Walnuts have been shown to have wide-ranging health benefits — from strengthening our hearts to reducing our risk of cancer. Now, new research sheds light on the mechanisms that may explain these benefits.
walnuts
New research explains why walnuts are so great for our all-round health.

In case you didn’t know, walnuts are a veritable treasure trove of health benefits.

A rich source of antioxidants, this delicious snack has been shown to reduce the risk of colon cancer, lower cholesterol levels, keep heart disease at bay, and even strengthen our brains.

But what is it that makes walnuts so potent in the fight against disease? Researchers at the University of Illinois at Urbana-Champaign set out to uncover the health benefits hidden under the walnut’s wrinkly shell, and they published their results in The Journal of Nutrition.

Specifically, the researchers — led by Hannah Holscher, an assistant professor of food science and human nutrition at the university — looked at how walnuts affect our gut microbiota, or the trillions of (mostly beneficial) micro-organisms that inhabit our gut.

Nuts are known to be a precious source of fiber, and dietary fiber helps to diversify and strengthen our gut bacteria. But the scientists’ findings go beyond that, showing how walnuts may benefit our cardiometabolic and gastrointestinal health.

Walnuts raise levels of beneficial metabolite

To uncover walnuts’ secret health benefits, Holscher and her colleagues fed 18 participants either 42 grams of walnuts or no walnuts at all over two 3-week periods.

To study how walnuts impacted the composition of the microbiota, the researchers collected and analyzed blood and fecal samples from the participants both at the beginning and the end of the study period.

The analysis revealed that walnut intake increased levels of three main bacteria: Faecalibacterium, Roseburia, and Clostridium. The researchers explain that these three bacteria produce a metabolic byproduct called butyrate, which has been shown to improve colon health.

However, Holscher cautions, these findings must be taken with the caveat that butyrate levels were not actually measured in this study. “[S]o,” she says, “we can’t say that just because these microbes increased that butyrate did increase. We still need to answer that question.”

She goes on to add that Faecalibacterium “has also been shown in animals to reduce inflammation. Animals with higher amounts also have better insulin sensitivity.” Faecalibacterium may also be a probiotic bacterium, Holscher suggests.

Walnuts may lower carcinogenic bile acids

The study also revealed that people who consumed walnuts had a decrease in so-called secondary bile acids.

While bile acids are normal compounds that help to absorb lipids, cholesterol, and some vitamins in the gastrointestinal tract, some of them have been shown to be toxic.

Bile acids can be primary or secondary, and the latter have recently been suggested to contribute to colon cancer.

“Secondary bile acids have been shown to be higher in individuals with higher rates of colorectal cancer,” Holscher says.

“Secondary bile acids can be damaging to cells within the [gastrointestinal] tract, and microbes make those secondary bile acids. If we can reduce secondary bile acids in the gut, it may also help with human health.”

Holscher explains that the way in which microbes absorb and process the energy derived from walnuts may hold the key to how the nuts affect our health.

Walnuts’ energy absorbed by gut microbes

“When you do calculations to determine how much energy we predicted we would get from eating walnuts, it didn’t line up with the energy that was absorbed,” Holscher notes. “You’re really only absorbing around 80 percent of the energy from walnuts that labels say.”

“That means that the microbes get access to that extra 20 percent of calories and the fats and fiber left in them, and so what happens then? Does it produce a positive health outcome, or a negative health outcome?”

Our study provides initial findings that suggest that the interactions of microbes with the undigested walnut components are producing positive outcomes.”

Hannah Holscher

However, she advises, “We need more research to look at additional microbial metabolites and how those are influencing health outcomes, instead of just characterizing the changes in the microbiome.”