Intermittent fasting is a popular technique for weight loss that centers around creating a daily eating window for oneself and only eating during a specific time period (and ‘fasting’ during others, such as the hours you sleep). We checked in with registered dietitians and other health and nutrition experts to learn more about the practice, and find out how many hours one should fast per day. Read on for tips, suggestions and insight from Brenda Peralta, RD, registered dietitian of Purely Cookware and Stephanie Hnatiuk, RD, CDE, PTS, registered dietitian, fitness coach, and owner of Stephanie Hnatiuk Performance Nutrition.
Timing & Healthy Intermittent Fasting
Intermittent fasting is an eating pattern that provides you with eating windows, Peralta explains. "It doesn’t tell you what you should be eating, but when you should be eating," she notes. Peralta points out that there are different types of intermittent fasting, and "usually given in a ratio."
For example, in the popular 16:8, you "fast for 16 hours and have an 8-hour eating window," she says. (This could mean eating all of your meals and snacks from 8 a.m. to 4:00 p.m., for example, and fasting for the remaining hours). When fasting, Peralta notes that drinking water is a great way to fuel your body.
If you are new to intermittent fasting, she says the healthiest way to do it is to start slowly. "If you are not used to fasting, start with 12 hours every day," she advises, "Once you feel comfortable, you can add more hours (like trying to reach 14). You start adding hours each week until you get to the desired fasting time."
Steps To Take As A Beginner
The first step according to Peralta is to "determine how many hours you are naturally fasting." This, she clarifies, means from "the last meal you have the night before until the next meal the following day." Usually, this is a 12-hour window. "Start by doing those hours consistently every day. For example, do 12 hours every day," she adds.
Once you feel comfortable with this, Peralta suggests to start alternating. "Do 14 hours every other day and 12 hours the rest of the day," she says. "You can keep this pattern or increase the hours until you reach the desired fasting hours."
Fasting & Exercising
Hnatiuk says that the best way to choose your ideal intermittent fasting window is to look at the time throughout the day that you're most active, "or when you have your workouts scheduled and ensure your eating window overlaps with this time frame." This, she stresses, is because "we want to ensure adequate and proper fueling for our workouts or training sessions." If there is a big discrepancy between when we need the fuel and when we're consuming it, we can wind up "frustrated with our lack of results and poor performance," she warns.
"For example one of the mistakes I often see people make is they fast all morning, but also exercise in the morning before work," Hnatiuk continues. This, she says, can lead to low energy availability, increased risk of injury, and slower results (such as building muscle or losing body fat).
"Individuals falling into this trap can also experience excessive hunger later in the day, which can lead to overeating in the evenings. It can become a bit of a vicious cycle and the solution is really just proper fueling around our workout schedule." So, she concludes that we want to time our eating window around our activity, "so we can get in the fuel we need to perform our best, and get the best results from our training."
