The current study revealed that an elevated serum uric acid level is associated with an increased risk of incident macroalbuminuria in people with type 2 diabetes. Previous research in animal models has suggested potential biological mechanisms for how hyperuricemia causes chronic renal dysfunction. Uric acid may impair nitric oxide production to induce endothelial cell dysfunction [10]. Moreover, hyperuricemia has been shown to trigger the renin-angiotensin system and damage vascular smooth muscle cells [6–8], leading ot the development of renal arteriolopathy, arterial hypertension, and then microvascular injury [5, 9, 19]. Another probable mechanism is the elevation of serum uric acid inducing inflammatory cytokine production, including tumor necrosis factor-α, C-reactive protein, interleukin-1β, and interleukin-6 [20].
Previous studies in different settings with various study designs have reported similar findings in individuals with type 2 diabetes [13–16]. In both cross-sectional and cohort researches, uric acid was reported to be related to diabetic nephropathy. In cross-sectional studies, serum uric level was related with microalbuminuria in Korean, Taiwanese, and Chinese people with type 2 diabetes [13, 21, 22]. A cohort study in Japan reported that diabetic patients with increased serum uric acid levels had increased risk of deterioration of diabetic nephropathy defined as shifting from microalbuminuria to albuminuria. However, this study also showed that uric acid was not associated with the risk of shifting from normoalbuminuria to microalbuminuria or macroalbuminuria [23]. This may be because the follow-up period of the study was only 2 years, which was too short for the occurrence of diabetic nephropathy. A cohort study with a 4-year follow-up duration in Italy revealed that serum uric acid was significantly associated with risk of incident macroalbuminuria only if the estimated glomerular filtration rate < 60 ml/min per 1.73 m2 [16]. Another cohort study with a follow-up duration of 5 years reported that uric acid was related to risk of incident diabetic nephropathy defined as an estimated glomerular filtration rate less than 60 ml/min per 1.73 m2 or an albumin-creatinine ratio greater 30 mg/mmol [15]. However, most of the study subjects were white, and race has some impact on the risk of kidney function progression [24].
As far as we know, few researches have reported the uric acid cutoff for an increased risk of impaired renal function. A cohort study in Italy reported that hyperuricemia is associated with incident diabetic nephropathy, and the definition of hyperuricemia was a serum uric acid level greater 7.0 mg/dL in men and greater than 6.6 mg/dL in women [15]. These cutoff values were those generally used to define hyperuricemia in their medical laboratory. The current study showed that the uric acid level cutoff for an increased risk of incident diabetic nephropathy was 6.9 mg/dL, which was calculated by the area under the ROC curve.
Previous studies had also reported that reducing the serum uric acid concentration can reduce kidney damage and recover renal function in diabetic mice, which suggested that the nephropathy caused by a high serum uric acid concentration might be reversible [25]. Momeni et al. thus conducted a double-blinded randomized controlled trial recruiting people with diabetic nephropathy, reporting that the serum uric acid concentration decreased after allopurinol management for 4 months, and the 24-hour urine protein excretion also decreased [26]. Another single-blind, randomized, controlled trial showed that allopurinol therapy for 2 years enhanced the estimated glomerular filtration rate and decreased cardiovascular risks [27, 28]. However, to date, despite some preliminary pilot studies of randomized clinical trials of allopurinol [26–28], there is still limited evidence regarding pharmacological intervention to reduce serum uric acid levels and thus prevent the deterioration of renal function.
One strength of the current study is the highly detailed demographic information of the participants collected by well-trained interviewers, including socioeconomic status, smoking habits, and alcohol consumption, which were proven to be potential confounders. Laboratory tests, including fasting blood glucose, HbA1c, uric acid, creatinine, lipid profiles, and urine albumin-creatinine ratio were achieved by standardized processes at the hospital with high accuracy. Moreover, there were few missing values in our dataset. The study was a cohort survey with longitudinal follow-up for 4.5 years, resulting in adequate relationships for causal inference. However, there are also some limitations to be borne in mind. The study was performed at the outpatient department of the Puli Branch of Taichung Veterans Hospital, which is a local hospital. Individuals with disabilities, those who were bedridden, and those who were unable to answer the questionnaire were excluded from the study, which might limit its generalizability. Moreover, recall bias on the self-reported questionnaire might be another concern. Only uric acid levels at baseline were collected, and the change over time was not measured; thus, the influence of uric acid on the incidence of diabetic nephropathy over time was not estimated. Medications, including anti-diabetic drugs, hypertensive drugs, and urate-lowering drugs were not included in the study because we believe that the effect of medication could be estimated by the laboratory data, including HbA1c, uric level, lipid profiles, and blood pressure measurements.