Oral vitamin C may reduce hyperuricemia

Oral vitamin C supplementation could lead to a reduction in serum uric acid (SUA) levels, a meta-analysis suggests.

SUA plays an important role in the development of metabolic syndrome. [Biomed Res Int 2015;2015:369179] “[Evidence has also shown that a] higher SUA level was associated with an increased prevalence of cardiovascular diseases, diabetic kidney disease, and a variety of diabetic complications. Therefore, it is urgent to explore a way to control SUA,” said the researchers.

“[In our study,] overall results from the random effects model have demonstrated that vitamin C supplementation played an important role in reducing SUA,” they said.

After pooling data from 16 randomized controlled trials (n=1,013), the random effects model revealed that vitamin C supplementation had a significant SUA-lowering effect (effect, –0.55; p=0.001). [Complement Ther Med 2021;doi:10.1016/j.ctim.2021.102761]

Subgroup analyses

The effect of vitamin C supplementation on SUA was stronger among individuals aged <65 years (effect, –0.63; p=0.007), in studies that used placebo or blank controls (effect, –0.66; p=0.003), and with a shorter duration of the regimen (ie, <1 month; effect, –0.84; p=0.001).

“The intervention was less effective in individuals aged >65 years. This may be because the elderly are often accompanied by other diseases,” the researchers explained. [Nutrients 2018;10:975] The stronger effect in younger individuals appears relevant in view of the increasing prevalence of hyperuricemia among younger individuals. [Clin Rheumatol 2018;37:2529-2538]

The duration factor aligns with evidence showing that long-term and high-dose vitamin C supplementation is not always effective for disease prevention, and may even be a detriment to health. [Adv Pharm Bull 2016;6:467-477]

Dose did not appear to influence the effect of vitamin C supplementation. “The intervention doses of the included studies all exceeded the recommended daily allowance [of] 75 mg/day for women and 90 mg/day for men (ie, between 200 and 2,000 mg/day),” said the researchers.

“[When] we divided the included studies into two groups according to intervention dose of 500 mg/day to explore the effect of different intervention doses … there was no difference,” they continued. This suggests that dose increases might have little impact.

Further validation warranted

With inconsistent evidence reflecting the effect of oral vitamin C supplementation on SUA, the issue has remained controversial, noted the researchers. There are studies showing inverse associations or nonsignificant findings, [Asia Pac J Clin Nutr 2018;27:1271-1276, Arthritis Rheum 2013;65:1636-1642] but there are also data reflecting favourable results, which correlate with the current findings. [Arch Intern Med 2009;169:502-507, Arthritis Care Res (Hoboken) 2011;63:1295-1306]

“[Our results suggest that] oral vitamin C supplementation had a significant [SUA-reducing effect]. The use of placebo, duration of intervention, and age of subjects … had an impact on the results of this study,” said the researchers.

Given the study limitations (ie, high heterogeneity, male predominance, lack of evaluation of SUA measurement methods) which could have led to bias, they called for more high-quality trials to validate the results.

“[Moreover,] our results show that the effect of vitamin C intervention alone is better than that of vitamin C combined with other drugs. Therefore, the interaction between different drugs must be considered [in future studies],” they added.