Intermittent Fasting Beats Calorie Cutting for Weight Loss

Intermittent fasting (IMF) with behavioral support may be more effective and better tolerated by patients than standard daily caloric restriction (DCR) in weight-loss programs, a randomized study found.

A 4:3 IMF program produced modestly superior weight loss than DCR of 2.89 kg over 12 months in the context of a guidelines-based, high-intensity, comprehensive behavioral weight-loss program, according to Danielle M. Ostendorf, PhD, MS, co-lead author and an assistant professor at The University of Tennessee Knoxville, and Victoria Catenacci, MD, study principal investigator, co-lead author, and an associate professor located at the University of Colorado Anschutz Medical Campus, Aurora, Colorado.

The study, published in Annals of Internal Medicine, found that objectively measured percentage caloric restriction was greater in the 4:3 IMF group, whereas there was no between-group difference in change in total moderate-to-vigorous physical activity, suggesting that differences in weight loss may have been due to greater adherence to 4:3 IMF. The 4:3 IMF program was well tolerated and attrition was lower in this group: 19% for IMF group vs 30% for DCR group.

The authors noted that alternative patterns for restricting dietary energy intake are gaining attention owing to the difficulty of adhering to a reduced-calorie diet daily, with most adults who lose weight through DCR showing significant weight regain a year later.

According to Ostendorf and Catenacci, fasting strategies “come in two different flavors and oftentimes get confused in the lay press and by patients and researchers. And there is a difference between IMF and time-restricted eating (TRE),” they told Medscape Medical News. “TRE involves limiting the daily window of food intake to 8-10 hours or less on most days of the week — for example, 16:8 or 14:10 strategies. TRE is done every day, consistently and involves eating in the predefined window, and fasting outside of that window.” 

IMF is a more periodic and significant fast and involves cycling between complete or near-complete (> 75%) energy restriction on fast days and ad libitum energy intake on nonfast days.

An appealing feature of IMF is that dieters do not have to focus on counting calories and restricting intake every day as they do with DCR, the authors wrote. Furthermore, the periodic nature of fasting is simpler and may mitigate the constant hunger associated with DCR.

Some said the diet was dreadful, but many said it was the easiest diet they had ever been on. “But it did take time for people to adjust to this strategy,” Catenacci said. “It was reassuring to see no evidence of increased binge-eating behaviors.”

Although objectively measured adherence to the targeted energy deficit (percentage caloric restriction from baseline) was below the target of 34.3% in both groups, the 4:3 IMF group showed greater percentage caloric restriction over 12 months. This suggests that, on average, the 4:3 IMF group may be more sustainable over a year than the DCR group. However, weight loss varied in both groups. Future studies should evaluate biological and behavioral predictors of response to both 4:3 IMF and DCR groups in order to personalize recommendations for weight loss.

Study Details

The investigators randomized 165 patients at the University of Colorado Anschutz Medical Campus, with a mean age of 42 years (18-60), a mean baseline weight of 97.4 kg, and a mean baseline body mass index (BMI) of 34.1 to IMF (n = 84) or DCR (n = 81). Of these, 74% were women and 86% were White individuals, and 125 (76%) completed the trial.

The 4:3 IMF group restricted energy intake by 80% on 3 nonconsecutive fast days per week, with ad libitum intake on the other 4 days (4:3 IMF). The 80% calorie reduction fasting corresponded to about 400-600 kcals/d for women and 500-700 kcals/d for men.

“Participants were only required to count calories on their fast days, which is part of the appeal,” Ostendorf said. Although permitted to eat what they wanted on nonfast days, participants were encouraged to make healthy food choices and consume healthy portion sizes.

For its part, the DCR group reduced daily energy intake by 34% to match the weekly energy deficit of 4:3 IMF.

Both groups participated in a high-intensity comprehensive weight-loss program with group-based behavioral support and a recommended increase in moderate-intensity physical activity to 300 min/wk.

On the primary endpoint, the 4:3 IMF group showed a weight loss of 7.7 kg (95% CI, –9.6 to -5.9 kg) compared with 4.8 kg (95% CI, -6.8 to -2.8 kg, P =.040) in the DCR group at 12 months. The percentage change in body weight from baseline was –7.6% (95% CI, –9.5% to –5.7%) in the 4:3 IMF group and –5% (95% CI, – 6.9% to –3.1%) in the DCR group.

At 12 months, 58% (n = 50) of participants in the 4:3 IMF group achieved weight loss of at least 5% vs 47% (n = 27) of those in the DCR group. In addition, 38% (n = 26) of participants in the 4:3 IMF group achieved weight loss of at least 10% at 12 months vs 16% (n = 9) of those in the DCR group. Changes in body composition, BMI, and waist circumference also tended to favor the 4:3 IMF group.

On other 12-month measures, point estimates of change in systolic blood pressure, total and low-density lipoprotein cholesterol levels, triglyceride level, homeostasis model assessment of insulin resistance, fasting glucose level, and A1c level favored 4:3 IMF. Point estimates of change in diastolic blood pressure and high-density lipoprotein cholesterol level favored DCR.

Currently lacking, the authors said, are data on safety in children and older adults, and adults affected by a long list of conditions: Diabetes, cardiovascular disease, kidney disease (stage 4 or 5), cancer, and eating disorders. Also, people of normal weight or only mild overweight, and pregnant or lactating women. “There have been concerns about IMF causing eating disorders, so we did not include people with eating disorders in our study,” Ostendorf and Catenacci said.

Offering an outside perspective on the findings, James O. Hill, PhD, director of the Nutrition Obesity Research Center and a professor at The University of Alabama at Birmingham believes IMF is a viable option for people trying to lose weight and has prescribed this approach for some in his practice. “But there is no one strategy that works for everyone,” he told Medscape Medical News. “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.” But as it won’t work for everyone, “we need to consider both metabolic success and behavioral success. In other words, would it be more effective if people could do it and how easy or hard is it for people to do?”

As for the influence of metabolic factors that may be associated with better weight loss, Ostendorf said, “Be on the lookout for future publications in this area. We are analyzing data around changes in energy expenditure and changes in hunger-related hormones, among others.” A colleague is collecting biological samples to study genetics in this context. “However, in general, it appeared that the difference in weight loss was due to a greater caloric deficit in the 4:3 IMF group.”

Ostendorf and Catenacci are currently conducting a pilot study testing 4:3 IMF in breast cancer survivors. “We think this is a promising strategy for weight loss in breast cancer survivors who struggle with overweight/obesity in addition to their cancer diagnosis,” Ostendorf said.

This study was funded by the National Institute of Diabetes and Digestive and Kidney Diseases. Ostendorf and Catenacci and colleagues disclosed no relevant financial conflicts of interest.