Intermittent Fasting—Latest Diet Fad or the Holy Grail of Weight Loss and Longevity?

Look around you: Obesity is a global health concern that affects the health, wellbeing, and longevity of individuals, and is considered an epidemic of our modern times.

According to 2014 statistics from the World Health Organization (WHO), 39% of adults are overweight, and 13% are considered obese. More alarmingly, there are more than 42 million children under the age of 5 who are overweight or obese as of 2013.

In Canada, 54% of adults are considered overweight, with 1 in 4 of these adults being obese.

Since obesity is correlated with increased prevalence and risk factors for nearly every chronic disease (think diabetes, heart disease, cancer etc.), it’s no wonder that people and practitioners alike are always in search of the next latest tool, strategy, or supplement which can help tackle this huge problem.

We know that lifestyle interventions (i.e. diet, exercise, stress management) play a fundamental role in the success of both short- and long-term weight management, and daily caloric restriction (DCR) is perhaps the most prevalent strategy for weight loss. While there are many continuous restrictive diets that manipulate macronutrient content, like the low-carbohydrate Atkins Diet or the more balanced Zone Diet, all of them have two things in common: 1) reduced caloric intake; and 2) a single active phase of weight loss. While people are typically able to achieve rapid short-term weight loss and improvement in their metabolic profile using this strategy, long-term dieting and body-weight loss are also associated with a decreased resting metabolic rate, which may contribute to weight-loss plateaus and weight regain. As a result, long-term weight-loss success rates remain abysmal, leaving us searching further for the holy grail of weight loss and maintenance programs.

Another nutrition strategy that has been emerging in recent years and has quickly become the next popular diet is intermittent fasting (IF), a broad term that refers to the practice of going extended periods of time without eating. And while there are specific feeding and fasting protocols, we all practice some form of IF every single day—we call it sleeping. Assuming a typical sleep-wake cycle, the time from your last meal at night until your first meal in the morning is considered your fasting window, and the time from your first meal in the morning until your last meal at night is considered your feeding window. So if we are all doing this every day anyway, why is this “new” diet receiving so much hype?

First off, intermittent fasting is nothing new. Humans throughout history have experienced restricted caloric intake in some form, from hunger strikes to food scarcity and famine, and to fasting for religious reasons. What is new and exciting is that the research is just beginning to bring to light the numerous benefits of intermittent fasting for health and longevity. In the grand scheme of evolution, humans and most domesticated animals ate intermittently, and many species adapted themselves to forms of quiescence with the onset of food shortage. Interestingly, many of the genes responsible for quiescence are also involved with the control of lifespan. For instance, rodents fasting either for 24 hours every other day or twice weekly extended their lifespan by 30%, independent of both weight loss and total food intake. They also experienced other health improvements, such as lowered inflammation and oxidative stress, as well as enhanced cellular function and improved responses to stress. The proposed benefits from the studies all fall into the categories of “look and feel better” and “live longer” physiological changes, which include:

• better blood sugar control—better insulin sensitivity, which means nutrients from food going into cells (where they can get utilized), and decreased risk/prevention of type 2 diabetes;
• better cell functioning—less damage to DNA, and better repair of damage to cells if it does happen;
• better brain functioning—including memory, decision-making, and reduced risk of Alzheimer’s disease and dementia;
• lowered blood pressure and cholesterol levels—decreasing the risks of heart disease and stroke;
• reduced incidence of prostate, breast, and pancreatic cancers; and
• increased metabolism and fat-burning capacity—better body composition and leaner physique.

While all of this data sounds impressive and compelling, you should take it with a grain of salt, as rodent and monkey experimental results are hard to extrapolate to human subjects. So before we start skipping meals or entire days of eating, let’s take a look at the human research, as well as the specific intermittent fasting protocols.

The majority of popular intermittent fasting protocols fall into one of three categories: alternate-day fasting, whole-day fasting, and time-restricted feeding. Alternate-day fasting (ADF) involves alternating between fasting days where you consume only one meal consisting of 25% of your daily caloric needs (so a modified fast in this sense), and ad libitum days, meaning you eat as much of anything you want. Whole-day fasting (WDF) involves selecting 1–2 days of complete fasting per week, followed by ad libitum eating on the other 5–6 days of the week. Some programs also allow for food intake up to 25% of daily caloric needs on the fasting days, where men consume approximately 600 kcal and women consume 500 kcal, making it a modified fast. Time-restricted feeding (TRF) involves eating the same way daily, with certain number of hours allotted for feeding and fasting windows, ranging from 12 to 20 hours of daily fasting and 4-⁠ to 12-⁠hour feeding windows. Many of the time-restricted feeding protocols also vary on the types of foods consumed, as well as on the timing of macronutrients according to exercise schedule.

Alternate-Day Fasting

Similar to rodent studies, short-term (two weeks or less) alternate-day fasting studies in humans showed no effect on body weight in normal-weight human subjects. However, when the studies extended to at least three weeks duration, a weight reduction of approximately 2.5% body weight was noted, and researchers proposed the effect may have been due to the inability of subjects to consume enough calories on refeeding days to maintain their body weight.

In a review study by Barnosky et al., findings from seven ADF studies resulted in subjects losing 3–8% of their body weight after 3–24 weeks of treatment, with the most significant weight loss occurring in the subjects that were provided meals on the fasting days. On average, studies on ADF also show a faster rate of weight loss as compared to other intermittent fasting methods, with the average rate of weight loss being 0.75 kg per week versus 0.25 kg weight loss per week. More importantly as it relates to chronic disease risk factors, visceral fat (the fat around your midsection and internal organs) was reduced by 4–7% in five ADF studies, over the course of 6–24 weeks. Unfortunately, the studies measured visceral fat indirectly with waist circumference, and not directly with MRI or DEXA scans. However, waist circumference is a widely used objective measurement that highly correlates with visceral adiposity, as well as increased risk for heart disease, diabetes, and other chronic diseases, so it does offer some clinical insight even here. Regardless, body-weight reduction and fat-mass reduction have been consistently shown in ADF protocols, and have been demonstrated in obese, overweight, and normal-weight individuals. Several ADF studies have also demonstrated reductions in cardiovascular disease risk, with reductions in total cholesterol, triglycerides, and LDL-cholesterol levels, although inconsistently.

Dietary changes and exercise with the goal of weight loss are considered first-line therapy for those individuals with prediabetes, and clinical trials have consistently demonstrated that intensive dietary and exercise interventions can completely prevent type 2 diabetes in high-risk prediabetic people. Not surprisingly, these studies also showed reductions in fasting glucose and insulin levels in these subjects. Several ADF studies on prediabetic individuals have also assessed these parameters and found consistent, yet minor (3–7% reduction) decreases in fasting glucose levels, and moderate reductions (20–30%) in fasting insulin levels over 8–12 weeks, with the greatest reductions observed in the longer-duration studies. Finally, the Barnosky et al. review compared ADF to the daily caloric restriction (DCR) strategy, and found the results for weight loss, visceral-fat loss, reduced fasting insulin levels, and type 2 diabetes risk reduction all comparable in effectiveness.

Whole-Day Fasting

Similar to ADF trials, whole-day fasting (WDF) studies consistently showed reductions in body weight and body fat. Interestingly though, all of these studies combined either 1–2 days of fasting (or modified fasting where the subjects consumed a small amount of food on fasting days) and overall caloric restriction, that is, the total weekly caloric intake was lowered, and not ad libitum as the original definition of WDF implies. When compared to daily caloric restriction (DCR), the losses in body weight and body fat were no different between groups, i.e. they were equally as effective. And when compared to control subjects consuming their regular eating pattern, WDF showed significant reductions in body weight and fat.

WDF studies have also been shown to help lower coronary heart disease risk in overweight and obese women, although with varying results. Many of these studies reported significant reductions in blood pressure, fasting triglyceride levels, total cholesterol, LDL cholesterol, blood pressure, and high-sensitivity C-⁠reactive protein, and the results were comparable to daily caloric restriction control subjects. It’s important to note that not all of the studies used the same number of fasting days per week, the studies that required two fasting days per week differed in whether the days were consecutive or nonconsecutive, and not all of the studies reported blood lipid or blood pressure changes, which limits the ability to compare the studies as a whole.

Time-Restricted Feeding

To date, 11 studies have been completed that evaluated the effects of TRF on body weight: two looked at the effects of a 4-⁠hour TRF, three tested the 7–8-⁠hour TRF, and six studied the 10–12-⁠hour TRF. The studies which assessed the 4-⁠hour TRF showed no changes in body weight in the subjects, as they were instructed to eat ad libitum to maintain their body weight. Of the three studies that examined the 7–8-⁠hour TRF, only one showed a 5% reduction in body weight after four weeks of intervention, despite similar study design. It’s possible that energy intake was lower in this one study compared to the other two, but it’s uncertain to know for sure, as energy intake was only measured in one of the other studies. Trials that looked at the 10–12-⁠hour TRF demonstrated consistent reductions in body weight of 1–3%. However, it should be noted that a majority of these trials were Ramadan trials, where the feeding window occurred at night, and thus, the feeding window would actually be limited, as 7–8 hours would be typically used for sleeping.

Stote et al. conducted a pilot trial using the 4-⁠hour TRF, which is the only study included in the Tinsley & La Bounty 2015 review, because it met the inclusion criteria of not being a Ramadan trial. This study involved a randomized crossover design with two 8-⁠week periods of consuming either 1 meal/day or 3 meals/day, separated by an 11-⁠week washout period. Subjects were allotted a 4-⁠hour TRF window in the evening during the 1 meal/day phase of the trial. After the 1 meal/day phase, results showed mixed positive and negative outcomes. The subjects lost body weight as well as fat mass and gained fat-free mass after the 1 meal/day phase. However, they also increased total cholesterol, LDL cholesterol, and blood pressure, which are known risk factors for cardiovascular disease. Due to the limited data on TRF, as well as the mixed results in health markers, it’s difficult to draw any hard conclusions about the effectiveness or safety of TRF.

The research for IF is promising, but obviously has many limitations. When taken as a whole, the studies have huge variations in types of protocol, duration, subject sample size, and measurement outcomes. As a result, it’s difficult to compare the effectiveness of the IF protocols. Also, assessing whether a nutritional strategy is effective for long-term weight-loss success, that is weight loss followed by weight maintenance, requires lengthy clinical trials, which are often difficult to conduct. Any claims on improving longevity also require following large groups of people following this nutritional strategy over the long haul, something that has yet to be done in humans. However, the preliminary evidence for IF has shown that it’s at least as effective as daily caloric restriction for weight loss, fat loss, and a host of other health markers. So for people who find traditional methods of daily caloric restriction more challenging, IF may be considered an alternative approach for weight loss and improved health. And beyond the scientific research, the anecdotal reports of weight-loss success, striking before-and-after photos, and social-media buzz surrounding IF as of late make this nutritional strategy incredibly appealing to the mainstream market. So, are you convinced you should take the plunge and give IF a try? Not so fast (excuse the pun!)…

There are a few other important considerations to take into account. Firstly, when it comes to long-term weight loss success, the issue of compliance comes into play. If one can’t stick to the regime, it doesn’t really matter how good the program reportedly is or that it has a mountain of evidence to support it. Fasting or prolonged energy restriction will test hunger and satiety signals, and people could experience many unwanted side effects from energy deficits that make adherence over the long haul difficult or impossible. And while short-term weight loss is achievable with IF, many people just may not be cut out for this kind of long-term dietary technique.

Another thing to consider is that IF may indirectly lead to developing poor overall dietary habits, especially if one follows an ad libitum style of eating on the nonfasting days. The studies reviewed showed that most of the protocols had subjects following IF alongside caloric restriction, so people were consuming less overall calories on a weekly basis. And that’s fine and dandy in a controlled setting, but out there in the real word, people may interpret IF as a free-for-all on nonfasting days and just binge-eat nutritionally devoid junk food—which certainly doesn’t help instill healthy relationships with food, and is certainly not a strategy to consider for anyone with a history of any eating disorder.

The final thing to consider before you give IF a try is whether or not this kind of strategy is safe in both the short and long term. Anytime you restrict calories significantly with the aim of weight loss, you need to consider what you’re eating as well. A thousand calories of donuts, chips, and beer is not the same thing as eating a thousand calories of vegetables, fruits, healthy fats, and lean proteins. Sure, you’ll probably lose weight with both, but chances are you will feel and function very differently with each of these protocols. A sensible approach to effective short- and long-term weight loss must consider caloric restriction without malnutrition or deficiencies, and in controlled settings, this is accounted for as researchers usually provide the diets or at least provide dietary guidance for subjects. Seeking the help of a trained professional, such as a registered dietician, holistic nutritionist, experienced nutrition coach, or naturopathic doctor who can monitor you while you try IF as a weight-loss strategy is strongly recommended, especially if you have any preexisting medical conditions that could be affected. And although most of us are lured with the appeal of rapid weight loss and extreme body transformations, remember that most successful long-term weight loss often comes from small, incremental changes to nutrition, exercise, and lifestyle habits. So if you’re going to commit to fasting, remember to take it nice and slow!