The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting

by Jason Fung

When body fat falls below 4 percent, the body is forced to use protein in order to feed itself.

An elite marathoner, despite looking very lean, might have approximately 8 to 10 percent body fat.)

I generally advise against any type of fasting at a BMI below 20, as the risk of complications rises dramatically, but it’s especially important to avoid longer fasts.

can be used as a kind of recalibration or resetting if someone feels they’ll benefit from that, but it shouldn’t be used as a quick fix to atone for perceived dietary sins.

DOES FASTING CAUSE ANOREXIA?

Anorexia is a psychiatric disorder of distorted body image.

It is a psychological disease and not one caused by undereating.

This is the symptom, not ...

Arguing that fasting causes anorexia is like arguing that washing your hands will lead to obsessive-compulsive disorder.

rather than doing a true fast, try eating a handful of salad greens at regular intervals. This preserves most of the benefits of the fast while alleviating the symptoms of heartburn. A “fat fast,” in which only fat is eaten during the fasting period, may also be effective—

malnourished women should not fast, since excessively low body fat can lead to amenorrhea (loss of menstrual cycles) and difficulty conceiving.

different kinds of fasts in terms of what may be consumed.

Most definitions of fasting allow noncaloric drinks only. This means that water, tea, and black coffee are all allowed during fasting,

but sugar, honey, fructose, agave nectar, and other suga...

There is some disagreement about artificial sweeteners like stevia, aspartame, and sucralose. Since there are no calories in these, technically, they could be allowed. However, the use of chemicals in these artificial sweeteners defeats...

not only from unwanted sugars and fats but also from chemicals and other artificial agents. The same argument applies to artificial flavors, such as those in Crys...

The water-on...

generally includes zero salt.

Without salt, the body cannot hold onto water, and therefore there is some risk of dehydration.

Juice fasting permits the consumption of juice as well as water.

this is not technically a true fast, but the word is often used in this context.

The “fat fast” is a newer variation of fasting.

Relatively pure fats, such as coconut oil, cream, and butter, are allowed during this fast, so it, too, is not a true fast.

The high fat content of the coffee gives this drink a substantial number of calories (400 to 500 per cup, depending upon the recipe),

more accurately termed a meal replacement.

In a “dry fast,” no fluids of any kind are allowed.

Ramadan.

The “fasting-mimicking diet” is a diet created by researchers to re-create the benefits of fasting without actual fasting.

reduced caloric

intake over five days ev...

There is sparse data to support the claim that this diet provides all the benefits of fasting, and I don’t recommend it because of its unnecessary complexity. To me, it is far simpler to follow five days of regular fasting per month.

experiment and adjust, as there are no hard and fast rules.

The IDM fast permits water, tea, and coffee. Sugar, honey, agave nectar, and other sweeteners are not permitted. Artificial sweeteners or flavors are not allowed, but natural flavors such as lemon juice, mint, cinnamon, or other spices are.

homemade bone...

makes fasting easier and may help prevent salt deficiency d...

Aim to drink two liters of water and other fluids daily.

lemon or lime to flavor the water,

add orange slices, berries, or cucumber slices to a pitcher of water for an infusion of flavor.

Diluted apple-cider vinegar in water may help lower ...

artificial flavors and sweeteners are prohibited. Kool-Aid, Crystal Light, or Tang should ...

green, black, oolong, and herbal.

Green tea

catechins in green tea are believed to help su...

Spices such as cinnamon or nutmeg...

Herbal teas are not true teas because they do not co...

However, they also are great for fasting. Cinnamon tea and ginger tea have both been used for their reputed appetite-suppressing power. Mint tea and chamomile tea ...

herbal teas contain no...

Adding a small amount of cream or milk to your tea is acceptable (see below), but sugar and artificial sweeteners and flavors are not allowed. However, if you are finding progress slow, you may want to eliminate all calories completely during fasting. We often advise patients to go back to a classic water-only fast if they hit a stubborn weight plateau.

We also permit a small amount of cream or coconut oil to be added to coffee or tea. While this does technically mean it’s not a true fast, the effect is small enough that it seems to make no difference to the overall outcome of the fast.

this flexibility may improve your ability to stick to the program.