Shorter fasts29 (<24hrs)
Intermittent fasting offers plenty of flexibility. You can fast for as long or short as you like, but fasts longer than a few days may require medical supervision.Here are some popular regimens. Generally, shorter fasts are done more frequently.
This way of doing intermittent fasting involves daily fasting for 16 hours. Sometimes this is also referred to as an 8-hour eating ‘window.’ You eat all your meals within an 8-hour time period and fast for the remaining 16 hours. Generally, this is done daily or almost daily.
For example, you may eat all your meals within the time period of 11:00 am and 7:00 pm. Generally, this means skipping breakfast, but some people prefer to skip dinner instead. Typically this involves eating either two or three meals within this 8-hour period.
This involves a 4-hour eating window and a 20-hour fast. For example, you might eat between 2:00 pm and 6:00 pm every day and fast for the other 20 hours. This would involve eating either one large, lengthy meal or two smaller meals within this period.
Longer fasts (>24 hours)
This way of doing intermittent fasting involves fasting from dinner to dinner (or lunch to lunch). If you eat dinner on day 1, you would skip the next day’s breakfast and lunch and eat dinner again on day 2. This is also known as “one meal a day,” or OMAD. It is generally done two to three times per week.
This is the version of intermittent fasting that has the most scientific support, as most studies on intermittent fasting have featured similar advice.Dr. Michael Mosley popularized this variation of intermittent fasting in his book ‘The Fast Diet.’
The 5:2 fast involves five regular eating days and two fasting days. However, on these two fasting days, you are allowed to eat 500 calories on each day. These calories can be consumed at any time during the day – either spread throughout the day or as a single meal.
Another related approach to 5:2 is to have “fasting” days with 500 calories not just twice a week, but every other day.
This involves fasting for the entire day. For example, if you eat dinner on day 1, you would fast for all of day 2 and not eat again until breakfast on day 3. This is generally 36 hours of fasting. This might provide a more powerful weight-loss benefit and may help avoid the temptation to overeat dinner on day 2.
However, there are some risks involved with fasting more than 24 hours.
The first rule of extended fasts is to always check with your health care provider to ensure you are not at risk for fasting complications.
Generally, for fasts greater than 48 hours, I recommend a multivitamin to avoid micronutrient deficiency. The world record for fasting is 382 days (although we don’t recommend this!), and going 7-14 days may be possible for some people.
I discourage people from fasting for more than 14 days due to high risk of refeeding syndrome, a dangerous shift in fluids and minerals that can occur when food is re-introduced after a long fast.