Just two ounces - about 28 walnut halves, or a small bag - protects the digestive system.
The tasty nut has been hailed a 'superfood' for years - but exactly why it is so good for us has been a mystery.
Now experiments in rats has found walnuts improve digestive health by increasing the amount of probiotic bugs.
Good intestinal bacteria not only improves digestion but also our brain health, mood, emotions, energy levels and weight loss.
What is more, our microflora has also increasingly been linked to many aspects of health including ageing, arthritis, depression, cancer and heart conditions.
Physiologist Professor Lauri Byerley, of Louisiana State University, said: "The health of the gut is related to overall health in the rest of the body.
"Our study is showing walnuts change the gut, which could help explain why there are other positive health benefits to eating walnuts such as heart and brain health."
She said walnuts do this by acting as a prebiotic to help nourish and grow the bacteria that keeps the digestive system healthy.
The study published in found a diet with walnuts led to an overall significant increase in the diversity of bacteria in the gut.
Explained Prof Byerley: "Gut health is an emerging research area, but we are seeing greater bacterial diversity may be associated with better health outcomes, whereas low diversity has been linked to conditions such as obesity and inflammatory bowel disease."
The lab rodents were randomly given either a diet containing ground walnuts, equivalent to about two ounces a day in humans, or an alternative with a similar amount of nutrients and calories for up to ten weeks.
Rats that ate the walnut-enriched diet saw a rise in beneficial bacteria including Lactobacillus, Roseburia and Ruminococcaceae.
Prof Byerley said walnuts are the only nut that contain a significant amount of omega-3 fatty acid, 2.5 grammes an ounce.
They also offer protein (4 grammes) and fibre (2 grammes).
She said animal research is provided as background and used to inform future studies needed to understand the effect on humans.
The results of this study shed light on a new way that walnuts may be beneficial for health, but more research is needed to understand how these outcomes translate to humans.
Funding for this research was provided by the California Walnut Commission and American Institute for Cancer Research.