Evidence Based Research To fulfill our commitment to bringing our audience accurate and insightful content, our expert writers and medical reviewers rely on carefully curated research.
Read Our Editorial Policy
Intermittent fasting can be both safe and effective, but it’s not for everyone, because it can throw off hormones and electrolytes in certain cases.
The harder a trend hits, the louder the evidence has to be to drown out all the buzz.
Intermittent fasting typifies this concept to the letter, as the diet’s somewhat polarizing debut (or rather, comeback) in recent years played over a clattering din of celebrity support on social media, a wave of fitness and nutrition books, and other highly visible mentions.
For anyone trying to determine whether intermittent fasting is safe and effective, the best approach is to mute all the noise and defer to the research.
First, what exactly is intermittent fasting?
Intermittent Fasting Explained
Intermittent fasting is the practice of cycling through periods of fasting and eating throughout the day and/or week.
This type of “diet” is more restrictive about when you eat than what you eat.
As the name implies, intermittent fasting is the practice of cycling through periods of fasting and eating throughout the day and/or week.
There are many scheduling variations intermittent fasters follow, including the following more popular approaches:
- Sixteen-hour fasting period, eight-hour eating period
- 24-hour fasts throughout the week (usually 2-3)
- Partial or complete fasting every other day
- Partial fasting until afternoon/evening, then large meal(s)
Though it’s tempting when first adjusting to the diet, it’s vital dieters don’t binge-eat the same amount of calories in their smaller time window, especially if those calories don’t come from nutritionally dense foods.
Proponents of intermittent fasting claim the diet helps to stimulate hormonal and metabolic shifts that encourage fat loss and muscle growth.
This method and keto are related in that, by fasting, you’re encouraging the body to switch from carb metabolism to fat metabolism.
One key difference is that intermittent fasting makes more frequent use of this metabolic track switcher, whereas the more carb-weary keto diet strives to keep you in a fat-burning state.
Opponents of fasting claim that the diet can increase stress, throw off your electrolyte balance, sour your mood, and destabilize blood glucose.
Let’s hear from both sides, starting with those in favor.
As usual, the surface-level merits of intermittent fasting as a weight-loss tool have eclipsed everything else, but researchers are revealing the clinical potential of this method in several convincing ways.
For example, intermittent fasting may help with part of the body’s natural detoxification process by prompting the death of damaged or old cells.
This immunotherapy potential isn’t the only door slowly creaking open when it comes to intermittent fasting and health, but let’s address the aesthetically motivated elephant in the room first.
Each dieter’s results will vary based on a range of circumstances, including which fasting schedule they choose, whether or not they overeat, underlying health conditions (especially hormonal imbalances), and more, but in almost every case, intermittent fasting promotes weight loss.
A study from Baylor University in Texas found that participants who fasted intermittently for trial periods between 3-24 weeks reduced their body weight (3%-9%), body fat, cholesterol, and triglycerides.
One of the least controversial mechanisms echoed throughout the evidence supporting this effect is insulin’s connection to weight gain.
Whether it’s endogenous or supplemented, too much insulin in the body will create a surplus of energy as excessive amounts of glucose enter the cells, and energy surpluses lead to fat storage.
Dieters can keep their insulin levels lower for longer by fasting, which hampers fat accumulation by keeping the influx of glucose low enough for the body to prioritize burning it over storing it.
To offer a simpler connection between fasting and weight loss, slamming the door shut on snacking for two thirds or more of the day is a powerful technique in its own right.
Human Growth Hormone
You don’t have to be an avid weightlifter to appreciate the importance of retaining as much muscle as possible while slimming down, a capability that intermittent fasting appears to support.
According to this University of Virginia Medical School study, participants who fasted for five days demonstrated clinically significant increases in growth hormone “pulse frequency.”
Put simply, growth hormone release in humans is highly unpredictable, as “pulses” can range in frequency and amplitude without clear cause, so anything that can raise levels is deserving of a closer look.
Some loss of muscle mass is always to be expected when dropping more than a pound or two, but the ability to minimize that loss can make the difference between a skinny fat person and a more optimal body composition.
From the Greek words for “self” and “eating,” autophagy refers to an immune process in which old or damaged cells destroy themselves.
When our cells are temporarily deprived of outside energy sources, they focus more on “housekeeping” tasks such as this one, a benefit of intermittent fasting that has perked up the ears of more than one cancer researcher.
As this Federal University of Sao Paulo study explains in much greater detail, many cancers rely on “hacking” the cell’s ability to target its own organelles when compromised.
Promoting autophagy with intermittent fasting was found by the researchers to stimulate “antiproliferative effects in breast, colorectal and lung cancer cell models, as well as the inhibition of tumor growth in an in vivo model of lung cancer.”
Diabetes Prevention and Treatment
When practiced under stricter conditions, intermittent fasting is surprisingly potent in its ability to prevent or even reverse type 2 diabetes.
Chief among those conditions in importance is the placement of the eating window as close as possible to the 8 AM – 2 PM time slot, a technique this study from Louisiana’s Pennington Biomedical Research Center refers to as “early time-restricted feeding.”
In the study, men with prediabetes who practiced early time-restricted feeding demonstrated increased insulin sensitivity, lower blood glucose levels, and decreased desire to eat in the evening, three very powerful deterrents to the development and progression of diabetes.
The increased insulin sensitivity and lowered blood glucose play off of each other to compound the effectiveness of this method, as these benefits improve the efficiency of glucose uptake into your tissues while also lowering the amount of glucose in the blood.
For all its merits, intermittent fasting isn’t perfect.
It’s true that humans are capable of ameliorating the effects of fasting on the cellular level, but nothing is free when it comes to human physiology—the bill eventually comes around.
For this reason, fasting has been shown to have some short-term and long-term side effects of varying nature and severity.
We’ll start with one of the most commonly issued rebuttals: stress hormone overproduction.
The Cortisol Drip
An academic review by the John B. Pierce Laboratory of New Haven, CT, found that “caloric restriction significantly increased serum cortisol level” in thirteen of the studies they included.
Cortisol is a stress hormone produced by your adrenal glands; it’s what your system gets flooded with (along with epinephrine, i.e., adrenaline) when you encounter a potentially life-threatening situation.
Of course, skipping a meal or two isn’t going to be as taxing on your brain and body as an all-out cortisol/adrenaline dump, but this “slow drip” of cortisol over a few dozen or hundred hours can still take a toll.
Symptoms of a high cortisol level include swelling, irritability, weakness, and, ironically, weight gain.
Electrolytes are charged minerals (sodium, magnesium, etc.) essential to several executive-level functions in the body.
These minerals rely just as much, if not more, on fluid as they do on nutrition to maintain suitable blood pH levels and more.
It’s completely possible for conscientious dieters practicing intermittent fasting to keep their electrolytes at optimal levels, but this study out of the AOMA Graduate School of Integrative Medicine in Texas found that some fasters tend to dehydrate themselves without knowing it.
During the study, multiple “serious adverse events (SAEs)” were observed among elderly patients, both related to dehydration, one of which related to sodium deficiency (hyponatremia).
In other words, forgetting to hydrate during a fast because you normally drink with meals will increase your vulnerability to electrolyte imbalances.
Yes, intermittent fasting can lessen blood glucose and improve insulin sensitivity, but for diabetics who are already on sugar-lowering medications, this method may push them into a hypoglycemic state.
As elucidated by this randomized trial from Wellington Hospital in New Zealand, intermittent fasting “of any type increased the rate of hypoglycemia.”
In the trial, 37 type 2 diabetics fasted every day or every other day for twelve weeks, after which the “mean hypoglycemia rate” was calculated as 1.4 events, even after antidiabetic medication was reduced.
This is why it’s critical that diabetics speak with their physicians before trying intermittent fasting; the metabolic changes associated with this method are strongly connected to the effects of hypoglycemic medications and vice versa.
Irritability and Overeating
Everyone gets “hangry” at one point or another, a playful term that hints at the relationship between appetite-regulating hormones and mood.
As it turns out, there is indeed a biological basis for irritability caused by hunger, as evidenced in this study from the University Hospital of South Manchester in the UK.
After four weeks of “intermittent energy restriction,” i.e., fasting, study participants demonstrated “increased feelings of hunger, worse mood, heightened irritability, difficulties concentrating,” and other problems related to mood and focus.
Most clinicians agree that these effects are related to the stress hormones our bodies release to counteract drops in glucose levels.
In other words, when well-fed, the brain can commit some resources to politeness and other pleasantries, but when fasting triggers the fight-or-flight response, manners make way for survival instincts.
Defining the Right Circumstances
Finding success with intermittent fasting is a matter of setting the proper context before you start.
For example, if you are a child under the age of 18, a pregnant or breastfeeding woman, or someone who struggles with stress and/or hormonal imbalances, intermittent fasting probably isn’t a great idea because these populations are more vulnerable to health concerns when fasting.
Also, remember to properly hydrate during fasting periods and resist the hormonally driven urge to overeat when the fasting period ends.
It will always be a struggle when first transitioning to this kind of eating pattern, but by taking these precautions and speaking with your doctor first, you can create a safer and smarter approach to fasting.