Walnuts may boost heart health by “feeding” gut bacteria, research suggests.
Scientists from Penn State University looked at 42 overweight or obese adults.
The participants were put on a diet that included walnuts or other foods with “equal” amounts of “good fats”.
Six weeks later, only those in the “walnut group” had significantly higher levels of certain gut bacteria.
This corresponded with a decrease in cholesterol and blood pressure, risk factors for heart disease.
“Replacing your usual snack, especially if it's unhealthy, with walnuts is a small change you can make to improve your diet”, study author Dr Kristina Petersen said.
“Substantial evidence shows small improvements in diet greatly benefit health.
“Eating 2-to-3oz [56g-to-85g] of walnuts a day as part of a healthy diet could be a good way to improve gut health and reduce the risk of heart disease.”
Heart disease kills one in four people in the UK and US, statistics show.
Studies have shown walnuts help lower cholesterol and blood pressure when eaten alongside a diet low in saturated fat.
When it comes to the power of the microbiome, research increasingly shows it may strengthen the cardiovascular system.
“There's a lot of work being done on gut health and how it affects overall health,” study author Professor Penny Kris-Etherton said.
“So, in addition to looking at factors like lipids and lipoproteins, we wanted to look at gut health.
“We also wanted to see if changes in gut health with walnut consumption were related to improvements in risk factors for heart disease.”
The scientists looked at a group of adults aged 30-to-65, who were placed on a typical “American diet” for two weeks before the study started.
The participants were then introduced to one of three diets, which all contained less saturated fat than the American eating regimen.
Some were told to eat walnuts, while others were given food that contained the same amount of the good fat alpha-linolenic acid (ALA) as the nut.
The remainder were “partially substituted” the fatty acid oleic acid for the same amount of ALA in walnuts, but without the nut.
Faecal samples were collected at the start and six weeks later.
“The walnut diet enriched a number of gut bacteria that have been associated with health benefits in the past,” Dr Petersen said.
“One of those is Roseburia, which has been associated with protection of the gut lining.
“We also saw enrichment in Eubacteria eligens and Butyricicoccus.”
Results, published in the Journal of Nutrition, show those with higher Eubacteria eligens levels had lower blood pressure.
Greater amounts of the bacteria Lachnospiraceae was linked to both reduced blood pressure and cholesterol.
These results only occurred in those on the walnut diet, not the other two eating regimens.
“Foods like whole walnuts provide a diverse array of substrates - like fatty acids, fibre and bioactive compounds - for our gut microbiomes to feed on,” study author Dr Regina Lamendella said.
“In turn, this can help generate beneficial metabolites and other products for our bodies.”
Future research will look at exactly how walnuts may benefit the microbiome.
“The findings add to what we know about the health benefits of walnuts, this time moving toward their effects on gut health,” Professor Kris-Etherton said.
“The study gives us clues that nuts may change gut health, and now we're interested in expanding that and looking into how it may affect blood sugar levels.”