Intermittent Fasting 101: All You Need to Know [The Definitive Guide]

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…

Intermittent Fasting 101

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 101 begins with understanding what it is.

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

• 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 intermittent 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.

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

Does Intermittent Fasting Get Results?

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 from intermittent fasting.

How 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. Do 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?

You, absolutely sleep counts as fasting.

In fact, sleeping 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.

Nonetheless, you burn more fat during sleep and you’re not eating during that time. So indeed, sleep counts as fasting time if you are intermittent fasting.

What Are the Expected Intermittent Fasting Results?

Intermittent fasting results are 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 Intermittent 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 Intermittent 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 Intermittent 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 Intermitent 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 of 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.

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

Why?

Because it’s realistic with many people’s schedules 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 keto meal plan on Monday, Tuesday, and Thursday.
• 16 hour fast into Wednesday, Friday, and Sunday.
• I follow a normal keto meal plan from approximately 1 PM onward on fast days.
• On Saturday, I follow a normal keto 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 Intermittent Fasting Keto Combo?

Combining the two diets for an intermittent fasting 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 an intermittent fasting keto meal plan.

Even during the 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. Nonetheless, low carb intermittent fasting might have its place in someone’s arsenal.

If you’re more concerned about intermittent fasting vs keto and looking for some direction, read on.

My suggestion is to start with intermittent fasting since it’s easier to maintain than a keto-based diet. Besides, the improvements in glucose tolerance will improve your health more than being in a ketogenic state.

But, if you’re looking for rapid weight loss, a keto diet will work faster. Personally, I don’t think it is sustainable and leads to a relapse of weight gain afterward.

Intermittent fasting is a more long term solution.

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 for women were positive.

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

Intermittent Fasting for Men

Intermittent fasting for 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

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.

Intermittent fasting and testosterone

I had to do a literature review to see how intermittent fasting affects testosterone levels. I was surprised to find that the jury’s still out regarding calorie restriction and testosterone levels.

Regardless, by incorporating strength training in one’s regimen while intermittent fasting, testosterone levels will remain at peak.

But it’s imperative that during your fed state, you consume enough good fat to keep your testosterone levels up. Testosterone is made from cholesterol, which we make from fat sources. If it’s too low, our sex hormones will also be too low.

This is one more reason I can’t stress enough how important it is to eat a well-balanced diet during your “feeding window.”

Intermittent fasting and diabetes

Intermittent fasting is a promising weapon in the arsenals for diabetics.

Despite the effort required to intermittent fast for diabetics, it’s worth the outcome.

It can be as or more effective than some medications.

Physicians who deal with diabetes should take the time to educate themselves about Intermittent fasting. Especially if a patient begins an IF protocol without modifying their medication.

Vegan intermittent fasting

There’s no difference in the outcome of intermittent fasting if you’re a vegan. The only critical aspect is a minimized timeframe to obtain enough protein and nutrients within your “feeding window.”

Anyone who is well versed in vegetarianism can design a dietary regimen that allows for enough protein within a shorter time. It just means you have to be more aware and critical of your food choices.

Black coffee intermittent fasting

If you already consume coffee, drinking it during your fast will not cause any issues.

In fact, some studies show that black coffee improves glucose tolerance. This is a good thing!

So, bottom line, if you’re fasting, it’s morning and you have two hours of fasting left, feel free to have some black coffee. You won’t “screw things up.”

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.

Is intermittent fasting safe?

You bet it is! In fact, because of the improvement in glucose tolerance, I recommend intermittent fasting.

If you start your fasting window well before bedtime, you’ll also sleep better. This is a huge benefit in so many ways.

Our metabolism works so well at keeping us alive under either a feast or famine circumstances, you have nothing to worry about.

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 intermittent fasting (16-hour fast, 8-hour feeding) variation works well. It’s realistic and sustainable.

We can take this one step further and do a 6-day version of the 16:8 style. Use the seventh day as a break day to eat the foods you enjoy in moderation. This is the method I’ve tried for almost a year now with outstanding results (lean with no muscle loss). And I feel darn good!

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.

17 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 intermittent fasting 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.

What can you drink during intermittent fasting?

Water, unsweetened tea or coffee, or any unsweetened seltzer water is fine. Zero calories and no artificial sweeteners is the key.

Can you chew gum while intermittent fasting?

Yes, as long as it’s sugar-free. The amount of artificial sweeteners is negligible, so it has little to no effect.

Green tea intermittent fasting?

As mentioned previously, unsweetened green tea is fine and will not disrupt your fast.

Can you drink alcohol intermittent fasting?

No. Alcohol will break your fast.

Question for You?

What have you experienced with intermittent fasting?

Are you more concerned with weight loss or health benefits?

Let me know in the comments below.

References on Intermittent Fasting

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