12 January - 20 June 2016

Walnut consumption may improve health by altering gut microbiome composition

A study, conducted by a research group from the University of Illinois and published in The Journal of Nutrition, suggests that eating walnuts can provide a wide range of health benefits by changing the gastrointestinal microbiota.

A study, conducted by a research group from the University of Illinois and published in The Journal of Nutrition, suggests that eating walnuts can provide a wide range of health benefits by changing the gastrointestinal microbiota.

Nuts are known to be good sources of dietary fibre and unsaturated fatty acids, both of which are known to improve gastrointestinal health.  Previous epidemiologic data has suggested that walnuts can have a range of beneficial health benefits, including reducing the risk of cancer and heart disease-related death. There is accumulating evidence that walnuts improve gastrointestinal microbiota and gut and metabolic health. The fact that this link had not previously been investigated in humans provided the basis for the current study, investigating the changes in gastrointestinal microbiota following consumption of walnuts.

Holscher et al. used two three-week periods where diet was controlled in 18 healthy participants, and 1.5 servings (42g) of walnuts were added. The rest of the diet-controlled intake was reduced by equal amounts on addition of walnuts to achieve isocaloric balance. Participants in the study also had their diets tailored based on their energy requirements in order to maintain their weight. Blood samples were analyzed before and after walnut treatment to gauge changes in cholesterol, glucose and lipid levels, among others. Fecal samples were also taken before and after each controlled dietary week, and DNA and bile extracted.

From the fecal samples, it was found that the levels of Firmicutes species, including Faecalibacterium, Roseburia, and Clostridium, increased after walnut consumption. These species have been shown to increase concentrations of butyrate, a metabolic byproduct which is important in colon health. The scientists note however that butyrate levels were not monitored in this study, and the reduction in abundance of Bifidobacteria, a species which produces a precursor to butyrate, suggests further work needs to be undertaken before the link between walnuts and increased butyrate can be made.

Previous studies have revealed that secondary bile acids are found in increased levels in people consuming diets high in saturated fat and low in dietary fibre. High levels of these acids have been found in individuals with colorectal cancer, and reduced levels have been linked with reduced gut inflammation. The synthesis and regulation of these has been shown to be controlled by microbiota present in the gastrointestinal tract. The Holscher group here showed that after walnut consumption, levels of secondary bile acids dropped. The group also found total and LDL cholesterol concentrations decreased after walnut consumption, which agreed with observations from previous work. However, the group noted that these changes weren’t thought to be related to changes in the microbiota.

Holscher et al. claim that this study is the first of its kind to examine the fecal microbiota after walnut consumption in humans. It showed that participants with diets supplemented with the nut had altered gastrointestinal microbiota, along with a reduction in other markers of health, including secondary bile acids and cholesterols. The researchers note in conclusion that the results “suggest that the gastrointestinal microbiota may contribute to the underlying mechanisms of the beneficial health effects of walnut consumption.”

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