What Is Intermittent Fasting, and Should You Try It for Weight Loss?
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There was a time when most people viewed voluntarily going without food as self-punishment (and possibly a warning sign of a potential eating disorder). Today, however, billionaires like Twitter CEO Jack Dorsey and millions on social media have made intermittent fasting a pop culture phenomenon.
But before you decide to fast, learn the facts on what it can—and can’t—do for you.
What Is Intermittent Fasting?
“Intermittent fasting simply means not eating for short stretches of time, typically somewhere between 16 to 24 hours, in the hopes of triggering positive physiological changes,” explains Denis Faye, OpenFit’s executive director of nutrition content.
These goals may include weight loss or health changes.
While many religions include fasting practices, in recent years intermittent fasting has become a secular trend.
How to Intermittent Fast
Some of the more common types of intermittent fasting are:
Most allow water and unsweetened coffee and tea during periods of fasting. Although some think of one meal a day (OMAD) as intermittent fasting, it technically isn’t and has negative implications for health.
16-8 intermittent fasting (also called Leangains) is a type of time-restricted fasting. You eat only during an eight-hour window (say, between 10 a.m. and 6 p.m.) and fast the other 16 hours of the day.
The 5:2 diet calls for following your normal eating plan five days of the week and restricting your intake to 500 to 600 calories the other two days (or the consumption of 20–25% of energy needs).
Also known as a 24-hour fast, the eat-stop-eat method is when you go an entire 24 hours without eating. Most who advocate this plan recommend fasting once or twice a week.
Alternate-Day Intermittent Fasting
Alternate-day fasting is exactly what it sounds like: fasting every other day. Some recommend zero caloric intake on fasting days, while others say to adhere to a modified fast by restricting your eating to about 500 calories on these days.
This “modified alternate-day fasting” may be easier to stick with, says Courtney Peterson, Ph.D., assistant professor of nutrition sciences at the University of Alabama at Birmingham, because you’re not as hungry.
Are There Any Benefits to Intermittent Fasting?
Why go hungry if there are no intermittent fasting benefits? While you can read plenty of anecdotal reports of all sorts of intermittent fasting results, the science doesn’t support everything. Here is what we know.
“Atherosclerosis is the buildup of damaged fat in heart vessels, and time-restricted feeding reduces oxidative stress, or the molecular damage to fat, so it may help with heart health,” Peterson says.
Most human studies, however, have only included overweight or obese subjects. While these indicate that intermittent fasting may help reduce triglycerides and blood pressure, experts caution that more research is necessary.
Similarly, intermittent fasting has been shown to improve insulin sensitivity in animals and overweight and obese adults and are being investigated as a tool to help maintain healthy blood sugar levels.
“It may be that because intermittent fasting gives your body a longer repair period, this improves the body’s ability to process sugar,” Peterson explains.
However, very few trials have examined the effects of intermittent fasting on insulin levels in adults with healthy weights, and in a small study published in the American Journal of Clinical Nutrition, intermittent fasting didn’t affect the insulin sensitivity of eight lean men. So, once again, more research is necessary.
Intermittent fasting has been shown to improve learning and memory in mice, according to one study. Over the years` several studies have focused on the positive effect of intermittent fasting on brain health, showing that it appears to reduce oxidative damage, increase cellular stress resistance, and increase the production of neuroprotective proteins.
Other research looks at potentially using intermittent fasting to aid in the formation of new neurons and the repair of existing ones. But again, the vast majority of this research is in rodents, not humans.
“Such studies should be viewed with healthy skepticism,” Faye says. “Humans, after all, are entirely different animals.”
Does Intermittent Fasting Result in Weight Loss?
Weight loss is probably the main reason anyone tries intermittent fasting.
According to a 2017 review in the journal Circulation, alternate-day fasting leads to about a 1.6-pound reduction to body weight each week, and fasting one or two days a week leads to about a half-pound reduction to body weight each week.
One possible reason is that you simply end up eating less, as long as you don’t overeat during eating times. Another reason is that fasting appears to lower levels of ghrelin, the so-called “hunger hormone.”
“We found that intermittent fasting lower hunger levels and that time-restricted feeding made participants feel a little fuller and slightly reduced their desire to eat,” says Peterson of a study she authored.
Other research, however, is inconclusive. In a study published in the American Journal of Clinical Nutrition in 2018, researchers divided 150 overweight and obese nonsmokers into three groups for a 12-week trial:
- One group followed the 5:2 diet
- Another group cut its daily caloric intake by approximately 20 percent every day
- The third group was a control
The scientists continued following the study participants for another 38 weeks after the trial concluded. After all that time, both intervention groups experienced similar weight loss and health improvements, showing that intermittent fasting isn’t superior.
A review of 11 trials published at the end of 2018 comparing intermittent and continuous energy restriction came to the same conclusions.
“Thus, despite the statistical significance of weight loss results, the clinical significance and practicality of sustaining an intermittent fasting regimen are questionable,” the authors of a review in Current Obesity Reports state.
“If you want to lose weight, there’s a far more common-sense solution,” Faye says. “Considering that most dietary transgressions occur after dinner, when television watching and mindless eating go hand-in-hand, try a 12-to-15 hour daily fast, meaning that the kitchen closes after dinner and doesn’t open again until breakfast.”
Although for most people an 8-to-10 hour eating window (and 14-to-16 hour fast) leads to the best results; even cutting to 11 or 12 hours (and fasting 12 to 13 hours) may lead to benefits, Peterson adds.
Are There Any Side Effects to Intermittent Fasting?
Other than hunger (and any subsequent side effects of that, such as feeling lightheaded or irritable), there aren’t many side effects of intermittent fasting. However, some people should not try intermittent fasting. This includes:
- Those with a history of an eating disorder or disordered eating
- People with diabetes
- Women who are pregnant or breastfeeding
- Those who are underweight
Also, if you take any medications, talk to your doctor before fasting.
Should Women Try Fasting?
The reviews of intermittent fasting for women are mixed. Anecdotally, some women report positive outcomes, while others report irregular menstrual cycles.
“There are no published studies suggesting that intermittent fasting affects menstrual cycles,” Peterson says. “However, there is data suggesting that not eating enough calories does.”
In a study of 54 obese women published in Nutrition Journal, combining intermittent fasting with caloric restriction led to reductions in body weight, fat mass, and visceral fat mass. However, these women were eating no more than 1,080 calories a day on non-fasting days.
And in a recent Australian study, overweight women who followed an alternate-day fasting program — and also reduced their caloric intake to 70 percent of their required energy intake — lost more weight than women who fasted but did not reduce their intake, women who only cut calories, and a control group. However, risk markers for diabetes increased in those who only fasted and the study authors did not look at reproductive health.
While there isn’t enough scientific evidence of how fasting affects women, if you notice any changes in your cycle while intermittent fasting, stop fasting and talk to your healthcare provider.
Also keep in mind that based on animal studies, “we think it may be a little be harder for women to fast, or they may need a shorter fasting duration,” Peterson says.
Should I Try Intermittent Fasting?
Your diet is ultimately your decision.
As long as you do not fall into any of the categories listed under side effects above, intermittent fasting may work for you. We continue to learn more about meal timing and may in the future find more conclusive human evidence that fasting is valuable.
“Intermittent fasting may one day become for nutrition what high-intensity interval training became for fitness—a technique used for years that science eventually catches up to and validates,” Faye says. “But for the moment, nothing is concrete, so you’re better off staying cautiously optimistic. Dig into the research, and make sure the model you have in mind is sane.”