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 so we set out to review the most recent evidence on the topic and whether intermittent fasting will yield the best results for weight loss and your health.
What is Intermittent fasting?
As you can probably guess from the name, Intermittent Fasting involves a fast, but it’s the “intermittent” part that sets this fast apart from the annual religious fasts you may have partaken in at some point in your Intermittent Fasting. 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” (when you’re sleeping), and range from 16 hours to a maximum of 1.5 days (but usually no more than 24 hours).
Intermittent Fasting was introduced as a more accessible diet, since 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, and on other days you would have the freedom to eat and meet energy requirements, so this diet was seen as a WAY 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!
But are all of those Intermittent Fasting claims 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 and 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 looksy at each.
This type is the most-studied form of INTERMITTENT FASTING, which 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” which has different durations/periods of fasting from anywhere between 30-40 hours based upon the individual’s schedule.
2. Whole-day Fasting (here, here)
Compared to alternate-day fasting, this kind of fasting may seem “more extreme” to some because out of 1 to 2 days per week, you need to either severely restrict your calories or completely abstain from food. This is commonly known as the “5:2 diet” in which the 5 represents the number of ad libitum days you eat normally, and 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” (if you want to call it that) 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 that lasts for 20 hours, followed by an “overeating” phase lasting for 4 hours that takes place every 24-hour period. This form of fasting is followed as a type of routine.
Of course, it’s important to consider both the PROS and CONS of something like Intermittent Fasting, so 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, and whether Intermittent Fasting is 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, and with a more flexible diet like Intermittent Fasting, you could yield the same weight loss results. One thing to keep in mind when analysing these results, is that many of these studies are short term so whether individuals were able to keep that weight off, is unclear.
Diets that continually restrict calories are known to reduce body fat, but ALSO something called Fat Free Mass (FFM – 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. However these results have not yet been 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 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 with an Intermittent Fasting diet 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 (the artery-clogging, BAD cholesterol!), blood pressure and triglyceride levels, which is super 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!
Other studies have explored the powerful effects of this time-restricted diet on cognitive performance (such as memory), which 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 among overweight and obese people.
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 (even though 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 (compared to the traditional calorie restriction that may be hard to follow in the long-run)! For some people, it may be easy to incorporate into your current routine and you don’t have to worry about limiting the types and amount of food you eat on feasting days. 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, leaving you feeling more full and satisfied, and you wouldn’t have to worry about eating later (because you probably wouldn’t even be hungry!). In a way, Intermittent Fasting can actually help prevent you from the typical binge on food at night after not eating all day at work, or the sooner-or-later binge-eating resulting from those calorie-restricted diets that are difficult to keep up with for long run.
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 and the enjoyment of eating with the people we care about. Since this new style of eating is very different from the typical daily eating patterns of most people, it may interfere with your social hangouts which usually involve tons of food. This is because of the shortened time frame you have for eating and 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
We are all well-aware that life is unpredictable and can throw something at you out of the blue! So when changes to our agenda occur, we might get hungry and be much less productive, especially if you are used to eating lots of snacks or meals throughout the day, and all of sudden can’t. 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. For those of us who are gym-goers, 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!
Since you only have a limited amount of time reserved for eating, 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, where it focuses mainly on how much calories and not the type of calories. Check out my rant on why calories are not created equal HERE and why we should be thinking about quality over quantity.
Some of us may experience digestion problems with these larger portioned-sized meals that we consume within a shorter amount of time. Larger volumes of food translates to more time needed to digest, which causes additional stress on your GI tract, leading to indigestion and bloating. This can have huge implications for those with IBS, who already have a more sensitive gut, inflammation of the GI, and disturbed bowel movements, and are 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.
So I know we just mentioned one particular study that showed greater lean muscle retention with INTERMITTENT FASTING, but the reality is, the research here is mixed. Other studies have found no huge difference between continuous calorie restriction and fasting on parameters like weight loss, fat mass, fat-free mass, glucose homeostasis, cardioprotection, and reducing appetite. 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 because it may have the opposite effect on your health. For instance, those who are diabetic or hypoglycemic 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 causing you to eat more food than your body can handle, or a “restrictive” mindset that is caught up with Intermittent Fasting because of that desire to limit food-intake and become skinnier through restriction. 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, so this could be a serious trigger for individuals with a history of disordered eating.
8. Potential Long-term Health Consequences (especially for women!)
If Intermittent Fasting is taken too seriously and excessively restricts energy and protein, there’s a real risk of nutrient deficiencies, electrolyte abnormalities, and issues with fertility and reproduction in women.
To expand more on the fertility issues, Intermittent Fasting is possibly linked to issues with 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 may impact not only reproductive health, but appetite-regulation as well. 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 but 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) which is responsible for triggering the lining of the uterine wall to thicken and begin the reproductive cycle process. And if that wasn’t enough, 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 as well (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. So, if your reproductive health is at stake, your overall health is too as your hormones become out of whack, resulting in a decline in other body functions.
9. Potential Weight GAIN!
Research on Intermittent Fasting’s effects on METABOLISM, has shown that there is decreased reliance on carbohydrates as the fuel source, as 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, can gain even 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 to keep our bodies fueled throughout the day. If you are able to follow and stick to Intermittent Fasting, it can actually aid you in making successful changes in weight loss by reducing 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, but others may develop a habit of overeating as a result of fasting, which would result in increased body weight and issues involving insulin’s ability to regulate blood glucose which is 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, which stabilizes our insulin levels and blood glucose, preventing any sort of ravenous meal gorging at the end of the day. It has been found that eating 6 times a day may even 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?
Many potential health benefits have been observed from Intermittent Fasting, but it is very important to remember that there are current limitations in Intermittent Fasting research and mixed results found in the different types of Intermittent Fasting. As of now, Intermittent Fasting can be seen as a more flexible alternative to the traditional calorie restricted diet. In the end, each person has their own way of eating and their own “diet personality,” which 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 and ultimately 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, but to nurture and show it some love by taking care of it, starting with a foundation of a nutritious diet.
Research by AK RD2B Rachel Shim and Sofia Tsalamlal, RD, MHSc
Abbey Sharp is a Registered Dietitian, an avid food writer and blogger, a cookbook author and the founder of Abbey’s Kitchen Inc.