Intermittent Fasting: Benefits, Drawbacks and Important Considerations

Intermittent Fasting Benefits Drawbacks and Considerations

Intermittent Fasting: Benefits, Drawbacks and Important Considerations

By: Julie Mancuso, founder of JM Nutrition, nutritional counselling service by registered nutritionists and dietitians. Learn more about JM Nutrition


In this post:

  • What is intermittent fasting and how does it work?

  • Common fasting methods or schedules

  • What are the benefits of intermittent fasting?

  • What are the drawbacks of intermittent fasting?

  • Important considerations of intermittent fasting

  • What you should eat when doing intermittent fasting

  • Other tips


Humans have been fasting for years. Hunter-gatherer societies had to adapt to fasting for one simple reason: food was scarce. For centuries, intermittent fasting has also formed a part of many cultures and religions. Reason being, intermittent fasting benefits are many and varied. Even medicine has turned to fasting in various forms, with intermittent fasting schedules playing a role.

Given this historical popularity, does intermittent fasting work? Should you give it a try?

Let’s take a look:


What is intermittent fasting?

Intermittent fasting is cycling between eating and fasting. In other words, you fast or abstain from eating for a set period of time, eat during a specific timeframe, and then fast once again (John Hopkins Medicine).


Common intermittent fasting schedules

Various fasting methods or schedules exist. Here a few common ones:


Intermittent fasting schedule: 12-8 fast

The 12-8 fast calls for the consumption of food only between 12p.m. and 8p.m., effectively creating a 16-hour window of fasting.

Other variations of the 12-8 fasting model exist. There is the 9-6 fasting method where food consumption only takes place between 9a.m. and 5 p.m. and the 10-6 fast where eating occurs between the hours of 10 in the morning and 6 in the evening. Both of these still adhere to the 16-hour fast rule.


Intermittent fasting schedule: 5:2 fast

The 5:2 method calls for a restriction of calories to about 500-600, 2 days per week.


Intermittent fasting schedule: 24-hour fast

This fast naturally requires a 24-hour fast once or twice per week. In other words, you eat only 1 meal every 24 hours, once or twice per week.


Intermittent fasting benefits


Weight loss

Fasting has a number of benefits, not the least of which is weight loss. By simply reducing caloric intake during the fasting periods and cumulatively throughout the week, many people tend to lose weight. This is probably the most common benefit of intermittent fasting.

Related: Nutritionists and dietitians for weight loss

According to Harvard Health, “there is evidence to suggest that the circadian rhythm fasting approach, where meals are restricted to an eight- to ten-hour period of the daytime”, is effective.


Insulin and blood sugar levels drop

There are other benefits of intermittent fasting.

Taking extended breaks from eating result in the insulin levels going down, observed Dr. Jason Fung in The Obesity Code. When this occurs, the fat cells release the stored sugar, which is used as energy. This contributes greatly to weight loss.

With lowered insulin levels, blood sugar levels improve, helping those stricken with type 2 diabetes.

Related: Registered dietitian for diabetes


Reduction of insulin resistance

Speaking of insulin, intermittent fasting has been shown to reduce insulin resistance, effectively lowering the risk of type 2 diabetes. This is a very important finding and a significant intermittent fasting benefit.


Metabolic switching

Furthermore, during the fasting stage people slowly burn through the glucose stored in the liver, according to Mark Mattson, a neuroscientist with Johns Hopkins Medicine.

Because it takes 10-12 hours to use this stored energy in the liver, beyond this time, our bodies use fats for energy. Burned fats often lead to lost weight–precisely the reason why intermittent fasting works. Mattson calls this “metabolic switching”. Typically, he continues, the 3-meals-per-day eating pattern found in many societies does not allow for this fat burning to occur.

Related: Metabolism and Nutrition


Resting of the digestive system

Intermittent fasting can also assist those afflicted with digestive distress. Abstaining from food for an extended period of time allows the digestive system to rest, recuperate and repair because it’s not continuously working and straining.


Hard to Digest Foods: Are You Ready For Some Surprises?

Gut Health 101: Digestive Health Basics

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Reduced inflammation

In addition, when we fast, we take a break from eating and this includes the consumption of various inflammatory foods that may otherwise find their way into our daily diet:

  • sugar,
  • saturated fats,
  • trans fats,
  • refined carbohydrates,
  • alcohol
  • and others.


Drawbacks of intermittent fasting

It’s not all roses in the fasting land. Intermittent fasting does have a handful of disadvantages that require some examination.


Not for disordered eaters

Intermittent fasting is not recommended for those who are suffering from disordered eating patterns or eating disorders such as binge eating disorder, anorexia, bulimia, ARFID et al. Further restrictions that accompany fasting schedules may potentially wreak havoc on sufferers mentally and physically, exacerbating an already volatile situation. We recommend avoiding such stress.

Related: Stress relief with food and supplements.


Fasting can lead to food overindulgences

Some people tend to follow a fast with food indulgences, often unhealthy ones. They feel they earn treats and sweets by fasting for many hours. Others may follow a fast with a feast and overeat food, period.

The end result being weight gain or the inability to lose weight which, for many, is one of the primary reasons for attempting intermittent fasting. Overindulgences such as these are counterproductive and can lead to swift disillusionment with fasting, leading to abandonment.

Related: Common Weight Loss Obstacles


Social barrier

Intermittent fasting can also be a social hindrance, particularly for those who frequent restaurants in the evenings or stay out late into the night at bars and clubs.

Because these social activities take place during the fasting hours of the most common fasting methods, they can be hard to stick to in the long-term. Attending such functions may be an enormous challenge because food restrictions are difficult to maintain when we are compelled to watch others enjoying their meals without restraint.


Fasting and stress

A recent study showed that, for some, restricting calories can result in increased levels of cortisol–the body’s stress hormone.

Learn more about foods for stress management.

This can be particularly harmful to those who already feel crippling levels of stress in their lives.

Related: How To Balance Hormones Naturally


Important considerations of intermittent fasting


Not for people with certain health conditions

It’s important to exercise caution when attempting intermittent fasting because severe caloric or food reductions can be dangerous for those with heart problems or diabetes, amongst others.

As a safeguard, it’s wise to check in with a doctor and a nutritionist or dietitian before undertaking such drastic changes to one’s eating habits.


Fasting for women

Women should avoid overextending the fast past 14 hours because doing so may negatively affect hormones, potentially causing a hormonal imbalance. Generally speaking, men should do fine with a longer fast of 16 hours.

Learn more about Intermittent Fasting For Women.


Does not fit everyone’s schedule

The 12p.m.-8p.m. eating window does not suit the schedule of some. This is fine and the timing of the fast can be adjusted accordingly, so long as there is a sufficient fasting period at some point.


Avoid inflammatory foods during eating periods

According to our dietitians and nutritionists who specialize in anti-inflammatory diets, an equally important consideration is to avoid negating the benefits of the fast with eating periods that consist of unhealthy and inflammatory foods. The eating stage needs to have healthy, nutrient-dense foods to fuel your body properly in order for intermittent fasting to be sustainable. 


Endurance athletes beware

Fasting can also have a negative impact on endurance athletes who typically require sufficient fuel to be able to perform highly strenuous activities, according to our fitness and sports dietitian.

Related: Strength Training Diet Myths and Misconceptions

For example, an endurance athlete who fasts from 10a.m. to 6p.m. may not be properly fuelled for the following morning’s long distance run, after such an extended period without eating.

This is why if you’re going to do intermittent fasting, it’s best to tailor it to your own schedule.

If you need help navigating this area, get in touch with our nutritionists who work with long-distance runners and cyclists


Fasting and children

Although many adults can and do benefit from fasting, children may not fall under the same umbrella.

Since studies in this area are virtually non-existent at the time of writing, our pediatric dieititians raise a couple of questions.

One, will children who fast be less able to concentrate at school? Two, do children need food more frequently than adults on account of their higher energy demands of growth?

Perhaps these questions will be answered in future studies.


Beware if you have low body weight

In addition, fasting could be detrimental for those who have a dangerously low body weight and can ill afford food restrictions. Doing so can worsen the existing health condition.


It takes time to adapt

Another point to keep in mind is that the intermittent fasting schedule might take a while to adapt. You may not be comfortable, especially at first, potentially leading to doubt. As with any other significant change, it takes time to get accustomed to new ways. It is commonly believed that it takes around 4 weeks to break a deeply-entrenched habit.


Hard to break old habits

No matter which intermittent method you choose or which schedule you follow, the ingrained eating habits are hard to shed, especially when we’re bombarded by cues that urge us to eat: advertisements on restaurant windows, treats in the staff room and the general availability of and easy access to food everywhere we turn.

It takes a strong will to be able to resist many of the temptations that beckon.


Can lead to guilt

Some people experience a great deal of guilt when they fail to adhere to the strict regimen of fasting and end up breaking it. Having failed, feelings of shame can be overwhelming for some, leading to self-laceration. 

Related: Intuitive Eating: Principles, Challenges and Important Considerations 

Dietitian for brain and mental health


Can make you hungry, if not fuelled properly

The last consideration is perhaps the most obvious.

Fasting can certainly lead to hunger and weakness, especially at the onset when the body is adjusting. These feelings can be particularly intense if the foods consumed during the eating periods are devoid of essential nutrients. Fuelling your body properly and consistently is key.

For this reason, it is vital for anyone attempting to undertake intermittent fasting to consult a nutritionist or dietitian to avoid deficiencies, and in turn, hunger.


What to eat for intermittent fasting

Okay, so you decided to try intermittent fasting, but you’re keen on doing it in a healthy, nutritionally-balanced way.

So what should you eat?

Here’s a brief overview:


1. Drink water throughout the day, and plenty of it. Water is essential for the body, particularly when you’re not eating. Discover how much water you should drink.

Related: Hydrating Foods For The Summer

2. Eat high fibre, high-energy foods: nuts, seeds, vegetables, salads, beans, etc.

3. Include lean sources of protein: chicken, turkey, fish, eggs.

4. Consume good fats: avocado, olive oil, again–nuts and seeds, and so on.

5. Turn to whole grain sprouted breads and pastas, brown rice, quinoa, et al.


Other tips

  • Make sure you learn about proper portions and serving sizes of foods. This will go a long way in helping you avoid overeating, even when you think you’re not overeating.


  • What you don’t eat sometimes matters more than what you eat. Eat as healthily as possible.


  • Avoid processed foods as much as you can: they are stripped of nutrients, often high in sodium and/or sugar, and loaded with additives and preservatives.


  • Find ways to relieve tension and stress to avoid breaking the fast with an impulsive, stress-induced reach for an unhealthy but highly palatable snack. Related: Emotional eating dietitians


  • When eating, stop when you no longer feel hungry rather than when you’re full.




Keep in mind that if you’re going to try intermittent fasting, find a schedule that works for you. Don’t fret about following a suggested cookie-cutter schedule just because it worked for your colleague at work.

Personalize by adapting it to your own lifestyle because that’s the only way it’ll work for you.

If you require assistance in this area or any other, for that matter, contact us for an appointment or book a free consultation. We offer in-person appointments, as well as the services of online dietitians.


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Author: Julie Mancuso

Julie Mancuso

Julie Mancuso is a graduate of the University of Toronto, founder and owner of JM Nutrition, a nutritional counselling service by registered dietitians and nutritionists. For 15+ years, JM Nutrition has helped thousands reach their health, wellness and nutrition goals. Julie and her team regularly lend their expertise to a variety of health publications such as Reader's Digest, Livestrong, Business Insider, Food Network, Today's Parent, MyFitnessPal, Toronto Star, Elle Magazine, Best Life, Weight Watchers and many more.