Over the past decade, intermittent fasting — where people do not eat for a set period 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 from eating can still shed the pounds unless you over-consume when it does come time to eat.
While some researchers have argued that traditional calories tracking is a more effective, proven way to lose weight, a new study published this week in the Annals of Internal Medicine shows that time-restricted eating may produce similar results.
“We found that people can just count time instead of counting calories and achieve the same reduction in calories and the same weight loss,” says Krista Varady, PhD, a professor of nutrition at the University of Illinois in Chicago.
“For some people, it's just much easier, and hopefully in the long term, they may be able to stick to that plan better than tedious daily calorie counting.”
To see how intermittent fasting compared with calorie restriction, Dr. Varady and her colleagues followed 90 obese adults from around Chicago, 77 of which completed the study. Subjects were racially diverse (Black, Hispanic, Asian, and white), about 80 percent female, and with an average age of 44.
Participants were randomly assigned to one of three plans:
The calorie-restricted and fasting groups met weekly with a dietitian for the first six months of the investigation, and then biweekly for the following six months. The dietary counseling was intended to improve eating habits, providing information like the benefits of increasing fruit and vegetable intake and lowering soda and alcohol intake. All participants were asked not to change their levels of exercise.
After a year, both the fasting and calorie-restricted groups had similar results. Compared with the controls, those fasting and not tracking their calories cut an average of 425 calories per day and lost about 10 pounds more weight. Meanwhile, those in the calorie-counting group reduced daily calories by an average of 405 and dropped almost 12 pounds compared with the controls.
On average, participants in both groups lost 5 percent of their body weight.
According to Varady, shortening the eating window just naturally cuts calories.
“The amount of weight lost indicates that behavioral approaches can work,” says David Creel, PhD, RD, a psychologist and registered dietitian at 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 the Washington University School of Medicine in St. Louis has indicated that a 5 percent drop in pounds may lower the risk for diabetes and heart disease, as well as improve metabolic function in liver, fat and muscle tissues.
Creel adds, however, that in many cases, more interventions — such as medications or bariatric (weight loss) surgery — may be needed to help obese patients attain more substantial slimming and health benefits.
The study authors also noted an improvement in insulin sensitivity in the fasting group compared with 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 impaired insulin sensitivity, their muscle, fat, and liver cells don’t respond appropriately to insulin, which means they can’t efficiently take up glucose from the blood or store it. This can lead to higher blood sugar levels, and ultimately prediabetes or type 2 diabetes, according to the American Diabetes Association.
“We know from prior studies that when one loses weight, your insulin sensitivity tends to improve,” says Christina C. Wee, MD, an 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 worse insulin sensitivity at baseline. Varady believes that’s why improvement was only seen in the fasting group and not the calorie restricted group. “When somebody is more unhealthy at baseline, it's easier to see changes in that group compared to the others,” she says.
Because the study population was small, Wee says that it’s “premature” to determine whether either dietary approach provides more benefit when it comes to insulin sensitivity.
Creel would like to see more extensive, deeper analysis comparing the two diet plans.
“We must 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 long term? It is clear that the body is well equipped to go longer periods without food, but how does this work for the majority of us in the context of our lifestyles?”