Everything You Need to Know Before You Try Alternate Day Fasting

There are plenty of things you'd do every other day, in a perfect world. Perhaps take a trip to a beach. Wear your favorite outfit. Feel absolutely invincible on a run. But what about eating? Alternate day fasting (ADF) — a type of intermittent fasting that's rising in popularity — will have you eating one day and fasting the next.

The diet allows all foods on non-fasting days and unlimited calorie-free beverages on fasting days. Compared to other eating strategies, alternate day fasting does not restrict calories, macros or any one specific food group. The only restriction? No food ... every other day.

ADF reduces your overall energy intake throughout the week by only allowing you to eat on certain days, similar to the 5:2 fasting diet which allows five days of normal eating and two fasting each week. When it comes to reducing overall calories, this diet has the potential to reduce calories in half due to fasting days.

​Read More:​ The Beginner's Guide to the 5:2 Fasting Diet for Weight Loss

An October 2014 review published in ​Translational Research​ examined weight loss when participants fasted one to three days per week compared to participants who restricted calories everyday. Fasting three days per week would be similar to alternate day fasting. The study found that this strategy for calorie restriction was just as effective for weight loss as daily calorie restriction.

Another study with 122 participants published in the July/August 2016 issue of ​Obesity Research & Clinical Practice​ found alternate day fasting my be more effective for middle aged people (50 to 59 years of age), as these participants had a greater weight loss during an eight-week alternate day fasting trial.

While there are weight-loss benefits shown by several studies, some populations may be more at risk when using alternate day fasting as a weight loss method. Specifically, people with obesity who engaged in eating disorder behavior may be at risk when following a strict alternate day fast.

When adjustments are made, risks may be reduced. A study published May 2015 in ​Nutrition Journal​ examining 59 adults with obesity found that allowing participants to get 25 percent of their daily calorie needs on fasting days reduced symptoms of binge eating and depression. While these results are promising if you have an eating disorder or depression, you should always consult with a qualified health care professional before taking on any fasting diet.

Sure, ADF can be good for weight loss, but what about your overall health? Research shows that it may have the potential to improve heart health in adults without obesity. A small August 2019 study of 30 people published in ​Cell Metabolism​ found that certain markers of cardiovascular health were improved in those using ADF for several months.

Hangry, anyone? Reducing calories down to zero on fasting days can have some uncomfortable side effects. "Energy levels and mood may be effected on fasting days due to a lack of fuel on fasting days," says Christy Alexon, PhD, RD, associate professor at Arizona State University and sport nutrition coach for Renaissance Periodization.

Alexon's concerns fall in line with common adverse symptoms which include: Headache, hunger, weakness, fatigue and dizziness, according to a 2015 study published in ​Nutrition Journal.​ For some, these symptoms resolve over time, but if they continue it's important to discontinue fasting and consult a medical professional.

​Read more:​ Why Intermittent Fasting May Help You Lose Weight

Just like running a marathon or taking a dance cardio class, alternate day fasting isn't for everyone. Perhaps you have a super active lifestyle, and need more calories to maintain energy levels. While ADF is flexible as it doesn't require calorie counting or restricted food groups, it does restrict your calories multiple days a week. Depending on your obligations, restricting your food intake can be difficult on fasting days. Planning workouts or personal obligations over non-fasting days can be difficult making the days your are restricting your food difficult.

Past injury and disease history can be important to take into account as well. If you have a history of kidney disease, diabetes, heart disease, hypoglycemia, are pregnant or breastfeeding or have had an eating disorder, it isn't recommended to take on an alternate day fasting diet without medical approval and supervision.

ADF may also have adverse effects on body composition during weight loss. Body composition — regarding retention of muscle mass — is rarely measured in studies conducted on alternate day fasting.

"Fasting does not support the recovery time needed to build new muscle in athletes or anyone training to increase muscle mass," says Alexon, who adds that while fasting may be effective for weight loss in the short-term there could be long-term metabolic effects of a potential decrease in muscle mass. "Muscle is built during recovery times and fasting does not provide the fuel needed for muscle recovery."

It's also important that a fast is taken on for no more than three months at a time, according to registered dietitian Robert Santana, CSCS, owner of Weights & Plates Strength & Nutrition Center in Phoenix, Arizona. "Staying within this parameter will help you avoid rebound weight gain, disordered eating and other potentially negative side effects of long-term calorie restriction."

You don't need to dive into fasting every other day per week. Begin with one fasting day per week if you've never fasted before. It may also be safer to begin with an adjusted version of ADF. An adjusted version allows for up to twenty-five percent of your daily calorie needs on a fasting day. This is recommended for those who with obesity who are looking to lose weight.

Taking on an adjusted alternate day fast may reduce common symptoms like headache, fatigue and low blood sugar. When following this method space your low-calorie meals throughout the day. This allows for the best chance to reduce symptoms as all calories are not eaten at one point in the day.

Alternate day fasting doesn't have any restrictions on calories or foods that you eat on non-fasting days. That doesn't mean you should live off highly processed foods or common binge items packed with sugar, salt and fat. Eating nutrient dense foods is important to ensure adequate nutrition and to avoid any potential nutrient deficiencies. Foods like whole grains, fruits, veggies, lean protein and healthy fats are all excellent choices for non-fasting days.

​Read More:​ The Beginner's Guide to 16:8 Fasting for Weight Loss

When choosing nutrient dense foods try to maintain a balance of protein, fat and carbohydrate. Balancing meals with all three macronutrients will keep you feeling fuller longer and will provide your body with the nutrients it needs to keep you feeling your best.

Staying hydrated is super important all the time, but especially when fasting. ADF allows unlimited non-caloric beverages on fasting days. Drinking water, unsweetened coffee or tea is recommended on fasting days, as it could reduce symptoms of hunger while fasting. The amount of water you need is influenced by your local climate, activity level and personal needs, according to the USDA. If you are feeling thirsty, you are already slightly dehydrated.