The word “intermittent” is just a fancy way of saying “sometimes”.
And “fasting” is just a fancy way of saying “not eating”.
So, intermittent fasting just means, “Sometimes, you don’t eat.”
Early research into fasting often used what we might call “straight” fasting, in other
words — taking in zero energy from food.
So, for instance, in one protocol known as alternate-day fasting, or ADF, one day of
eating might be a normal maintenance intake — in other words, the amount of
calories from food that someone’s body required for normal functions — and
another day might be zero intake from food.
Another protocol such as a weekly fast might have one fasting day — again, a day
of zero calories — per week.
Time restricted feeding, or TRF, might involve fasting for a certain period of the
day, and eating only within a particular, prespecified time period, which we might
call an “eating window” — for example, eating only during a few hours in the
afternoon and evening.
Some studies have also looked at periodic fasting, or a period of zero energy intake
over more than one day — in human studies, anywhere from 2 to 21 days.
While fasting may be part of laboratory studies, in fact all of us fast every day when
we sleep, so it’s perfectly normal to go for 8 hours or more without eating.
As research on fasting developed, researchers started looking at whether they
could get the same results from energy reduction — in other words, taking in
significantly less energy from calories than the body’s maintenance needs, but not
This protocol came to be known as intermittent energy reduction, or sometimes
eating much less.
So, for instance, if we use an alternate-day protocol, one day a person might eat
their normal maintenance level of calories, and then the next day, eat much less,
perhaps 500 calories — but again, not zero.
So what makes fasting potentially something “special”?
Intermittent fasting has a number of features that might make it useful for some
First, it allows people to restrict energy — in other words, eat less — in a way that
isn’t chronic. So, if someone wants to lose weight, they might not have to eat less
every single day, as they might on a typical so-called “diet”.
Second, it shifts our metabolism from using sugar-based fuels to using fat-based
fuels, which may have some metabolic benefits for some people.
Fasting can change our metabolic & hormonal environment.
Fasting changes cell signaling & gene expression — some processes are
upregulated or increased, while others are downregulated or decreased.
Fasting can be a temporary “good stress” on the body, almost like a metabolic
Fasting promotes “cell cleanup”, otherwise known as autophagy.
And finally, fasting can potentially teach us that “hunger is not an emergency”.
These features of fasting can then offer some people some possible benefits.
First, if people are eating less, they may be able to lose weight or fat more easily,
without, again, having to “go on a diet” and chronically restrict their intake.
They may not see some of the common metabolic problems with chronic dieting, and
they may get to lose weight or fat without a lot of complex diet rules.
Second, fasting may improve insulin sensitivity — in other words, the ability of our
cells to respond to the action of insulin — and glucose tolerance.
Fasting may change our metabolic & hormonal environment… potentially in
positive and negative ways.
And, fasting MAY improve longevity. Many animal studies suggest that restricting
energy may help us live longer.
Fasting may improve our immunity, and lower inflammation — again, especially if
It may lower our risk of many chronic diseases, such as heart disease, type 2
diabetes, and some types of cancer.
And, it may help people be less afraid of being hungry, which can help with the
psychological aspects of losing weight or fat.
But, importantly, none of these benefits will always happen for everyone.
So, as always, whether any dietary protocol is a good idea depends on many
Since intermittent fasting or energy restriction is a stressor, it works better for
people whose overall stress load is lower. Someone already under high stress
should probably not add more.
It’s important that a person have a balanced, objective, rational mindset — that
they can easily review data, take feedback, and think about things logically.
For someone who is already anxious, emotionally tied to food and eating, this may not
be a good fit, and it’s definitely not a good fit for someone with an existing
disordered eating condition.
Any plan should fit easily into someone’s lifestyle. If it causes problems, it’s likely
not a good match.