Intermittent fasting?(IF),?one?of the most talked about diets right now,?is a way of eating that designates periods of time for eating and for fasting. And no signs suggest interest is waning. “IF, in its different forms, is holding a steady pace,” says?Kimberley Rose-Francis, RDN, CDCES, based in Sebring, Florida.
There are a?few approaches, but the two most popular are?16:8, which calls for squeezing all the day’s meals into an?eight-hour window and fasting for the remaining 16 hours, and 5:2, in which five days of the week are spent eating normally and two are spent?fasting?(usually defined as eating only 500 to 600 calories per day).
Why would someone opt for this way of eating versus a standard?diet, such as going?low carb?or low fat? Some people say fasting has loads of health benefits. “The research so far proves the benefits of IF to the extent that it is worthwhile as a method to lose weight,?manage your blood sugar,?and slow down the aging process,” says?Sara Gottfried, MD, of Berkeley, California, author of?Women, Food, and Hormones, The Hormone Cure, The Hormone Reset Diet,?and?Brain Body Diet.
But not everyone’s on board. “From my standpoint and the standpoint of a lot of other people, it does tend to fall into the next?fad diet?category,” says?Elizabeth?Lowden, MD, a Warrenville, Illinois–based physician who is board-certified in obesity medicine; endocrinology, diabetes, and metabolism; and internal medicine. A lot of the data is conflicted, she says, and many studies done on animals have not yet been repeated in people. “For every study that shows there's no change, there are some studies that show maybe there is improvement,” Dr. Lowden says.
So rather than take popular claims about IF at face value, we decided to dive into them and explore whether 12 touted benefits of the approach are legit or the science doesn’t yet stack up.
1. Weight Loss
Most people start IF to lose weight. And that claim seems to hold up, at least in the short term. According to an?article published in February 2020 in the Canadian?Family Physician, IF may contribute to weight loss for overweight or obese individuals. The researchers looked at data from 27 studies and found that IF helped participants lose up to 13 percent of their weight.
That’s probably welcome news if you’re hoping to fast for weight loss, but the fact that those studies were short term means it’s unclear if IF is sustainable and can help you keep off extra pounds in the long run.
What’s more, not all studies have found IF resulted in weight loss. A?study published in September 2020 in JAMA Internal Medicine involving 116 overweight or obese people who ate between the hours of 12 and 8 p.m. for 12 weeks found that they did not have significantly more weight loss than the control group.
The other catch: The amount of weight lost doesn't seem to be any?more than?what you’d expect from?another calorie-restricted diet. A?review published in October 2019 in Nutrients found that similar amounts of weight and fat loss were achieved through IF and continuous energy restriction, such as a low-calorie diet. And depending on how many calories you’re eating each day, you could even end up gaining weight.?After all, the diet doesn’t restrict high-calorie foods or total calories — it only restricts when you can eat.
When the diet is done properly, IF can be as effective as normal caloric restriction, Lowden says.?Some people, especially busy people who don’t have time to devote to?meal planning,?might even find a time-restricted diet easier to follow than something like the keto diet or the paleo diet, she says.
2. Reduced Blood Pressure
IF may help?lower high blood pressure?in the short term. A pilot study published in June 2018 in?Nutrition and Healthy Aging?found that 16:8?significantly decreased systolic blood pressure in the 23 study participants. As the Mayo Clinic notes, systolic blood pressure is the top number in your?blood pressure reading and indicates the force of the heart against your artery walls each time it beats.
The link between lower systolic blood pressure and IF appears in both animal and human studies,?according to a?review published in March 2019 in?Nutrients.?And a?study published in September 2020 in the European Journal of Nutrition?found that IF led to even greater reductions in systolic blood pressure than another diet that didn’t involve defined eating times.
Having a healthy blood pressure is important — unhealthy levels can hurt your heart, brain, kidneys, and eyes, according to the?Centers for Disease Control and Prevention (CDC).
But so far the research shows that these blood pressure benefits last only while someone is doing IF. Once the diet ended and people returned to eating as was normal for them, researchers found that blood pressure readings returned to their initial levels.
3. Reduced Inflammation
Animal studies suggest that both IF and general calorie restriction?can?reduce inflammation?levels, though clinical trials are few and far between. The authors of?research published?in?Nutrition Research wanted to know if that link existed among humans, too, so they analyzed a group of 50 participants who were fasting for Ramadan, the Muslim holiday, which involves fasting from sunrise to sunset and eating overnight. The study showed that during the fasting period, pro-inflammatory markers were lower than usual, as was blood pressure, body weight, and body fat.
4. Lower Cholesterol
According to a review published in?October 2021 in the Annual Review of Nutrition, various forms of IF, including alternate-day fasting and 5:2, can help lower LDL (“bad”) cholesterol, among other markers of cardiometabolic health, such as blood pressure.
LDL cholesterol can raise your risk of heart disease?or stroke, according to the?CDC. The?researchers also noted?that IF reduced the presence of?triglycerides, which are fats found in the blood that can lead to stroke, heart attack, or heart disease,?according to the Mayo Clinic.
Yet not all researchers agree that IF significantly reduces cholesterol levels. A?review published in the summer 2020 issue of the Journal of Patient-Centered Research and Reviews concluded that there wasn’t a difference in cholesterol levels between those who did IF and those who followed a low-calorie diet.
5. Better Outcomes for Stroke Survivors
Healthier?cholesterol levels?and lower blood pressure (two benefits noted above) play a major role in helping reduce your?risk of stroke. But that’s not the only possible stroke-related benefit of IF. An?article published in October 2019 in Nutrients found that fasting may provide a protective mechanism for the brain and enhance recovery from a stroke, partly because of IF’s anti-inflammatory effect. That was the conclusion based on animal studies — the researchers noted that human studies regarding the effects of IF on stroke are lacking.
6. Boosted Brain Function
Dr. Gottfried says that IF may improve mental acuity and concentration.?And there’s some early research to support that idea: A?study published in November 2021 in Molecular Psychiatry found that fasting every other day?may enhance memory. This study was done only in animals, though. A?review published in September 2021 in Nutrients took a look at how IF affects human brains and found that it doesn’t seem to lead to short-term cognition benefits among healthy people, but it may guard against the development of a neurological disorder.
7. Cancer Protection
A fasting diet may reduce?cancer risk?by slowing the ability of cancer cells to adapt and spread and by improving the effects of cancer treatment, according to a?review published in?November 2018?in Nature Reviews: Cancer. This is another area where more research is needed: An?article published in November 2021 in CA: A Cancer Journal for Clinicians notes that more high-quality randomized clinical trials are needed to confirm this association. Of course, it’s also important that people undergoing cancer treatment consult their healthcare team before making changes to their diet.
8. Increased Cell Turnover
Gottfried says that the period of rest involved in intermittent fasting increases autophagy, which is “an important detoxification function in the body to clean out damaged cells,” she says. Put differently, a break from eating and digestion gives the body a chance to heal and get rid of junk inside the cells that?can accelerate aging, she explains.
A?study published in?May 2019 in?Nutrients?found that time-restricted feeding, which the researchers defined as eating between 8 a.m. and 2 p.m., increased the expression of the?autophagy?gene?LC3A?and the protein?MTOR, which regulates cell growth. This study was small, involving only 11 participants for four days.
9. Reduced Insulin Resistance
Gottfried proposes that intermittent fasting may help stabilize blood sugar levels in people with diabetes because it “resets insulin,” though more research is needed. The idea is that restricting?calories may improve?insulin resistance, which is a marker of type 2 diabetes,?according to a?study published in April 2019 in?Nutrients. Fasting, such as the kind of fasting associated with IF, encourages insulin levels to fall, which may play a role in reducing the risk for type 2 diabetes, the study notes.?“I?have colleagues at other facilities who have seen positive results especially in improvements in insulin [medication] needs for diabetics,”?Lowden?says.
The Nutrition and Healthy Aging?study investigated this effect in humans, and while a 16:8 approach did result in reductions in insulin resistance,?the results were?not significantly different from the control group.?And again, this study was small.
Registered dietitians advise people with diabetes to approach intermittent fasting with caution. People on certain?medications for type 2 diabetes?or those on insulin (whether to manage blood sugar for type 2 or?type 1 diabetes) may be at a greater risk for low blood sugar, which can be life-threatening. Check with your doctor before trying intermittent fasting if you have any type of diabetes, they advise.
10. Lower Risk of Cardiovascular Issues
Per the?Nutrients?study, when insulin levels fall, so does the risk of dangerous cardiovascular events, such as congestive heart failure, which is important for people with type 2 diabetes to consider because they are twice as likely to develop and die from heart disease than adults without diabetes,?according to the?American Heart Association.
The Nutrients study noted that while there aren’t prospective human studies demonstrating this effect, observational studies have shown that IF may deliver both cardiovascular and metabolic benefits.?Lowden?suspects that changes to metabolic parameters, such as lower levels of?triglycerides and a decrease in blood sugar levels,?are the result of losing weight and would be achieved no matter how the weight was lost, whether through IF or a low-carb diet, for example.
For instance, a?review published in January 2021 in the Cochrane Database of Systematic Reviews found that there wasn’t a significant difference between IF and reduced calorie intake in lowering risk factors for cardiovascular disease.
11. Increased Longevity
There have been a few animal studies that have shown IF may extend?life span, possibly because?fasting?seems to build resistance to age-related diseases. A study published in September 2021 in?Nature?found that IF slowed the aging process within cells of fruit flies, helping the fruit lies live longer. A?review published in?June 2018 in?Current Obesity Reports?noted that while these findings are promising, they haven’t been replicated in human studies. Until that happens, it’s best to be skeptical about this potential benefit.
12. A Better Night's Sleep
If you’ve ever felt like you slipped into a food coma after a big meal, you know that diet can have an impact on wakefulness and sleepiness. Some IF followers report being able to sleep better as a result of following this way of eating. “IF and mealtimes may have an impact on?sleep,” Rose-Francis says.
Why? One theory is that IF regulates circadian rhythm, which determines sleep patterns. A regulated circadian rhythm means you’ll fall asleep easily and wake up feeling refreshed, though research to support this theory is limited,?according to an?article published in 2018 in Nature and Science of Sleep.
The other theory centers on the fact that having your last meal earlier in the evening means you’ll have digested the food by the time you hit the pillow.?According to the?Sleep Foundation, going to sleep with a full stomach (especially if that last meal involved heavy or spicy foods) can interfere with digestion or give you?heartburn, which can make it hard to fall asleep.