A mere five years ago, skipping meals was a top
diet taboo. Now it's the core of an increasingly popular (and increasingly
research-backed) weight-loss approach.
Intermittent
fasting—periodically eating very little—is not only not bad for you, it may
lower blood glucose levels and insulin resistance and reduce inflammation and
cardiovascular risk. Why? How? Theories abound, but some experts believe
fasting puts your cells under mild stress, just as exercise taxes your muscles
and heart, ultimately strengthening them and making them more resistant to
disease.
And that's not
all, says Courtney Peterson, Ph.D., an assistant professor in the department of
nutrition sciences at the University of Alabama at Birmingham. "Studies
suggest you keep more muscle and lose more fat than on other diets, even if you
lose the same number of pounds." That's because after about 12 hours of
fasting, you run out of stored energy from carbs and start burning
stored fat.
But a troubling flaw has
popped up in this system. (You knew there was a "but" coming, right?)
In a recent study, people on an alternate-day fasting plan for six months lost
about 6 percent of their body weight—the same as those on a conventional
low-cal diet—but 38 percent of fasters dropped out, nearly 10 percent more than
in the other diet group. A similar problem has surfaced in other trials.
Why try a plan with a high
dropout rate and hours of hunger? Beyond the health benefits,
some people actually like it—and find it the easiest way to control their
weight. Mark Mattson, Ph.D., a neuroscientist at the National Institute on
Aging, has studied intermittent fasting since the 1990s and himself
been on a plan for years. "Once you get used to it, it's not a big
deal," he says. "You adapt." Other fans? Reportedly, trendsters
from Beyonce to Silicon Valley techies; and Jimmy Kimmel has said he lost 25 pounds
fasting too. So... should you experiment?
THE SPECIFICS
To be clear: The new fasting
is not about deprivation, but about divvying up your calories differently than
the three-square-meals-plus-snacks pattern—which some scientists say is a
mismatch with the way we evolved to eat, when food was sporadic.
Whether a regimen calls for
two fasting days a week or eating your meals in a smaller "window" of
time in the day, all plans share a near-freedom from calorie counting, a
big plus for weary food diarists. Once you have planned your fasting-period
menu—say, a 500-calorie day of chicken and veggies—you're set. And in your
nonfasting periods, you eat normal, healthy meals (even that steak!) without
worrying about every bite.
The key, of course, is to not
go overboard on these "normal" days. Researchers have found, though,
that fasters ate at most 10 to 15 percent more on their nonfast days, so
overall they took in fewer calories.
THE HUNGER FACTOR
Yes, you'll be hungry at times—but it's not
necessarily overwhelming or constant. "Hunger doesn't seem to get worse as
the day goes on, and some of our studies report increased fullness and
satisfaction," says Kristin Hoddy, Ph.D., R.D.N., a dietitian in private
practice who has researched fasting. "Some subjects remarked that they'd
get distracted and 'forget' they were hungry."
Feelings over
hunger may also become more tolerable over time. Studies of alternate-day
fasting have shown that people rate their hunger at 8 on a scale of 1 to 10 for
their first few fast days, but after two weeks, that number drops to 3. Mattson
puts the window at three to four weeks, after which, he says, "you're not
hungry on fasting days."
THE BEST CANDIDATES
There is no long-term fasting research yet, but the
benefits are promising and the risks low: You can always just quit. A
limited-time fast might bump you off a plateau or out of a rut, says
Keri Glassman, R.D., who advised our fasters during their diets, though she
says that for some, fasting, even short-term, may be too rigid. That hints at
the larger takeaway: Perhaps more than for traditional diets, these plans won't
work for everyone.
Hoddy suggests trying a plan for two weeks and keeping a
record of how you feel and what you eat. You can switch to another one if it's
making you grumpy, or you can modify it; when Hoddy fasted, she split one meal
into two smaller ones on fast days so she could have dinner with her husband.
Or follow a plan partway, says Peterson. "Any form of fasting helps burn
fat, and extending your overnight fast a little—say, eating dinner earlier—is
an overall health benefit."
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