Diet May Have No Short-Term Effect on Renal Function

In a study, unhealthy eating did not hasten decline in estimated glomerular fi ltration rate.
In a study, unhealthy eating did not hasten decline in estimated glomerular fi ltration rate.

SAN DIEGO—Unhealthy diets are not associated with more rapid declines in kidney function, according to data presented at Kidney Week 2012.

The data are from a longitudinal cohort study of 5,405 participants (mean age 62 years) who had baseline dietary data from a 120-item food frequency questionnaire and at least two measures of serum creatinine, cystatin C, or urine albumin-creatinine ratio (UACR).

The researchers, led by Julie Lin, MD, MPH, of Brigham and Women's Hospital in Boston, divided subjects into four groups based on dietary pattern: “fats and processed meats” (similar to a “Western” pattern); “beans, tomatoes, and refined grains; “vegetables and fish”; and “whole grains and fruit” (similar to a “prudent” pattern. 

The investigators calculated subjects' estimated glomerular filtration rate (eGFR) using serum creatinine or cystatin C and defined rapid eGFR decline as 3 mL/min/1.73 m2 or greater per year decrease over five years of follow-up.

The racial composition of the cohort was 41% white, 25% black, 21% Hispanic, and 13% Asian. Adherence to each of the four dietary patterns varied markedly by race and ethnicity. The researchers observed no significant associations between any dietary pattern either eGFR decline or change in UACR in either unadjusted or adjusted analyses, the latter controlling for demographics, body mass index, hypertension, diabetes, lipids, and physical activity.

Dr. Lin noted that she and her team sought to assess the generalizability of the previously reported associations between dietary patterns and progression of kidney function decline in the almost entirely Caucasian study population of older women in the Nurses' Health Study (NHS).

Their study included an ethnically diverse and younger population represented by the Multi-Ethnic Study of Atherosclerosis (MESA), a cohort study of cardiovascular disease funded by the National Heart, Lung, and Blood Institute, National Institutes of Health. They hypothesized that less healthy eating patterns would be associated with faster decline of kidney function, whereas healthy patterns would be inversely associated. They also postulated that unhealthy dietary patterns would be associated with greater increases in albuminuria over time, whereas healthy dietary patterns would be associated with less progression of albuminuria.

“To our surprise and disappointment,” Dr. Lin told Renal & Urology News, “we did not observe any statistically significant associations between any of the dietary patterns and either of the kidney outcomes of interest with the exception of higher adherence to the ‘vegetables and fish' pattern being associated with an increase in albuminuria over time, which was the opposite of what we expected.”

She pointed out that the study was limited by a relatively short follow-up time of five years for the kidney parameters. In contrast, the NHS study, which reported that an unhealthy “Western Pattern” diet was associated with more rapid kidney function decline had 11 years of follow up for changes in eGFR. 

Moreover, the markedly different dietary patterns in the various ethnic groups may represent an important confounding factor. she said. “It is still possible that associations between dietary patterns and kidney outcomes will become evident with longer follow-up time in MESA, which is an on-going cohort study that continues to collect measures of eGFR and albuminuria every few years,” Dr. Lin said.

Importantly, she added, published data from the NHS and MESA studies as well as other studies support the benefits of healthy eating patterns, such as a reduced risk of chronic medical conditions such as diabetes and cardiovascular disease (both of which are major risk factors for progressive kidney disease), even when analyses are adjusted for important confounders such as age, gender, and body mass index.

“Our experience with this study highlights the importance of long-term follow-up of several years for better assessment of progressive kidney dysfunction as well as challenges in measuring changes in albuminuria in populations where the majority [of subjects] are in the traditional ‘normal' range,” Dr. Lin said.

“While large randomized, interventional dietary studies would be ideal, the reality is that adherence to treatment assignment by study participants over the many years needed to assess for kidney outcomes would be very difficult to maintain.”

In a separate study of diet and kidney disease presented at the conference, Orlando M. Gutierrez, MD, of the University of Alabama in Birmingham and colleagues found that a “southern dietary pattern” characterized by high intake of fried foods, organ meats, and sweetened beverages was significantly associated with an increased risk of death in patients with chronic kidney disease. The study included 3,971 CKD patients, of whom 611 died during a median five years of follow-up.

Compared with patients in the first quartile of consumption the foods in the southern dietary pattern, those in the fourth quartile had more than a 50% increased risk of death, after adjusting for age, gender, race, geographic region, and other potential confounders.

Dr. Gutierrez's team found no significant association between diet and mortality among subjects classified as adhering to a “convenience” diet—characterized by high intake of Chinese and Mexican foods, pizza, pasta, and other mixed dishes—and a healthy diet characterized by high intake fruits and vegetables.

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