Maybe you’ve tried intermittent fasting (IF) to shed a few (pandemic!) kilos, since the hope and potential for weight loss is what this eating plan is best known for. And yes, scientists are looking into whether or not it really is effective at helping people slim down. But some studies show that IF—in which you only eat during a specified time period—may have other possible long-term health benefits as well.
“The goal with IF is improving metabolic health, reducing the risk of certain conditions such as diabetes, and increasing longevity,” says advanced genomic nutritionist Laura Kelly. “One theory as to why fasting may be beneficial is that during the fasting period, the body’s cells are under mild stress, similar to exercise. The cells respond to this stress adaptively by enhancing their ability to cope with stress and to resist disease.”
The most popular IF methods include:
- Alternate day fasting
- 5/2 fasting, which involves eating regularly for 5 days, with 2 non-consecutive fast days.
- Time-restricted fasting, which limits eating within a certain window of time each day. This method appeals to many people because you can tailor the timing to your lifestyle and schedule, says Kelly. For example, some people may fast from 8pm until lunch around midday the next day, or they may eat only between the hours of 9am to 3pm.
“Intermittent fasting doesn’t have a standard definition or regimen,” says dietitian Deborah Cohen. “Because of the number of different methods, it’s difficult to study and compare them, and we have limited evidence beyond six months about its benefits. Many of the studies also involve a very small number of participants.”
Because of this, there's a lot we don't know yet about IF—but here’s what researchers have found out about the emerging science behind the potential health benefits of intermittent fasting, and answers to your top questions about it:
Is intermittent fasting safe?
For most healthy individuals, it’s fine to follow a fasting regimen. “There’s nothing really harmful in trying it if you have no underlying health issues, such as diabetes,” says Cohen. “And if it promotes weight loss, that can have great psychological benefits, which may spur you on to other healthy behaviors such as making regular exercise part of your life, too.”
Can anyone try intermittent fasting?
Still, most experts agree that some people should steer clear of IF altogether. That includes kids and adolescents because they have higher kilojoule needs due to ongoing growth and development; women who are pregnant or breastfeeding; people with pre-existing conditions such as diabetes; and anyone with a history of eating disorders, says Cohen.
Can you eat whatever you want when you’re not in a fasting period?
Intermittent fasting isn’t a free pass to eat anything you want, contrary to what you may have heard (or wish were true!). You still should avoid processed foods, eat more whole foods from plants and animals, and get moving for at least 150 minutes per week. “Your eating plan should be something you can follow the rest of your life to promote good health,” says Cohen.
What are the benefits of intermittent fasting?
Here’s what the science says about the ways IF might help you get healthier.
One study with 150 participants for 50 weeks showed IF to be as effective as (though not more effective than) a diet that restricts kilojoules; other small studies have shown that IF will help you lose weight in the short-term. A review of 27 trials found that weight loss from 0.8 percent to 13 percent of baseline body weight occurred with IF, though the researchers pointed out that larger studies of longer duration are needed. In 16 of the studies that measured BMI, the participants’ BMI decreased by 4.3 percent, says Kelly. Reducing the daily eating window may also decrease kilojoule intake for the whole day, resulting in weight loss without restrictive kilojoule counting.
There’s some research showing potential benefits to heart health. Short-term studies show IF may be beneficial for regulating blood glucose levels and lipid panels (meaning cholesterol and other blood fats), though those effects may be partially related to the weight loss itself. A small 12-week study showed a decrease in waist circumference and visceral fat in people with metabolic syndrome. In addition, a small short-term study found that following an 8-hour time restricted feeding resulted in a slight reduction in systolic blood pressure (decrease of 7mmHg) over a 12-week period in obese subjects. However, it’s important to remember that while short-term gains are good, if you’re trying to prevent complications with conditions such as blood pressure, it’s important that those gains stick around long term, says Cohen.
Chronic inflammation is associated with a long list of health conditions, including dementia, diabetes, stroke, and heart disease, says Cohen. While mice studies have demonstrated that short periods of fasting (24 hours) show a reduction in inflammatory markers, a small human study showed IF reduced the levels of pro-inflammatory factors such as homocysteine and C reactive protein, which contribute to the development of plaque in the arteries. Another small eight-week study in overweight patients showed an improvement of asthma symptoms including better pulmonary function.
Some research has shown that IF induces a process called autophagy, which plays a role in the functions of your immune system, including cell survival, cell defense, and regulation of immune responses, says Kelly. For example, autophagy is necessary for T cell production and survival in fighting off bacterial and viral infections. Preliminary research is exploring how to harness this process as a strategy for treating diseases such as Long COVID.
Decades of research has shown that rodents on fasting diets live longer. Human research also is exploring how fasting may influence circadian rhythms to increase longevity, says Kelly. Circadian rhythms, which affect physiological functions such as sleep and metabolism, are regulated by clock genes. These genes may become disrupted by age, illness, and environmental factors such as poor diet and stress. Studies have shown that fasting may optimise and “reset” these clock genes.
All of these studies show that scientists are on the hunt to figure out the real benefits of IF, whether the potential benefits last long-term, and which people would get the most out of trying out this way of eating. As the research continues, we’ll learn more about how IF may be one more useful tool for helping us live longer, healthier lives.