Curious about intermittent fasting for weight loss benefits? We go through the evidence based pros and cons of this popular fad diet.
Intermittent fasting is among a variety of diets right now that has stood the test of time. Even though intermittent fasting has been around for a while, it has gained some momentum over the past year. We set out to review the most recent evidence on the topic. We’ll discuss whether intermittent fasting will yield the best results for weight loss and your health.
What is Intermittent fasting?
Intermittent Fasting involves a fast. It’s the “intermittent” part that sets this fast apart from common annual religious fasts. In most intermittent fasting diet regimes, you have a smaller time window when you can only eat (usually fewer than 8 hours), and you fast for the remainder of the day. These fasts are longer than a typical “overnight fast”, and range from 16 hours to a maximum of 1.5 days. These fasts are usually no more than 24 hours.
Intermittent Fasting was introduced as a more accessible diet. The typical Continuous Energy Restriction diet was becoming too difficult to follow because of its total restriction. With Intermittent Fasting, you are restricting food intake, but only on certain days. On other days you would have the freedom to eat and meet energy requirements. This diet is a more flexible approach to dieting.
Many have claimed that Intermittent Fasting diets that involve prolonging this fasted state have a multitude of health benefits such as improving glucose homeostasis, boosting energy, increasing growth hormone (GH) production, reducing inflammation decreasing oxidative stress, lowering triglyceride (TG) levels (here and here) and blood pressure, increasing and protecting brain function (here, here) increasing resistance to age-related diseases like immune disorders, cancer, heart disease, stroke, eye disease, Alzheimer’s (here, here) and promoting longevity!
are the claims on intermittent fasting benefits true?
Well, a lot of those claims have been made based on animal studies. And of course, although rodents like Ratatouille are clever ones, a rat’s body and a human’s body don’t work the same way, and it’s harder to conduct those studies on humans due to many influencing factors. The ones that do exist out there are pretty limited, but do show some exciting results that puts IF out there as a possible approach to benefiting human health as well! But it is important to mention that sometimes these studies had mixed results, so we can’t make super clear cut conclusions.
Types of Intermittent Fasting
Intermittent Fasting is seen as an umbrella term, because there are a variety of types of fasting. This also makes it difficult to study Intermittent Fasting because it’s harder to compare diets when they are executed in different ways.
Generally, Intermittent Fasting involves restricting calories 1-3 days a week. Then being able to freely eat on ad libitum days (days where you can eat as you wish or without restriction!).
The most popular Intermittent Fasting programs include: Alternate-day fasting, Whole-day fasting and Time-restricted feeding. Let’s take a look at each.
This type is the most-studied form of INTERMITTENT FASTING. This requires alternating between feeding and fasting days. On a typical fasting day, there is 1 meal at lunch that consists of approximately 25% of caloric needs based on the individual. This form of fasting involves “modified fasting”. This has different durations/periods of fasting from anywhere between 30-40 hours based upon the individual’s schedule.
2. Whole-day Fasting (here)
Compared to alternate-day fasting, this kind of fasting may seem “more extreme” – 1 to 2 days per week, you need to either severely restrict your calories or completely abstain from food. This is also called the “5:2 diet”. The 5 represents the number of ad libitum days you eat normally. The 2 represents the number of non-consecutive days you have to restrict calorie consumption to 25% (500-600 calories) of your total daily energy expenditure calories.
This is the most “lenient” type of Intermittent Fasting that requires you to fast for a specific number of hours each day. A very popular form of this time-restricted feeding program consists of an “under-eating” phase. This phase lasts for 20 hours, followed by an “overeating” phase lasting for 4 hours that takes place every 24-hour period.
Of course, it’s important to consider both the PROS and CONS of Intermittent Fasting. Let’s go through them together.
PROS of INTERMITTENT FASTING
1. PROMOTING Health & Weight Loss
Results in several human studies have found that alternate-day and whole-day Intermittent Fasting has been associated with a significant DECREASE in body weight, body fat, and waist circumference both short and long term, but has also been frequently observed in some time-restricted Intermittent Fasting studies.
In a 2016 systematic review, all of the studies reviewed experienced weight loss. However, they noted that individuals tended to lose MORE weight earlier on in the study compared to the final follow up point. This likely due to the study’s dropout rates. Is Intermittent Fasting a diet that can be sustained in the long term? We’ll discuss that in a bit.
On top of that, in a more recent 2017 systematic review and meta-analysis, weekly Intermittent Fasting interventions were JUST AS EFFECTIVE as Continuous Energy Restriction (CER) for weight loss. This proved that it was not necessary to starve yourself every single day. With a more flexible diet like Intermittent Fasting, you could yield the same weight loss results. One thing to keep in mind, is that many of these studies are short term so unclear whether they could keep the weight off.
reduces fat free mass
Diets that continually restrict calories, reduce body fat, but ALSO Fat Free Mass (FFM). Fat Free Mass is basically everything other than fat. This is no good, because that means you are losing lean muscle mass. However, studies have shown that with sufficient protein intake and resistance training, Intermittent Fasting may help to retain lose fat mass while RETAINING MORE of their lean mass (aka. our fat-burning lean muscle!) compared to daily calorie restriction-type of diets. These results are not proven in long term studies. You may also be thinking: wouldn’t it be difficult to exercise on fasting days. You may be right! We’ll discuss that in a bit.
Also, Intermittent Fasting may help improve the symptoms of individuals with asthma. It may help by lowering airway resistance, oxidative stress and inflammation.
Not to mention, researchers found improvements in insulin sensitivity and glucose homeostasis. In a 2017 systematic review, a few studies were able to report a reduction in percentage body fat and HbA1C compared to a continuous energy restriction diet. On top of that an Intermittent Fasting diet can lead to significant DECREASES in total cholesterol and LDL. Also a reduction in blood pressure and triglyceride levels. These are all important for preventing and reducing the risk of various diseases – notably, cardiovascular disease (in normal weight, overweight and obese individuals)! (here, here, here, here, here).
2. BRAVO to Increased Brain Functioning!
This is one of the common benefits of intermittent fasting. Studies have explored the powerful effects of this time-restricted diet on cognitive performance (such as memory). IF has been found to be beneficial especially for athletes whether they are exercising or at rest (here, here). A 2017 systematic review found that weight loss in general is associated with improvements in cognitive function.
3. No calorie restriction and No change in diet?!
That’s right! You can still eat the same number of daily calories and don’t have to take away or change the actual foods you eat. However, we believe you would reap MUCH BETTER results for your health with a whole-food, well-balanced diet from each of the 4 food groups.
4. It’s Simple.
This eating pattern is easily implemented and for those who like routine, it can be adhered to fairly easily. For some people, it may be easy to incorporate into your current routine. For example, did you know that the common “Time-restricted feeding” type of Intermittent Fasting is often unintentionally practiced by those who skip breakfast and do not eat after an early dinner each day?
5. Larger portions in a shorter period of time
Some people may like this part a lot because you get to consume more food at once. This would leave you more full and satisfied. In a way, Intermittent Fasting can actually prevent you from the typical binge on food.
CONS of intermittent fasting:
1. Interference with the SOCIAL aspect of Eating
Eating is very much a social activity. When you think about it, all of our celebrations, milestones and special occasions revolve around food. This new style of eating is very different from the typical daily eating patterns of most people. This is because of the shortened time frame you have for eating. It can be difficult for you in social gatherings where everyone else is eating and sipping on beverages, making you awkwardly stand out from the crowd.
Not to mention, you might be missing out on those late night romantic dinners, home-made family suppers, birthday dinners, lunch meetings with your boss and co-workers, and maybe even sharing a meal with your spouse and kids. Not so fun.
2. Getting HANGRY, Low in Energy & Unproductive
In a 2016 systematic review, a few studies found that some Intermittent Fasting participants experienced minor adverse physical ailments including: feeling cold, constipation, headaches, lack of energy, bad temper and lack of concentration. We might not even feel like we will have the energy or motivation to be active and do the things we actually like!
However in a 2017 review, a 12-week trial found that Intermittent Fasting did not appear to limit an individual’s ability to exercise. Again, this was a short term study, so whether you can maintain your exercise habits on Intermittent Fasting is still unclear. Studies on breakfast consumption have shown positive outcomes such as promoting cognitive and academic performance as well as weight-loss maintenance in various individuals. Getting enough fuel throughout the day can help keep us in good spirits, energetic and productive.
3. Feasting = BINGE!
Some people may take the “Feasting” periods as an opportunity to eat more calories than they really need. When you’re hungry, or you anticipate a period of fasting coming up, it can be very tempting to go hog wild at the first sight of food. If the fasting element in Intermittent Fasting were to create some sort of caloric deficit, it’s very possible that the feasting period easily undo it. Let’s also remember that the foods we choose to eat can have a significant impact on our health. This binging strategy of the diet reminds me of the It Fits Your Macros diet. This diet focuses mainly on how much calories and not the type of calories. Check out my rant on the IFYM diet here.
Some of us may experience digestion problems when they eat large amounts of food in a short amount of time. Larger volumes of food translates to more time needed to digest. This can cause additional stress on your GI tract, leading to indigestion and bloating. This can have huge implications for those with IBS. They already have a more sensitive gut, inflammation of the GI, and disturbed bowel movements. Therefore they are more susceptible to cramping, abdominal pain and bloating. Especially with IBS, you may already have difficulty obtaining all your nutritional requirements due to the uncomfortable symptoms that come along with it. That’s why people with digestive issues are recommended to eat at regular times, take time when eating, and not skip meals in order to have regular bowel functions.
Other studies have found no huge difference between continuous calorie restriction and fasting. Many of the recent reviews have not found that one strategy is better than the other, and at the end of the day both yield short term weight loss. It is also difficult to compare these strategies because of the different study methodologies and study duration. Clearly we need more research, longer term studies and a larger sample size with a more diverse group of participants.
6. Unclear Impact on Heart
For cardiovascular markers such as total cholesterol, some mixed results were also observed in alternate-day intermittent fasting, in which both LDL (BAD cholesterol) and HDL (GOOD cholesterol) increased, while triglyceride levels decreased. However, other studies show that total cholesterol and LDL decreased (here, here) or HDL remained the same. In an animal study, alternate day fasting reduced total plasma cholesterol and triglyceride concentrations. Clearly we need more human studies on Intermittent Fasting.
7.It isn’t for everyone!
If you have a medical condition, it is the best to avoid this type of fasting. For instance, individuals with diabetes or hypoglycemia need glucose throughout the day and going without can have dangerous effects. If you are one of those people who feel nauseous or just don’t feel great going too long without eating, Intermittent Fasting may not suitable for you. It’s also important to note that if you have ever had a history of an eating disorder – Intermittent Fasting is definitely not for you.
Since Intermittent Fasting causes you to eat more food in a short amount of time, it may exacerbate potential disordered eating patterns such as a “binge-eating” mentality. This could cause you to eat more food than your body can handle, or a “restrictive” mindset to become skinnier. This will definitely have adverse effects on your relationship with food and your body’s physical health. A 2016 review found that some Intermittent Fasting participants experienced a preoccupation with food. This could be a serious trigger for individuals with a history of disordered eating.
8. Potential Long-term Health Consequences (especially for women!)
Since IF excessively restricts energy and protein, there’s a real risk of nutrient deficiencies, electrolyte abnormalities, and fertility and reproductive issues in women.
To expand more on the fertility issues, Intermittent Fasting may be linked to menstruation, fertility, metabolism disruptions and early menopause in woman. According to animal studies, fasting led to decreases in body weight, blood glucose levels and more shockingly, reduced ovary size – significantly impacting fertility. In fact, this is because fasting interferes with hormone levels of LH (luteinizing hormone), estradiol and ghrelin, which impacts appetite-regulation. In a 2017 review, a study involving normal weight, normal cycling women found that three consecutive days of a total fast during the mid-follicular phases affected the luteinising hormone. However, it did not interfere with follicle development or menstrual cycle length.
Another study in this review found that obese and overweight women reported longer average menstrual cycle length following Intermittent Fasting for six months compared to the continuous energy restriction group. Researchers from this study discussed the need for further study especially which include longer periods of energy restriction.
Additionally, women typically eat less protein compared to men, and fasting women, even less. Low protein consumption means less amino acids that are needed to activate estrogen receptors and produce IGF-1 (Insulin-like Growth Factor). IGF-1 is responsible for triggering the lining of the uterine wall to thicken and begin the reproductive cycle process. Since estrogen receptors are found throughout our bodies (such as our GI tract, bones and even brain), changed estrogen balance will change metabolic activity everywhere in your body. This includes: digestion, protein turnover, bone formation, recovery, growth, cognition and mood.
Although there are limited human studies on intermittent fasting’s effects on fertility, these significant results suggest that there may be similar effects on human females as well. With your reproductive health at stake, so is your overall health. Your hormones get out of whack, and there’s a decline in other body functions.
9. Potential Weight GAIN!
Research on Intermittent Fasting’s effects on METABOLISM. There is a decreased reliance on carbohydrates as the fuel source because fatty acids are mainly used in its place. According to studies on short-term fasting, Intermittent Fasting protocols can cause glucose concentrations to decrease (reduced glucose oxidation) and lipolysis (fatty acid oxidation) to increase significantly during the first 24 hours. Therefore, Intermittent Fasting can be beneficial as it promotes the breakdown of stored fat.
However, reducing energy intake too severely can lead to the body responding with physiological adaptations. That can cause weight regain after losing the weight during fasting. This means that it is very likely that individuals will not maintain their weight loss after extreme restriction of food and in fact. They can even gain more weight. Obviously, that’s not ideal. However, it is difficult to confirm this claim, because there have not been any long term studies testing the sustainability of the diet. This may also be because of its high drop-out rates. In a 2016 review, the average dropout rate was 31%. That should tell you something!
Now to really get a BIGGER picture, let’s compare the research on the impact of spreading calories throughout the day vs. large meal binges. If you are able to follow and stick to Intermittent Fasting, it can actually aid you in making successful changes in weight loss. It can help to reduce overall caloric intake from up to 20 to 25% over the course of a week (as long as you do not binge on your normal ad libitum days). Some studies have shown that Intermittent Fasting is an easy method of cutting calories if made a habit. However, others may develop a habit of overeating as a result of fasting. This results in increased body weight and issues involving insulin’s ability to regulate blood glucose. These are all strongly tied to weight maintenance.
Rather than overeating, Intermittent Fasting can make you have a slower metabolism as your body goes into starvation mode and begins to use your muscle protein as a source of fuel. Even a short 24 hour fast can lower your BMR (Basal Metabolic Rate), having negative health implications in the long-run!
For many of us, a better weight-loss approach may be eating several small meals throughout the day. This stabilizes our insulin levels and blood glucose. It also prevents any sort of ravenous meal gorging at the end of the day. Eating 6 times a day may help maintain your lean muscle mass (which means faster metabolism!) compared to eating less frequently.
Now with all this info, the BIG QUESTION here is – Should I start Intermittent Fasting?
There are many potential health benefits of Intermittent Fasting. However, it is very important to remember that there are current limitations in Intermittent Fasting research and mixed results. Intermittent Fasting is a more flexible alternative to a calorie restricted diet. In the end, each person has their own way of eating and their own “diet personality”. That means that there is no one-size-fits-all, perfect diet that is suitable for everyone. What’s most important is that you assess your own eating habits and find out what approach works best for you.
Personally, I’m not convinced by the research. I believe that any “diet” that requires you to disregard your body’s innate hunger and satiety cues is not likely to be sustained. But if you do decide to try Intermittent Fasting, always speak with a Registered Dietitian for personalized advice and supervision. Ultimately, the goal here is not to cause your body to go under any sort of chronic stress. Instead, to nurture and show it some love by taking care of it, starting with a foundation of a nutritious diet.
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Research by Rachel Shim and Sofia Tsalamlal, RD, MHSc
Updated on October 8th, 2020
Abbey Sharp is a Registered Dietitian (RD), regulated by the Ontario College of Dietitians. She is a mom, YouTuber, Blogger, award winning cookbook author, media coach specializing in food and nutrition influencers, and a frequent contributor to national publications like Healthline and on national broadcast TV shows.