The method is not appropriate for everyone, however, particularly pregnant women and those with medical conditions such as diabetes or eating disorders. And though studies are promising, it remains to be seen if this type of eating can become a sustainable lifestyle habit instead of a drop-a-dress-size-in-a-week fad.
"The jury is still out, especially on how healthy, sustainable and realistic this approach is," said Samantha Heller, a registered dietitian and senior clinical nutritionist at NYU Langone Health in New York. "Eating very low calories, or none, on alternate days feels punitive to many and may exacerbate an already difficult and complex relationship someone has with food."
If you are still hoping to give intermittent fasting a try, there are some things to consider that can increase your chances of reaching your goals. Also, be sure to check with your doctor before starting a fasting regimen.
The appeal of intermittent fasting may be related to its simplicity, as it seems to require less effort than restricting calories every day. But there are different approaches to the eating pattern that vary in intensity.
One regimen known as time-restricted feeding advises consuming all of your calories within a range of three to 12 hours a day, whereas an approach known as the "5:2" diet advises eating 500 to 600 calories for two days of the week and eating whatever you want on the other five days. A third, much more stringent type of regimen involves alternating 24-hour fasting periods (in which no foods or calorie-containing beverages are consumed) with non-fasting days, during which anything can be eaten in any amount.
Though time-restricted feeding might not seem that different from a typical eating day, especially if you tend to skip breakfast, alternate-day fasting may cause intense hunger on fasting days and can be much more challenging to sustain.
"People who are accustomed to eating regularly scheduled meals or suffer from 'head hunger' may find alternate-day fasting extremely difficult," said Kristen Smith, a registered dietitian and spokeswoman for the Academy of Nutrition and Dietetics. Head hunger is the desire to eat when you are not physically hungry but are triggered to eat by emotions and social factors.
For the greatest success, Smith recommends choosing a fasting regimen that best suits your lifestyle, your health history and your ability to sustain fasting. For example, alternate-day fasting may not be appropriate for someone with an active job that requires manual labor or a job that centers around food, such as a chef.
Focus on protein and fiber-rich foods on semi-fasting days
Protein and fiber-rich foods will keep you feeling full and satisfied, which is important when calories are strictly limited. Healthy fats such as nuts and peanut butter can also curb hunger but provide a lot of calories for a small portion size.
If you are limiting your calories to about 500 to 600 per day, you might focus on one or two mini meals and snacks. Smith recommends a small piece of grilled salmon with half a cup of steamed veggies, grilled chicken over spaghetti squash, an egg-white omelet with veggies or black beans with a cup of veggies. For snacks, she favors fat-free plain Greek yogurt, two cups of plain popcorn, a cup of raw veggie sticks or a medium piece of fruit.
High-volume foods such as soups may also help on semi-fasting days, as they can fill you up on fewer calories than solid foods. Additionally, meal replacement bars may provide a convenient option, especially if you don't feel like preparing food on fasting days.
Don't gorge on non-fasting days
Our bodies want to keep us alive. If we are consuming very few calories on certain days, when we do eat, our bodies are going to do everything they can to encourage us to eat a lot because they don't know when the next meal will be available, Heller explained. "We also feel that as a reward for sticking to 500 calories or less, we are allowed and 'deserve' to eat large portions of whatever we want," she said.
That kind of reward can bring unintended consequences. "Overdoing it on a non-fasting day and eating excessive calories may actually lead to consuming an amount of calories you would consume in two days and defeat the purpose of a fasting day," Smith said.
In other words, having pancakes with bacon for breakfast, a fried chicken sandwich for lunch and a cheeseburger with fries for dinner on a non-fasting day can stall weight loss and prevent you from reaching your goals.
Don't go overboard with exercise on fasting days
Strenuous exercise, such as cycling, running, high-impact aerobics and weight lifting, is not advised on fasting days because the body does not have the fuel available to sustain rigorous activity, Heller explained.
If you want to do some form of physical activity on those days, Smith recommends sticking with walking, stretching or gentle yoga.
Drink enough water
If you decide to avoid food for any length of time, adequate hydration is important and should be maintained. "On a fasting day, someone may be more likely to become dehydrated because they aren't eating regularly scheduled meals, which is a common time people hydrate," Smith said.
Foods such as fruits, vegetables and broth-based soups contain significant amounts of water and can help you stay hydrated. Those fasting should be cautious with caffeine-containing beverages, as they may contribute to dehydration, Smith said.
Know when to stop
If you experience lightheadedness, dizziness, fatigue, nausea, headaches,difficulty concentrating or any other symptoms that regularly interfere with your daily functioning, you should stop fasting.
"If any of these symptoms occur, it might be a good time to meet with a registered dietitian to discuss other dietary interventions that might be better suited for you," Smith said.