Intermittent fasting, which restricts your snacks and meals to a certain timeframe during the day, may have some benefits depending on your goals. But when it comes to weight loss—if that’s your goal!—a new study in the journal Science Translational Medicine suggests the approach isn’t more effective than simply reducing the total daily calories you eat, and eating them at unspecified times.
In fact, intermittent fasting may even be detrimental to muscle mass, according to lead researcher James Betts, Ph.D., co-director at the Center for Nutrition, Exercise, and Metabolism at the University of Bath in the U.K.
Researchers recruited 36 participants and assigned each to one of three groups for three weeks. The first group didn’t intermittently fast at all and just reduced their daily calories by 25%. The other two groups fasted on alternate days, with one group following a fast day by eating 50% more than usual and the other group eating 100% more.
At the study’s conclusion three weeks later, the last group showed no weight loss, while the first group and the 50% group each lost about the same amount of weight but with significant differences, Betts told Runner’s World. Those who fasted and then ate 50% more lost much more muscle mass than the non-fasting group.
“Because of this, anyone who wants to do intermittent fasting needs to keep in mind that they may need to do more physical activity in order to maintain muscle mass,” he said.
Another factor to consider here is quality of foods, Kristin Gillespie, M.S., R.D., a dietitian and certified nutrition support coach, told Runner’s World.
“You may help stimulate weight loss if you’re consuming fewer calories simply because you’re reducing the number of hours that you’re eating,” she said. “However, it is certainly possible to offset this calorie deficit if you’re consuming calorie-dense foods during your eating window.”
The premise of intermittent fasting is that you’re meant to be eating nutrient-dense, healthy foods in relatively normal amounts during the eating timeframe, she added. When you stray away from that with a junk food binge, it’s doubtful you’ll see benefits just from going without eating for 12 hours or more.
Another caveat is that the study did include men and women, but didn’t differentiate those results. Gillespie said that women generally don’t do as well with this strategy because they tend to be more adaptable to periods of fasting and energy conservation, so it would be interesting to see a study focusing on only women.
Finally, there’s the size of the study—36 participants is considered a modest sample—and the fact that all of the participants were technically classified as “lean” with a body mass index in the low 20s. However, Betts said that the results should not dissuade anyone from trying the tactic, especially if they’re willing to play around with it.
“I do think that different fasting strategies may have different effects,” he said. “For example, whether to vary fasting times each day, how you re-feed, how long you fast, and so on.”
The bottom line? Eat in a way that works best for you and lines up with your own nutrition and training goals. And, as always, consulting with a sports dietitian or other medical expert is the best way to personalize your diet for best results.
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