natural tendency to [want to] eat more
as a result of your higher metabolism.”
Jensen also notes that it’s difficult
for people to sustain the workouts
required to keep the muscle mass they
gained. “For most, it’s kind of imprac-
tical,” he added.
Overall, he says, “There’s not any
part of the resting metabolism that you
have a huge amount of control over.”6
DIETING CAN SLOW DOWN
YOUR METABOLISM.
While it’s extremely hard to speed up
the metabolic rate, researchers have
found there are things that can slow it
down—like drastic weight-loss.
“[Crash diets] probably have the
biggest effect on resting metabolism,”
says Jensen. But not in a good way.
For years, researchers have been
documenting a phenomenon called
“metabolic adaptation.” As people lose
weight, their basal metabolic rate actu-
ally slows down to a greater degree than
would be expected from the weight loss.
To be clear, it makes sense that los-
ing weight will slow down metabolism.
Slimming down generally involves
muscle loss, which, in turn, means
the body doesn’t have to work as hard
to keep running. But the slowdown
after weight loss, researchers have
found, often appears to be substan-
tially greater than makes sense for a
person’s new weight.
Sandra Aamodt, a neuroscientist and
author of the book Why Diets Make UsFat, believes this may be the body’s
way of vigorously defending a certain
weight range, called the set point.
Once you gain weight and keep the
weight on for a period of time, the body
can get used to its new, larger size.
When that weight drops, a bunch of
subtle changes kick in—to the hormone
levels, the brain—slowing the resting
metabolism and having the effect of
increasing hunger and decreasing sati-
ety from food, all in a seeming conspir-
acy to get the body back up to that set
point of weight.
“I don’t think most people appreci-
ate how big these metabolic changes
can be when they lose a lot of weight,”
Aamodt said. “Weight gain and loss are
not symmetrical: the body fights much
more strongly to keep weight from
dropping than it does to keep weight
from increasing.”7
RESEARCHERS DON’T FULLY
UNDERSTAND WHY THIS
METABOLIC SLOWDOWN HAPPENS.
There are some interesting hypotheses,
however. One of the most persistent is
an evolutionary explanation.
“Over hundreds of millennia, we
evolved in an environment where we
had to confront frequent periods of
undernutrition,” Columbia’s Rosen-
baum says. “So you would predict that
human DNA would be full of genes
that favour the storage of extra calories
as fat. That ability would to some
extent increase our ability to survivereader’s digest
58 september 2019