Alternate day fasting (ADF) is a type of intermittent fasting in which you alternate between “fast days,” when you consume very few or zero calories, and “feed days” when you eat as much as you like. When done correctly, ADF is a powerful tool for weight loss and longevity.
How an Alternate Day Fast Is Done
The basics of alternate day fasting are relatively straightforward. You just eat every other day. On fast days, you water fast or consume other zero-calorie liquids like tea and detox water. On feed days, you can eat whatever you want (within reason).
If zero calories seems like an impossible goal, many experts recommend a modified approach. In the modified form, instead of consuming zero calories on fast days, you consume about 25% of your normal energy requirements. Exact calorie requirements differ from person to person, but if we assume a 2000 calorie diet, that means you consume 500 calories on fast days. These calories should be consumed in a single meal between noon and 2 p.m.
Researchers theorized that people who ate only 25% of their calorie requirements on fast days would compensate by binge-eating 175% of their needs on feed days. Surprisingly, that doesn’t seem to be the case. Participants in the study only ate slightly more than normal on feed days. What that means is that their bodies absorbed fewer calories over the two-day period. This type of energy restriction means that you can lose weight while still eating what you want half the time.
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Alternate Day Fasting vs Conventional Dieting
The standard type of weight loss plan is caloric restriction (CR). CR is simply reducing the number of calories you consume every day. You usually have to restrict your calories by 25-40% to achieve noticeable results. As a weight loss tool, caloric restriction seems to make sense. Take in fewer calories than you consume every day, and you have to lose weight. It’s basic math, right?
A concern with traditional caloric restriction is adherence. People just don’t stick to it for very long. Generally, dieters are very good about rigorously following their meal plan for a couple of weeks. However, there’s a dramatic drop-off after the two month mark. Eight weeks seems to be about the limit that most people can endure daily calorie restriction. What’s more, your metabolism doesn’t keep chugging along despite running a deficit. It starts to conserve energy where it can, meaning that your metabolism slows dramatically, and weight loss slows to a halt.
For many, alternate day fasting is a more manageable option than conventional CR. With feed days never more than a day away, the fast days don't seem quite so bad. On a traditional restrictive diet, you must exercise extreme self-control and deny yourself any treats, and that can leave you feeling defeated, depleted, and frustrated. With ADF, you know that you can eat what you want tomorrow.
With this comforting knowledge, many people find alternate day fasting easier to stick to than conventional calorie restriction. In fact, studies have found that ADF has an adherence rate of about 87%. The first couple days are the hardest. Most people feel hungry during the first few days of the fast, but, eventually, their leptin and ghrelin levels stabilize and their metabolism adapts to the new schedule.
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The Health Benefits of Alternate Day Fasting
Humans have known about the benefits of fasting for thousands of years, but conventional medicine has often ignored this knowledge. Fortunately, recent research now confirms much of what our ancestors already knew — fasting, when done correctly, can have a tremendously positive effect on the body.
Promotes Weight Loss
Multiple studies, both animal and human, have reported significant weight loss for ADF participants. The results? An average loss of about 8% of total body weight over an eight week period and a measurable reduction in belly fat.
What’s more, ADF preserves muscle mass more effectively than conventional dieting. After a successful conventional diet, about 75% of weight loss comes from body fat; the remaining 25% is lost from lean muscle. With ADF, studies show that approximately 99% of lost weight is in the form of fat. This makes for a much healthier body composition after the fast is complete.
Improves Insulin and Blood-Glucose Levels
ADF may have beneficial effects for individuals with type 2 diabetes. Studies have found that ADF reduces blood glucose levels in animals and improves insulin sensitivity in humans.
Supports Heart Health
In animal testing, ADF was found to reduce heart rate, decrease blood pressure, and improve cholesterol levels. Further testing is necessary to determine if these results are replicable in humans.
Cells become stronger if you put them under mild stress and allow them the time to recover from it. That’s essentially why exercise works. Exercise stresses muscle tissue, which then grows back stronger after recovery. “There is considerable similarity between how cells respond to the stress of exercise and how cells respond to intermittent fasting,” says Mark Mattson, senior investigator for the National Institute on Aging. Intermittent fasting has been confirmed to extend lifespan in animal studies, but more research is necessary to see if this benefit carries over to humans.
Making the Most of Your Fast
Alternate day fasting has many benefits, but it needs to be done the right way. You must still make healthy decisions. All forms of dieting work best when paired with exercise. ADF is no excuse to skip hitting the gym, so find an exercise regimen that works for you. Likewise, if you spend feed days eating toxic, processed food, your health will suffer. Make an effort to eat a healthy diet with plenty of fruits and vegetables and a reasonable amount of healthy fats.
Alternate Day Fasting Alternatives
While many people find the alternate day fast easier to follow than other dieting options, some may find it more difficult. In particular, frequent snackers and people who get irritable when they don’t eat regularly every few hours typically find the ADF difficult to adhere to. That’s perfectly fine. We are all unique individuals with different dietary needs, metabolisms, activity levels, and preferences. ADF is far from the only type of fasting regimen. If you are interested in different types of fasts, ADF can also be an excellent introduction to fasting in general.
Which specific nutritional plan you follow is less important than the fact that you have a plan. There are many other types of diets and fasts (like the ketogenic fast), each with their own advantages and disadvantages. Find a nutrition plan that suits your body and make it part of your healthy lifestyle.
Have you tried alternate day fasting? Any other kind of fast? What was your experience? Let us know in the comments below.
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†Results may vary. Information and statements made are for education purposes and are not intended to replace the advice of your doctor. If you have a severe medical condition or health concern, see your physician.