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Brain may subvert efforts to diet
By Janet Raloff
In obese people, even when the
brain knows the body isn’t hungry, it
responds to food as if it were, new brain-scan data show. That means that when
obese people try to shed weight, they
may find themselves on the losing side of
a battle with neural centers that unconsciously encourage eating.
For instance, in normal-weight people
a neural reward system that reinforces
positive feelings associated with food
turns off when levels of the blood sugar
glucose return to normal after a meal — a
signal that the body’s need for calories
has been sated. But in obese people, that
reward center in the central brain turns
on at the sight of high-calorie food even
when blood sugar levels are normal.
“The regulatory role of glucose was
missing in the obese,” says Elissa Epel
of the University of California, San
Francisco, an obesity researcher not
involved with the new study. She says
the data might “explain the drive to eat
that some obese people feel despite how
much they’ve eaten.”
For the study, nine lean and five
obese adult volunteers viewed pictures
of foods such as ice cream, french fries,
cauliflower or a salad while undergoing
brain scans. Throughout the procedure,
researchers asked the recruits to rate
their hunger and how much they wanted
a particular item.
Volunteers arrived for their brain
scans several hours after eating, and
the researchers used insulin pumps to
establish volunteers’ blood sugar levels
at either normal background values
(roughly 90 milligrams per deciliter), or
at the “mild” end of low (around 70 milli-
grams per deciliter). That low value can
occur briefly in some people during the
day, especially in people with diabetes or
metabolic conditions that precede dia-
betes, notes Yale endocrinologist Robert
Sherwin, a coauthor of the new study.
The surprise, Sherwin says, was that
the part of the brain that allows people
to consciously exert willpower over food
intake largely turns off in obese people.
Such changes “may perpetuate obesity.”
Though small, this study was so well
designed and controlled that it “lets us
see some clean results in a relatively
small cohort,” says obesity scientist
Dianne Lattemann of the Veterans
Affairs Puget Sound Health Care System in Seattle. The research team’s ability to discern differences in the desire
for foods by body weight — and based
on a not-too-dramatic drop in blood
sugar — “is extremely interesting,” she
adds. The results pull together trends
seen in animals and findings reported
in more limited human trials.
If that’s a TV,
this is the den
To the brain, a scene may be
just the sum of its objects
By Laura Sanders
To one part of the brain, a bathroom
equals tub plus toilet. In mental terms,
certain scenes are sums of their objects,
researchers report online September 4
in Nature Neuroscience. The results help
explain how people quickly and accurately recognize complicated scenes such
as playgrounds and traffic intersections.
Much of what is known about vision
comes from studies of how people see
simple objects in isolation, such as a
line floating on a white screen, says
Dirk Bernhardt-Walther of Ohio State
University. The new work, in contrast,
deals with messy, real-world scenes. “It’s
an awesome study,” he says.
A number of brain systems help tell
people where they are, each relying on
different information. When the outlines
of a place offer little info, the brain homes
in on specific objects. “A bathroom and a
kitchen may have similar three-dimen-
sional shapes ... but the objects will tell
you a big difference,” says study coauthor
Sean MacEvoy of Boston College.
