Medically reviewed by Shahzadi Devje, Registered Dietitian (RD) & Certified Diabetes Educator (CDE)
Fasting is something that has been practiced for centuries. Many of us associate fasting with the sacred month of Ramadan and other religious traditions. But what is intermittent fasting (IF)? Why has this diet trend become so popular? What makes it different from other fad diets? Is it safe? But before you ask, how to do intermittent fasting, let’s talk through the claims (and more importantly the research), to shed some light on whether it’s worth trying.
What is Intermittent Fasting?The key difference between IF and other fad diets, is that IF focuses more on when to eat and less on what to eat (I’ll talk more on this later). When you are fasting, you’re abstaining from consuming some or all foods and beverages. IF simply means, you are are doing this for a defined period of time.
How To Do Intermittent Fasting?
Intermittent Fasting Protocols: What Are They?
IF is actually a broad term; used to describe a wide range of different eating patterns or “protocols”. All of these patterns involve some variation of alternating periods of fasting and eating, “ad libitum” (which in a literal sense means as much as you want).
Let’s break down the three most common types of intermittent fasting.
Alternate Day Fasting (ADF)
Alternate day fasting involves restricting your intake every other day. This means a full 24 hours of fasting, every other day of the week. In one version of ADF, known as modified ADF, you consume about 500-600 calories (or 25% of your daily calorie requirements) on fasting days – perhaps a little less daunting.
Whole Day Fasting
The most popular (and perhaps less-rigid) version of whole day fasting, is the “5:2 Diet”. Essentially, for two non-consecutive days of the week, you fast for a full 24 hours. This could also mean consuming little (about 25% of your daily calorie requirements or 500-600 calories) to no calories during this time period. For the remaining five days of the week, you eat as you usually would. This approach to IF, allows for a little more flexibility, because you choose to fast on the two days of week, that are most convenient for you - versus every other day. Another variation for some, is to practice whole day fasting - once per week (6:1, if you will).
This approach to IF, is a bit different from the other categories. In time-restricted feeding, there is a designated “feeding time” (usually less than 12 hours) each day. A common example of this is the 16/8 protocol. This involves fasting for 16 hours of the day - leaving an 8-hour window for eating.
In pretty much all of the IF protocols, calorie-free beverages (i.e. water, coffee, tea) are allowed. In fact, if intermittent fasting is something that you practice, drink plenty of water during your fast - to help prevent dehydration. It may also help (somewhat) to control hunger pains.
Why is It Popular?
IF has become popular because it’s an appealing (and less agonizing) alternative to conventional “continuous energy restriction” (aka, low calorie) diets. Some of us can relate to the struggle of following a low calorie diet for weight loss; they’re challenging and unsustainable. Low calorie diets require you to restrict your intake 24/7 (hello ‘hanger’). IF, on the other hand, is a way to reduce your total intake, by offering a window to eat “as usual”, at least some - if not most of the time.
There are heaps of claims out there about intermittent fasting. Proponents of this approach to eating, boast numerous benefits, including: weight loss, increased muscle mass, better blood sugar control, decreased inflammation, higher metabolism, reduced risk of certain chronic diseases (like diabetes and cardiovascular diseases) and even improved longevity.
Going extended periods of time without food sounds pretty appealing, right?!
But the important question, if you're considering how to do intermittent fasting, is whether the above claims hold up to scientific scrutiny. Let’s find out.
Is Intermittent Fasting Effective?
Can You Lose Weight With Intermittent Fasting?
If weight loss is something you’re after, the science suggests that IF is equally as effective as low calorie diets, at helping to shed a few pounds. Most studies on IF show some degree of weight loss.
But what’s more important, is whether or not, you are able to maintain this - in the long term.
When we adopt a new approach to eating - in hopes of losing weight, being able to sustain that change is key. I'd even go a step further, and argue that making positive health changes should be realistic, enjoyable and sustainable - to improve our quality of life.
Now back to the evidence...
In studies that compared low calorie diets and IF, dropout rates were similar - regardless of which diet type the study participants were following. This suggests that, although IF is thought to be a potentially more appealing approach to weight loss - relative to typical low calorie diets, the results, in terms of sustainability - may be similar. BUT, so far, studies of IF aren’t long enough, to tell us whether people are able to stick to the diet, and keep the weight off in the long run.
Is Intermittent Fasting Good For Fat Loss?
Several studies (here, here and here) have shown that IF can help to reduce body fat, body mass index and waist circumference to some degree. These measures don’t necessarily tell us a lot about changes to our lean muscle mass (something that folks tend to lose when following low calorie diets).
So, is it true that intermittent fasting can help to preserve lean muscle mass, when we lose weight (an ideal scenario)?
A 2015 review showed mixed results, of the impact of alternate-day fasting regimens on muscle mass. Some showed decreases, while others showed no change. Alternate reviews of the research suggest, that in weight loss trials, in individuals of higher weight, the evidence isn’t strong enough to indicate that IF is any better at preserving lean muscle than low calorie diets. Higher protein intakes, and adequate exercise (resistance training in particular), during IF have shown to help in the preservation of fat free "muscle" mass.
Does Intermittent Fasting "Kill" Muscle?
Why is muscle mass so important? Besides strength, stamina and mobility, preservation of fat free muscle mass helps to limit decreases in your metabolic rate. This is how fast your metabolism works and is measured in calories. Muscles help to burn calories (yup those muscles!).
Research suggests, our bodies adapt to reduced caloric intake, by decreasing resting metabolism. This is the amount of energy your body uses when it is in a resting state - to carry out basic life functions; like breathing, your heart beat, wound healing and processing nutrients. Muscle burns more calories than fat, so the more muscle you have - the higher your metabolic rate. And if you lose muscle, this can slow your metabolic rate; making it hard to lose weight, and easy to gain it back.
So, does IF help to prevent or reduce the changes in Resting Metabolic Rate (RMR)? There isn’t a lot of research on how intermittent fasting impacts RMR. Most of the research looks at RMR on non-fasting days of IF, typically showing decreases. But this could be different on fasting days.
We simply don’t have enough evidence to support the notion, that intermittent fasting is any better than low calorie diets, at preventing a reduction in our RMR.
Can Intermittent Fasting Improve Blood Sugar Control
Improved blood sugar control and sensitivity to insulin are key goals in the management of type 2 diabetes. We know, keeping blood sugars in check, can help reduce risk of diabetes-related complications (e.g. cardiovascular disease, neuropathy, nephropathy, retinopathy). And we also know, that for those carrying a little extra weight (especially in the tummy area), weight loss can help improve insulin sensitivity. So, there is a lot of interest into whether or not IF is an alternative (and more effective) approach to low calorie diets.
Research in rodents suggests that IF can improve glucose tolerance (your ability to handle glucose), and insulin sensitivity (how sensitive your body is to insulin). In the case of humans, this systematic review found, that IF had similar effects to low calorie diets, in improving blood sugar control.
Research on IF, in individuals living with type 2 diabetes, is very sparse. The research that does exist, suggests that those of higher weights - who also have type 2 diabetes, intermittent fasting can result in improvements in HbA1c measurements (indicator of longer-term blood sugar control) similar to those seen with conventional low calorie diets.
With that being said, with the limited research on intermittent fasting in type 2 diabetes, we do not have a comprehensive understanding of what is the safest and most effective intermittent fasting regimen for those with diabetes. If you have type 2 diabetes, and are taking antihyperglycemic medications and are interested to learn how to do IF, it is best to seek medical advice on how to manage your medication - to prevent adverse symptoms.
Remember, if you're using IF, as an approach to weight loss, ongoing medication adjustments will be required. First and foremost, if you're taking insulin or insulin secretagogues (i.e. sulfonylureas) and/or you have hypoglycemia unawareness, you should NOT try intermittent fasting - without appropriate ongoing medical advice and supervision. Intermittent fasting can increase risk of hypoglycemia.
You should also avoid IF, if you are pregnant or breastfeeding, planning to get pregnant, or have a history of disordered eating. There's insufficient evidence in the young, the elderly and individuals who are underweight. Thus, IF is not recommended in these groups, and may not be safe.
Does Intermittent Fasting Reduce Inflammation?
Chronic inflammation is known to increase the risk of various diseases and conditions such as heart disease, cancer and rheumatoid arthritis.
IF seems to show promise in this area; with improvements in inflammatory markers in asthma. Animal studies also show reductions in inflammatory markers (e.g. TNF-a, IL1-B, and IL6) and oxidative stress associated with intermittent fasting. Nonetheless, we still require more robust research in humans, in order to better understand the effects of intermittent fasting on inflammation - in specific disease states.
Can Intermittent Fasting Delay Ageing?
There's a lot of excitement surrounding intermittent fasting in helping to delay the aging process. This stems from animal studies (here and here). Despite the fact these studies show evidence of reductions in oxidative stress, improved longevity and resistance to diseases, linked to aging, we do not know what these results mean in the case of humans.
Limitations Of The Evidence
Despite some exciting and promising results, there are some issues with the current research, that limit our understanding of intermittent fasting.
First of all, there isn’t enough research. We can't say with enough confidence, that intermittent fasting leads to any of these potential benefits.
As well, many of the studies on intermittent fasting have included a small number of participants. This limits the strength of the results, and how much they apply in other settings and circumstances. Not to mention, some of the studies aren’t in humans. We agree, research in animals is important in informing future studies, but we can’t draw conclusions about the impact of intermittent fasting in humans from animal studies alone.
On the other hand, we don’t have a good understanding of what the best method of intermittent fasting is, and who would benefit from it the most. With so many different approaches to intermittent fasting, it makes it challenging to compare studies. Currently, there is more research on alternate day and whole day fasting than there is on time restricted feeding.
Finally (and perhaps most importantly), we don’t know the long-term impacts (potential benefits and harms) of intermittent fasting. Most of the existing studies have taken place over short time periods.
It’s important to remember, that if you want to learn how to do intermittent fasting - do not abandon basic healthy eating practices. Indeed, intermittent fasting is about timing of meals, but this doesn’t mean we should consider it as an all access pass to overindulge in foods that have little or no nutritional value, during our “feeding periods”. This would severely limit any intended benefit of intermittent fasting.
My Top 5 Takeaways
● Pick the best approach for your schedule. Like any lifestyle change, the best results are achieved (and maintained) when we stick to them long-term. Choosing an intermittent fasting protocol that’s well-suited to your schedule, internal hunger cues, and long-term nutritional requirements will help ensure greater success.
● Don’t abandon basic healthy eating principles. Although intermittent fasting places more emphasis on when to eat, it doesn’t mean we should neglect what we’re eating. Changing the timing of our meals won’t change the quality. Again, we’ll only serve to reduce any sort of benefit achieved through intermittent fasting, if we don’t also try to incorporate the basic principles of a balanced, eating pattern as well.
● Stay hydrated. Drinking plenty of water during fasting periods will help (to a degree) with feelings of hunger and help ensure you stay hydrated.
● Load your meals with fibre and protein. This will help you feel satisfied and keep your body fuelled for longer. Carry a snack with you in case you need to break your fast for any reason (e.g. feelings of extreme fatigue or low blood sugars).
● Be safe. Intermittent fasting is not for everyone, because it is not well-researched in a number of groups, and can impose risk (see above). If you have type 2 diabetes and are on insulin or are taking antihyperglycemic agents, you should seek medical advice before learning how to do intermittent fasting.
I'd love to hear from you? Are you looking to learn how to do intermittent fasting? Have you tried it before? How did you feel? Comment below!
By Alexandra Burghardt RD, BASc & Shahzadi Devje RD, CDE, MSc.
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Shahzadi is an award-winning registered dietitian (RD) regulated by the College of Dietitians of Ontario and certified diabetes educator (CDE), approved by the Canadian Diabetes Educator Certification Board. A YouTuber and notorious foodie, she's dedicated to helping you end your cooking wars, transform your health, and be the best version of yourself! Shahzadi is an on-air nutrition expert for CTV Your Morning and a regular contributor for Global News and other national media outlets.