A study of 6,348 women who were part of the Nurses' Health Study (NHS) and who were diagnosed with breast cancer between 1976 and 2004 found that women who consumed more protein, especially from animal sources, had a modestly lower risk of recurrence and death resulting from breast cancer. The study, “Protein Intake and Breast Cancer Survival in the Nurses’ Health Study,” published in the Journal of Clinical Oncology (JCO) online ahead of print, November 7, mirrors the findings of several previous studies, including a study from 1999 that looked at 1,982 women in the NHS.

Michelle D. Holmes, MD, DrPH, MPH, first author of the current JCO study, as well as the study from 1999, said this is the largest study to date looking at the connection between protein consumption and risk of breast cancer recurrence and death. The question of whether high-protein diets, especially animal protein, poses a risk for cancer survivors, is an ongoing debate in the research and clinical community.

“There’s been a concern among patients and their clinicians that eating animal products might be bad for breast cancer,” said Dr. Holmes. “This study provides evidence that patients can be much less worried about that association. Protein consumption does not seem to be bad for these patients, and it might even be good.”

Caveat: high-protein diet might still pose risk

Dr. Holmes cautioned that the study’s findings should not be seen as a green light to dismiss all concerns about eating high-protein foods. First, the lowest risk of recurrence and death was observed among women who had the equivalent of a modest two servings of low-fat dairy per day. In addition, while protein intake was not found to be correlated with higher rates of recurrence and death for breast cancer, high intake of some animal products is correlated with increased risk for other chronic illnesses, which may develop years down the line.

“Our study looked at women with early stage breast cancer, most of whom live a really long time after their diagnosis. This longevity means they’re still at risk of developing diseases like heart disease and colon cancer, for which we know we have to be more careful in choosing the animal products. So if there’s a take-away from this study it is that patients should make healthy choices of animal sources of protein, such as poultry and fish and low-fat dairy, which might help prevent other chronic diseases as well.” 

The association between high-protein intake and lower risk of breast cancer recurrence and death was not significant among breast cancer survivors who consumed diets that were high in vegetable protein. However, Dr. Holmes said the absence of this correlation might be due to the fact the vegetables contain very low amounts of protein, making it more difficult to detect an effect of vegetable protein.

Can insulin explain the association between protein and lower risk of recurrence?

The study also looked for a mechanism to explain the association between a high-protein diet and a lower risk of recurrence and death from breast cancer. The authors hypothesized that women who had diets high in certain kinds of proteins, including essential amino acids, branch-chain amino acids, and leucine intake, would have improved survival after breast cancer since these proteins have been shown to improve usage of insulin and increase lean muscle mass, a beneficial effect that also occurs with physical activity. The authors also hypothesized that the association between a high-protein diet and a lower risk of recurrence and death from breast cancer would be strongest in women with tumors expressing insulin receptors (IR). Analysis of the data did not support either of these hypotheses.

Michelle D. Holmes, MD, DrPH, MPH, is an associate professor in the Department of Epidemiology at the Harvard T.H. Chan School of Public Health, in Boston. She has been an ASCO member since 2016.

Abstract of the original JCO article.

PDF of the original JCO article.

Holmes MD, Wang J, Hankinson SE, et al. Protein intake and breast cancer survival in the nurses’ health study. J Clin Oncol. Epub Nov 7, 2016.

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@ 2016 American Society of Clinical Oncology