In a recent study published in the journal Antioxidants, researchers reviewed current literature to examine walnuts' potential antioxidant and anti-cancer properties and the health benefits of incorporating walnuts into the diet.
Study: Antioxidant and Anti-Inflammatory Properties of Walnut Constituents: Focus on Personalized Cancer Prevention and the Microbiome. Image Credit: SOMMAI/Shutterstock.com
Background
An essential part of the Mediterranean diet consists of nuts, especially walnuts, which have been associated with a decreased risk of cardiovascular diseases, type 2 diabetes, cancer, and coronary heart disease, and have beneficial effects in slowing aging.
Various studies have reported the association between walnut consumption and reduced total and cardiovascular disease-linked mortality, as well as improvements in cardiac function and lipid profiles.
Among tree nuts, including hazelnuts, pecans, and almonds, walnuts have the highest amount of polyunsaturated fatty acids, such as omega-3 alpha-linolenic acid, and are a good source of fiber and protein.
Studies on animal models have reported improvements in gut microbial diversity, tumor multiplicity, volume, and gene expression changes associated with nut consumption.
Furthermore, cohort studies and clinical trials in humans have indicated that walnut consumption and treatment with phenolic extracts from walnuts can alter expression profiles of genes and bring about anti-inflammatory changes.
Anti-cancer properties
Animal studies have reported that walnut consumption was linked to reductions in breast cancer cell proliferation and mammary gland tumor size, multiplicity, and incidence.
Additionally, walnut consumption was associated with alterations in apoptosis, cell differentiation, and proliferation pathways.
A study that compared the benefits of a high-fat diet rich in walnuts with a micronutrient-matched soybean oil-rich diet also reported decreased growth and weight of prostate tumors in mice models, accompanied by lower levels of resistin, low-density lipoprotein, and plasma insulin-like growth factor-1.
Diets consisting of flaxseeds and walnuts were also found to decrease angiogenesis significantly.
A high-fat diet supplemented with walnuts was also found to decrease the tumor burden by 32% in mice models, with histological observations revealing reductions in adenocarcinomas and tubulovillous-type adenomas.
Human studies involving breast cancer patients reported that a walnut-rich diet was linked to a significant change in tumor gene expression with inhibition of cell proliferation-related signaling pathways.
Additionally, increased consumption of tree nuts was also found to decrease the risk of hepatocellular carcinomas and esophageal squamous cell carcinoma.
The review also reported the results from human studies that found improvements in other parameters such as gut microbiome diversity and abundance, urolithin levels, and reductions in cholesterol, apolipoprotein B, triglyceride, and low-density lipoprotein levels associated with walnut consumption.
Influence on the gut microbiome
The incorporation of a substantial portion of walnuts in the diet was found to increase the abundance of butyrate-producing and probiotic bacteria and a reduction in pathogenic bacteria such as Clostridium species.
Walnut consumption also decreased the levels of microbially derived pro-inflammatory secondary deoxycholic, bile, and lithocholic acids and increased the abundance of Roseburia and Faecalibacterium. Which ferment insoluble fiber into short-chain fatty acids that suppress inflammation.
Animal studies reported that while exposure to carcinogens reduced the richness and diversity of the gut microbiome, a diet rich in walnuts granted partial protection against the detrimental effects of carcinogens, courtesy of the ability of walnuts to influence the gut microbiome towards a more anti-tumor profile.
Studies on rat models, with protein and fiber content matched diets for comparison, reported increased probiotic bacteria such as Roseburia, Ruminococcaceae, and Lactobacillus in the gut after walnut consumption.
Anti-inflammatory properties
In animal models of ulcerative colitis induced by dextran sodium sulfate, urolithin A exhibited significant protection against chronic and acute colitis. This occurs by reducing mediators of inflammation and up-regulating the expression of tight junction proteins and ligand-activated transcription factors.
While walnut consumption has been found to increase urolithin levels, treatment with phenolic extracts from walnuts and walnut oil was also seen to down-regulate pro-inflammatory cytokines and stabilize tight-junction proteins.
Metabolic changes induced by a walnut-rich diet, such as increased polyunsaturated fatty acids and S-adenosylhomocysteine, were thought to protect against inflammatory tissue injury induced by dextran sodium sulfate.
Studies also reported cancer-preventative and gastro-protective properties of walnuts against inflammation induced by alcohol and in gastric cancer animal models.
Conclusions
Overall, the findings indicated that walnut consumption was associated with various health benefits, including protective effects against carcinogens, down-regulation of pro-inflammatory signaling pathways and cytokines, and increasing the diversity and abundance of probiotic bacteria in the gut.
Incorporation of substantial portions of walnuts and other tree nuts in the diet, as is followed in the Mediterranean diet, could provide a wide range of health benefits.
New Lancet Commission on Breast Cancer: Transforming breast cancer care globally