Intermittent fasting is a popular trend, but you may not understand how it impacts your metabolism and muscle mass. 

In today’s blog post, let’s take a closer look at a case study of an individual who’s metabolism was severely affected by intermittent fasting. 

Before we dive in, let’s define intermittent fasting. It’s an eating pattern that cycles between periods of fasting and eating with a focus on when you eat instead of what you eat. 

In this case study, the patient followed the 16/8 method, which involves a 16-hour fasting period and 8-hour eating period each day. Specifically, she ate between the hours of noon and 8 p.m. Like many individuals, she wasn’t a big breakfast eater, so she figured it was no problem to cut out this meal. In truth, it meant she wasn’t eating enough calories. 

As dietitian Elaina Efird, RDN, CD, CEDRD, CSSD explains in a video on our YouTube channel, “In that noon to 8 p.m. window, she was only eating about 1,500 calories per day, which is not enough to fuel her body.” Plus, she was exercising six days per week for an hour at a time, usually in the morning during her fasting period. 

Combined, her total caloric intake and her exercise habits ultimately had a huge impact on her metabolic rate and her muscle mass. Her resting metabolic rate was only 877 calories per day when it should’ve been 1,438 calories per day (or even higher). “This happened because she wasn’t fueling her body enough,” Elaina reveals. Her body’s not going to keep burning more calories than it’s being given, so her metabolism adapted. 

Additionally, her body was breaking down 25 percent more muscle mass than it should. That means her body was relying on lean mass and muscle to get those additional calories it needed to function. 

This individual was adamant about sticking with intermittent fasting, so we recommended an intake of 2,300 calories per day in her 8-hour eating window. 

This caloric recommendation was meant to fuel her body’s needs at rest, to fuel her workouts, and to restore her metabolism. At her next visit, she admitted that it was really hard to eat 2,300 calories in such a limited window of time. As a result, she incorporated breakfast back into her day.

“That is the case for so many people. They just cut out breakfast and don’t eat enough during the remainder of the day,” Elaina says. “When they try to increase their calories, they can’t do it in that small period of time.”

Oftentimes, with intermittent fasting, individuals are just eating less calories than they previously did. Research has shown that there is no difference in weight loss when following intermittent fasting or a normal calorie deficit. In fact, some studies have shown that people lose muscle mass with intermittent fasting, as we saw in this case study. 

The lesson here? Listen to your body!

This individual noticed poor energy during her workouts but brushed it off as a normal part of fasting and working out. In truth, she was severely under-fueling her body because of her intermittent fasting habits. 

She continued to eat 2,300 calories throughout the full day and almost immediately noticed better energy and better performance during exercise. Overall, her body functioned much better for her. 

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