The Effect of Dietary Protein Intake on Kidney Function in Women with Normal or Mildly Abnormal Kidneys

Summaries for Patients are a service provided by Annals to help patients better understand the complicated and often mystifying language of modern medicine.

Summaries for Patients are presented for informational purposes only. These summaries are not a substitute for advice from your own medical provider. If you have questions about this material, or need medical advice about your own health or situation, please contact your physician. The summaries may be reproduced for not-for-profit educational purposes only. Any other uses must be approved by the American College of Physicians-American Society of Internal Medicine.

The summary below is from the full report titled "The Impact of Protein Intake on Renal Function Decline in Women with Normal Renal Function or Mild Renal Insufficiency." It is in the 18 March 2003 issue of Annals of Internal Medicine (volume 138, pages 460-467). The authors are EL Knight, MJ Stampfer, SE Hankinson, D Spiegelman, and GC Curhan.

What is the problem and what is known about it so far?

Doctors think that high-protein diets have a bad effect on people with moderate or severe kidney disease because they cause kidney function to decrease more rapidly than do low-protein diets. However, it is not known whether high-protein diets are also bad for people with normal or only mildly abnormal kidney function or whether particular kinds of protein (such as meat protein) are worse than others (such as dairy or vegetable protein).

Why did the researchers do this particular study?

To find out whether eating a high-protein diet over a long period of time was bad for kidney function in people with normal or mildly abnormal kidneys and whether type of protein was important.

Who did the researchers study?

1624 women who participated in the Nurses' Health Study and had provided a blood sample for evaluation of kidney function in 1989.

What did the researchers do?

After being selected for the study, participants were asked to fill out questionnaires (in 1990 and 1994) about their eating habits to determine their usual amount and type of dietary protein. They were also questioned about other factors that could affect kidney function, such as age, weight, height, diabetes, high blood pressure, smoking habits, alcohol use, elevated cholesterol level, use of certain types of pain medication, and use of blood pressure medication. The researchers used the blood samples to evaluate how well the kidneys were working at each time point. This helped them to determine if kidney function was getting worse and, if so, how fast.

What did the researchers find?

In women who started out with normal kidney function, there was no association between the amount or type of protein and a decrease in kidney function over time. On the other hand, women who started out with mildly decreased kidney function showed some worsening with increasing amounts of protein intake, particularly meat protein. Dairy or vegetable protein was not associated with worsening kidney function.

What were the limitations of the study?

Since participants were not randomly assigned to follow diets with specific amounts of protein, some factor other than protein intake might have been responsible for the decrease in kidney function seen in people who started out with a mild abnormality. Furthermore, since dietary habits were evaluated at only two points in time, actual protein intake may have been different than the researchers thought. Since most of the study participants were white, the results may not be accurate for black women.

What are the implications of the study?

To decrease the rate of further loss of kidney function, it may be appropriate for women with mild kidney abnormalities to avoid a high-protein diet, particularly excess meat intake.