But science has also discovered health benefits linked to whole-day, alternate-day, and time-restricted fasting, says Carolyn Williams, Ph.D., the registered dietitian that helped develop the new book The Mens Health Guide to Intermittent Fasting .
Scientists speculate that the benefits of short-term fasts may come from the structured break they provide to around-the-clock eating.
Even if you dont change the content of your diet, by controlling the time period in which your calories are consumed, you give your body a pause from a constant onslaught food, says Williams.
Maybe youre skeptical . But Williams says that, at first, she was too.
She studied the research. She looked at the data. She even tried a time-restricted fast herself. I expected the fast to affect my blood sugar because Im prone to low blood sugar and I know how I get without eating, Williams says.
But Williams says she was surprised to find that she had no trouble going 16 hours without eating. Her method: She stopped eating after dinner and fasted from 7 p.m. to 11 a.m., following the popular 16:8 intermittent fasting pattern, which leaves an 8-hour-long window for eating.
I find Im really not hungry; in fact, sometimes I have to remind myself to eat lunch, Williams says.
While more research is needed to determine if fasting is effective for long-term dieting, theres no debate that it works in the short-term.
By refraining from eating for at least 12 hours (ideally 16), your body starts burning through glucose and can begin tapping fat for fuel, explains Williams. Studies show that you can expect to lose between 3 and 8 percent of your bodyweight in as few as three weeks.
Compared to calorie-restriction diets, intermittent fasting tends to trigger more belly fat loss, the research suggests. Anecdotally, Williams says she senses greater energy and improved clarity of thought.
Here are some other potential upsides of intermittent fasting, each supported by research.
Intermittent fasting may help maintain muscle .
Whenever you restrict calories and lose weight, some of that weight comes from a reduction in muscle mass. That goes for intermittent fasts as well as traditional calorie-restriction diets.
However, at least one study conducted by the Department of Kinesiology and Nutrition at the University of Illinois suggests that intermittent fasting may be more effective for retaining muscle mass.
The study compared overweight and obese adults who followed a calorie restriction diet with similar-weight subjects who restricted calories through intermittent fasting. After 12 weeks, the researchers found both diets to be equally effective in trimming body weight and fat mass, but less muscle was lost by the group that fasted.
Intermittent fasting may target belly fat.
Overweight people who could choose any 10-hour timeframe to eat as long as they refrained from eating the other 14 hours of the day saw a reduction in waist circumference and visceral abdominal fat after 12 weeks, according to a report in the journal Cell Metabolism.
Intermittent fasting may reduce diabetes risk.
The study in Cell Metabolism referenced above also demonstrated the potential of intermittent fasting to reduce risk of metabolic diseases, such as type 2 diabetes and heart disease.
All the participants in the study were diagnosed with metabolic syndrome, a cluster of health conditionsincluding high blood sugar, excess abdominal fat, high blood pressure, and abnormal cholesterol or triglycerides levelsthat occurring together boost the risk of type 2 diabetes, stroke, and heart disease.
After 12 weeks, every participant experienced improvement in all of the common markers of metabolic syndrome.
A similar study in the journal Translational Research found that alternate-day fasting, in which participants restricted calories by 75 percent on a fast day, followed by a feed day without calorie restriction, resulted in clinically significant reductions in blood sugar and insulin resistance.
Intermittent fasting may lower high blood pressure.
A study published in Nutrition and Healthy Aging demonstrated that participants who practiced 16:8 intermittent fasting without calorie counting significantly reduced their systolic blood pressure compared to a control group after 12 weeks.
Intermittent fasting could fight inflammation.
Inflammation is your bodys natural way of fighting off infection, illness, and injury. But theres another type of inflammation, a chronic inflammation that can silently trigger heart disease and diabetes.
Smoking, mental stress, and a regular diet of fatty, fried, or sugary foods are common causes. Several studies have shown that intermittent fasting may induce an anti-inflammatory effect that reduces risk of those metabolic diseasesand even improve pulmonary function in people with asthma.
Whats more, a reduction in inflammation due to short-term fasting appears to protect the brain from memory disorders and depression, according to a study in Obesity.
Intermittent fasting may reduce oxidative stress.
Even when you dont lose weight while on an intermittent fasting routine, your cells may benefit from extra protection, according to a study in Cell Metabolism.
The study assigned men with prediabetes to either a 6-hour early eating period, where they could eat only from 8 a.m. until dinner before 2 p.m., fasting the rest of the day, or a 12-hour feeding period.
At the end of five weeks, the researchers found that the men on the early time- restricted fast improved blood pressure and insulin sensitivity (as expected), but also improved resistance to oxidative stress, where unstable molecules called free radicals can damage proteins and DNA.
Intermittent fasting may help you live longer.
Rodent studies suggest that intermittent fasting, which is much easier to maintain than extreme calorie cutting, may boost lifespan, too. In one study comparing rats who were given unrestricted access to food to rats who were fed every other day, the rats who fasted lived 83 percent longer than those who gorged themselves.
For a quick primer on how to start a health-boosting fast yourself, plus dozens of delicious recipes for brunch, dinner, and even Keto-diet-friendly meals, check out The Mens Health Guide to Intermittent Fasting here.