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Intermittent Fasting May Not Help Weight Loss Directly, Study Suggests

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Key Takeaways

  • Researchers looked at popular fasting strategies and noted they weren’t more effective than traditional diets that involved reducing calories.
  • This type of fasting may have an indirect effect on weight loss, however, such as prompting more mindful eating.
  • Intermittent fasting doesn’t need to be ditched completely, a dietitian says, but it shouldn’t be the only weight-loss strategy you’re employing.

Although intermittent fasting (IF)—an approach that involves restricting your eating time frame—has garnered attention recently for anecdotal reports of weight loss, a study in Science Translational Medicine suggests the strategy is not more effective than the traditional tactic of reducing calories.

Researchers put 36 participants into three groups for three weeks:

  • Group one fasted on alternate days with the fast day followed by a day of eating 50 percent more than usual
  • Group two reduced calories across meals every day by 25 percent
  • Group three also fasted on alternate days but followed their fast day by eating 100 percent more than usual

All participants ate about 2,000 to 2,500 calories per day on average at the start of the study. During the three weeks, group three maintained that amount while groups one and two reduced their calories to between 1,500 and 2,000.

At the study’s conclusion, participants in groups one and two lost about the same amount of weight. However, those in the first group had a higher percentage of lost muscle mass in addition to body fat compared to the second group, which didn’t fast.

“Anyone trying intermittent fasting may need to think about this finding, because it’s not helpful to lose muscle,” according to the study’s lead author, James Betts, PhD, co-director at the Centre for Nutrition, Exercise and Metabolism at the University of Bath, U.K. “It may be worth thinking about consciously increasing physical activity levels during intermittent fasting, driving more fat loss and hopefully limiting muscle loss.”

Potential Benefits

Although this study may serve as a caveat to people trying intermittent fasting, there are a couple of advantages to the strategy, according to Kristin Gillespie, RD, a dietitian and certified nutrition support coach.

First, this type of fasting—in whatever time frame you choose, because there are many options—tends to make you more aware of what you’re eating in general, she says. That often causes a shift from mindless snacking to more deliberate preparation and consumption, and can increase the amount of nutrient-dense foods versus highly processed choices.

Mindful eating includes practices like:

  • Eating slowly and chewing thoughtfully
  • Experiencing food with all five senses
  • Minimizing distractions while eating
  • Paying close attention to hunger and fullness cues

Research published in Eating Behaviors found that people who received mindfulness training were able to reduce emotional eating behaviors, including binge eating.

Second, even if you take a “free for all” approach to your eating window, in which you don’t count calories at all, that time restriction tends to cause people to eat less, Gillespie says. That means they intake lower calories overall compared to an expanded eating time frame. In part, that may be related to being more thoughtful, as well as recognizing fullness cues more reliably.

Metabolic Benefits

Intermittent fasting has also been shown to have a potentially indirect result on weight loss. For example, a study in The Journal of Nutrition, Health and Aging compared IF to caloric restrictions in people with obesity and found, much like the recent research, that it’s calorie reduction that causes weight loss. However, those researchers noted that even without a change in calories, IF enhanced cellular resistance to disease, most likely because of shifts in metabolic rhythm.

“One way IF is thought to stimulate weight loss is by regulating some of the metabolism-related hormones in the body, primarily insulin and norepinephrine,” says Gillespie. “This is thought to change the way the body metabolizes food and nutrients.”

She adds that this applies less to women than to men. Although the recent study didn’t differentiate results based on gender, Gillespie says women don’t do as well with IF because their bodies are more wired to adapt to periods of energy conservation. For both men and women, the best approach here might be to try IF as part of a weight-loss plan or for possible metabolic advantages, but not have it be the main tactic for either, Gillespie suggests.

“Intermittent fasting shouldn’t be seen as a sole weight-loss strategy, but rather, should be utilized in combination with other strategies,” she says. “That includes calorie deficit, healthy food choices, portion control, and regular physical activity.”

What This Means For You

Intermittent fasting may not take the place of calorie reduction if you're trying to lose weight, but there may be some indirect benefits that might prove helpful.

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  1. Templeman I, Smith HA, Chowdhury E, et al. A randomized controlled trial to isolate the effects of fasting and energy restriction on weight loss and metabolic health in lean adults. Sci Transl Med. 2021;13(598):eabd8034. doi:10.1126/SCITRANSLMED.ABD8034

  2. Katterman SN, Kleinman BM, Hood MM, Nackers LM, Corsica JA. Mindfulness meditation as an intervention for binge eating, emotional eating, and weight loss: a systematic review. Eat Behav. 2014 Apr;15(2):197-204. doi:10.1016/j.eatbeh.2014.01.005

  3. Aksungar FB, Sarıkaya M, Coskun A, Serteser M, Unsal I. Comparison of intermittent fasting versus caloric restriction in obese subjects: a two year follow-up. J Nutr Health Aging. 2017;21(6):681-685. doi:10.1007/s12603-016-0786-y