Intermittent Fasting May Be More Effective Than Daily Calorie Restriction

“Calories in, calories out” is a well-known phrase in the world of dieting that emphasizes the balance between energy consumed and energy expended. While maintaining a calorie deficit does result in weight loss, emerging research suggests that the timing of calorie intake may play an equally, if not more, important role.

Fasting methods of intermittent Fasting (IMF) and Time Restricted Eating (TRE) often become conflated with one another. IMF refers to cycling through longer periods of fasting where some days involve greater than 75% calorie restriction and on the other non-fasting days, an unrestricted calorie intake is permitted. On the other hand, TRE is focused on limiting food within a daily window. For example, someone on a TRE diet may consume food for an 8 hour window and then restrict all calories for 16 hours when they can break their fast the next day. The TRE fasting method is done within a 24 hour window, whereas IMF is done on a weekly basis.

IMF may be a more sustainable and appealing method to some dieters due to the fact that it does not involve daily restrictions. Another popular and non-time restricted dieting method is Daily Calorie Restriction (DCR) which involves eating in a calorie deficit on a daily basis. Both TRE and DCR involve being vigilant about caloric intake every day. Additionally, the on and off fasting that IMF provides may feel more simplified and sustainable than having to continually ration calories every single day.

“IMF may be a more sustainable and appealing method to some dieters due to the fact that it does not involve daily restrictions.”

A new study led by Danielle M. Ostendorf, PhD, MS and Victoria Catenacci, MD, and published in the Annals of Internal Medicine compared the weight loss effects of 4:3 Intermittent Fasting (IMF) to Daily Calorie Restriction (DCR) over the course of 12 months. They found that the 4:3 IMF group had significantly more success with losing weight than the DCR group. The study took place at the University of Colorado Anschutz Medical Campus and consisted of 165 participants between the ages of 18-60, with a mean age of 42. Of the 165 participants, 74% were women, 86% were White, and 76% (125) completed the trial. Both groups received behavioral support and completed the same amount of moderate to vigorous physical activity of 300 minutes per week.

The IMF group was instructed to restrict their calorie consumption on 3 nonconsecutive fasting days per week and then allowed unrestricted calorie consumption the other 4 days of the week. While they had no limitations on calorie consumption during these 4 days, they were still encouraged to adhere to a healthy diet and consume balanced portion sizes.

During the 3 fasting days, participants of IMF were instructed to limit their daily calorie consumption by 80%. This resulted in consuming only around 500 calories per day for women and 600 calories per day for men. In order to match the weekly calorie intake, the DCR group reduced their daily calorie consumption by 34%. Since the IMF group only had to count calories for 3 days, this diet had more appeal to participants and researchers saw less IMF participants drop out of the study compared to the DCR group.

At the end of the 12 months, the IMF group reported a weight loss of 7.7 kg, whereas the DCR reported a weight loss of 4.8 kg. In addition to a greater reduction in weight loss, body composition, BMI, and waist circumference all favored the IMF group. They also found better health markers in the IMF group. After 12 months, researchers saw a more positive change in systolic blood pressure, total and LDL cholesterol levels, triglyceride levels, insulin resistance, and blood sugar levels (including A1c) for the IMF group. However, the DCR group had greater benefits in health markers such as diastolic blood pressure and HDL cholesterol levels.

“In addition to a greater reduction in weight loss, body composition, BMI, and waist circumference all favored the IMF group.”

It is important to note that while participants found success through IMF, there were certain criteria that these participants did have to meet, meaning IMF may not be safe or effective for everyone. Researchers specifically did not include people with eating disorders in their study as there has been concern that IMF may cause eating disorders. There is also currently no evidence of how this diet may affect older adults, young children, pregnant or lactating women, and adults who may be affected by: diabetes, cancer, advanced kidney disease, and cardiovascular disease.

James O. Hill, PhD Director at UAB Nutrition Obesity Research Center, recommends IMF as an effective dieting method, but expressed that he does not believe there is a universal approach to weight loss. He says, “I recommend IMF as a science-based strategy that can be effective for some people, and I think it should be on the list of science-based tools that people can consider using.” He adds that the component of behavior towards the diet plays as role, as well as metabolic factors, as these can vary quite drastically between individuals.

Overall, it appears that the more significant weight loss seen in the 4:3 IMF group was due to a greater total calorie deficit. A primary appeal of IMF is that it does not come with the constant hunger often associated with DCR. The nature of fasting is simple and only required strict adherence for 3 days of the week, whereas DCR is a daily commitment. While it did take participants time to adjust to the 4:3 IMF diet strategy, researchers were relieved to see no evidence of binge eating behaviors. With the IMF diet, researchers will continue to explore changes in energy expenditure, the role of hunger related hormones, and metabolic variances.

“A primary appeal of IMF is that it does not come with the constant hunger often associated with DCR.”

Although IMF may be beneficial to some, this diet may not work for everyone. Excessive calorie restriction can also result in failing to meet adequate nutritional requirements. That being said, dietitians and other medical professionals, can work with patients and provide guidance on maintaining balance, meeting nutritional needs, feeling satiated, and creating a better relationship with food through IMF diets.

It is important to remember that losing weight is just one step in the path to normalizing body weight. Equally important is the maintenance of a normal weight after a successful weight reduction. Unfortunately, for the majority of people who are successful in the first step, there is a rebound effect (“jojo effect”) which results in regaining the lost pounds, often ending up with a higher weight than in the beginning of the weight loss intervention. The combination of IMF with regular physical exercise and behavioral techniques can prevent this rebound effect. Alternatively, the combination of IMF with one of the new weight loss drugs with exercise and behavioral strategies may currently be the most effective and lasting strategy to return to and maintain a healthy weight.