In the first study of its kind, , compared with those who said they had not eaten nuts.
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There was also an improvement in overall survival, but this was not statistically significant.
The finding comes from a study of more than 3,000 patients conducted in China, published online in the International Journal of Cancer. Patients were queried about nut consumption on only one occasion, 5 years after their breast cancer diagnosis.
The investigators report a dose-response pattern between nut eating and the risk of both breast cancer recurrence and overall mortality, with those consuming the largest amounts having the lowest risks.
“Nuts are important components of healthy diets. Promoting this modifiable lifestyle factor should be emphasized in breast cancer survivor guidelines,” conclude Xiao-Ou Shu, MD, PhD, of Vanderbilt University, Nashville, Tenn., and colleagues in the study.
“The association for disease-free survival is quite strong and robust,” Dr. Shu told this news organization.
However, as with all observational studies, this report shows an association and not causation.
“Based upon this study alone, the evidence is weak,” said Wendy Chen, MD, MPH, a breast oncologist at Dana-Farber Cancer Institute in Boston, who was approached for comment.
The people who consumed nuts generally had more education, higher income, lower body mass index, earlier-stage cancers, and more physically active lives – all factors associated with better breast cancer survival, she observed. “The authors tried to control for these factors,” Dr. Chen acknowledged. But it’s hard to know whether nut consumption was “truly” the difference maker, she said.
Furthermore, the study population is also “a bit unusual” because people had to survive 5 years after diagnosis to be included in the analysis – and thus is not representative of breast cancer survivors, she noted.
Erin Van Blarigan, PhD, an epidemiologist at the University of California, San Francisco, described the overall evidence of the beneficial relationship between nut eating and breast cancer – including this study – as “limited.” She previously led a study that observed benefits of nut intake for patients with colon cancer.
Dr. Van Blarigan also noted that nut intake in this study was “very low” – with the median intake less than one serving per week.
She also offered some general advice about eating nuts.
“Nuts are an energy-dense food, so portion sizes should be kept small,” she said, explaining a portion should be about 1 ounce or 1/4 cup of nuts or 1-2 tablespoons of nut butter.
A little may go a long way, she suggested, as research to date “suggests only small amounts may be needed to gain potential benefits.”
The level of nut consumption was low in the Chinese study population (median = 17.3 grams/week) compared with the 42.5 grams/week recommended by the American Heart Association, the study authors acknowledge.
“Nuts, particularly tree nuts, are expensive in China. Traditionally, nut consumption level has been low among Chinese, particularly in the old generation,” commented Dr. Shu.
Study authors did an adjusted analysis
The new study was conducted among 3,449 participants of the Shanghai Breast Cancer Survival Study.
Nut consumption (including peanuts and tree nuts such as walnuts) was assessed with a food questionnaire at 5 years post-diagnosis.
An analysis was conducted at 10 years post-diagnosis (and 5 years after the diet questionnaire). At this 10-year mark, there were 252 breast cancer-specific deaths. Among 3,274 survivors without previous recurrence at the dietary assessment, 209 went on to develop breast cancer-specific events – either recurrence, metastasis, or breast cancer mortality.
Nut consumers had higher overall survival (93.7% vs. 89%; P = .003) and disease-free survival (94.1% vs. 86.2%; P <.001) rates than nonconsumers.
However, the two groups had many differences, as noted by the authors and outside experts.
The consumers had a younger age at diagnosis, lower BMI, higher total energy intake, higher diet quality score, and higher soy food intake. In addition, nut consumers were more likely to have a higher education, personal income, and physical activity level (≥7.5 metabolic equivalent of task-hour/week) as well as to have received immunotherapy.
So the investigators adjusted for many of those variables and found that nut consumption was associated with significantly better disease-free survival (hazard ratio, 0.52; 95% confidence interval, 0.35-0.75), but a nonsignificantly improved overall survival (HR, 0.90; 95% CI, 0.66-1.23), as noted above.
Analyses by amount of nut intake showed a dose-response relationship for both overall survival (P trend = .022) and disease-free survival (P trend = .003).
The authors say that “there has been no strong evidence to support individual food items in favor of breast cancer survival,” citing a 2018 report entitled “Diet, Nutrition, Physical Activity and Breast Cancer Survivors” from the World Cancer Research Fund/American Institute for Cancer Research.
The new study provides evidence that nuts may be such a food, they say, while also calling for studies to confirm their findings.
Study limitations include that fact that the statuses of recurrence and metastasis were self-reported. Misclassification, particularly regarding the event date, is likely, the team says.
The study authors and Dr. Van Blarigan and Dr. Chen have disclosed no relevant financial relationships.
A version of this article first appeared on Medscape.com.