The term alternate day fasting seems to be self-explanatory; you simply alternate between fasting and non-fasting days. Two common variations add a twist by allowing you to eat some food on your fasting days. In this post, I show you how to follow complete and modified alternate-day fasting routines.
Alternate day fasting is worth a look whether you are new to intermittent fasting or experienced and looking for ways to vary your routine. Let’s start with the straightforward method.
Alternate Day Fasting Summary
- A complete Alternate Day Fasting routine requires you to completely abstain from calories every other day. (Non-fasting days allow you to eat normally)
- A Modified Alternate Day Fasting routine allows you to consume 500 calories every other day. (Non-fasting days allow you to eat normally)
- 5:2 Fasting involves limiting your calories to 500 for two days of the week and eating as you normally would for five days.
- On fasting days you are allowed to consume non-caloric drinks (water, coffee, tea).
3 Ways to Do Alternate Day Fasting [Video]
In this video, you’ll learn…
- The three different ways to do alternate day fasting.
- The advantages and disadvantages of both.
- The best foods to eat during these diets.
Alternate Day Fasting (Complete Fast)
Performing a full alternate day fast means that you consume no calories on your fasting days. Depending on your interpretation, this means fasting anywhere from 24 to 36 hours.
A 24-hour fast is very similar to One Meal A Day (OMAD) fasting in which you are eating every day, but only for one meal.
With a 36-hour fast, you take a true day off from eating. For instance, you could finish eating dinner at 7PM on Monday, avoid food on Tuesday, and then resume eating at 7AM on Wednesday.
During the fasting period, you can consume non-caloric drinks, like water, coffee, and tea. During the eating window, you can eat whatever you want.
Advantage of Complete ADF
The advantage of complete Alternate Day Fasting is that it results in weight loss even though food choices are not restricted on eating days.
It would seem that fasting for a day would lead to binge eating the next day. While studies show that participants tended to eat more than normal on non-fasting days, the amount that they consumed was not enough to overcome the reduction of calories from fasting days. The weight loss that is seen with consistent alternated day fasting is due, in part, to the fact that calorie intake is naturally decreased over time.
Disadvantage of Complete ADF
Going without food for more than a day may cause a loss of muscle mass. There have been studies showing that the excretion of urea nitrogen increased after one full day of fasting (1).
That is important because urea nitrogen is a waste product that is excreted in the urine when protein, like that found in muscle, is broken down. It is possible that if you are not refueling properly on your eating days, that you could be sacrificing muscle.
Another disadvantage is that it is hard to completely go without calories every other day, which makes it hard to envision this strict form of fasting as a long-term strategy (2).
Modified Alternate Day Fasting (500 Calorie Fasting Days)
Dr. Krista Varady is one of the leading researchers on alternate day fasting. She found that allowing the consumption of 20-25% of your body’s energy needs on the fasting days increased adherence, without sacrificing the health and weight loss benefits.
To make this modified form easier to do, it is acceptable to eat 500 calories on your fasting days. The alternate days are looked at as feasting days, meaning you can eat whatever you want.
Advantages of Modified ADF
It is much easier to stick with a fasting routine long term if you get to consume some calories on the “fasting days.” There may also be a mindset advantage.
By knowing that you can eat to your satisfaction level the next day, you eliminate the common panic-driven thoughts that make us feel deprived and cause us to throw in the towel.
Studies also show that the allowance of some calories on fasting days prevents the loss of muscle and protects against disease. (3)
Disadvantages of Modified ADF
This method still requires discipline. Even though you are allowed to eat 500 calories on your fasting day, hunger may become an issue. You’ll do best if you track your nutrient intake to make sure that you’re getting proper nutrition in your overall diet.
The research that has been done on this and other forms of alternate-day fasting took place over many weeks or months, so consistency may be needed for full benefits to show.
5:2 Diet (5 days of normal eating – 2 hypocaloric days)
Another way that you can alternate between fasting and non-fasting days is to follow a 5:2 routine. This method was popularized by Dr. Michael Mosley and is also known as the Fast Diet.
The 5:2 diet protocol involves limiting your calories for two days of the week and eating as you normally would for five days.
The fasting day calorie limit is 500 calories. Some sources promote a slight difference for men and women with men allowed 600 calories and women 500. Those fasting day calories can be consumed in a single meal or spread out into smaller meals.
The days that you choose to fast are up to you as long as you do not do them back to back. Fast on Monday and Thursday, for example, but not on Monday and Tuesday.
Advantages of 5:2 Fasting
5:2 fasting is effective at weight loss. This weight-loss advantage may be due to the fact that alternating between fasting and non-fasting days creates a calorie deficit over time.
One six-month study compared two groups of overweight women. One group was placed on a daily diet that cut their normal calorie intake by 25%. The other group restricted their calories by 75% on two days a week and then ate normally on the other five days.
Analysis at the end of the study showed that the two groups took in about the same level of calories and macronutrients, despite their different patterns of eating. This again seems to indicate that alternate-day fasting does not cause binge eating as we might expect (4).
5:2 fasting requires you to eat a low-calorie diet on two days of the week rather than every other day. You have the freedom of choosing the days to fast as long as they are not consecutive days of the week.
There is no assurance that these alternate day fasting routines will be right for your metabolism, but 5:2 would be the least restrictive and, therefore, the easier entry point for this form of fasting.
Disadvantages of 5:2 Fasting
As with any extreme calorie restriction, you may get side effects like irritability, hunger, or difficulty sleeping.
Five days of “normal” eating could be a slippery slope for some people. Even if there is an overall calorie deficit, it is human nature to want to reward yourself after doing hard work. This could translate into eating more junk food than you normally would on your feasting days.
Does It Matter What You Eat When Alternate Day Fasting?
Lastly, it is truly ok to eat whatever want on non-fasting days? I certainly understand the appeal of this idea. Having regularly scheduled days to feast implies that you could regularly enjoy the Standard American Diet with boxed snack cakes, fast food meals, and sugary coffee drinks.
Dr. Varady’s research shows that even when study participants are allowed to select their food items on their feasting days, they consumed fewer calories throughout the study (5).
This finding suggests that participants did not overdo it on calories when allowed to eat freely.
One thing that I find missing from alternate day fasting studies is the reporting of foods consumed on feasting days. It is not clear if being in a study caused a conscious or unconscious improvement in food choices.
Eating with abandon has the potential for health consequences and makes your fasting experience less enjoyable. Regular consumption of fast foods and processed foods, even if it is every other day, has little health value. It can also lead to unstable blood sugar levels that could result in enhanced cravings on fasting days.
If a modified fast is something that interests you, you’ll be happiest with your health and weight loss results if you choose whole, unprocessed foods.
I have a free report on Intermittent fasting if you’d like more information. It shares common mistakes to avoid and ways to make fasting easier.
(1) Elia, Marinos. “Metabolic response to starvation, injury and sepsis.” Artificial Nutrition Support in Clinical Practice: Second Edition.
(2) Heilbronn, Leonie K., et al. “Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism.” The American journal of clinical nutrition 81.1 (2005): 69-73.
(3) Varady, Krista A., et al. “Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults.” The American journal of clinical nutrition 90.5 (2009): 1138-1143.
(4) Harvie, Michelle N., et al. “The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women.” International journal of obesity 35.5 (2011): 714-727.
(5) Varady, Krista A., et al. “Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults.” The American journal of clinical nutrition 90.5 (2009): 1138-1143.
About the Author
Dr. Becky Gillaspy, DC graduated in 1991 with a Doctor of Chiropractic degree from Palmer College.