Thursday 19 July 2018

Walnuts beneficial for the intestine and brain

 Walnuts beneficial for the intestine and brain

Research has often shown that walnuts are beneficial for the brain. But the intestinal microbiome has also been found to know what to do with these. Interactions of microbes with the undigested walnut components have a positive effect on metabolism, the immune system, the mood and they give a feeling of fullness.

Walnuts are high in fibres, antioxidants and alpha-linoleic acid, an essential omega-3 fatty acid. Despite the level of calories, walnut consumption can help with weight management. Several studies have shown that walnuts give a feeling of fullness. One study examined the neurocognitive effect behind this (Mitchell 2017). More about this later on under ‘Study 1’. 
An interesting fact is that only about 80 percent of the calories present in walnuts are absorbed. This means that the intestinal microbiome has access to the remaining 20 percent of calories, fats and fibres. These serve as a source of food for the intestinal microbiome. Partly thanks to these fibres, microbes are able to produce butyrate, important for energy metabolism and a nutrient for colonocytes. This enables the number of micro-organisms and the diversity to increase. 

Diversity of intestinal flora and diseases of affluence

A diet that is high in nuts is, in contrast to the Western diet, associated with a reduced risk of death from cancer and heart conditions and improved brain health. More and more studies are indicating that a greater diversity of micro-organisms in the intestines reduces the risk of diseases (of affluence). A healthy intestinal microbiome helps to digest complex foods, provides nutrients and gives us a feeling of fullness. Interactions of microbes with the undigested walnut components have a positive effect on our metabolism, our immune system and our mood (Byerley 2017; Holscher 2018). More about this under ‘Study 2’.

Study 1. Neurocognitive effect and walnuts

Our brain activity is influenced by our eating habits. Recent research has shown that the craving for food and the so-called ‘sense of hunger’ is regulated by the consumption of walnuts. 
Study design 
During a randomised double-blind study, ten obese participants stayed at a research centre for two five-day sessions. The food intake was monitored. One session required the consumption of smoothies containing 48 grams of walnuts. During the second session, a walnut-free smoothie was served, similar in terms of nutritional value and flavour. 
On the fifth day of the experiment, fMRI tests were performed to study how the brain works in this process. Whilst the participants were in the MRI scanner, images were shown of desirable, unhealthy foods, neutral objects such as flowers and stones and less desirable, healthy foods. 
Results and conclusion
After the walnut-rich diet, a certain area of the brain, the insula, showed increased activity when the photos of food were viewed that were considered to be "very desirable". Activation of the insula is related to cognitive control, less hunger and a higher level of satiation.
The choice of food and the fMRI test showed that after consuming walnuts, the participants made more conscious food choice. They opted for the less desirable, but healthier options. It is not yet known whether eating more walnuts led to improved brain activation. (Mitchell 2017)

Study 2. Human intestinal microbiome and walnuts

Research involving rodents has already demonstrated that the quantity and quality of the intestinal microbiome changed following walnut consumption, whilst reduced diversity is associated with obesity and other illnesses, such as inflammatory bowel diseases. A significant increase in useful bacteria was found, including Lactobacillus. This suggests that walnuts are prebiotics, nutrients that selectively boost the numbers and activity of useful bacteria. (Byerley 2017)
Study design
As part of this study, 18 healthy participants consumed a walnut-free diet for a period of three weeks and then a diet containing 42 grams of walnuts a day for a three-week period. Blood and faeces were tested at the start and end of the two periods, to investigate the effect of walnut consumption on the intestinal microbiome, bile acids and metabolic markers.

Results and conclusion
The consumption of walnuts resulted in a reduction in the LDL cholesterol level.In the intestines, the presence of Faecalibacterium, Roseburia and Clostridium increased, which are all capable of producing butyrate. This is a beneficial metabolite for the health of the large intestine. It also protects the intestinal cells, because the microbes that are present produce fewer secondary bile acids. It has been proven that in cancer of the large intestine, higher levels of secondary bile acids occur more often. Faecalibacterium is possibly a probiotic bacteria. In animals, it helps to reduce an increased presence of inflammations and it improves insulin sensitivity (Holscher 2018).


Not all calories from walnuts are absorbed, furthermore walnuts supress the appetite. That is why they could play a role in combatting obesity. Furthermore, the walnut residues interact with the microbiome, and the metabolites that these produce. This means that even more health effects could be achieved.


Byerley LO et al, Changes in the Gut Microbial Communities Following Addition of Walnuts to the Diet, The Journal of Nutritional Biochemistry (2017).
Holscher HD, Guetterman HM, Swanson KS, An R, Matthan NR, Lichtenstein AH, Novotny JA, Baer DJ; Walnut Consumption Alters the Gastrointestinal Microbiota, Microbially Derived Secondary Bile Acids, and Health Markers in Healthy Adults: A Randomized Controlled Trial, The Journal of Nutrition (2018) Mitchell, J. Beth Israel Deaconess Medical Center. 2017