Christmas cookies don’t
count
Cell Press
Anyone watching their
waistline this holiday season may want to pay attention to what their gut
bacteria are eating. It's not just calories that matter in a healthy diet --
it's fiber that resists digestion by the body but is readily eaten by bacteria
in the gut.
The amount of fiber in
someone's diet can influence weight gain, blood sugar, insulin sensitivity, and
colon health. Two studies with mice, publishing December 21 in the
journal Cell Host & Microbe, help shed light on how and why
fiber has such a powerful effect on the entire body.
"Once the
mechanism is understood, it can be exploited in different ways to promote
health," says Andrew Gewirtz, who studies the intestinal epithelium at
Georgia State University's Center for Inflammation Immunity & Infection and
is senior author of one of the papers. "This will allow ways to modify
diets to maximize those benefits."
Fiber in its various
forms is found in fruits, legumes, vegetables, and whole grains. So-called
Western diets, which are high in fats and sugars but low in fiber, have been
linked to an increased risk of inflammatory bowel diseases, weight gain, and
diabetes.
"It is becoming increasingly clear that the average person's fiber intake in a Western country has drastically reduced over the past few decades," says Fredrik Bäckhed, who studies the role of the normal gut microbiota in metabolic diseases at the University of Gothenburg, Sweden and is a co-senior author of the other paper.
Both studies started
by feeding a group of mice a diet that was extremely low in fiber. The
low-fiber diets rapidly led to weight gain, high blood sugar, and insulin
resistance in the mice.
The Bäckhed study
found that mice developed problems with the protective mucus layer in the colon
after just 3-7 days of eating the low-fiber diet: this mucus layer became more
penetrable and bacteria encroached upon the epithelial cells of the colon.
The Gewirtz study
observed that the colons of mice on the low-fiber diet shrank significantly in
thickness. And not only did large amounts of gut bacteria die off after mice
ate a low-fiber diet, mice developed unhealthy imbalances of different gut
bacteria strains.
"These papers
show the importance of the inner mucus layer in separating bacteria and human
host. It nicely illustrates how dynamic and quickly this responds to diet and
bacterial alterations," says Gunnar C. Hansson, a co-senior author with
Bäckhed and a professor in the Mucin Biology Group at the University of
Gothenburg, Sweden.
"Both our paper
and the Bäckhed paper are essentially reaching the same conclusion that the
lack of fiber results in bacteria encroaching into the mucus layer, and those
bacteria are promoting low-grade inflammation, contributing to metabolic
syndrome," says Gewirtz.
After the fallout from
the low-fiber diet, the two research teams tried different treatment
approaches.
Bäckhed, Hansson, and
colleagues found that a transplant of gut bacteria from a healthy mouse could
undo some of the harmful changes to the colon mucus layer.
Supplementing the mice
with Bifidobacterium (a beneficial bacteria) helped colon mucus growth issues,
but not mucus penetrability issues. Supplementing the mice's diet with inulin
(a type of fermentable fiber) lowered the mucus penetrability but did not help
mucus growth issues.
By switching mice to a
diet comprised of 20% inulin, Gewirtz and colleagues found that they could
reverse some aspects of metabolic syndrome in the mice -- like promoting weight
loss and greater blood sugar control -- but not others, such as elevated triglycerides
levels.
The fiber inulin also
helped restore colon mass, increased the number of intestinal absorption cells
enterocytes, and restored some number and diversity to the gut bacteria.
However, the fiber
supplements could not completely return the mouse gut bacteria to its original
bacterial diversity after eating the low-fiber diet. This may mean using
supplements as a treatment could cause complications if a person doesn't have a
healthy mix of gut bacteria.
"Diets that lack
fiber alter the bacterial composition and bacterial metabolism, which in turn
causes defects to the inner mucus layer and allows bacteria to come close
[encroach], something that triggers inflammation and ultimately metabolic
disease," says Hansson. "It is not enough just to add fiber to your
diet; it also depends on which bacteria you carry."
This likely means more
study is needed before fiber supplements can be used as a treatment.
"Simply enriching
processed food with purified fibers might offer some health benefits, but we're
not ready to recommend it until we understand more of the very complex
interplay between food, bacteria, and host," Gewirtz says. Bäckhed and
Hansson agree.
