Researchers have found a seasonal variation in urinary albumin-to-creatinine ratio (UACR) in Japanese patients with type 2 diabetes and early nephropathy, with the highest values in winter and the lowest in summer.

Systolic blood pressure (BP) also followed this pattern, raising the possibility of a causal link. The investigators suggested that clinicians consider this seasonal variation in UACR when evaluating the effects of drugs on diabetic nephropathy.

A group led by Yoshiharu Wada, MD of the Center for Diabetes and Endocrinology at the Tazuke Kofukai Foundation Medical Research Institute Kitano Hospital in Osaka, Japan, recruited a study cohort of 430 patients (275 male, 155 female, mean age 64.8 years) with type 2 diabetes, early nephropathy, and microalbuminuria. Subjects visited the clinic every three months from 2006 to 2009. The researchers defined microalbuminuria as a UACR of 30-300 mg/g of creatinine. They excluded patients with advanced nephropathy with elevated creatinine.

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Reporting the study findings during a poster session at the 46th Annual Meeting of the European Association for the Study of Diabetes, Dr. Wada said readings were categorized according to season: winter (December through February), spring (March through May), summer (June through August), and fall (September through November).

Although UACR and systolic BP were highest in the winter, HbA1c peaked in spring. Estimated glomerular filtration rate (eGFR) and diastolic BP showed no significant seasonal variations.

UACR and systolic BP showed a significant correlation, with UACR rising as systolic pressure rose. The investigators observed no correlation between BP and HbA1c or between HbA1c and UACR.

The fact that both UACR and systolic pressure were highest in winter and lowest in summer “suggest that the seasonal change in systolic blood pressure… may contribute to the variations in UACR,” Dr. Wada said.

Some attendees at the poster session raised questions about possible explanations for the observations. One suggested the seasonal variations could be related to the amount of exercise people engaged in. Another wondered if the findings could be related to seasonal variations in serum vitamin D levels. Dr. Wada said he had not measured exercise levels or vitamin D in his subjects.

Previous work based on data from the U.S. Third National Health and Nutrition Examination Survey (Am J Kidney Dis. 2007;50:69-77) demonstrated an increased prevalence of albuminuria with decreasing quartiles of serum 25-hydroxyvitamin D concentration. Compared to the highest quartile of serum vitamin D level, the lowest quartile was associated with a 37% increased relative risk of albuminuria. As this study was retrospective and observational, no conclusions could be drawn as to causality.