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Does Intermittent Fasting Boost Your Metabolism?

Intermittent fasting is an eating pattern that involves periods
of food restriction (fasting) followed by normal eating.

This pattern of eating could help you lose weight, reduce your
risk of disease and increase your lifespan (1, 2).

Some experts even claim that its beneficial effects on
metabolism make it a healthier way to lose weight than standard
calorie restriction (3).

Intermittent Fasting Is Highly Effective for Weight Loss

Annoyed Girl With Empty Plate

Intermittent fasting is a simple,
effective approach to fat loss
that is relatively easy to
stick to (4).

Studies have shown that when it comes to weight loss,
intermittent fasting can be just as effective as traditional
calorie restriction, if not more (5, 6, 7, 8).

In fact, a 2014 review found that intermittent fasting could
help people lose an impressive 3–8% of their body weight in
3–24 weeks (9).

Moreover, a recent review concluded that in overweight and
obese people, intermittent fasting may be a better approach to
weight loss than very-low-calorie diets (10).

Interestingly, this approach to eating may also benefit your
metabolism and metabolic health (1, 11, 12, 13).

There are a
few different ways
to try intermittent fasting. Some people
follow the 5:2
diet
, which involves fasting for two days a week. Others
practice alternate-day
fasting
or the 16/8 method.

If you are interested in trying intermittent fasting, you can
read more about it in this
detailed guide for beginners
.

Bottom Line: Intermittent fasting is a
powerful weight loss tool. It can also improve your
metabolism and metabolic health.

Intermittent Fasting Increases Several Fat Burning Hormones

Hormones are chemicals that act as messengers. They travel
through your body to coordinate complicated functions such as
growth and metabolism.

They also play an important role in the regulation of your
weight. This is because they have a strong influence on your
appetite, the number of calories you eat and how much
fat
you store or burn (14).

Intermittent fasting has been linked to improvements in the
balance of some fat burning hormones. This could make it a
helpful tool for weight management.

Insulin

Insulin is one of the main hormones involved in fat metabolism.
It tells your body to store fat and also stops your body from
breaking fat down.

Having chronically high levels of insulin can make it much
harder to lose weight. High levels of insulin have also been
linked to diseases like obesity, type 2 diabetes, heart disease
and cancer (9, 15, 16).

Intermittent fasting has been shown to be just as effective as
calorie-restricted diets for lowering
your insulin levels
(17, 18, 19).

In fact, this eating style could reduce fasting insulin levels
by 20–31% (9).

Human Growth Hormone

Fasting can cause a rise in blood levels of human
growth hormone
, an important hormone for promoting fat loss
(20, 21).

Some studies have shown that in men, levels of human growth
hormone may increase by as much as five-fold while fasting
(22, 23).

Increases in blood levels of human growth hormone not only
promote fat burning, but they also preserve muscle mass and
have other benefits (24).

However, women don’t always experience the same benefits from
fasting as men do, and it’s not currently clear if women will
see the same rise in human growth hormone.

Norepinephrine

Norepinephrine, a stress hormone that improves
alertness and attention, is involved in the “fight or flight”
response (25).

It has a variety of other effects on your body, one of which is
telling your body’s fat cells to release fatty acids.

Increases in norepinephrine generally lead to larger amounts of
fat being available for your body to burn.

Fasting leads to a rise in the amount of norepinephrine in your
bloodstream (26, 27).

Bottom Line: Fasting can help decrease
insulin levels and boost blood levels of human growth hormone
and norepinephrine. These changes can help you burn fat more
easily and help you lose weight.

Short-Term Fasts Boost Metabolism by up to 14%

Many people believe that skipping meals will cause your body to
adapt by lowering its metabolic rate to save energy.

It’s well established that very long periods without food can
cause a drop in metabolism (28, 29).

However, studies have shown that fasting for short periods can
actually increase
your metabolism
, not slow it down (30, 31).

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One study in 11 healthy men found that a 3-day fast actually
increased their metabolism by an impressive 14% (26).

This increase is thought to be due to the rise in the hormone
norepinephrine, which promotes fat burning.

Bottom Line: Fasting for short periods can
slightly boost your metabolism. However, fasting for long
periods may have the opposite effect.

Intermittent Fasting Decreases Metabolism Less Than Continuous
Calorie Restriction

When you lose weight, your metabolic rate goes down. Part of
this is because losing weight causes muscle loss, and muscle
tissue burns calories around the clock.

However, the decrease in metabolic rate seen with weight loss
can’t always be explained by the loss of muscle mass alone
(32).

Severe calorie restriction over a long period can cause your
metabolic rate to drop, as your body enters so-called starvation
mode
(or “adaptive thermogenesis”). Your body does this to
conserve energy as a natural defense against starvation
(33, 34).

This has been demonstrated dramatically in a study of people
who lost large amounts of weight while participating in the Biggest Loser show on
TV.

Participants followed a calorie-restricted diet and intense
exercise regimen to lose large amounts of weight (35).

The study found that six years later, most of them had regained
nearly all of the weight they had lost. However, their
metabolic rates had not gone back up and remained around 500
calories
lower than you would expect for their body size.

Other studies investigating the effects of calorie restriction
on weight loss have found similar results. The drop in
metabolism due to weight loss can amount to hundreds of
calories per day (36, 37).

This confirms that “starvation mode” is real and can partly
explain why many people who lose weight end up regaining it.

Given the short-term effects of fasting on hormones, it’s
possible that intermittent fasting may reduce the drop in
metabolic rate caused by long-term calorie restriction.

One small study showed that losing weight on an alternate-day
fasting diet did not reduce metabolism over 22 days (17).

However, currently there is no quality research available
looking at the long-term effects of intermittent fasting diets
on metabolic rate.

Bottom Line: One small study suggests that
intermittent fasting may reduce the drop in metabolic rate
that’s associated with weight loss. More research is needed.

Intermittent Fasting Helps You Hold on to Muscle Mass

Muscle is metabolically active tissue that helps keep your
metabolic rate high. This helps you
burn more calories
, even at rest (38, 39, 40).

Unfortunately, most people lose both fat and muscle when they
lose weight (41).

It’s been claimed that intermittent fasting could preserve
muscle mass better than calorie restriction due to its effect
on fat burning hormones (42, 43).

In particular, the increase in human growth hormone observed
during fasting could help preserve muscle mass, even if you’re
losing weight (44).

A 2011 review found that intermittent fasting was more
effective at retaining muscle during weight loss than a
traditional, low-calorie diet (45).

However, results have been mixed. A more recent review found
intermittent fasting and continuous calorie restriction to have
similar effects on lean body mass (5, 46).

One recent study found no difference between the lean body mass
of people who were fasting and people on continuous calorie
restriction after eight weeks. However, at 24 weeks, those in
the fasting group had lost less lean body mass (6).

Larger and longer studies are needed to find out if
intermittent fasting is more effective at preserving lean body
mass.

Bottom Line: Intermittent fasting may help
reduce the amount of muscle you lose when you lose weight.
However, the research is mixed.

Conclusion

Although research has shown some promising findings, the
effects of intermittent fasting on metabolism are still being
investigated (3).

Early research suggests that short-term fasts boost metabolism
as much as 14%, and several studies suggest that your muscle
mass doesn’t decrease much with intermittent fasting (6, 26, 45).

If this is true, then intermittent fasting has several
important weight loss advantages over diets based on continuous
calorie restriction.

At the end of the day, intermittent fasting can be a highly
effective
weight loss tool
for many people.

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