What is the impact of consuming walnuts on the dietary intake of total polyphenols?

By Neha MathurMar 7 2023Reviewed by Danielle Ellis, B.Sc.

In a recent study published in Nutrients, researchers performed secondary analyses of the Walnuts and Healthy Aging study (WAHA) data, a two-year prospective, randomized controlled trial (RCT) carried out among the 63 to 79-year-olds of Loma Linda University (LLU) in California, United States of America (USA) and Barcelona in Spain, between 2014 and 2016.

The parent study examined the effect of daily dietary walnut supplementation of 30 to 60 grams/day on the aging outcomes compared to a walnut-free customary diet. However, in the present secondary data analyses, researchers used data from LLU participants only. They examined the effects of dietary supplementation with walnuts at 15% of energy, i.e., the consumption of 30 to 60 grams of walnuts every day.

Background

Dietary polyphenols are excellent antioxidants and anti-inflammatory phytochemicals that offer multiple health benefits. For instance, they increase high-density lipoprotein (HDL) and lower low-density lipoprotein (LDL) in the blood (or plasma) lipids, modify inflammation and endothelial dysfunctions, and increase antioxidant defenses, thus, reducing the risk of cardiovascular diseases (CVDs).

Among the nuts, walnuts contain the highest concentrations of polyphenols and have a favorable nutrient and fatty acid profile. Chromatography and mass spectrometry have identified that most polyphenols reside in the seed coat or pellicle of the edible walnut kernel, averaging 2,500 gallic acid equivalent (GAE) per 100 grams. Walnuts increase the concentration of plasma polyphenols within 30 minutes of ingestion.

About the study

In the present study, researchers first performed nutrient analysis using the Nutrition Data System for Research, for which trained research dietitians obtained 24-hour dietary recall data via telephonic conversations or face-to-face interviews from all eligible participants. Next, they conducted these interviews over two years at varying intervals to capture seasonal variations of food intake wherein they noted data on the food, beverages, and dietary supplements consumed by these individuals in the past 24 hours.

Further, the team used the Phenol-Explorer database to estimate the polyphenol content of consumed foods and beverages from 24-hour dietary recall data. The rapid Folin–Ciocalteu (F–C) and chromatographic methods helped them estimate cumulative polyphenol content and polyphenol subclasses, respectively.

Polyphenol subclasses comprised phenolics; flavonoids consisted of flavonols, flavones, and anthocyanins; additionally, there were the subclass lignans. The team entered how consumed foods contributed to all subclasses into the dietary database to match food composition data and compute the intake of aggregate dietary polyphenols and phenol subclasses.

Furthermore, the team used spot urine samples of the WAHA participants to estimate the aggregate urinary polyphenol concentrations in mg GAE/L. In the second year of the study, they adjusted for urine dilution due to creatinine concentration.

Results

From 356 LLU participants, the present secondary analyses used only 300 subjects. Both walnut-consuming and control groups had more women participants. In 1242 sessions, research dieticians collected five 24-h dietary recalls from most participants.

They noted that the walnut group had higher total fat, energy, and dietary fiber intake than the control group over the two-year study period. Accordingly, the aggregate polyphenol intake in the walnut group (due to mean walnut consumption) was higher than the control group (632 vs. 40 mg/d). Walnut consumption also markedly contributed to more aggregate polyphenols, flavanols, flavonoids, and phenolic acids but not lignans. Thus, both study groups had comparable lignan intake.

Compared to the baseline, the polyphenols urinary excretion in the walnut group approached statistical significance only in the first year (p-value 0.066). However, in the control group, the values remained comparable at all time points, indicating no significant inter-group variations throughout the two-year study duration.

The analyses of spot urine samples did not show any correlation between dietary polyphenol consumption (via walnuts) and urinary polyphenol concentrations but an inverse correlation with flavonoid intake. The estimation method (the F-C assay) or the short half-life of bioavailable polyphenols and their metabolites could have caused this.

Around 5 to 10% of the total dietary polyphenols reach the small intestine and are absorbed there due to their structural complexity and solubility. Since the gut also absorbs and eliminates some polyphenols, it explains their reduced urinary excretion.

Walnuts are rich in ellagitannins which hydrolyzes to ellagic acid, upon which the gut microbiota acts to produce urolithins. These are excreted primarily via urine and could serve as valuable biomarkers of walnut consumption. Yet, future studies should utilize 24-h urine collection following the walnut consumption to capture most polyphenols in the urine samples.

Conclusions

The study analysis showed that a single food, i.e., walnuts, increased the total dietary polyphenols and their subclasses (except lignans) in healthy older adults. These individuals also showed higher energy, total fat, fiber, and unsaturated fatty acid intake. Thus, including nuts, such as walnuts, in daily diet could have significant health benefits. Nuts protect from age-related chronic illnesses, like CVDs and neurological disorders. Since they also have high amounts of polyphenols, they could synergistically reduce the disease risk.