Walnuts seem to offer little protection from age-related cognitive decline, new research suggests.
A community-based study of older adults showed a walnut-enriched diet conferred no cognitive benefit in healthy seniors, but did offer some modest benefit in individuals with a history of smoking and in those from disadvantaged backgrounds.
"Although there were no statistically significant differences between those who ate walnuts for 2 years vs controls who did not eat walnuts, we did find that there were protective trends in subjects at higher risk — meaning, those with lower baseline consumption of omega-3 fatty acids in their diet, smokers, or those with lower baseline cognitive scores," study investigator Joan Sabaté MD, DrPH, Loma Linda University, California, told Medscape Medical News.
The study was published online January 7 in the American Journal of Clinical Nutrition.
Good for the Heart
The current study is an extension of previous studies conducted by Sabaté's group showing walnuts have a beneficial effect on serum lipid levels and blood pressure, coronary heart disease, and cardiovascular disease.
"We studied many nuts and found that walnuts lowered total and LDL cholesterol and, given that dementia is a prevalent issue and vascular dementia in particular has some of the same risk factors and mechanisms as heart disease, we thought to study the connection between walnuts and prevention of age-related cognitive decline," said Sabaté.
"Besides fatty acid composition, walnuts are the only nuts that contain alpha-linolenic acid — an omega-3 fatty acid — and also contain polyphenols and antioxidants, and we thought some of the constituents of this matrix of nutrients contained in walnuts may decrease or reduce the rate of age-related cognitive decline."
To investigate, researchers at two research sites — Loma Linda University in California and Hospital Clínic in Barcelona, Spain — compared the effects of a walnut-enriched diet (at 15% of the energy, based on a 2000-calorie diet) to an eating pattern that did not include walnuts in healthy seniors over a 2-year period.
Participants from Barcelona were younger, less educated, smoked more, and displayed lower scores in the Hamilton Depression Rating Scale (HAM-D), compared with their Loma Linda counterparts.
At baseline, participants underwent physical examination and provided medical history. The researchers also obtained blood samples that were used in genetic testing for APOE genotypes.
At baseline and at the end of the study period, participants completed a battery of neuropsychological tests, with the primary study outcome being the mean 2-year change from baseline in a global cognitive composite score.
Secondary outcomes included mean change from baseline in composites for the cognitive domains of memory, language, perception, and frontal function.
Brain functional magnetic resonance imaging (fMRI) was conducted on a subset of 120 participants at baseline and after 2 years.
Encouraging, Not Conclusive
Of 708 eligible subjects, a total of 657 at both sites completed the full study and were included in the intention-to-treat (ITT) analyses (n = 336 [66% female, mean age 69.4 years, 83% never-smokers in the walnut group; n = 321 [67% female, mean age 68.9 years, 84% never-smokers] in the control diet group).
Adherence to the walnut diet was good and, at the end of the 2-year study period, participants in the walnut group showed increases in fiber, linoleic acid, and alpha-linolenic acid intake, which were not observed in the control group.
None of the study participants developed clinically significant cognitive impairment during the study period.
There were also no significant differences in change from baseline in the global cognition score between the walnut and the control groups (unadjusted mean change −.072 [95% confidence interval [CI], −0.100 to −0.043] and −0.086 [95% CI, −0.115 to −0.057], respectively, P = .491).
Similarly, no differences were observed in the individual cognitive domains of secondary outcomes.
However, post-hoc analyses showed significant differences in global cognition score between the walnut diet and control groups at 2 years, specifically in the Barcelona participants (unadjusted changes −0.037 [95% CI, −0.077 to 0.002] and −0.097 [−0.137 to −0.057], respectively, P = .40).
A calculation derived from the cognitive trajectory of controls showed that 1 year of aging was associated with a mean decline of .049 (95% CI, 0.029 - 0.069) in the global composite score.
Applying that calculation, the researchers discovered that the mean difference in changes between groups in the Barcelona subjects was 0.060 (95% CI, 0.005 - 0.115), which translated into an equivalent of approximately 1.24 years of aging.
In particular, the perception score improved significantly in the walnut group vs the control group. However, there were no differences in fMRI results between the two groups in the subset of Loma Linda participants.
In Barcelona participants who underwent fMRI, the control group displayed greater functional network recruitment, "a common fMRI finding associated with response to cerebral damage in aging," the authors note.
By contrast, participants in the walnut group did not show this type of increased functional recruitment, "suggesting greater brain efficiency."
"The take-home message is that 2 years is probably not enough of a follow-up to see an effect, which is why we are now advocating doing a longer follow-up because consumption of walnuts may have a delayed protective effect," Sabaté stated.
The authors describe the findings as "encouraging but not conclusive." Further research should be "directed to disadvantaged populations in whom greatest profit could be expected."
No Harm in Cracking the Occasional Nut
Commenting on the study for Medscape Medical News, Keith Fargo, PhD, director of scientific programs and outreach at the Alzheimer’s Association, called it "fundamentally, a negative study."
However, "that said, we learn from every clinical trial and this research adds to our understanding of the relationship between diet and dementia risk," said Fargo, who was not involved with the study.
"Eating walnuts or taking walnut supplements alone is not sufficient to improve cognition in healthy older adults" and "research suggests that individuals concerned about their dementia risk should instead focus on eating an overall healthy diet — lower in fat and higher in vegetables and fruit — combined with other brain-healthy behaviors, such as regular physical activity, not smoking, and lifelong learning."
Also commenting on the study for Medscape Medical News, Henry Brodaty, AO, MBBS, MD, scientia professor of Ageing and Mental Health and codirector of the Centre for Healthy Brain Ageing at the University of New South Wales in Sydney, Australia, said that the statistically significant result for global cognition shown in the post-hoc analysis "was small and probably not clinically significant."
"On the basis of this study, it is premature to recommend that everyone should eat 30-60 g/day of walnuts, although, as the authors point out, it may be worth investigating in those from more disadvantaged communities," stated Brodaty, who was not involved with the study.
Sabaté added that, based on this study, evidence is insufficient to recommend eating walnuts specifically for cognitive protection — even though it might be helpful for individuals at high risk.
"But since it is well demonstrated that walnuts lower cholesterol and prevent the risk of cardiovascular disease, just on this basis, I don’t see reason not to continue recommending eating small quantities on a regular basis," he said.
Sabaté reports receiving research funding through his institution from the California Walnut Commission, being a nonpaid member of the California Walnut Commission Scientific Advisory Council, and having received honoraria from the California Walnut Commission for presentation. The other authors' disclosures are listed on the original paper. Fargo has disclosed no relevant financial relationships. Brodaty reports being on the advisory board for Nutricia Australia.
Am J Clin Nutr. Published online January 7, 2020. Abstract
Medscape Medical News © 2020
Cite this: Walnuts Shell Out Little Protection AgainstAge-Related Cognitive Decline - Medscape - Feb 07, 2020.