Alternate Day Fasting – The Complete And Definitive Guide

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By Thomas Youngerman

Reviewed by Juliana Tamayo, MS, RDN - Last Updated

Guide to Alternate Day Intermittent Fasting

Fast as in the verb to abstain from food – not the adverb, quickly. Although, as you’ll see in this article, fasting, specifically intermittent fasting, can lead to significant weight loss, fast. Intermittent fasting, in its various forms, is all the rage, with celebrities such as Kourtney Kardashian, Jennifer Aniston, Halle Berry, and Hugh Jackman incorporating a form of fasting in their nutrition and exercise regimens. What exactly is fasting? How do you do it? What is the most effective form of fasting for weight loss and overall health? 

We’ll answer those questions and more in this guide to alternate day fasting. Although weight loss is one of the more highly touted benefits of fasting, IF (Intermittent Fasting) is not a diet – it’s an eating pattern focusing on when you eat, not what you eat. We’ll explore the different types of IF, feeding versus fasting times, and explain what happens to your body while fasting. We do a deep dive on (ADF) Alternate Day Fasting, citing clinical studies regarding its benefits, effectiveness and comparing ADF to other types of calorie restriction to help you determine which kind of fasting might be right for you. 

Intermittent Fasting

What Is Intermittent Fasting And Why Fast?

When practicing intermittent fasting, you alternate between periods of fasting and eating on a set schedule. Many people combine IF with diets such as the Ketogenic, Paleo, slow-carb, or reduced carb diet to amplify results. The benefit of intermittent fasting is that it replicates metabolic responses similar to a (CR) calorie restriction diet – in laboratory studies, CR has been shown to have significant benefits relative to lifespan, disease avoidance, and weight loss.

For background, a calorie-restricted diet means reducing your average daily caloric intake below what is typical or routine, without suffering malnutrition or the deprivation of essential nutrients. Most clinical studies restrict intake by about 30%. In these experiments, CR feeding has delayed the onset of age-related disorders and extended the lifespan of lab animals, including worms, crabs, snails, fruit flies, and rodents.

Intermittent fasting diet

What Happens To Your Body While Fasting?

Following the typical Western diet, you eat throughout the day, often snacking in addition to your meals. As a result, your body burns sugar (glucose) as its primary fuel for energy. Over time, your body becomes more insulin resistant, leading to weight gain, obesity, type 2 diabetes, and potentially other diseases. 

The goal of intermittent fasting (and CR) is to flip the metabolic switch from glucose to fatty acid-derived ketones as your primary fuel. Limiting your eating window gives your body time to digest, repair cells properly, and produce beneficial hormones that make stored body fat more accessible. When in a fasted state for more than twelve hours, the body’s glycogen stores are depleted, fatty acids are mobilized, and fatty acid-derived ketones are produced. Your body begins to use the energy reserves stored in fat for your primary source of fuel. 

Alternate Day Fasting For Fit Body

The benefits of intermittent fasting are related to changes in hormones and cell function. Your insulin levels drop, facilitating fat burning. Human growth hormone (HGH) secretions increase dramatically, beneficial for gaining muscle and burning fat. Older, dying cells (senescent cells) are removed, and autophagy (a cellular detox process) increases.

Popular Types Of Intermittent Fasting

The primary differentiator in the types of intermittent fasting is the length of time you abstain from food. In addition to IF, there are other fasts that last for extended periods, such as “purge” fasts and “detox” fasts, that last for multiple days. For our purposes, we will concentrate on the more popular, more frequently used intermittent methods of fasting for weight loss:

Types of Intermittent Fasting

The 12-Hour Method – is a popular method for many people as most fasting is done while sleeping. For example, you would have your last meal at 7 p.m. and your next meal at 7 a.m. This is the shortest fasting duration, as studies indicate that a fast must last at least twelve hours to be beneficial.

The 16/8 Method – the most common method practiced due to its ease of compliance. You fast after your evening meal, skip breakfast, and don’t start eating until sixteen hours have passed. Your next meal is then consumed within eight hours. 

The Fast 5 Method – as I’m sure you guessed, beginning at bedtime, you fast for nineteen hours, followed by five hours of eating as much as necessary to satisfy hunger. 

One Meal a Day Method – this is another popular regimen for the more disciplined faster. You have one meal per day, consumer within a one-hour window, and fast for twenty-three hours of the day.

The 5:2 Method – although the 5:2 method features time restrictions and is often referred to as an IF, it is not truly a fast and is closer to a CR diet strategy. In 5:2, you follow your normal diet for five days of the week. On the other two days, you reduce caloric intake to about one-third of your usual calories, approximately 500 calories for women – 600 for men. 

In the next segment, we discuss the final type of IF, the Alternate Day Method.

Alternate Day Fasting Method

As the name implies, during alternate-day fasting (ADF) you eliminate, or dramatically reduce your food intake one day, and you feast the next, eating whatever you want. There are two versions of ADF. You are only allowed unsweetened beverages in the strictest interpretation, such as water, coffee, and tea on your fasting day. 

A more common version of this diet involves “modified” fasting, where you consume about 20-25% of your daily energy requirements, or about 500 calories on fasting days, typically during a two-hour window at lunchtime. There is no need to count calories or manage your macronutrients on feeding days – you eat whatever you desire. The modified approach may be more sustainable than abstinence on fasting days but seems to be just as effective.

Two additional strategies to amplify or accelerate weight loss while on ADF are endurance exercise and combining ADF with Keto, Paleo, low carb, or slow-carb diets. 

Obviously, either method meets the requirement of at least twelve hours of fasting to flip the metabolic switch. A UIC (University of Illinois, Chicago) Professor of Nutrition, Dr. Krista Varady, is one proponent of alternate day fasting and has written “The Every Other Day Diet,” extolling its benefits and efficacy. Dr. Varaday has conducted extensive studies on the eating pattern, and we review some of her results in the Clinical Studies segment. 

Alternate Day Method of Intermittent Fasting

Alternate Day Fasting Clinical Studies

In addition to Dr. Varady, Harvard Health, WebMD, and the NCBI (National Center for Biotechnology Information), a branch of the National Institutes of Health has reported on alternate day fasting studies. Not surprisingly, as is the case in so many areas related to diet and nutrition, the results are mixed. In this segment, we identify areas of concurrence and where these institutes disagree as to the effectiveness and advisability of alternate day fasting. 

Without boring you with details regarding methodologies, suffice it to say all tests were conducted following appropriate testing protocols; we will go straight to results, findings, and conclusions.

Dr. Varaday on the Every Other Day Diet

Quoting Dr. Varady, “When people want to lose weight, the most common way is daily calorie restriction. You reduce calories by about 25 percent, which works for a little while. The problem is that you probably can’t stick to it for more than two months. You’re hungry and uncomfortable throughout the day. Fasting every other day works better. You have a day in between where you can feel normal again.” 

Seven separate studies validate Dr. Varaday’s results, and she is in the process of a yearlong study number eight in concert with the NIH. Her research shows that alternate day fasting can shed ten to thirty pounds in about eight weeks, with additional positive impacts on cholesterol, blood pressure, and cardiac health. Exercise combined with ADF can accelerate weight loss and amplify the other benefits, and one study found that a diet high in fat also boosted weight loss.

Dr. Frank Madeo on Alternate Day Fasting

WebMD quotes Dr. Frank Madeo, a professor of molecular biology at Karl-Franzens University of Graz, in Austria. In a one-month study of ADF, participants lost, on average, more than seven pounds. “This is an easy regimen — no calculation of calories — and the compliance was very high. Folks in the fast and feast group lost an average of 4.5% of their body weight. The reason might be due to evolutionary biology, as our physiology is familiar with periods of starvation followed by food excesses.”  In addition to weight loss, the alternate day fasting group also saw healthy heart disease risk factors, such as lower cholesterol. 

Harvard Health’s Position on ADF

Harvard Health, reporting on a yearlong study of ADF versus more traditional calorie restriction. Just a bit of background to clarify the study. Researchers divided 100 obese volunteers into three groups, an alternate day fasting group, a group on a daily calorie restriction diet, and a control group eating a normal diet.

Both diet groups lost 12 pounds, or about 5.5% of their body weight by month six, and significantly improved blood pressure, blood sugar, insulin, and inflammatory proteins. Both groups regained about 1.8% (four pounds) by month 12.

Ironically, at the end of twelve months, the only difference between the two diet groups: the ADF group had elevated low-density lipoprotein (LDL), an increase of 11.5 mg/dl as compared to the daily calorie restriction group. LDL, the bad cholesterol, is known to be a risk factor for heart attacks and strokes.

NCBI Report on ADF Studies

The NCBI synopsis of a Clinical Trial studying the effect of alternate day fasting as published in the JAMA (Journal of American Medicine Association). The study’s objective was to compare the effects of alternate day fasting versus daily calorie restriction on weight loss, weight maintenance, and risk indicators for cardiovascular disease. The primary outcome of the trial was weight loss. This randomized clinical trial demonstrated that although both groups lost significant weight and improved cardiac health, the alternate day fasting group did not produce superior adherence, weight loss, weight maintenance, or improvement in risk indicators for cardiovascular disease compared with daily calorie restriction.

Areas Of Consensus In Clinical Trials

Each trial observed weight loss and additional benefits in cholesterol, blood pressure, and cardiac health. The drop out rate among participants was highest in the alternate day fasting groups. 

Areas Of Conflict

Both Harvard Health and NCBI assert that compliance is vastly more difficult in ADF than daily caloric restriction – Dr. Varaday disagrees. Harvard Health and NCBI both cite an increase in LDL (the bad cholesterol) at twelve months.

We should note that these tests monitored very specific outcomes, not detailing all the myriad benefits of calorie restriction or intermittent fasting. Separate studies have confirmed many other possible benefits of these eating strategies:

Additional Benefits Of CR And Intermittent Fasting 

The anti-aging and life extension potential of CR and intermittent fasting are well documented and validated by long-term laboratory studies. Some of the additional benefits of intermittent fasting include:

  • Reduced fasting insulin levels and insulin resistance
  • Reduced risk factors for type 2 diabetes
  • Increased lifespan in laboratory animals 
  • Decreased blood pressure and improved cardiovascular health
  • Reduced oxidative damage (free radical damage) and chronic inflammation 
  • Initiates “autophagy” – cleaning out senescent cells
  • May help prevent Alzheimer’s 
  • Brain cells work at full capacity
  • Proven effectiveness in animal laboratory studies to reduce the risk of cancer

Alternate Day Method of Intermittent Fasting Benefits

Is Alternate Day Fasting Safe?

Studies indicate that alternate day fasting is safe for most people. In alternate day fasting, compensatory hunger, and weight regain over time is on par with other calorie-restricted diets. Compensatory hunger, in response to fasting and weight regain over time, is on par with other calorie-restricted diets. Although nearly all forms of calorie restriction cause a slight drop in resting metabolic rate (adaptive thermogenesis), ADF does not seem to do so. 

The most common issue encountered when practicing ADF is hunger. Of those who dropped out of clinical studies, most cited being uncomfortably hungry and irritable and were unable to continue.  In those studies, scientists theorized that compensatory hunger, which refers to an increased level of hunger due to calorie restriction, would cause people to overindulge when they were finally allowed to eat. In Dr. Varaday’s tests, people only consumed about 110 percent of their energy needs on feed days – they didn’t binge, and they ended up losing weight.

Our recommendation – if you become hungry while fasting, have some water, tea, or black coffee (unsweetened). If the hunger is intolerable, eat. Other side effects of IF include:

  • Brain Fog
  • Headaches
  • Fatigue
  • Dizziness
  • Dehydration

These are the same symptoms you may experience when entering ketosis and encounter the Keto-Flu, as the body adapts to using ketones for energy versus glucose. 

Alternate day fasting is not appropriate for pregnant and lactating women, children, and those with certain medical conditions. As always, we recommend you discuss fasting with your healthcare provider before attempting this eating pattern, particularly if you have a pre-existing condition or are currently taking prescription medications. More research on the safety and effectiveness of alternate day fasting is needed for those with eating disorders.

In Summary – Is Alternate Day Fasting For You

What is the most effective form of intermittent fasting for weight loss? Whatever method you find the easiest to adhere to. Compliance, the ability to stick with your eating pattern, is crucial. Be it 16/8, the Fast Five, or alternate day fasting, whichever program you find works best with your schedule, lifestyle, and discipline should be your fasting method. Science tells us that the calorie restriction associated with fasting is the key and that the benefits of one method versus another are minimal. As to the hunger associated with alternate day fasting, Dr. Varaday is confident your body will adapt to the strategy, and the compensatory hunger will fade with experience. 

So, what are you waiting for? It’s time to get started. Fast.

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Thomas Youngerman

Thomas Youngerman is an entrepreneur and author in the health and wellness space with extensive experience in the supplement industry. He has owned multiple sports nutrition stores and served as the Category Manager and Director of Business Development for a regional chain of nutrition stores. Thomas created a successful line of men’s healthy aging supplements that was distributed in GNC, The Vitamin Shoppe, and Kroger, and later sold to a West Coast corporation. Thomas was previously a certified nutrition coach. He has a strong understanding of nutrition, supplement formulations, DSHEA, cGMP, and FDA regulations.