Everything You Need To Know About Intermittent Fasting

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Intermittent Fasting has become hugely popular in the last few years. You may have heard that Intermittent Fasting can help you lose weight, improve health markers like cholesterol and blood sugar, and even extend your life. With those kind of proposed benefits, it’s worth investigating isn’t it?

I’m a big supporter of the concept of individuality, when it comes to both nutrition and exercise. What’s right for one person isn’t right for another. Your optimal nutrition plan depends on your genetics, psychology, and personal preferences. There’s no one diet that’s right for everyone. Whenever I talk nutrition, I try to break it down into that vital question, “Is it right for YOU?”

While it appears that Intermittent Fasting may have benefits, deciding whether this style of eating is right for you is what this article is really about.

What Is Intermittent Fasting?


Intermittent Fasting (IF) is a way of eating that involves defined periods of eating and fasting (not eating).

We fast every day (when we’re sleeping), but IF means extending that period a little wider. What generally marks IF is that the periods of fasting are greater than 12 hours (the usual eating pattern in modern western society).

Three Main Types Of Intermittent Fasting:

24hr Fasting – Skipping a day of food, once or twice a week. This is the traditional form of fasting, often associated with some religious practices. This is also advocated by the Eat-Stop-Eat program.

Alternate Day Fasting – You eat for a period of time (often 24hrs), fast for a period of time (24hrs) and then keep repeating the cycle.

Feeding Window (Time Restricted Feeding TRF) – Eating within a certain time window ONLY, generally between 4-8 hours.

The times are variable, but in practice most people skip breakfast and then eat between noon-8pm or 11am-7pm. That’s actually fairly doable for most people and is dubbed the 16:8 Diet. It’s also the schedule recommended by the Leangains program.

Limiting eating to a 4-hr period normally means from 2-6pm or 3-7pm, which is a bit more of a challenge. This is the IF system recommended by The Warrior Diet and normally involves eating just one large meal a day.

Bulletproof IF is a Feeding Window system with the addition of coffee and butter in the morning. The coffee and butter just make it a bit easier to get through the fasting period of the morning that’s required of IF. 

If one of your friends tells you they’re doing IF, chances are they’re doing some form of the Feeding Window system, which seems to be most popular.

How Intermittent Fasting Works


What Happens When You Eat

Every time you eat, your food gets broken down into various molecules for your cells to use. These molecules are released into your bloodstream. At the same time, insulin is released in order to shuttle nutrients into your cells. Depending on what you eat, and how much, your insulin levels can remain elevated for several hours (somewhere between 3 and 8 hours).

When your body is digesting and absorbing what you’ve eaten, it’s in a postprandial state.

When it’s finished absorbing the meal, your insulin levels will drop to their baseline level and your body is in a fasted state.

Intermittent Fasting aims to extend the amount of time you spend in a fasted state compared to a postprandial (fed) state.


Intermittent Fasting triggers a physiological process called autophagy, which is a natural, regulated process through which the body’s cells get rid of defective parts. Autophagy is important in maintaining muscle mass and is the primary mechanism behind the anti-aging effects of calorie restriction (more on that later!).

Fasting gives your body a chance to perform autophagy, a way to “clean house” and make sure that cells are functioning optimally. The more often you fast, the more opportunity your body has to keep things in order. This could theoretically help extend your lifespan and reduce your risk of certain cancers.

The Way We Used To Eat?

One of the ideas behind Intermittent Fasting, as with Paleo and Primal eating plans, is that it’s closer to traditional human eating patterns than what we currently do in modern society. Cavemen would hunt for animals and forage for vegetation, then have a big meal of deer or sabre toothed tiger with a side of berries and call it a day.

Even so, these behaviours would have been driven by necessity and are not necessarily optimal for modern humans. Just because early humans had to go a day or two without eating doesn’t mean that it’s the right way for you to get through your busy workday. We’ve changed a lot since those days (Living longer, not dying in childbirth, flushing toilets, smartphones) and what worked way back then doesn’t always work well now.

Health Benefits Of Intermittent Fasting


What The Evidence Says

Fasting has been shown in studies to improve a wide range of age-related disorders including diabetes, cardiovascular disease, cancers and neurological disorders such as Alzheimer’s disease, Parkinson’s disease, and stroke. The majority of these studies were performed on rats though, and the human evidence is still pretty scant. Time restricted feeding in normal and overweight human subjects has shown some potential  for weight loss, improvements in insulin resistance, and reductions in risk factors for cardiovascular disease.

Periodic fasting for religious purposes (Ramadan) was found in one study to reduce inflammation and improve risk factors for cardiovascular diseases (homocysteine, CRP, TC/HDL ratio).

A small recent study found that Intermittent Fasting improved insulin levels, insulin sensitivity, β cell responsiveness, blood pressure, and oxidative stress levels in men with prediabetes, even though food intake was matched to the control arm and no weight loss occurred. This study was only 5 weeks long though and consisted of only 8 men, but it does give direction for further research.

While some small studies show interesting results, the largest systemic review of Intermittent Fasting was conducted at The University of Sydney in Australia. After analyzing 40 studies on Intermittent Fasting (with 12 comparing it directly to traditional dieting), researchers found no significant benefits related to body composition, fat loss, insulin sensitivity, or hormonal balance.


Cardiovascular Benefits

Intermittent Fasting has been shown to increase insulin sensitivity, reduce abnormal fat accumulation, and reduce the incidence of stroke and diabetes…in rats. Clinical studies in humans began around 2005 and most of that evidence suggests that the benefits of IF are mostly or completely the result of weight loss.

So it doesn’t matter how you lose the weight, as long as you do.

Any diet that allows you to lose weight will improve insulin sensitivity, reduce fat levels, and reduce cardiovascular risk. Most human evidence so far seems to show that Intermittent Fasting provides only the benefits of the weight loss it provides. Meaning if you lose weight another way, like with conventional calorie reduction, you’ll be equally as successful.

The American Heart Association has a position statement related to Intermittent Fasting and they concluded that fasting may be effective for weight loss. IF can decrease fasting insulin and reduce insulin resistance and may lower blood pressure when weight loss of at least 6% occurs.

Calorie Restriction & Long Life


Calorie Restriction, the ongoing reduction of calories while taking in essential nutrients, has been shown to extend lifespan in many species. A recent study looked at calorie restriction in humans for a period of two years. Restricting calories (in this case, by 15%) showed evidence of persistent metabolic slowing with reduced oxidative stress, a marker of aging. Fairly promising stuff on the anti-aging and increased longevity front, but not everyone wants to be on a lower calorie diet for long periods just to get a few extra years of life (and even then, it’s not guaranteed).

Intermittent Fasting is another way to promote calorie restriction over time and both methods primarily show benefit through autophagy. Cleaning up your cells more frequently seems to make the body function better over time.

Does Intermittent Fasting Help Fat Loss?


Our primary goals in building a lean, athletic physique is to lose fat and build muscle.

Intermittent Fasting is one of many strategies that can help people lose fat. It primarily works because it makes calorie restriction easier and sets limits that reduce the quantity of food consumed. Less time spent eating generally means fewer calories consumed overall.

IF will help you with fat loss in as much as IF allows you to restrict your calorie intake. Most of the studies cherry-picked into articles promoting Intermittent Fasting tend to be on rodents and don’t often refer to IF itself, but calorie restriction in general.

The fat loss benefits attributed to Intermittent Fasting in studies tend to occur because of calorie reduction. 

Will You Lose Muscle With Intermittent Fasting?


In order to build and maintain muscle, you need to eat adequate protein and calories and do weight training that’s challenging enough to produce a training response.

The usual way of losing weight is to reduce your calorie consumption, generally by about the same amount every day. A review study compared the literature for a reduction in calories on a daily basis (traditional dieting) with alternate day fasting (eating 24hrs and fasting 24hrs). While the amount of weight lost and fat lost was similar, fasting was slightly more effective in maintaining lean mass. The study only looked at obese individuals so the same effects may not occur in people who are overweight or of normal weight.

An interesting study with resistance trained males showed that Intermittent Fasting could improve health-related biomarkers, decrease fat mass, and at least maintain muscle mass. Only 34 men were included in this study, however, so it’s not entirely convincing.

A lot of other things influence how much lean mass you retain while losing weight, including your protein intake, how you train (moderate-heavy weights provide more muscle retention than light weights), and your hormonal status. Based on the available evidence, if you’re able to meet your calorie and protein needs then IF shouldn’t effect your ability to retain muscle.

Who Probably Shouldn’t Do Intermittent Fasting

People With GERD/Acid Reflux

Most people with digestive issues like reflux (like me!), probably won’t benefit from having only one or two large meals each day. If you have GERD or just occasionally suffer from acid reflux, you’ll want to eat smaller meals more frequently to keep acid levels in check. Smaller meals also minimize pressure on the lower esophageal sphincter, which prevents acid from coming up from the stomach into the esophagus.

History of Eating Disorders

If you have a history of eating disorders like anorexia, bulimia, or binge eating, intermittent fasting might not be for you. It’s best to talk to a trained counsellor about making changes to your nutrition plan. She may not feel that Intermittent Fasting is right for you.


Pregnancy is one of those times when we’re supposed to avoid giving advice to women. Unless something has been investigated with large-scale placebo-controlled studies with pregnant women we’re supposed to err on the side of caution and tell pregnant women to avoid it. Since most foods or methods of eating haven’t been well investigated with this population (there just isn’t the time or money for all those potential studies!) a pregnant woman needs to use her own judgment. As for everyone, getting the right quantity and quality of nutrients is the most important thing. Eat heathy food whenever you’re hungry is probably the advice most doctors would give pregnant women. Seems sensible to me.

Skinny People

If you’re slim and looking to put on muscle, Intermittent Fasting probably isn’t for you. Putting on muscle size and strength requires a certain amount of calories and protein. You need to be in a calorie surplus in order to gain muscle and it’s harder to do that when you’re trying to squeeze everything into a couple of large meals each day or skipping food altogether every other day. If you want to bulk up, you need to eat a lot of food – frequently – and IF just makes that harder.

Diabetics Or Anyone With Blood Sugar Regulation Issues

Do you get dizzy and feel weak if you haven’t eaten for a while? Then Intermittent Fasting probably isn’t for you. Some people talk about having “low blood sugar” or being hypoglycemic, but true hypoglycemia can be a dangerous condition that occurs with diabetes or with diabetic medication (insulin). For some people, low blood sugar can cause serious side effects like clumsiness, confusion, and seizures.

If you have any issues with glycemic control, you might want to pass on Intermittent Fasting.

Psychological Benefits of Intermittent Fasting


Many people have a negative relationship with food. They have feelings of guilt or get anxious about eating certain foods or they use food to deal with stress or emotional upset. Others have a habit of compulsive constant eating, which means they’re always thinking about food and overly concerned that they’re not getting enough nutrients and/or protein to stay healthy or build muscle.

Some people will find that Intermittent Fasting gives them periodic breaks from thinking about food, which can be helpful. For other people, taking time away from food, whether it’s a few extra hours or an entire day, may create an intensified interest/obsession with food. Decide which of these people you are to determine whether IF is a good fit.

Will You Be Hungry?


How frequently you eat can have an impact on how hungry you are throughout the day. While the International Society of Sports Nutrition position stand states that more frequent meals can reduce hunger and improve appetite control, there’s evidence to the contrary as well. Some research shows that increasing meal frequency from 3 meals to 6 meals a day can lead to increased hunger.

That’s more evidence of individuality. Be aware of how eating more frequently affects YOU. If eating more regularly increases your appetite, eat less frequently (if you’re trying to lose weight). If eating less often increases your appetite then eat more frequently. Simple!

Personal Experiences With Intermittent Fasting

My Experience

As I mentioned, I suffer from acid reflux and large meals are not my friend. Smaller meals of non-acidic foods keep my heartburn symptoms under control. I have tried single day fasting a few times and I’ve had to fast for certain routine blood tests. One thing I certainly noticed was how much extra time I had when I didn’t have to think about eating! Certainly a useful benefit for those of us who are always busy.

Client Experiences

Some of my online coaching clients have tried Intermittent Fasting. Many have found IF works well at the beginning, especially if you have a lot of weight to lose. Once you get closer to your target weight, it’s harder to get the right amount of calories and protein into fewer meals.

My client Steve came to me for advice after he’d initially lost about 20 pounds with IF. He got stuck. I got him off IF, increased his protein (he wasn’t managing enough in two meals), and optimized his post-workout nutrition. He was training really hard five times a week and struggling to get leaner until he stopped IF. Adding more protein and calories was necessary because he was already lean and needed to tighten up his calories and macros to get leaner. If someone does IF and still meets all their protein and calorie needs, then it shouldn’t be harmful.

My husband Ryan, a former semi-professional rugby player who has been my client for nearly ten years, decided to try IF a couple of years ago. He did the “One Meal A Day” version within a two-hour window. While he enjoyed the free time he gained from not eating, he was ready to fall asleep most nights by 7pm! The large meal made him so tired that he couldn’t function in the evening.

Can You Stick To Intermittent Fasting?

If you’re the type of person who makes up ravenous then Intermittent Fasting probably won’t be for you. Why torture yourself if you enjoy waking up and eating a satisfying meal? It may very well help you make better decisions later in the day if you eat a good breakfast with enough protein and fibre.

If you don’t like eating early then you may already be skipping breakfast and eating a bit later. That’s fine as long as the choices that you make later on in the day are sensible and provide you with the right amount of calories and the ideal balance of macros (protein, carbs, fat) for you.

It’s a rare person who skips meals all day and then eats the right quality and quantity of food in the evening. In fact, that’s one of the most common patterns that I see among people who are overweight and struggling.

Getting Enough Protein For Muscle Growth

If you want to maximize your muscle mass, it isn’t just the total protein you get each day, but how you consume it. Although this study is on rodents, it appears to show that distributing your protein throughout the day, rather than consuming it all at once in one meal is better for optimizing muscle mass.

If you’re still committed to doing Intermittent Fasting, you can restrict your carbs and fats during the fasting periods and just eat your protein to ensure you’re still supporting muscle growth.

What Really Works

The most important factor determining success with a diet is how well you can stick to it. Success with fat loss/weight loss comes down to dietary adherence. It’s one of the reasons that Clean Eating is so hard for most people. We all want a few treats once in a while…and maybe not just a once a week cheat meal!

Contrary to popular belief, you don’t need to eat frequent meals to stoke your metabolism. Most research seems to indicate that the number of meals you eat each day (1-2 compared to 5-8) does not effect your metabolic rate. It also doesn’t mean that fewer meals is superior…only that it doesn’t matter much. So whether you use IF and only eat a few meals or eat six meals a day, it doesn’t seem to affect your metabolic rate.

The Bottom Line

In terms of fat loss, IF can be a strategy that helps people lose weight. It can work because it makes calorie restriction easier and sets limits that reduce the amount of food consumed. That’s why most diets work. They make it easier for you to reduce your calorie consumption and then you lose weight.

Is Intermittent Fasting optimal for putting on muscle mass and staying lean for someone who’s already in good shape?

Less likely.

No matter how many anecdotes you hear from people who say that they did a particular diet (whether it’s IF, The Zone, Vegan, Paleo, The Dukan Diet, 16:8), it doesn’t really count in the scientific world. A case study (or several case studies), which is one person’s experience with an intervention, is the lowest form of research.

When other people are REALLY, REALLY excited about something, it’s hard not to get caught up in the hype. This explains the success of CrossFit, Paleo, Clean Eating, and most cults. Step back, look at the actual evidence carefully, and see if it makes sense to you.

If you’re one of the individuals who functions optimally on IF, great! Keep doing what you’re doing. Just don’t try to convince everyone else that it’s the best thing for them…because it might not be.

Ivana Chapman

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Ivana Chapman

Ivana Chapman

Ivana Chapman BSc BA CSCS is a Canadian fitness and nutrition coach, happy wife, and mom to an energetic 8-year-old boy. She is a YouTuber, writer, published fitness model, speaker, 3rd Dan black belt in Shotokan Karate, former World Cup Karate Champion, one-time marathoner, and CBBF National level Natural Bikini competitor. She loves weight training, chocolate, mountain biking, and ice cream...not always in that order of preference.
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