Over the past decade, intermittent fasting — where people don’t eat for a set amount of time each day or week — has become an increasingly popular weight loss plan. For many of these fasting strategies, no calorie counting is required, which many dieters prefer. Taking long breaks to eat can still lose weight, unless you overeat when it comes time to eat.
While some researchers have argued that traditional calorie tracking is a more effective and proven way to lose weight, a new study published this week in the Annals of Internal Medicine shows that time-restricted feeding can produce similar results.
“We’ve found that people can simply count time instead of count calories and achieve the same calorie reduction and weight loss,” says Krista Varady, PhDprofessor of nutrition at the University of Illinois at Chicago.
“For some people it’s just a lot easier, and hopefully in the long run they can stick to this plan better than the tedious daily calorie count.”
Lose weight without tracking calories
To see how intermittent fasting compares to calorie restriction, Dr. Varady and his colleagues followed 90 obese adults from around Chicago, 77 of whom completed the study. The subjects were of various races (black, Hispanic, Asian and white), approximately 80% female and aged 44 on average.
Participants were randomly assigned to one of three plans:
- Eat as much as you want but limit yourself to an eight-hour window (between noon and 8 p.m.)
- Count calories and reduce usual calorie intake by 25%
- As a control group, stick to usual eating habits (consumption over a period of 10 hours or more per day)
The calorie-restricted and fasting groups met weekly with a dietician for the first six months of the survey, then every two weeks for the next six months. The dietary advice aimed to improve eating habits, providing information such as the benefits of increasing fruit and vegetable consumption and reducing soda and alcohol consumption. All participants were instructed not to alter their exercise level.
After one year, the fasting and calorie-restricted groups achieved similar results. Compared to controls, those who fasted and did not track their calories cut an average of 425 calories per day and lost about 10 pounds more. Meanwhile, those in the calorie counting group reduced daily calories by an average of 405 and lost nearly 12 pounds compared to controls.
On average, participants in both groups lost 5% of their body weight.
According to Varady, shortening the eating window naturally reduces calories.
Even minor weight loss lowers risk of diabetes and heart disease
“The amount of weight lost indicates that behavioral approaches can work,” says David Creel, PhD, RDpsychologist and dietitian at the Cleveland Clinic.
Even this relatively small weight reduction can have significant health benefits, according to Dr. Creel, who was not involved in the study. Research from Washington University School of Medicine in St. Louis reported that a 5% drop in weight can reduce the risk of diabetes and heart disease, as well as improve metabolic function of the liver, fat and muscle tissue.
Creel adds, however, that in many cases, more interventions — such as medication or bariatric (weight loss) surgery — may be needed to help obese patients achieve more substantial slimming and health benefits.
Intermittent Fasting May Boost Insulin Sensitivity
The study authors also noted an improvement in insulin sensitivity in the fasting group compared to the control group, but not in the calorie-restricted group.
Impaired insulin sensitivity is also known as insulin resistance. Insulin is a hormone produced by the pancreas and plays a vital role in regulating blood sugar (glucose) levels. When a person has reduced insulin sensitivity, their muscle, fat, and liver cells do not respond appropriately to insulin, which means they cannot effectively take up glucose from the blood or store it. This can lead to higher blood sugar and ultimately prediabetes or type 2 diabetes, depending on the American Diabetes Association.
“We know from previous studies that when you lose weight, insulin sensitivity tends to improve,” says Christina C. Wee, MD, associate professor of medicine at Harvard Medical School and director of the obesity research program at Beth Israel Deaconess Medical Center in Boston. Dr. Wee was not involved in the research.
Among the participants, the fasting group had lower insulin sensitivity at baseline. Varady thinks this is why the improvement was only seen in the fasting group and not in the calorie-restricted group. “When someone is more unhealthy to begin with, it’s easier to see changes in that group compared to others,” she says.
Because the study population was small, Wee says it’s “premature” to determine whether either dietary approach provides more benefit when it comes to insulin sensitivity.
Creel would like to see a deeper, more in-depth analysis comparing the two diets.
“We need to look at these approaches in the context of other important aspects of health,” he says. “What do people actually choose to eat during their eating window? How does it impact sleep and physical activity, and is it sustainable in the long term? Clearly the body is well equipped to go longer periods without food, but how does that work for the majority of us in the context of our lifestyles?”