Intermittent fasting (IF for short) is right up there with the Keto diet on the list of popular fad diets of the past decade. Which is a bit ironic considering fasting has been practiced for centuries — albeit mostly for religious and spiritual purposes, versus the potential health and weight loss benefits that have given rise to its mainstream appeal in recent years.

Despite IF’s lengthy track record and the growing scientific research supporting its safety and efficacy, many questions remain – many of which are answered below by one of the United States Military’s most knowledgeable dietitians.

Susan Lopez is a tactical performance dietitian who specializes in working with some of America’s bravest — from military athletes to firefighters, police officers, and first responders. Lopez is a military veteran and special operations spouse whose unique experience and knowledge help elite war fighters and community heroes stay fit and healthy. She is also the team dietitian for Bravo Sierra.

What goals and individuals are best served by intermittent fasting? And what specific IF protocols are best for achieving these goals?

The intention of any style of IF is to stimulate weight loss through calorie reduction or restriction, which is associated with improved hormone levels and other biomarkers. IF should not be considered a performance diet, but rather a health-optimization and weight-loss strategy. Individuals with physically demanding jobs or who consider themselves athletes may need to consider other strategies to maintain performance levels.

Popular IF strategies include fasting for 18 hours, including sleeping, and eating the remaining six hours of the day (aka 18:6). Another method is to eat normally five days a week, and then consume approximately 500 calories total on each of the other two days (aka 5:2). A third popular method includes eating normally six days a week, followed by fasting for a complete 24-hour period.

If an athlete chooses to use IF, carefully consideration around workout schedules or sporting competitions is recommended. And just like any other calorie-restricted diet, it should have a specific start date and end date with periods of maintenance eating, or “diet breaks,” planned to keep the metabolism working efficiently. Anyone with a medical condition such as diabetes, cardiovascular disease, or any other metabolic disorder should not use Intermittent Fasting and should see their health care physician before undertaking any nutrition program.

Of the three popular IF strategies, do you recommend one over the others for health optimization and weight loss? Or is it more a matter of picking which of the three best fits one’s lifestyle?

Assuming calorie restriction is equal across the different styles, any style that fits the person’s schedule and personal preferences can work. The easiest to manage in most cases is the 24-hour fast, as it really takes no thought other than just not eating for one full day each week. Many people find that being uncomfortable one day is easier than daily restriction.

You mention people with physically demanding jobs and athletes as those who may not want to do IF. What about people who have neither of those but are seriously into fitness and to some extent could be considered athletes?

Calories should be available when physical activity is highest in order to compensate for the higher energy demand at those times. IF can also be utilized on recovery or off days to avoid limiting energy availability on training days. Energy availability refers to the energy left over for metabolic functions outside physical training. Even for sedentary, recreational, fitness, or aesthetic-related individuals, IF should still be a purposeful and planned activity or cycle.

When you say “consideration around workout schedules” for athletes doing IF, are you talking specifically about making sure you get proper nutrition both before and after training? Do you discourage training in a fasted state?

In most cases, training in a fasted state has no validated benefits, particularly around weight loss, that simple calorie restriction can’t accomplish. I typically don’t encourage it, as it could negatively affect hormone levels.

Generally speaking, what type of “diet breaks” would you recommend? 

The steeper the calorie deficit, the shorter a “dieting” cycle should be and the longer a diet break may need to be. Low energy intake and availability for long periods of time has been shown in research to have negative impacts on resting metabolism; sex and endocrine hormone levels, such as thyroid; as well as mood, lean muscle retention, and bone health.

I typically won’t let an athlete do a hard “cut” (more than a 400- to 500-calorie deficit) for more than five to six weeks at a time using any diet style. Then, I’ll take them to maintenance calories for a week or two before entering another cut phase or training cycle. Lower deficits (around 300 calories or less) can be used for longer periods (i.e., 12 weeks) for someone who wants to remain highly active or experience less decrements in performance during a cut.

Man intermittent fasting and timing his meals and feeding windows

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