Intermittent fasting is an eating pattern that involves periods
of food restriction (fasting) followed by normal eating.
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
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).
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 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).
In fact, this eating style could reduce fasting insulin levels
by 20–31% (9).
Human Growth Hormone
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.
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.
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.
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
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
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
This has been demonstrated dramatically in a study of people
who lost large amounts of weight while participating in the Biggest Loser show on
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
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
Unfortunately, most people lose both fat and muscle when they
lose weight (41).
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).
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
Bottom Line: Intermittent fasting may help
reduce the amount of muscle you lose when you lose weight.
However, the research is mixed.
Although research has shown some promising findings, the
effects of intermittent fasting on metabolism are still being
If this is true, then intermittent fasting has several
important weight loss advantages over diets based on continuous
At the end of the day, intermittent fasting can be a highly
weight loss tool for many people.