Over the last decade or so, intermittent fasting (IF) has garnered a ton of traction and popularity. Yet it’s been around for much longer. In fact, periods of voluntary fasting—a key distinction from starvation—have been practiced since earliest antiquity by people around the globe. Personally, I was first introduced to IF around the year 2000 thanks to the controversial books Natural Hormonal Enhancement (by Rob Faigin) and The Warrior Diet (by Ori Hofmekler), highlighting all the health benefits of intermittent fasting.

At the time, most dietitians, nutritionists, and health gurus were advocating that people eat small meals every two to three hours throughout the day, spread over six to eight meals or more. On the flipside, these defiant authors suggested a radically different lifestyle that included how and when we eat. In direct opposition of the grazing mantra, they suggested that health benefits of intermittent fasting for extended periods “flips the switch” needed to optimize hormones, energy levels, fat burning, physique transformation, and youthfulness.

So, What is Intermittent Fasting?

So, what is intermittent fasting? Intermittent fasting is a broad term that refers to dietary approaches in which individuals go extended periods of time (typically, 12 – 48 hours) with little or no caloric intake, with intervening periods of normal intake, on a recurring basis.¹ Although that definition is fairly clear-cut, the truth is there’s quite a bit of confusion when it comes to IF. This is because it encompasses several different eating patterns or subclasses.

Here are some of the most common examples of intermittent fasting:²

• Time-restricted feeding (TRF). This is the most popular form of IF. It involves restricting food intake (often referred to as a “feeding window”) to specific time periods of the day, typically 8 hours or less. For example, one might fast for 16 hours followed by an 8-hour feeding window. The Warrior Diet is an example of TRF as is the popular IF method Leangains.
• Alternate-day fasting (ADF). This involves alternating “fasting” days (no calories) with “feast” days (unrestricted food intake). In other words, eat nothing one day, then eat to your satisfaction the next. Some refer to this method of IF as “zero-calorie” ADF to distinguish it from…
• Alternate-day modified fasting (ADMF). This variation of ADF restricts calories to about 75% of your baseline needs on “fasting” days (about 500 calories/day), which are alternated with unrestricted “feast” days (ad libitum food consumption). The Every Other Day Diet is an example of ADMF. It’s based on the research of Dr. Krista Varady (who’s also the coauthor). ADF and ADMF are the most studied forms of IF in humans.
• Periodic fasting (PF). This IF eating pattern, sometimes referred to as “whole-day fasting,” consists of one to two days of fasting along with ad libitum food consumption the other five to six days of the week. One popular example is Brad Pilon’s Eat Stop Eat program. A modified version is the 2-Day Diet (also known as the 5:2 Diet, or MPF), which involves two consecutive days of calorie restriction (about 500 – 700 calories/day) followed by five days of “normal” eating. PF can also include water fasts, which often last two to five days.
• Fasting mimicking diets (FMD). As the name implies, FMD is designed to mimic the physiological state of fasting and provide many of the health benefits of intermittent fasting—without actually fasting. FMD is based on research conducted by Dr. Valter Longo and colleagues. It hinges on a five-day period of calorie restriction during a monthly cycle. During this five-day period, you consume a very-low-calorie diet (about 34 – 54% or your normal intake, or about 800 – 1,100 calories per day) that’s also very low in protein (about 10% of your normal intake). The rest of the month, you eat normally, and this cycle is typically repeated at least three times. If you’re interested in learning more about FMD, The Longevity Diet (by Dr. Longo) and ProLonFMD.com are excellent resources.

Another key term that also gets bundled with IF is intermittent energy restriction (IER). This is defined as periods of caloric restriction interspersed with normal energy intake. As you can tell, pretty much all the IF protocols qualify as IER. However, it’s important to distinguish IER regimens that allow food on restricted days (such as ADMF and the 2-Day Diet, for instance) because complete abstinence (i.e., true fasting) may evoke greater metabolic fluctuations and be associated with hyperphagia (which is a scientific term for overeating) on non-restricted days.³

Also, the IER distinction is important for two other reasons:

1. Continuous energy restriction (CER). By now, most people know that weight gain is the result of excess energy intake—which is usually the byproduct of overeating and a sedentary lifestyle. Weight gain and excess body fat—especially belly fat—are consistently linked to illness, disability, and mortality. On the flip side, intentional weight loss reduces type 2 diabetes and all-cause mortality and improves cognitive and physical function. Generally speaking, the traditional approach to weight loss is daily caloric restriction, or CER. In other words, the focus has pretty much always been on creating a modest—albeit meaningful—energy deficit daily. However, we now know that IER is at least as effective as CER for weight loss. This has obvious implications, especially considering that many people find it difficult to stick with conventional weight-loss diets because food intake must be limited every day—or so at least it has traditionally been thought. The truth is that it’s about your behavior, on average, over time. Some days, you can eat more; others, eat less.⁴
2. Calorie restriction (CR). Besides weight loss, CR is one of the best-known anti-aging tools at our disposal. CR has been shown to prevent age-related disease including tumors, cardiovascular disease, type 2 diabetes, and dementia. It slows age-related functional decline. And it increases lifespan. Granted, much of the research on CR has been conducted in mice. Nonetheless, the findings are quite impressive. CR promotes metabolic and cellular changes that positively affect oxidative damage and inflammation, optimize energy metabolism, and enhance cellular protection. CR dramatically improves metabolic health, reduces body weight, improves mitochondrial function, and extends longevity. The problem with CR? Simply put, it’s hard, and quite frankly, it sucks.

   Think about eating 40% less than you normally do…all the time…indefinitely. That’s basically CR (as an anti-aging protocol) in a nutshell. Not surprisingly, folks who dabble with CR experience constant hunger. That makes the various IER protocols a potentially more useful tool. After all, compliance rules. It’s actually possible that the benefits of IER may be stronger than CR because of the more intense—albeit less frequent—degree of calorie restriction.

How Does Fasting Affect the Human Body? 3 Health Benefits of Intermittent Fasting

When attempting to answer this question, it’s important to highlight the human body. You see, despite its popularity, much of the scientific research on IF comes from animals. And without fail, many have taken the liberty to extrapolate those findings and health benefits to humans. Unfortunately, it’s not quite that simple, although research does suggest humans undergo strikingly similar molecular, metabolic, and physiologic adaptation as lab animals. Fortunately, there’s a growing body of evidence that highlights the key health benefits of intermittent fasting in humans.

• Weight loss. As mentioned above, IER is an alternative to daily caloric restriction (CER). Studies conclusively show that the various forms of IF are at least as effective as CER at reducing body weight, fat, and visceral fat.^1,2,4 However, despite the many bold claims out there, IF does not appear to be a magic bullet for weight loss—it’s just another tool in the toolbox. In other words, the weight loss is the result of caloric restriction. And even though IF protocols like TRF may not actively restrict calories, by and large, most people reduce how much they eat when they condense their feeding window.

  Having said that, there may be additional health benefits of intermittent fasting beyond weight loss. And certainly, some people find it’s easier to stick with IF compared to the traditional dieting approach. And when it comes to weight loss, adherence is king.

• Metabolic function. Research shows that IF (such as ADMF and MPF) leads to significant changes in metabolic function including increased insulin sensitivity, reduced levels of blood glucose, insulin, and leptin, increased adiponectin, increased fat burning, and elevated levels of ketones (more on this in a moment), which are known to have beneficial effects on cells with a high energy demand, such as neurons in the brain.⁵

  Interestingly, research shows that intermittent fasting increases insulin sensitivity to a greater degree than CER despite similar weight loss, and IF also appears to be a tool that can stimulate mitochondrial biogenesis, which is science speak for increasing the size and number of mitochondria.

• Heart health and longevity. The various IF regimens have been shown to improve a variety of cardiometabolic risk factors including:^1,2
• Increased insulin sensitivity and increased ketone production
• Improved blood lipids (e.g., triglycerides)
• Healthier levels of inflammatory markers (e.g., CRP, TNFα, IL-6)
• Reduced markers of oxidative stress and decreased levels of homocysteine
• Weight loss and reduced visceral/abdominal fat
• Reduced resting heart rate and blood pressure
• Increased parasympathetic activity

Intermittent Fasting and “Flipping the Switch”

Scientists now believe that many of the health benefits of intermittent fasting come back to something they call flipping the metabolic switch.² What the heck does that mean? Well, researchers define the metabolic switch as “the body’s preferential shift from use of glucose to fatty acids and fatty-acid derived ketones.” In other words, IF turns off sugar burning and turns on fat burning and ketone production.

Researchers believe this is “preferential” because there is a growing body of research indicating that ketones are the preferred fuel for both the brain and body during periods of fasting and extended exercise. [For more on ketones, ketosis, and the ketogenic diet, see these resources.] An increase in ketone production (called ketogenesis) is a natural, expected adaptation to fasting (and severe carbohydrate restriction).

Intermittent Fasting and Circadian Rhythms

There are some other potential mechanisms that may have a hand in the various health- and fitness-promoting benefits being discovered with IF. For example, the adaptive responses of the brain and autonomic nervous system (increased parasympathetic activity, reduced heart rate and blood pressure) to food deprivation may play a major role.¹

Researchers also believe IF may beneficially impact circadian rhythms and the gut microbiome.⁶ For example, IF may directly influence the diversity and activity of the gut microbiota, which is the complex, vast microbial community that resides in the intestinal tract. Research shows that changes in the composition and function of the microbiota can have a significant impact on body weight and metabolic function.

Like we’ve talked about before, food timing is one of the major variables that impacts circadian alignment. Along those lines, IF regimens (such as TRF) that limit food consumption to daytime/light hours may be particularly effective at promoting metabolic health. Ironically, most people who implement TRF eat the bulk of their food/calories at nighttime, which may disrupt circadian rhythms and lead to negative health and metabolic outcomes.

While there’s no question that fasting may account for some of the health benefits mentioned, it’s important to note that caloric restriction and weight loss may also play a role in mediating these positive physiological responses.

So, is it safe to fast?

Evolutionarily, there’s no question that hunter-gatherers went extended periods of time without food. This obviously doesn’t fall under the umbrella of voluntary fasting. Yet, it highlights the fact that humans are designed to withstand periods of abstinence. On top of that, we’re markedly efficient at storing energy (i.e., body fat). And during periods of prolonged fasting (and carbohydrate restriction), it’s very clear that we’re wired to flip the metabolic switch to produce and use ketones, which some argue are the body’s preferred fuel source.

Historically, fasting has been used as both a religious and medical practice for thousands of years. Fasting for medicinal purposes has been practiced since the time of ancient Chinese, Greek, and Roman physicians. Of course, Ramadan and Lent are two well-known periods of religious fasting for Muslims and Christians, respectively.

Heck, even Benjamin Franklin, one of America’s Founding Fathers, said, “The best of all medicines is resting and fasting.” And author Mark Twain wrote, “A little starvation can really do more for the average sick man than can the best medicines and the best doctors. I do not mean a restricted diet; I mean total abstention from food for one or two days.”

Okay, Franklin and Twain may not add a ton of credibility to the argument, but the point is fasting has been practiced therapeutically for centuries. That’s not to say that extended periods of fasting aren’t without potential risks. However, intermittent fasting protocols, which are considered short-term fasting protocols, including TRF, ADF, ADMF, MPF, and FMD, have been shown to be safe and not associated with any adverse events.² There are many longer-term studies—up to 1 year—with very few adverse events reported.

Still, there are concerns that IF could promote erratic eating patterns, binging, and poor mood states. However, research shows that intermittent fasting does not trigger binge eating in those not without previous history. Additional research demonstrates improved mood states, decreased fatigue, reduced feelings of anger and tension, and an increase in vigor.³

There is also some concern as to whether IF can affect women’s menstrual cycles (and female hormones). While further research is needed, if there is an effect, it’s likely related to the starting weight of the woman, overall caloric intake, and the duration of the fast/restriction.

And despite what many people presume, intermittent fasting does not hamper an individual’s ability to exercise. While IF may not be the best approach to gain weight, research shows that people incorporating IF protocols (including ADF and TRF) can increase strength and endurance and maintain muscle mass when combined with regular resistance training and cardiovascular exercise.^7,8

Simply put, in healthy, normal weight, overweight, or obese adults, there is little evidence that intermittent fasting regimens are harmful physically or mentally (i.e., mood).⁶ Having said that, if you are pregnant, nursing, at risk for, or are being treated for any medical condition, please consult with your physician prior to making any changes to your diet.

Does this count as fasting?

Believe it or not, one of the most frequently asked questions about fasting is what counts as fasting. Said differently, people want to know if they can eat x, y, or z when they are fasting. Simply put, if you put something in your mouth that’s not water, you’re not fasting—calorie-containing or not. That goes for coffee, zero-calorie sweeteners, chewing gum, branched chain amino acids, and so on.

Having said that, it’s hard to say what effect—if any—consuming no- or low-calorie drinks, foods, and supplements has. Also, remember that the health benefits of intermittent fasting may come back to caloric restriction and ketone production. In the case of the former, if you’re still restricting calories, then it’s likely benefits will ensue.

In the case of the latter, the body ramps up ketone production in response to depletion of carbohydrate stores (i.e., liver glycogen) and carbohydrate restriction (i.e., low, stable blood glucose levels). Essentially, if you’re not consuming carbohydrate (or a meaningful amount of protein) during the fasting window, then it seems that it wouldn’t have a negative effect.

My suggestion, however, would be to limit consumption to water as best you can when fasting to leave no doubt and maximize the health benefits of intermittent fasting, which include:

• Reduced levels of insulin and leptin
• Increased insulin and leptin sensitivity
• Reduced body weight, fat, and visceral fat
• Increased ketone levels
• Reduced resting heart rate and blood pressure
• Increased heart rate variability (a good thing)
• Reduced inflammation
• Increased resistance of the brain and heart to stress
• Improved metabolic function