this review. Participants were predominantly middle-aged men, and their severity of gout was representative of gout in general. Allopurinol reduced sUA levels more than vitamin C (MD 0.10 mmol/L, 95% CI 0.06 to 0.15), low-quality evidence. The study reported no adverse events; none of the participants withdrew due to adverse events. The study did not assess the rate of gout attacks, joint pain reduction, participant global assessment, or tophus regression.

Authors' Conclusions: While dietary supplements may be widely used for gout, this review found no high-quality that supported or refuted the use of glycomacropeptide-enriched skim milk powder or vitamin C for adults with chronic gout.