Alternate-Day Fasting: A Complete Guide to Fasting Every Other Day - Perfect Keto

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Alternate-Day Fasting: A Complete Guide to Fasting Every Other Day


Alternate-day fasting (ADF) is a form of intermittent fasting. When you practice ADF, you fast every other day.  

The protocol is simple. On fasting days, you eat 0-25% of your normal calories. On non-fasting days, you eat your fill. 

Alternate-day fasting shows promise for weight loss, body composition, inflammation, heart health, and a variety of other health measures. 

In this article, you’ll learn how to do alternate-day fasting, ADF benefits, who should (and shouldn’t) try ADF, and much more. 

What is Alternate-Day Fasting?

Alternate-day fasting is an easy-to-understand intermittent fasting protocol. You simply alternate fasting and feeding days. 

Fastings days, however, needn’t be 100% calorie restriction (CR) days. In fact, research shows you can eat up to 25% of your normal calorie intake during these non-feeding periods[*]. 

On a “fast day”, you consume less than 25% of your daily calories. A good rule of thumb is to not exceed 500 calories. 

Here’s the good news. On feeding days, you eat ad libitum, or to satisfaction. So you’re eating a little one day … a lot the next day … a little the next day … and so on. 

The total length of each fasting period is around 36 hours. 36 hours sounds like a lot, but remember: that chunk of time includes two nights. 

Broadly speaking, the regular oscillation between fasting and feeding is known as intermittent fasting (IF). There are many forms of IF, with alternate-day fasting being one of the more popular. 

Other forms of IF include 16/8 fasting, time restricted feeding, 24 hour fasting, fat fasting, and more. Check out this comprehensive guide to intermittent fasting for the inside scoop on this topic. 

This guide, however, is on alternate-day fasting. So how do you do it?

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How To Do Alternate-Day Fasting

You’ll learn about alternate-day fasting benefits soon, but first: time to get practical. 

Yes. Before embarking on your ADF journey, you’ll want to heed these basic rules:

  • Fast every other day: This is the protocol studied for weight loss, heart health, and more[*]. If ADF doesn’t work with your schedule, and you still want to try fasting, perhaps consider another form of IF.
  • Consume 0-25% of calories on fast days: In the ADF literature, some studies have employed zero-calorie fasting protocols, while others have allowed up to 25% of calories on fast days[*]. Unless you’re a seasoned faster, the second option will likely be easier to stick to. 
  • Eat ad libitum on feed days: Eat as much as you like on feeding days. Don’t hold back. But be careful not to overeat, as this can cause digestive problems,among other issues. 
  • Monitor your weight: Alternate-day fasting is a form of calorie restriction (CR). When you skip entire days of eating, that’s less calories in! Weighing yourself regularly not only helps you track weight loss goals, but also lets you back off if you’re losing too much weight. 

That’s it for ADF rules. Plain and simple. 

By now you’re probably wondering what, exactly, you should eat on fasting and feeding days. Does it matter?

What To Eat for Alternate-Day Fasting   

Most studies on ADF don’t control for food type, only calories. One interpretation of this data is that food choice doesn’t matter in the context of fasting. 

But if you want to optimize your health, this simply can’t be true. Of course it matters what you eat. Certain foods are healthier — and more conducive to fasting protocols — than others. 

Healthy foods don’t have added sugar, trans fats, or artificial ingredients. Eat whole foods — meats, fruits, vegetables, and healthy fats — and you’ll be doing better than most. 

On fast days, you must stay under about 500 calories, whatever the source. If you find yourself struggling with gnawing hunger, soup can help fill you up. 

That’s right. Calorie for calorie, soup is one of the world’s most satiating foods[*].

Feeding days have less restrictions. Just eat whole foods.  

To ease the transition into fasting, you might try a high-fat low-carb keto diet. Here’s how keto can help with ADF. 

After a couple weeks on keto, your body adapts to using fat for energy. This fat-adaptation is like a superpower. You can access stored body fat when you need it most: during a fast.

Without fat adapting first, going 16 or more hours without food can be a harrowing experience. But when you fat-adapt on keto, you more easily glide through the day on fatty acids. 

Keto expert and clinician Dr. Peter Attia uses the ketogenic diet to facilitate his quarterly week-long fasts. He calls it the “Nothingburger”: one week of fasting sandwiched between two weeks of nutritional ketosis[*]. 

The takeaway? Favor whole foods on ADF, and consider going keto to make fasting easier.  

Benefits of Alternate-Day Fasting

Done properly, fasting is a “good stress” for your body. When you fast, you activate specific pathways that positively affect your metabolism, hormones, cellular repair mechanisms, and more. 

These benefits have been studied in test tubes, animals, and humans. Human evidence is what you’ll see here, for the most part. 

And yes, researchers have specifically studied alternate-day fasting. Read on to learn what they found.  

#1: Weight Loss

When you alternate feeding days, you naturally eat fewer calories. 

Calorie restriction, in fact, is probably the biggest driver of weight loss on ADF. Fewer calories in, fewer pounds gained.

Another form of calorie restriction is to maintain a caloric deficit — day in, day out. No unlimited feeding days. Oh the hunger!  

So which is better for weight loss: alternate-day fasting or daily calorie restriction?

The answer might surprise you. They work about equally well[*].

But there are other considerations that give ADF a slight edge. 

For one, alternate-day fasting may be an easier protocol than daily CR. People are more likely to stick with it[*].  

Also, one meta-analysis found that ADF caused greater fat-mass (though not bodyweight) reductions than daily CR[*]. 

One final example: A 2013 randomized controlled trial on normal and overweight adults doing ADF[*]. 

At week 12, alternate-day fasters had lost significantly more weight (about 11 pounds) than controls. Most of that weight was fat. 

#2: Body Composition

When you fast, it sends a signal to your body: time to unleash those fat stores!

This is good news for body composition. It means the weight lost during intermittent fasting is mostly fat mass, not lean mass. 

In one study, researchers found that 8 weeks of CR and ADF achieved similar improvements in body composition and weight loss in obese adults[*]. Yet after 24 weeks (unsupervised, no less), the ADF folks had more muscle and less body fat than the CR folks.

Finally, exercise appears to supercharge fasting-induced fat loss. Obese people, it was shown, lost more fat when exercise and ADF were combined than for either treatment alone[*].

#3: Hunger 

Your appetite is a complex system controlled by various hormones, proteins, and chemical impulses that course through your body, moment by moment. 

Here’s the simple story: if you’re fasting, you’re going to get hungry. Hunger is part of the deal, and it’s not necessarily a bad thing. 

As a general rule, less calories in means more hunger. Research shows, in fact, that the two main appetite hormones — leptin and ghrelin — are equally affected by alternate-day fasting and calorie restriction[*]. 

Yet other researchers have found that ADF promotes more “fullness” and “satisfaction” than CR, even if hunger hormones aren’t affected[*]. In yet another study, ADF decreased hunger (after the fasting period) in 64 obese people, compared to other treatments[*]. 

Hunger is also tied to your level of fat-adaptation. If you’re already a fat-burning keto machine, the hunger of ADF shouldn’t be crippling. 

In fact, a ketogenic state is linked to[*]:

  • Lower ghrelin (your hunger hormone)
  • Lower neuropeptide Y, which stimulates appetite
  • Higher CCK, which promotes fullness

All these changes help you deal with the hunger of a fast. 

#4: Insulin Function and Diabetes

Type 2 diabetes is a metabolic disorder defined by high blood sugar, high insulin, high blood pressure, and obesity. According to the CDC, over 100 million Americans have diabetes or its precursor, prediabetes[*].

Fasting affects many metabolic markers linked to diabetes, including insulin. 

Insulin is your blood sugar boss, or the hormone that tells glucose to exit your blood and enter your cells. In the case of diabetes, however, your cells stop listening to insulin, and blood sugar rises to dangerous levels. Hello type 2 diabetes. 

Enter fasting. Regular fasting, it’s been shown, can improve insulin sensitivity in those with metabolic syndrome[*]. 

Other research suggests that ADF (and other forms of intermittent fasting) can decrease fasting insulin levels, visceral fat, and insulin resistance. Welcome tidings for diabetes risk reduction[*]. 

Strangely enough, fasting interventions don’t appear — at least in published studies — to significantly lower blood sugar levels[*]. 

More research is needed. It would be interesting, for instance, to combine a ketogenic diet with alternate-day fasting. Both interventions are encouraging therapies for type 2 diabetes[*].  

#5: Heart Health

Heart disease kills more people than any other disease. It’s always the elephant in the room. 

Can ADF help?

You already learned that alternate-day fasting promotes weight loss. Since obesity is a major risk factor for heart disease, this can — depending on the individual — be a very good thing. 

Reducing heart disease risk, of course, is more complex than losing weight. The arterial hardening that causes most cardiac events (atherosclerosis) is driven by inflammation, blood lipids (like LDL), blood vessel health, blood pressure, and many other factors. 

Alternate-day fasting, it’s been shown, can improve some of these factors. Here’s what researchers found by studying fasting in 32 overweight or normal weight people, randomized to either control or ADF groups[*]:

  • Reduced triglycerides: High triglycerides are red flags for other heart disease risk factors, like obesity or type 2 diabetes[*]. 
  • Decreased c-reactive-protein (CRP): A blood test for CRP measures inflammation in your body. High CRP indicates a chronic or acute immune response, and is positively correlated with heart disease risk[*]. 
  • Increased LDL particle size: Large, “fluffy” LDL particles are linked to lower heart disease risk[*]. On a related note, other studies have found that ADF decreases LDL-C, the more standard measure of LDL cholesterol[*].  
  • Increased adiponectin: Low levels of adiponectin (a fat-burning hormone) are linked to increased risk of obesity and heart disease[*]. 
  • Decreased leptin: High leptin may indicate leptin resistance, which is linked to obesity[*]

The bottom line is that ADF is a promising therapy — in conjunction with many other therapies — for reducing heart disease risk.

#6: Autophagy

Just like your desk gets messy, your cells get messy too. You clean your desk by putting away papers and wiping it down. You clean your cells by activating autophagy. 

Autophagy means “self eating”. It’s when your cells digest, recycle, and reuse damaged parts. 

Think of it this way. When you clean your desk at work, you can get more done. 

Same with your cells. When they clean themselves, they become healthier

When autophagy is impaired, however, bad things happen: increased risk of cancer, neurodegenerative disease, and heart disease — to name a few[*]. 

There are many ways to activate autophagy — exercise, coffee, the keto diet, and certain supplements — but the most reliable autophagy-activator is fasting. 

To be clear, most of this evidence comes from animals and test tubes[*]. Unfortunately, there isn’t a great way to measure autophagy in humans. Nobody wants to get biopsied. 

Because of this, no one knows the optimal fasting protocol for autophagy. 

That said, longer fasts probably induce more autophagy than shorter ones. But remember: fasting is stressful, so be careful not to push it. 

With that in mind, here’s who should be careful with alternate-day fasts. 

Who Shouldn’t Try Alternate-Day Fasting

Fasting is a stress similar to exercise. Too much can be hard on your body.  

If your stress bucket is already full, first make room before adding more stressors. This is how you avoid burnout. 

For instance, if you’re not sleeping well, exercising excessively, and super stressed out at work: it might be wise to forgo fasting until you get those areas handled. 

Also consider your level of fat-adaptation. If you can’t utilize stored body fat for energy, any form of intermittent fasting (including alternate-day fasting) will be excruciating. 

Here’s why. When you rely on sugar for energy, you need a steady supply of carbohydrates to fuel your day. 

Because of this, sugar burners have a hard time taking 16 (much less 36) hours off food. That’s what happens when you ride the blood sugar rollercoaster. 

To prepare for alternate-day fasting, consider going low-carb and possibly keto. After all, keto is a fasting-mimicking diet. On both a keto diet and a fasting protocol, you’re running on fat!

One more point. For various reasons, women may have more trouble with fasting than men:

  • Women have higher levels of ghrelin (the hunger hormone), which may increase fasting difficulty[*]
  • Female rats have decreased fertility during fasting[*]
  • Women may have a greater stress response to extended fasts[*]

Now that you know the basics, here are specific groups who should be extra-careful with fasting:

#1: Pregnant or nursing women

Calorie restriction or fasting during pregnancy is not recommended. The growing fetus needs additional nutrients. 

A similar principle applies to nursing women, whether or not they decide to breastfeed. Nursing women need more calories, not less. 

#2: Underweight people

Being underweight (having a BMI below 18.5) is linked to number of negative health outcomes like heart disease, muscle loss, brittle bones, cognitive issues and more[*][*][*]. 

About 1% of men and 2.4% of women are underweight, according to the CDC[*]. 

Alternate-day fasting is a bad idea for underweight folks. They need to gain weight — not lose weight — and calorie restriction won’t help with that goal. 

#3: People fighting infection 

If you’re fighting an infection, you need your immune system running on all cylinders. Extended fasting, unfortunately, may hamper your immune response. 

This has mostly been shown in animals[*]. It’s also been shown in humans — as increased susceptibility to infection during famine[*].  

Yet the picture is complex. For instance, researchers have shown that, in mice, fasting triggers a regeneration of immune stem cells[*]. 

And so fasting may help with long-term immune health, but impair it in the short-term. 

Should You Try Alternate-Day Fasting?

Not everyone should try alternate-day fasting. Check your stress levels and the above considerations first. 

ADF is a fairly simple protocol, however, and it’s been linked to a number of health benefits, including weight loss.

To make fasting easier, consider starting a low-carb ketogenic diet. You’ll fat-adapt, access stored fat more easily, and breeze more easily through your fasting days. 

Take care and thanks for reading.   


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