Used to be that fasting was relegated to religious practitioners and hunger strikers. Nowadays, we can’t go a week without another media report on the wonders of this once esoteric practice.
Like anything in nutrition nowadays, it’s no surprise that for every study showing health benefits of fasting, there’s another study showing detriments. Determining if fasting is good or bad is all the more confusing because there’s no universal definition of intermittent fasting. For some, fasting means not eating for at least a 24 hour period, whereas for others, it’s as simple as containing your eating to a 12-hour window daily.
Then there’s a popular practice called Alternate Day Fasting in which someone typically eats a low calorie diet on “fast” days, but normally on other days. In this article, the term “fasting” will be used generally to refer to all these phenomenon, with specific description of protocols when appropriate.
Another complication is the fact that fasting impacts unique populations differently. Fasting may be more appropriate for overweight folks and those with metabolic disorders than athletes or lean, healthy people. For certain, we know that men and women respond differently, and although the more restrictive fasting protocols may be harmful for women, more moderate options such as containing eating to a 10- or 12-hour daily window, may actually have an anti-stress effect for women.
The number one thing you need to know about fasting is that there’s no simple answer. What works for you might be a nightmare for your neighbor. The key is to do your research, and if you decide to give fasting a try, don’t be afraid to experiment. This article will help you get ahead of the game with a list of the pros and cons of fasting.
How This Article Works
Most articles about intermittent fasting start by presenting the benefits. They give you a laundry list of attractive reasons to abstain from eating, tacking on a few drawbacks at the end. Instead of using that approach, this article will give you the negatives to fasting right up front, and then review the pros, providing you with key tips to minimize the pitfalls.
Be aware that the negatives are more likely in certain populations (athletes, lean and active individuals, or those with high stress levels and poor eating habits), or with protocols that take fasting to the extreme. It’s completely possible the drawbacks can be avoided if you match your protocol to your specific situation and adjust as body composition, lifestyle, and needs change.
Cons of Fasting
#1: Suppression of Hunger-Reducing Hormones
Most people don’t realize that there are a plethora of hormones at play in the body that regulate hunger, appetite, and satisfaction after a meal. For optimal body composition over the long-term, you NEED your hunger hormones working for you:
Both insulin and leptin decrease hunger, giving your brain the message that you’re full, turning eating “off.”
Gut hormones like peptide YY, amlyn, and CCK all produce a short-term feeling of satisfaction and inhibit activity of hormones that turn eating “on”.
Ghrelin, NPY, and orexins all stimulate appetite and promote fat storage, while lowering energy expenditure and making you feel lazy.
A side effect of certain fasting protocols is that these hormones can get out of balance, leading you to become unresponsive to cues that tell us you’re full and should stop eating. For example, if cells become insensitive to leptin or insulin, your brain won’t register satisfaction and hunger will remain elevated.
One example is a short-term study that found that when college women at the University of Virginia fasted for 2 days, they experienced a 75 percent drop in leptin and a 50 percent increase in cortisol (1). Other studies show that the hormones that trigger eating get amplified in response to fasting (2).
In simple terms, this means that people who experience altered hunger hormones in response to fasting find themselves battling an uncontrollable appetite. Then, once they do eat, satiation cues that tell them to stop eating won’t register. Bingeing is a common side effect, and the sad thing is that once people realize they need to alter their fasting protocol or go back to more regular eating, their hunger hormones won’t be working for them.
Avoid It: Instead of opting for full-blown fasting in which you avoid food entirely, a more moderate, approach in which you contain your eating to a 10 to 12-hour window can convey the benefits of fasting, (fat burning and insulin sensitivity) without the suppression of hunger hormones.
#2: Excess Stress & Insomnia
Anytime you go without food for long periods, you activate the fight-or-flight sympathetic nervous system and increase cortisol secretion in order for your body to mobilize energy stores. For some people, such as the overweight, this can actually be beneficial because it improves the body’s ability to burn body fat (see #3 in the pros section).
But for people who suffer a lot of stress in their daily life or who have an elevated cortisol curve, skipping meals and abnormal eating patterns may increase the experience of stress.
High cortisol has a number of negative effects: First, it triggers food intake, and leads to strong cravings for high-carb foods (insulin is a cortisol antagonist and your body basically craves sugar as a mechanism to lower cortisol and reduce the stress).
Second, the combination of fasting and cortisol can produce obsessive thoughts about food, which in turn raises anxiety, causing a further release of cortisol. Preoccupation with food can turn into disordered eating pretty quickly, which is a nasty, but less talked about side effect of some fasting protocols.
Finally, the combination of high cortisol and activation of the hypocretin neurons, which incite wakefulness, can trigger insomnia and trouble sleeping.
Avoid It: Start by establishing a healthy eating pattern with frequent meals eaten at the same time each day. This should allow you to avoid hunger and cravings and get your cortisol in check prior to starting a fasting protocol. Once that happens, a 10- or 12-hour time restricted program may be beneficial: One study of healthy young women found that a 10-hour eating window (with a 14 hour overnight fast) had an “anti-stress” effect, lowering cortisol and increasing parasympathetic activity (3).
#3: Reproductive Hormone Imbalances
You may have heard about animal experiments that have found that females don’t respond well to fasting. For example, female rats that are deprived of food “masculinize,” become infertile and hyper alert, and sleep much less. Researchers suggest that when their bodies detect a starvation state, they develop traits that will help them find food.
In humans, there’s a lack of research on reproductive hormones and the effect of fasting. Studies of Muslim women who fast for Ramadan show that some women experience alterations in their menstrual cycle from fasting: One study found that prior to Ramadan 11 percent of women had an abnormal cycle, whereas during the month of Ramadan this number increased to 30 percent (4).
In regards to men engaged in Ramadan fasting, one study found significant alterations in testosterone release over Ramadan, suggesting alteration of circadian function (5). This isn’t a big surprise based on how sensitive the human biorhythm is to changes in eating and sleeping behavior. Anytime you fast all day and confine eating to nighttime hours, it’s natural to see changes.
Avoid It: Athletes, leaner individuals, and those with more stressful lifestyles are the populations who are most at risk for sustained alterations to reproductive function from fasting. A time-restricted eating model (10 to 12-hour eating window) is one alternative that may not negatively affect androgen hormones. Obese individuals are less likely to experience problems and may even benefit from fasting because a reduction in body fat can significantly improve reproductive capacity.
Pros Of Fasting
#1: Fat Loss
Easily the most compelling benefit of intermittent fasting is that when done correctly, it consistently produces fat loss. In fact, some studies show that losing fat through fasting protocols may be more sustainable than with conventional calorie-cutting diets.
This could be a game changer since conventional diets have poor success rates with a high likelihood of weight regain, especially when people try to employ them in real life out of the structure of a clinical trial setting.
One review of the issue found that across 8 studies that tested a variety of fasting protocols, overweight or obese subjects lost between 0.2 to 0.8 kg/wk over the course of trials ranging from 5 weeks to a year (6). A second review found fasting produced fat loss ranging of 4 to 15 percent, with most of these being on overweight subjects (7).
Dr. Krista Varady who has conducted a number of studies on the topic suggests that fasting is easiest with a calorie cycling protocol, such as alternate day fasting (ADF) or the 5:2 model adopted by some celebrities, including Jimmy Kimmel (8). With ADF, people alternate between days in which they eat normally as much as they want and days in which they restrict calories to about 500.
For example, a 12-week ADF study found that subjects lost an average 3.2 kg of body fat more than a control group (9). A moderately high-fat diet (to improve fat burning in the body) is recommended as is consuming high-quality protein (to sustain lean muscle) and plenty of phytonutrient-rich fruits and vegetables (for fiber and anti-inflammation).
#2: Metabolic Health: Restore Insulin Sensitivity & Lower Cholesterol
Besides possibly being easier to stick to than your typical low-calorie diet, fasting can be used to improve metabolic health by restoring insulin sensitivity, lowering glucose, and reducing belly fat.
This combination is a boon for health and diabetes prevention because when you’re cells are resistant to insulin, your body is much more likely to store the food you eat as fat. Insulin resistance also produces inflammation, causing health problems that no sane person would want to deal with.
A review of studies that compared ADF with calorie restricted diets found that in overweight people, insulin sensitivity improved between 20 to 31 percent, with small reductions in fasting glucose. LDL and triglyceride levels also tend to decrease by as much as 30 percent after alternate day fasting (10).
An 8-week trial that employed three “fast” days a week in which obese subjects ate one meal containing 450 calories alternated with four days in which they could eat normally resulted in a 21 percent decrease in LDL cholesterol and a 32 percent decrease in triglycerides, both of which were associated with loss of body fat (subjects dropped 5.6 kg by the end of the trial) (11).
Blood pressure and heart rate also decreased, likely due to the significant decrease in body size due to the weight loss. Eighty percent of the subjects completed this study, which is typical of ADF trials and a better completion percentage than typical diet studies that tend to report any where from 50 to 75 percent adherence.
#3: Fat Burning & Metabolic Flexibility
One of the best things about certain fasting protocols is that they allow you to become metabolically flexible so your body is easily able to burn body fat. Being metabolically flexible may be the missing link in many fat loss programs because it reduces hunger and food cravings, while improving metabolism and brain function.
A review of the use of intermittent fasting for athletes reported that after 18 hours without food, fat burning increases by 50 percent, which corresponds to a similar decrease in glucose utilization (7).
Besides improving metabolism and making fat loss more achievable, the greater metabolic flexibility associated with fasting may help prevent the loss of lean mass that often occurs with low-calorie diets. Dr. Varady reports that it appears that in obese populations, a lower proportion of lean mass is lost during intermittent fasting (about 10 percent) compared with your typical diet that uses daily caloric restriction (about 25 percent of weight lost is lean mass) (8).
Although alternate day fasting may be more effective for maintaining lean mass than your typical diet, the time-restricted eating model mentioned above may be ideal for leaner individuals and athletes. Limiting your daily food intake to a predetermined window of time will enhance fat burning and may promote optimal body composition.
Although trials are lacking, longer eating windows of 10 to 12 hours are likely more manageable, and for leaner people, may be favorable because they improve circadian function and the regulation of genes that regulate metabolic activity (12-14).
Bottom Line: Planning your eating in a structured, organized fashion so as to minimize hunger but optimize energy levels is a no-brainer if you want to achieve and maintain optimal body composition. Some people will get the best results from ADF, while others will want to adopt a time-restricted eating protocol. Others may find that longer fasts fit the bill. The key is to customize your plan to your unique goals and preferences.