Glynn’s Guide:
Takeaways that won’t fail you

  • I believe the 16/8 intermittent fasting every 3rd day is the most realistic style to follow alternating days. Meaning, 2 days of normal eating, followed by a 16 hour fast. Then repeat.
  • For this to be successful. It has to be sustainable with your schedule and abilities to avoid food at certain times.
  • The health benefits that stand out include:
    • Improved glucose tolerance (more so for men than women).
    • Weight loss without muscle loss.
    • Improved cholesterol ratios
    • Improved triglyceride levels
    • A drop in systolic blood pressure
    • Reduction in risk for coronary artery disease
    • Protects against age-related fibrosis by reducing inflammation and oxidative damage
  • I could not find anything that is unsafe about intermittent fasting.

Intermittent Fasting a Trend or Science?

Picture a salesperson knocking on your door. You open the door and their spiel goes something like this:

“Good day! What if I offered you something that will help you lose weight, reduce your risk for age-related diseases and improve your blood chemistry! Would you be interested? But wait, don’t answer yet. Because I also want to throw in improved glucose tolerance and reduced blood pressure! And best of all it will cost you less than you already spend! Act today and I’ll even throw in the preservation of lean muscle mass! What do you say?

Well, let me introduce you to the many variations of intermittent fasting.

And I can’t get over the amount of peer-reviewed research that supports intermittent fasting benefits.

Side note. I’m hypercritical of how research is done, funded and reported on. More so than the media. And I only review textbooks and credible research to form my opinions.

I have no doubt you’ve heard someone discussing this method at some point. Well, like you, I was curious. You see, even with all the years in college (studying the chemistry of our metabolism). Variations of diet fads were not discussed. Especially this one.

But I sure do like to dissect them! And I have to say, I’m impressed.

We covered calorie restriction of course. But not applying calorie restriction to intermittent fasting.

I reviewed over twenty peer-reviewed studies and will go over what I learned for you.

Let me fill you in. Onward…

What Does Intermittent Fasting Mean?

This seems to be a mixed bag.

My literature review covered six different variations. I cover them more specifically in the next section.

Intermittent fasting is when you abstain from eating or drinking calories for a fixed time period. Then resume normal dietary patterns. You then continue this in a repetitive cycle. This is true regardless of the frequency.

The definition of intermittent is:

Not happening regularly or continuously; stopping or starting repeatedly or with long periods in between.

So, I believe a better description for intermittent fasting is interval fasting. That denotes a plan rather than more spontaneous timing.

Not my decision. But I may rename my plan interval fasting.

Most Common Types of Intermittent Fasting

Types of Intermittent Fasting

  • 1 day fasting to 2 days eating as you desire. (I.e., fast every 3rd day)
  • Alternate day fasting. (I.e., 1 day on and 1 day off)
  • Alternate day fasting with 1-day eating as you desire, and 1 day with 25% of normal calories
  • 60% reduction of calories two days a week randomly spaced.
  • Limit feeding to an 8 hour period (16:8 fasting) every day.
  • Fast for 24 hours one day a week.

Carlson states

“the optimum amount of fasting appeared to be fasting 1 day in 3 and this increased the lifespan of littermate males about 20% and littermate females about 15%.”

A note on the type of animal used in research

Although this may apply to research on rats. Don’t let that distract you. Most of the research academia does is on rodents and wild primates. Their metabolism resembles ours very closely.

And many of the studies for other things used to set guidelines are done on rats and mice too.

Intermittent Fasting Results

So, whether the research is done on humans or rodents, both are viable.

Does Intermittent Fasting Work?

Yes, without a doubt! Based on the peer-reviewed research, of course. I’ve also had clients use one variation or another with great results.

How Does Intermittent Fasting Work?

Does Intermittent Fasting Work

Not all mechanisms are known for some of the improvements experienced. But that doesn’t change the fact that the positive changes really occur. Let’s summarize it first and then break it down by each change.

Since we become more sensitive to insulin due to the fasted state. Our bodies are better at using the calories eaten after a fast. So, once we acclimate to the new style of eating. The food eaten after a fast is more likely to be used as energy rather than stored as glycogen or fat.

If you exercise during a fast and the intensity is low. You’ll burn fat the majority of the time. But, if exercise is too intense and there is no glycogen left as a result of fasting. You may use some stored protein (muscle) to go through gluconeogenesis.

Strength training during the fast is great and encouraged!

What’s the message? If you exercise during a fast, keep the intensity low.

The following list briefly discusses “the how” of more specific improvements.

  • Improved insulin sensitivity: Less insulin is needed to do the same job.
  • Improved glucose tolerance: The body improves its ability to respond to carbohydrates by regulating its blood glucose concentration.
  • Weight loss without muscle loss: My best guess is that we become so efficient at making and using ketone bodies (derived predominantly from fat). That we do not need to use muscle tissue for gluconeogenesis. Elevated growth hormone levels may also play a role.
  • Improved cholesterol ratios: Unknown (or could not find reason in the literature).
  • Improved triglyceride levels: Unknown (or could not find reason in the literature).
  • A drop in systolic blood pressure: Also unknown.
  • Reduction in risk for coronary artery disease: A result in a combination of the prior three: improved cholesterol, triglycerides and a drop in systolic blood pressure.
  • Protects against age-related fibrosis by reducing inflammation and oxidative damage to cells.

How to fast safely

Consume normal meals for 1-day eating as you desired followed by:

  • A 16 hour fast beginning after your last meal of the day.
  • Then after 16 hours (includes sleep) resume normal meals from that time onward.
    • In other words, if it’s lunchtime that you break your fast, have a normal lunch meal.
    • This will keep you around 25-50% of normal daily calories.
  • Day four becomes day 1 for a complete cycle.

You can see a layout for what I’m currently experimenting with here.

What Does Fasting do to the Body

The benefits from intermittent fasting seem to result from both a reduction in oxidative damage. And increased cellular stress resistance.

I found one interesting tidbit. Heilbronn states:

Seventy-two hours of fasting did not change the 24‐hour diurnal rhythm of ghrelin in humans. These results call into question the role of ghrelin in initiating hunger drive.

If you want to learn more about the role of ghrelin, check out my article Feeling Hungry After Eating? Here’s Why (And How to Fix It).

I’m sure it adjusts after a longer variation. Any grad students want some great research? Look at the long-term results for leptin and ghrelin after intermittent fasting. Then let me know!

Does Sleeping Count as Fasting?

Sleeping Intermittent Fasting

You bet it does.

In fact, this is when our bodies burn the majority of our stored fat. You can learn more about that in my article, Weight Loss vs. Fat Loss: You’ve Got it All Wrong.

During exercise or physical activity, we typically burn more glucose. This, of course, does not apply if you’re following a ketogenic meal plan. You can learn more about how a ketogenic diet.

Nonetheless, because you burn more fat during sleep and you’re not eating during that time. Sleep definitely counts toward your fasting time.

What Are the Expected Results From Intermittent Fasting?

Intermittent fasting is better at retaining lean mass (muscle) than just calorie restriction. So, ultimately you will lose fat weight.

You will also:

  • Improve insulin sensitivity
  • Reduce the risk of coronary artery disease
  • Decrease the risk of age-related brain degeneration

That’s awesome if you ask me!

I go into more detail in the sections below about the different variations.

But first, why do 16 hours seem to be the magic number? Most of the research I looked at found all the positive results to occur with a minimum of a 16 hour fast.

12 Hour Fast Daily

This basically states that you eat within a 12-hour window outside of sleeping. I could not find any research to support this style.

I believe it’s a healthy way to eat, but not what you’re looking for in intermittent fasting.

16:8 Fast Daily

This basically states that you only eat within an 8-hour window during waking times.

This is something you would practice daily. I (oddly) can’t find a lot of research on the results of doing this daily.

That said, I’m certain it carries all of the same benefits.

But, is it realistic for your schedule and your ability to fast? If so, it certainly simplifies things for you and will be successful.

16/8 Alternate Day Fasting

This style states that you eat within an 8-hour window every other day. One day of normal food all day, and one day that allows consumption within an 8-hour window.

An example is to:

  • Eat normally all day Monday until 9 PM.
  • On Tuesday resume eating at 1 PM until 9ish.
  • Wednesday is a normal day again
  • Thursday repeat Tuesday on Thursday
  • And so on…

16/8 Fasting Every 3rd Day

This states you eat within an 8-hour window every third day. This is much like alternate day fasting but on a 3-day cycle rather than a 2-day cycle.

24-hour fasting once a week

This is self-explanatory. You would pick one day a week to simply skip eating for 24 hours. Sunday seems to be a popular day.

I only recommend this if you’re already following a healthy meal plan. A good scenario would look like this:

  • Monday-Friday: eat your regular healthy meal plan.
  • Saturday: break day to include your favorite foods without structure within moderation.
  • Sunday: Consume only water all day.

I like the simplicity of this one, but fear it’s unrealistic for most people. Besides, it was not mentioned with a lot of the research. So, it may not include all of the positive results from intermittent fasting.

What’s better, a 12 hour fast, a 16/8 fast variation or a 24 hour fast once a week?

I found several different variations discussed above. But these three seem to stand out amongst the blogging world.

Bottom line, 16:8 fasting every third day is number one in my book.

Why?

Because it’s realistic with many peoples schedule and includes all of the positive benefits.

I have to stand behind what I believe is superior. And that’s based on education and experience. The following is a great scenario because it’s realistic and effective. I covered this under the section “How to safely fast.

  • A 16 hour fast beginning after your last meal of the day.
  • Then after 16 hours (includes sleep) resume normal meals from that time onward.
    • In other words, if it’s lunchtime that you break your fast, have a normal lunch meal.
    • This will keep you around 25-50% of normal daily calories.
  • Day four becomes day 1 for a complete cycle.

I’m experimenting with the following variation because it works with my schedule.

  • Regular ketogenic meal plan on Monday, Tuesday and Thursday.
  • 16 hour fast into Wednesday, Friday and Sunday.
  • I follow normal ketogenic meal plan from approximately 1 PM onward on fast days.
  • On Saturday, I follow a normal ketogenic meal plan for the first half. And then eat whatever I like for the second half. I enjoy a break for a burger and some craft brews as much an anyone else!

So far, this works well. But I was already lean, so I’m doing it for the health benefits and to see how I feel. I’ll post my experience and results after a few months. It was only after reviewing the research that I decided to give it a try myself. My clients have done well with it prior to my trial.

What About the Keto and Intermittent Fasting?

Combining intermittent fasting with a keto meal plan carries a lot of benefits.

Harvie states,

“Reductions with the intermittent energy carbohydrate restriction diets were significantly greater compared with the daily energy restriction diet.” You can translate that into a ketogenic based intermittent fasting meal plan.

Even during a maintenance phase of one of the studies, there were benefits. One day a week of 16-hour fasting and restricting carbs reduced insulin resistance. This is a big deal and has prolonged effects!

Although, I’m skeptical of how long that benefit would last with only one day of fasting a week.

The duration of the maintenance phase was not disclosed.

Intermittent Fasting Health Benefits

Let’s take a look at some of the intermittent fasting benefits you’ll enjoy.

  1. Increased resistance of neurons to neurodegenerative diseases. Research showed 1 day of fasting. And two days of ad libitum feeding to be most beneficial for this particular benefit. (Anton)
  2. Mattson states, “interestingly, cellular and molecular effects of intermittent fasting and calorie restriction on the cardiovascular system and the brain are similar to those of regular physical exercise, suggesting shared mechanisms.” That’s enough of a reason to try intermittent fasting.
  3. Mattson also states, “Intermittent fasting (IF; reduced meal frequency) and caloric restriction (CR) extend lifespan and increase resistance to age-related diseases in rodents and monkeys and improve the health of overweight humans.”
  4. A reduction in oxidative damage to cells.
  5. Findings suggest that intermittent fasting for weight loss is a viable dietary option. To help obese individuals lose weight.
  6. Another important intermittent fasting benefit is a decrease in some coronary artery disease risk factors.
  7. It protects against aged related fibrosisThickening and scarring of connective tissue (Castello).

I believe that the initial drawback of intermittent fasting and hypoglycemialow blood sugar is negated after a brief time. So, I would not worry about this if you’re concerned.

What Changes Take Place Within Our Bodies From Intermittent Fasting?

  • Improved insulin sensitivity
  • Improved glucose tolerance
  • Weight loss without muscle loss
  • Improved cholesterol ratios
  • Improved triglyceride levels
  • A drop in systolic blood pressure
  • Reduction in risk for coronary artery disease
  • Preservation of lean body mass (muscle)
  • Protects against age-related fibrosis by reducing inflammation and oxidative damage

Intermittent Fasting for Women

This is an important point to discuss.

I did find one discrepancy regarding glucose tolerance in women. If you recall, glucose tolerance is our ability to respond to carbs by regulating our blood glucose concentration.

One study found that intermittent fasting women had impaired glucose tolerance after 22 days of intermittent fasting.

I only found one study with this result. I cannot base a claim on one study alone. Besides, all of the other results from intermittent fasting were positive for women.

So, unless you’re a female with diabetes, it is still worth a try.

Intermittent Fasting for Men

Intermittent fasting men showed a significant improvement in all aspects across the board.

The Difference Between Men vs Women?

The research all seems to agree that intermittent fasting is 5% better for males than females. Regardless of the style chosen.

I quoted this before, but it’s worth repeating.

Heilbronn states,

The optimum amount of fasting appeared to be fasting 1 day in 3 and this increased the lifespan of littermate males about 20% and littermate females about 15%.”

So, should women avoid intermittent fasting?

No.

I don’t believe the small difference makes this unproductive or dangerous for women. So, go for it!

Intermittent fasting and exercise

Intermittent Fasting Exercise

Strength training will play a major role in preserving muscle tissue. In fact, you can gain muscle with strength training while intermittent fasting.

Endurance exercise will also help you make improvements. Bhutani states, “our results suggest that the combination of ADF (alternate day fasting) plus endurance training results in greater body composition and lipid‐altering effects than that of each intervention alone.”

Bhutani used the 25% reduction in calories on the fasting day variation. If you’re curious.

With this stated. There’s definitely research to back adding strength training and endurance training to your intermittent fasting plan.

Is Intermittent Fasting Sustainable?

This is a tough one.

I believe the intermittent fasting benefits or intermittent fasting weight loss may be negated by one’s inability to fast. We’ve all experienced the “hangry” feeling. This will subside after a period of time whether you’re ketogenic or fasting.

But, can the average person get to that point? I believe enough of you are capable, making it a worthwhile trial.

Johnstone states,

This approach does seem to promote weight loss but is linked to hunger, which can be a limiting factor for maintaining food restriction.

I can’t put it any more simply.

Let’s look at reality…

It’s Saturday, your family or significant other is chowing down on a pizza. You still have two more hours to fast. Can you do it?

My other concern is scheduling.

Does your work schedule permit you to eat when you should typically break your fast?

These are realistic concerns, but nothing that can’t be overcome with a strong desire to improve.

What if Intermittent Fasting Doesn’t Work?

I could not find any research that concluded no positive change from intermittent fasting.

Although, one relevant note I made stated no or minimal glucose toleranceglucose tolerance is the body’s ability to respond to dietary carbohydrate by regulating its blood glucose concentration. improvement for women. This was mentioned previously.

I believe this is negated by the other positive benefits.

As we discussed in “is intermittent fasting sustainable”, I believe one’s schedule and ability to follow through will play a large role in the success rate.

A Very Important Point!

Regardless of which style of intermittent fasting you choose, it is not a green light to over consume calories during the feeding state!

So, keep the sugar to a minimum and stick to a solid meal plan.

Also, can diabetics benefit from this style of eating?

Technically, yes. But please discuss with a knowledgeable doctor first if you have issues regulating blood glucose levels.

Conclusion on Intermittent Fasting

Bottom line, it works.

There are just too many positives to not give intermittent fasting (interval fasting) a serious consideration.

Harvie sums it up by stating,

Intermittent energy restriction is as effective as caloric energy restriction with regard to weight loss, insulin sensitivity and other health biomarkers, and may be offered as an alternative equivalent to caloric energy restriction for weight loss and reducing disease risk.

And with all the other health benefits (more important in my book). I say it’s a no-brainer. And the maintenance of lean muscle tissue to boot absolutely rocks!

Best I can tell, the alternating day, 16:8 (16-hour fast, 8-hour feeding) variation works well. It’s realistic and sustainable.

To sum it up, the research legitimately shows the following positive improvements:

  • Improved insulin sensitivity
  • Improved glucose tolerance
  • Weight loss without muscle loss
  • Improved cholesterol ratios
  • Improved triglyceride levels
  • A drop in systolic blood pressure
  • Reduction in risk for coronary artery disease
  • Preservation of lean body mass (muscle)
  • Protects against age-related fibrosis by reducing inflammation and oxidative damage

So, if you ask me, give it a try.

Intermittent Fasting FAQs

Is intermittent fasting healthy?

Yes.

Is intermittent fasting easy?

If you’re disciplined, yes. And it saves money.

Can intermittent fasting make you sick?

Not likely once you’ve acclimated.

When intermittent fasting, can I drink coffee?

If you already habitually drink coffee, yes. I would not add it if you do not currently drink coffee. One drawback is the shakiness that may occur on an empty stomach. Exercise during this time can cut this problem.

Why intermittent fasting is popular?

First, it’s made intriguing by social media. And second, if applied correctly, it works well.

What intermittent fasting should I do?

Alternate day 16:8 seems to be the most effective and realistic.

Who should do intermittent fasting?

Anyone who wants to improve health or lose weight in the form of fat.

Is intermittent fasting keto?

No, but they work well together.

Is intermittent fasting anti-aging?

Going out on a limb here, but yes, based on the research.

Can you drink water while fasting?

You bet you can and I encourage drinking water while fasting.

Which intermittent fasting method works best?

Alternate day 16/8 seems to be the most effective and realistic.

Intermittent fasting for women over 50?

I can’t answer this without research. I believe the differences seen between men and women are sex hormone based. So, this one needs to be researched by academia.

Again, any grad students up for some amazing research?

Intermittent fasting vs ketosis?

Both are viable and effective. They work well together too.

Can intermittent fasting help you build muscle?

I can’t answer from experience. But I can say that muscle seems to be preserved better than when just restricting calories. I’ll let you know after a few months of trying my variation.

Question for You?

What have you experienced with intermittent fasting?

Are you more concerned with the weight loss or health benefits?

Let me know in the comments below.

References on Intermittent Fasting

Anton J. Carlson, Frederick Hoelzel; Apparent Prolongation of the Life Span of Rats by Intermittent Fasting: One Figure, The Journal of Nutrition, Volume 31, Issue 3, 1 March 1946, Pages 363–375

Laura Castello, et al., Alternate-day fasting protects the rat heart against age-induced inflammation and fibrosis by inhibiting oxidative damage and NF-kB activation, Free Radical Biology and Medicine, Volume 48, Issue 1, 1 January 2010, Pages 47-54.

Surabhi Bhutani, Monica C. Klempel, Cynthia M. Kroeger, John F. Trepanowski, Krista A. Varad, Alternate-day fasting, and endurance exercise combine to reduce body weight and favorably alter plasma lipids in obese humans, Obesity, A Research Journal, Volume 21, Issue 7, July 2013, Pages 1370-1379

Samira Eshghinia and Fatemeh Mohammadzadeh, The effects of modified alternate-day fasting diet on weight loss and CAD risk factors in overweight and obese women, Journal of Diabetes & Metabolic Disorders 2013 12:4

Guillaume Fond, Alexandra Macgregor, Marion Leboyer, Andreas Michalsen, Fasting in mood disorders: neurobiology and effectiveness. A review of the literature, Psychiatry Research, Volume 209, Issue 3, 30 October 2013, Pages 253-258.

Nils Halberg, Morten Henriksen, Nathalie Söderhamn, Bente Stallknecht, Thorkil Ploug, Peter Schjerling, and Flemming Dela, Effect of intermittent fasting and refeeding on insulin action in healthy men, Journal of Applied Physiology, Volume 99, Issue 6 December 2005, Pages 2128-2136.

M N Harvie, M Pegington, M P Mattson, J Frystyk, B Dillon, G Evans, J Cuzick, S A Jebb, B Martin, R G Cutler, T G Son, S Maudsley, O D Carlson, J M Egan, A Flyvbjerg & A Howell, 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 volume 35, pages 714–727 (2011).

Michelle Harvie, Claire Wright, Mary Pegington, Debbie McMullan, et al. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women, British Journal of Nutrition, Volume 110, Issue 8, 28 October 2013, pp. 1534-1547.

Leonie K. Heilbronn, Anthony E. Civitarese, Iwona Bogacka, Steven R. Smith, Matthew Hulver, Eric Ravussin, Glucose Tolerance, and Skeletal Muscle Gene Expression in Response to Alternate Day Fasting, Obesity A Research Journal, Volume 13, Issue 3, March 2005. Pages 574-581.

Benjamin D Horne, Joseph B Muhlestein, Jeffrey L Anderson; Health effects of intermittent fasting: hormesis or harm? A systematic review, The American Journal of Clinical Nutrition, Volume 102, Issue 2, 1 August 2015, Pages 464–470.

A Johnstone, Fasting for weight loss: an effective strategy or latest dieting trend? International Journal of Obesity volume 39, pages 727–733 (2015).

Monica C.Klempel, Monica C.Cynthia M.Kroeger, Krista A.Varady, Alternate day fasting (ADF) with a high-fat diet produces similar weight loss and cardio-protection as ADF with a low-fat diet, Metabolism Volume 62, Issue 1, January 2013, Pages 137-143.

Donald E. Mager, Ruiqian Wan, Martin Brown, Aiwu Cheng, Przemyslaw Wareski, Darrell R. Abernethy, and Mark P. Mattson, Caloric restriction and intermittent fasting alter spectral measures of heart rate and blood pressure variability in rats, The FASEB Journal 2006 20:6, 631-637

Mark P. Mattson & RuiqianWan, Beneficial effects of intermittent fasting and caloric restriction on the cardiovascular and cerebrovascular systems, The Journal of Nutritional Biochemistry, Volume 16, Issue 3, March 2005, Pages 129-137.

Mark P. Mattson Valter, D.Longo, Michelle Harvie, Impact of intermittent fasting on health and disease processes, Ageing Research Reviews, Volume 39, October 2017, Pages 46-58.

R. Michael Anson, Zhihong Guo, Rafael de Cabo, Titilola Iyun, Michelle Rios, Adrienne Hagepanos, Donald K. Ingram, Mark A. Lane, Mark P.Mattson, Intermittent fasting dissociates beneficial effects of dietary restriction on glucose metabolism and neuronal resistance to injury from calorie intake, Proceedings of the National Academy of Sciences May 2003, 100 (10) 6216-6220

Grant M. Tinsley, Paul M. La Bounty; Effects of intermittent fasting on body composition and clinical health markers in humans, Nutrition Reviews, Volume 73, Issue 10, 1 October 2015, Pages 661–674.

Rob M. van Dam, Wilrike J. Pasman, Petra Verhoef, Effects of Coffee Consumption on Fasting Blood Glucose and Insulin Concentrations, Diabetes Care Dec 2004, 27 (12) 2990-2992.

Varady, K. A. (2011), Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?. Obesity Reviews, 12: e593-e601

Krista A Varady, Surabhi Bhutani, Monica C Klempel, Cynthia M Kroeger, John F Trepanowski, Jacob M Haus, Kristin K Hoddy and Yolian Calvo, Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial, Nutrition Journal 201312:146.

Krista A Varady, Surabhi Bhutani, Emily C Church, Monica C Klempel; Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults, The American Journal of Clinical Nutrition, Volume 90, Issue 5, 1 November 2009, Pages 1138–1143.