Afvallen
kan wel eens moeilijk zijn.*
Het
gevecht van dikke mensen tegen de kilo’s kan ze wel eens moeilijk gemaakt
worden. De darmen van dikke mensen bevatten wel 20% meer zogenoemde
firmicutes-bacteriën, dan de darmen van dunne mensen. Deze firmicutes-bacteriën
kunnen voedsel veel efficiënter omzetten. Dat blijkt uit Amerikaanse studies
waarvan de resultaten in het gezaghebbende tijdschrift 'Nature' staan.
Het onderzoeksteam concludeert dat te veel eten en te weinig bewegen maar deels
de oorzaak zijn van overgewicht. De firmicutes-bacterië komt opvallend meer
voor bij mensen met overgewicht dan bij slanke mensen. De wetenschappers pleiten
in 'Nature' voor een nieuwe aanpak van overgewicht. De samenstelling van
darmflora kan wellicht met probiotica worden beïnvloed.
Tijdens
een van de onderzoeken zijn bacteriën van een te dikke muis bij dunne muizen
ingebracht, die vervolgens aankwamen. Toch zijn de gevolgen voor mensen met
overgewicht nog niet duidelijk. Naast experimenten met muizen is er ook naar
mensen gekeken. Vrijwilligers volgden een jaar lang een dieet en verloren 25
procent van hun lichaamsgewicht. Bij deze mensen nam het aantal
firmicutes-bacteriën af, hoewel het niveau niet zo laag werd als bij dunne
mensen.
An obesity-associated
gut microbiome with increased capacity for energy harvest
Peter
J. Turnbaugh, Ruth E. Ley, Michael A. Mahowald, Vincent Magrini, Elaine R.
Mardis and Jeffrey I. Gordon
Relative
Abundance Of Common Microbes Living In The Gut May Contribute To Obesity
A
link between obesity and the microbial communities living in our guts is
suggested by new research at Washington University School of Medicine in St.
Louis. The findings indicate that our gut microbes are biomarkers, mediators and
potential therapeutic targets in the war against the worldwide obesity epidemic.
In two studies published this week in the journal Nature, the scientists report
that the relative abundance of two of the most common groups of gut bacteria is
altered in both obese humans and mice. By sequencing the genes present in gut
microbial communities of obese and lean mice, and by observing the effects of
transplanting these communities into germ-free mice, the researchers showed that
the obese microbial community has an increased capacity to harvest calories from
the diet.
"The amount of calories you consume by eating, and the amount of calories
you expend by exercising are key determinants of your tendency to be obese or
lean," says lead investigator Jeffrey Gordon, M.D., director of the Center
for Genome Sciences and the Dr. Robert J. Glaser Distinguished University
Professor. "Our studies imply that differences in our gut microbial ecology
may determine how many calories we are able to extract and absorb from our diet
and deposit in our fat cells."
That is, not every bowl of cereal may yield the same number calories for each
person. People could extract slightly more or slightly less energy from a
serving depending upon their collection of gut microbes. "The differences
don't have to be great, but over the course of a year the effects can add up,"
Gordon says.
Trillions of friendly microbes reside in the intestine, where they help to
digest food that the body can't on its own, such as the complex sugars found in
grains, fruits and vegetables. As part of the digestive process, the microbes
break down nutrients to extract calories that can be stored as fat.
The researchers focused on two major groups of bacteria - the Bacteroidetes and
the Firmicutes that together make up more than 90 percent of microbes found in
the intestines of mice and humans. In an earlier study, they compared
genetically obese mice and their lean littermates. The obese mice had 50 percent
fewer Bacteroidetes and proportionately more Firmicutes. Moreover, the
differences were not due to a bloom of one species in the Firmicutes or a
diminution of a single or a few species of Bacteroidetes: virtually all members
of each group were altered.
In one of this week's Nature articles, Ruth Ley, Ph.D., a microbial ecologist in
Gordon's group, reports on her investigation into whether these findings also
held true among obese humans. She followed 12 obese patients at a Washington
University weight loss clinic over a one-year period. Half the patients were on
a low-calorie, low-fat diet and half were on a low-calorie, low carbohydrate
diet.
At the outset of the study, the obese patients had the same type of depletion of
Bacteroidetes and relative enhancement of Firmicutes as the obese mice. As the
patients lost weight, the abundance of the Bacteroidetes increased and the
abundance of Firmicutes decreased, irrespective of the diet they were on.
Moreover, not one particular species of Bacteroidetes but the entire group
increased as patients lost weight.
In a companion paper in the same journal, Peter Turnbaugh, a Ph.D. student in
Gordon's lab, compared the genes present in the gut microbial communities of the
obese and lean mice using the newest generation of massively parallel DNA
sequencers.
The results of these so-called comparative metagenomic studies revealed that the
obese animals' microbial community genome (microbiome) had a greater capacity to
digest polysaccharides, or complex carbohydrates. By transferring the gut
microbial communities of obese and lean mice to mice that had been raised in a
sterile environment (germ-free animals), he confirmed that the obese microbial
community prompted a significantly greater gain in fat in the recipients.
Gordon notes that these findings represent steps in a long journey designed to
understand the contributions of our microbial self to our health. "Our
microbial cells outnumber our human cells by as much as 10 fold and, and they
may contain 100 times more genes than our own human genome," Gordon says.
These studies raise a number of questions, according to Gordon. "Are some
adults predisposed to obesity because they 'start out' with fewer Bacteroidetes
and more Firmicutes in their guts?" he asks. "Can features of a
reduced Bacteroidetes-Firmicutes enriched microbial community become part of our
definition of an obese state or a diagnostic marker for an increased risk for
obesity? And can we intentionally manipulate our gut microbial communities in
safe and beneficial ways to regulate energy balance?"
Turnbaugh PJ, Ley RE, Mahowald MA, Magrini V, Mardis ER, Gordon JI. An
obesity-associated gut microbiome with increased capacity for energy harvest. Nature,
Dec. 21, 2006.
Ley RE, Turnbaugh PJ, Klein S, Gordon JI. Human gut microbes associated with
obesity. Nature.
Funding from the National Institutes of Health and the W.M. Keck Foundation
supported this research.
Washington University School of Medicine's full-time and volunteer faculty
physicians also are the medical staff of Barnes-Jewish and St. Louis Children's
hospitals. The School of Medicine is one of the leading medical research,
teaching and patient care institutions in the nation, currently ranked fourth in
the nation by U.S. News & World Report. Through its affiliations with
Barnes-Jewish and St. Louis Children's hospitals, the School of Medicine is
linked to BJC HealthCare.
Washington University in St. Louis
One Brookings Dr., Campus Box 1070
St. Louis, MO 63130
United States
(Jan. 2007)