By Andreas
von Bubnoff Feb. 27, 2021 at 8:00 a.m. CST
The world is getting fatter. More than 40 percent of U.S.
adults are obese — almost three times more than in 1980.
One reason for this weight gain is Americans are consuming
more: National figures suggest an increase of about 200 daily calories between
the early 1970s and 2010. Another is more snacking. In 2010, U.S. adults ate
about 20 percent more of their daily calories as snacks than they did 50 years
ago.
But there is more to rising obesity rates than endless
grazing. What also matters is timing, some experts believe. We eat when we
shouldn’t, and don’t give our bodies a long enough break in between.
We didn’t evolve to eat day and night, says neuroscientist
Dominic D’Agostino of the University of South Florida. Until the dawn of
agriculture about 12,000 years ago, we subsisted on hunting and gathering and
often had to perform those activities with empty bellies. “We are hard-wired,”
D’Agostino says, “to undergo periodic intermittent fasting.”
What’s more, people are now eating at times of the day when
historically they would have been asleep, says Satchin Panda, a circadian
biologist at the Salk Institute for Biological Studies in La Jolla, Calif., who
co-wrote an overview on the timing
of eating in the 2019 Annual Review of Nutrition. For
thousands of years, he says, our nightly fast probably started much earlier
than in these times of late-night television.
Although the research is still mixed, the timing of eating
seems to matter for body weight and health. Studies suggest significant
potential benefits from fasting every other day or so — or, on a daily basis,
eating only when we would normally be awake, within a window of 12 hours or
fewer — a practice known as time-restricted eating.
Such practices — referred to under the umbrella term
“intermittent fasting” — may help ward off obesity. More speculatively, there
are signs in animal studies that the regimens might enhance athletic endurance
and cognition, alleviate diabetes and perhaps even help combat other medical
conditions.
Intermittent fasting works for many — not only for weight
loss but also for heart health
How Americans became a nation of snackers is complicated,
but part of the problem is studies starting in the 1960s that found apparent
benefits from eating many small, nutritious meals throughout the day. Nibbling
advice became a health mantra, partly based on the belief that frequent eating
revs up the metabolism and makes the body burn more calories. This could be one
of the reasons “lots of people are now eating for 16 hours a day,” says
biochemist Valter Longo of the University of Southern California.
Newer human research finds no support for the notion that
endless nibbling increases metabolic rate, says
Antonio Paoli of the University of Padua in Italy. One 2017 study found
that people who ate three or more times per day gained more weight per year
than those eating just one or two meals per day.
So could breaks between meals actually be good for one’s
health? Studies as far back as
the 1940s reported that regimens akin to intermittent fasting
improved health and prolonged life span in animals like rats. But it was
unclear whether this was because of the breaks or simply because the animals
consumed fewer calories.
One of the first hints that pauses in eating might be
healthy came from a 2003 mouse study led
by neuroscientist Mark Mattson, at that time at the National Institute on
Aging. He and his colleagues compared a group of mice fed every other day with
mice fed every day. Those groups ate essentially the same amount of food
overall. Researchers also followed another group, which ate 40 percent less.
After 20 weeks, the mice in the first two groups weighed
about the same. But, strikingly, both the alternate-day-fasting mice and the
fewer-calories group were healthier (including lower blood sugar and insulin
levels), than the daily eaters.
The study also suggested that intermittent fasting might
improve brain function, which from an evolutionary perspective makes sense,
says Mattson, now at Johns Hopkins University School of Medicine. After all,
when an animal is in a state of hunger, it needs the brain to stay sharp so it
can find food.
Studies to replicate Mattson’s animal findings in people are
difficult, however. Trials in which people eat less or nothing every other day,
or every few days, consistently do show weight loss and improved cardiovascular
health. But you can get similar results simply by cutting calories. It has been
hard to prove that the periodic breaks from eating have added benefits.
In the longest human trial reported
so far, nutrition researcher Krista Varady of the University of
Illinois at Chicago and her team randomly assigned 100 otherwise healthy obese
people to one of three groups: Group one ate just 25 percent of their normal
intake every other day (and 25 percent more than normal on days in
between); group two ate 75 percent of their calorie needs every day; and group
three ate normally.
The study, published in 2017, found that both groups one and
two lost the same amount of body weight on average (about 7 percent) and
displayed similar measures for risk factors for heart disease and diabetes.
That the intermittent fasting offered no additional benefits beyond traditional
calorie restriction was “pretty disappointing,” says nutrition scientist
Courtney Peterson from the University of Alabama at Birmingham.
Meanwhile, another strand of research suggested that the
timing of meals also matters for health. A rodent study in 2009, led by sleep
researcher Fred Turek at Northwestern University, showed that mice fed a
high-calorie diet during the day (when the nocturnal animals would normally be
asleep) gained a lot more weight than mice fed the same diet during the night,
even though the animals consumed the same number of calories.
The same year, neuroscientist Frank Scheer of Brigham and
Women’s Hospital and a professor at Harvard Medical School, reported that
putting people on an artificial, 28-hour “day” in a lab for 10 days (so that
they often ate at times when they would normally be sleeping) led to increased
blood pressure and levels of blood sugar and insulin. “That,” Peterson says,
“was a huge watershed study because it showed, literally, that the time when
you eat affects your health.”
If eating when the body should be sleeping is unhealthy,
then it follows that restricting eating to waking hours might be healthy.
That’s what Panda’s team showed in studies published in 2012 and 2014.
They reported that mice fed a calorie-dense diet during windows of eight to 12
hours at night (the animals’ active time) were protected from becoming obese
and gained less weight than mice able to eat at any time — even though both
groups ate the same number of calories.
Restricting eating to just nine hours at night even caused
obese mice to lose weight. It also lowered their blood sugar and improved
glucose tolerance, suggesting that time-restricted eating might help alleviate
diabetes.
Panda and colleagues also found — again in mice — that
time-restricted eaters excrete more
sugar and cholesterol breakdown-products in their feces (a sign
of a more healthful metabolism) than the mice fed at the “wrong” times of the
day.
It’s unclear, though, whether Panda’s mouse findings apply
to people — mice studies often don’t translate to humans. “It sounds too good
to be true,” says research dietitian Michelle Harvie of Manchester
University NHS Foundation Trust in the United Kingdom. One human study
published in 2007, she says, even suggested that restricting eating
times too much can be bad. When people ate all their calories in a single meal
between 4 and 8 p.m., blood sugar levels rose, and glucose tolerance worsened,
both signs of ill health.
A recent, randomized, controlled study in
overweight and obese people also showed no improvement from restricting eating
times to the eight hours between noon and 8 p.m.
Still, some researchers think the problem with such
disappointments might lie in timing. Peterson suspects that in both these
studies, the last daily meal might have been consumed too late, when blood
insulin levels had dropped too low to process food properly.
In 2018, Peterson and her colleagues reported lower blood pressure
and better-controlled blood sugar levels in eight overweight
prediabetic men asked to eat all their food within a six-hour window — but with
dinner before 3 p.m. If confirmed with more people, the study would indicate
that restricting eating to a window of less than 12 hours while we are awake
can have health benefits independent of calorie reduction, as long as the
window isn’t too late in the day.
And what’s going on in the body to make it healthier to eat
at one time than another? Daily biological rhythms may be central here, says
Dorothy Sears, an obesity researcher at Arizona State University. It’s during
the day that your body is best able to process food, says Sears, who wrote
about the metabolic effects
of intermittent fasting in the 2017 Annual Review of
Nutrition. And just as the brain needs rest at night to do much-needed repair
and cleanup work, so does the body, Panda says.
All the unsettled points and lack of clear-cut evidence from
human trials hasn’t stopped thousands of enthusiasts from experimenting with
their own personal intermittent-fasting regimens, in large part because many
find it easier to count hours than calories. And there’s hope it might become
easier still, if what goes for mice goes for people: Panda’s rodent studies
suggest that skipping weekends doesn’t ruin the time-restricted health effect.
“You think, ‘What about Saturday night, when I go out for a
late dinner?’ In the mice, that is okay,” Sears says. “It’s very encouraging,
because it seems that you don’t have to ask people to be perfect every day of
the week. . . . And you don’t ever have to read a label.”
Andreas von Bubnoff is a freelance science writer and
professor of science communication based in Germany. He drinks his morning
latte in a beer mug and likes to have breakfast at noon. This article was
first published in longer form in Knowable Magazine.
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