Serum vitamin C concentrations have been reported to be low in diabetic patients (1). Diabetic nephropathy is known to develop in diabetic individuals, and decreased renal function and hypertension could reportedly accelerate atherosclerosis in patients with type 2 diabetes (2). Observational epidemiologic studies showed an inverse relation between the dietary intake or serum levels of vitamin C and blood pressure (3). Although reduced concentrations of vitamin C were reported in type 2 diabetic retinopathy (4), the concentrations of serum vitamin C in type 2 diabetic patients with diabetic nephropathy have not been previously reported. The aim of the present study is to determine whether the concentrations of serum vitamin C have any relation to type 2 diabetic nephropathy. Since diabetic nephropathy is associated with low-grade inflammation (5), we also examined the inflammatory marker of high-sensitivity C-reactive protein (hs-CRP).
Forty-one type 2 diabetic subjects (age 35–65 years [average 55]) and 15 age- and sex-matched control subjects participated in the study. Any subject who smoked, took vitamins or nonsteroidal anti-inflammatory agents, or was receiving hormone replacement therapy was ineligible for the study. In addition, any patient with a history of macrovascular disease was excluded. Although vitamin C intake did not differ between type 2 diabetic patients and control subjects, serum vitamin C levels were found to be low in type 2 diabetic patients compared with control subjects (4.9 ± 0.3 vs. 6.8 ± 0.4 μg/ml, P < 0.005). Serum vitamin C levels significantly correlated with serum creatinine (r = −0.43, P < 0.005), log hs-CRP (r = −0.36, P < 0.05), and diastolic blood pressure (r = −0.33, P < 0.05) in diabetic subjects. No correlation was found between serum vitamin C levels and urinary albumin excretion levels in patients with diabetes. A stepwise multiple regression analysis demonstrated that both creatinine levels (β = −1.232, F value = 9.09) and log hs-CRP (β = −6.335, F value = 4.12) were independent determinants of serum vitamin C levels.
In type 2 diabetic patients, low levels of serum vitamin C were closely associated with concomitant renal dysfunction and low-grade inflammation.