One Major Side Effect of Intermittent Fasting, Says Science

From consistent weight loss to improved blood glucose (sugar) levels, there are a lot of ways people can benefit from intermittent fasting. However, there are a few groups of people who should avoid intermittent fasting, (also known as IF) due to potential side effects—particularly this one major side effect among athletes.

IF is a diet that cycles through periods of fasting and eating, the most common of which involve a 16-hour fast and 8-hour eating window daily. While this style of eating works for some, people who are active should strongly reconsider doing IF. When you are consistently active, your body requires a lot more calories than someone who is sedentary or even someone who is moderately active (walks 1.5 to 3 miles each day, at a pace of three to four miles per hour), for example, which means you need to eat around the clock—not restrictive timeframes. If the body is not properly fueled, it can prolong muscle recovery.

So, who is considered active? According to the most current USDA Dietary Guidelines, an active individual is someone who walks more than three miles a day at a pace of three to four miles an hour. The guidelines also suggest that active females ages 26 to 40 need between 2,200 and 2,400 calories a day, whereas men among the same age range require between 2,800 and 3,000 calories. (Related: 15 Underrated Weight Loss Tips That Actually Work).

However, this doesn't account for athletes, who are routinely burning calories and likely need even more. An athlete, such as someone who regularly does CrossFit or is training for a marathon, may require even more calories than what the guidelines suggest. Plus, it's very important that athletes refuel immediately after they work out. In another Eat This, Not That! article, Kacie Vavrek, MS, RD, LD a sports medicine registered dietitian at The Ohio State University Wexner Medical Center, explains the importance of eating directly after exercise.

"During a hard workout, you will put little tears in your muscle and deplete your glycogen stores," she says. "A recovery meal within 1 to 2 hours [post-workout] plus regular meals every 3 to 4 hours after will help to replace glycogen stores and repair and rebuild muscle throughout the day."

She warned that skipping your post-workout meal could prolong your recovery and even suppress essential muscle-building and repair, which is a serious side effect of intermittent fasting if you're an athlete. Ultimately, this could begin to negatively affect your energy levels and you may begin to lose strength. In fact, one study found that IF could result in a decrease in muscle mass. For example, if you ran for 60 minutes at 9:00 am but didn't eat your recovery meal until 1:00 pm your muscles may be in danger of not fully recovering.

In short, those who are active and engage in intermittent fasting should be cautious. Be sure to talk to a registered dietitian before following this eating pattern to ensure it's safe for your body.