May 3, 2018
University of Illinois
College of Agricultural, Consumer and Environmental Sciences
Diets rich in nuts, such as walnuts, have
been shown to play a role in heart health and in reducing colorectal cancer.
According to a new study, the way walnuts impact the gut microbiome — the
collection of trillions of microbes or bacteria in the gastrointestinal tract
— may be behind some of those health benefits.
Diets rich in nuts, such
as walnuts, have been shown to play a role in heart health and in reducing
colorectal cancer. According to a new study from the University of Illinois,
the way walnuts impact the gut microbiome — the collection of trillions of
microbes or bacteria in the gastrointestinal tract — may be behind some of
those health benefits.
Walnuts are just one in a
line foods that contain dietary fiber and have interested scientists for their
impact on the microbiome and health. Dietary fiber acts as a food source for
gut microbiota, helping the bacteria to do their jobs — breaking down complex
foods, providing us nutrients, or helping us feel full, for example.
Fruits, vegetables, whole
grains, nuts, and legumes are important plant sources of dietary fiber. Eating
a variety of these foods helps promote a diverse gut microbiota, which in turn
helps to support health.
Findings from the study,
published in The Journal of Nutrition, show that consuming walnuts not
only impacted the gut microbiota and microbial derived secondary bile acids,
but also reduced LDL-cholesterol levels in the adults participating in the
study; good news for cardio, metabolic, and gastrointestinal health.
“We found that when
you consume walnuts it increases microbes that produce butyrate, a beneficial
metabolite for colonic health. So the interaction of walnuts with the microbiome
is helping to produce some of those health effects,” says Hannah Holscher,
assistant professor of food science and human nutrition at U of I, and lead
author of the study. “It is about getting to the ‘black box’ that is all
the microbes in our GI tract to see how they are interfacing with the food we
eat and having downstream health effects.
“Some of those
health effects are hypothesized to be related to the metabolites bacteria
produce,” she adds.
controlled-feeding study, 18 healthy male and female adults consumed diets that
either included 0 grams of walnuts or 42 grams — about a third cup or a
palm-full of walnuts — for two, three-week periods. Fecal and blood samples
were collected at the beginning and end of each period to assess secondary
outcomes of the study, including effects of walnut consumption on fecal
microbiota and bile acids and metabolic markers of health.
resulted in higher relative abundance of three bacteria of interest: Faecalibacterium,
Roseburia, and Clostridium.
“The microbes that
increased in relative abundance in this walnut study are from one of the
Clostridium clusters of microbes, and there’s increased interest in those
because they have the ability to make butyrate,” Holscher says.
“Unfortunately in this study we didn’t measure butyrate, so we can’t say
that just because these microbes increased that butyrate did increase. We still
need to answer that question.
“There is a lot of
interest in Faecalibacterium because it has also been shown in animals
to reduce inflammation. Animals with higher amounts also have better insulin
sensitivity. There is also growing interest in Faecalibacterium as a
potential probiotic bacteria, and so we are trying to follow up on foods that
help support Faecalibacterium.”
The findings also show,
with walnut consumption, a reduction in secondary bile acids compared to the
control. “Secondary bile acids have been shown to be higher in individuals
with higher rates of colorectal cancer,” Holscher explains. “Secondary
bile acids can be damaging to cells within the GI 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.”
Previous research that
prompted this microbial research showed that the amount of energy (calories)
derived from walnuts after we eat them is less than previously thought.
“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
says. “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.
“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,” Holscher says.