6/17/2015

by Parker Brown
Staff Writer, MedPage Today

Nut consumption was associated with a decreased risk of some types of cancer but not with type 2 diabetes in a large review.

When patients eating the most nuts were compared with those eating the least, those in the first group had a lower risk of colorectal cancer in three studies (RR 0.76, 95% CI 0.61-0.96; I²=51.3%), of endometrial cancer in two studies (RR 0.58, 95% CI 0.43-0.79; I²=0%), and pancreatic cancer in one study (RR 0.68, 95% CI 0.48-0.96; I² not available). Those results were reported in the meta-analysis of 36 observational studies, with a total population of more than 30,000 patients.

Nut consumption was also associated with a lower risk of cancer in general (RR 0.85, 95% CI 0.76-0.95;I²=66.5%), according to the authors. But it was not associated with other types of cancer or with type 2 diabetes (RR 0.98, 95% CI 0.84-1.14; I²=74.2%), found the researchers, who were led by Lang Wu, a PhD candidate at the Mayo Clinic. They published their results on June 16 in Nutrition Reviews.

“Overall, nut intake was associated with a decreased risk of cancer,” wrote Wu and colleagues. “Given the scarcity of currently available data, however, evidence from additional studies is required to more precisely determine the relationship between nut consumption and risk of individual cancer types.”

Evidence for the association between nuts and cancer has been mixed, according to the authors. Follow-up time in the studies ranged from 4.6 years to 30 years, found the review. Studies were included if they specifically investigated the association between nut consumption and cancer or type 2 diabetes. Sixteen cohort studies were included along with 20 case-control studies. Data about nut consumption was obtained from interviews in 42% of the studies and from self questionnaires in the rest of the studies.

The amount of nuts eaten ranged from none for some of the patients to eating nuts more than seven times a week. In total, 22 of the studies provided estimations of nut consumption with risk of disease, six provided estimations for peanuts only, four for nut and seed intake only, two for pulse, nut, and seed intake only, one for fruit and nut intake only, and one for beans, lentils, nuts, and seeds only.

The three studies examining colorectal cancer had separate analyses for males and females. No associations were found between nut consumption and acute myeloid leukemia, breast cancer, gastric cancer, glioma, hepatocellular carcinoma, leukemia, lymphoma, ovarian cancer, prostate cancer, or stomach cancer.

For acute myeloid leukemia, a case-control study found a significantly lower risk ratio than did the prospective cohort study (RR 0.35, 95% CI 0.19-0.65; P=0.001 versus RR 1.27, 95% CI 0.79-2.04; P=0.32). In addition, for breast cancer, a case-control study found a lower risk ratio than did a cohort study (RR 0.76, 95% CI 0.61-0.95 versus RR 1.03, 95% CI 0.88-1.20).

The authors noted that during the time they were doing the review, several studies were investigating the association between nuts and type 2 diabetes.”The findings of a null association with type 2 diabetes in the present review is largely consistent with the findings of those studies,” wrote the authors. But at least two previous studies have found an inverse relationship between nuts and type 2 diabetes, they added.

Nuts have been associated with improved glycemic responses in healthy people, according to the authors, but that relationships is not clear.

“Numerous mechanisms have been proposed to explain the potential effect of nuts on the risk of cancer,” wrote Wu and colleagues. “More investigations on the role of nuts in each of the cancers examined in the present review are warranted.”

Limitations of the study include the researchers not testing for publication bias “which is likely to exist when evidence consists of observational studies that do not require trial registration.” There may be studies that found no correlation that were not published, according to the authors.

In addition, the studies reviewed were observational, so it’s possible — perhaps even plausible in this case — that those who consumed more nuts than others were healthier in other ways not adjusted for in the analysis. In some of the case-control studies, there is the possibility of recall bias, which could affect the reliability of the data.

• Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco