Dear Doctor: My gym has been closed for months, so I’ve been doing online workouts during the pandemic. A guy in our session keeps talking about something called “fasted cardio,” and how it’s helping him lose fat and gain muscle. What is it? Does it really work?

Dear Reader: The term fasted cardio refers to doing a heart-pumping workout after not eating for a stretch of time long enough to be considered a fast. Often, it’s overnight, which means anyone who has gone for a run or taken a dance class before grabbing breakfast can say they’ve done fasted cardio.

The logic behind the practice lies in the way that our bodies use stored energy. The body’s top choice when it needs energy is glycogen, a form of sugar that is stored in the muscles and the liver. It’s made up of multiple linked glucose molecules, the simple sugars in our blood that are derived from digestion.

The body can burn either sugar or fat for energy, but using sugar is both faster and easier, so it uses that first. However, glycogen stores are finite, which is where fasted cardio comes in. The idea is that by the time you’ve gone 10 or 12 or more hours without eating, your body will have worked its way through a good portion of its existing glycogen stockpile. And, since those glycogen stores haven’t yet been replenished with a meal, the body will then be forced to turn to Plan B during your workout, which is to also burn stored fat.

The catch is that, despite the appealing and simple logic, the research into fasted cardio is scant, and the results have been mixed.

A study from 2017, which analyzed research into the practice, didn’t find the promised changes to body composition. Another drawback is that, when in a fasted state, the body has another energy option besides fat to make up for the lack of glycogen. We’re talking about protein, with the source being your own muscle tissue. Losing muscle mass is the opposite of what you’re trying to achieve. And, while the big picture of how the body uses energy appears straightforward, it’s actually quite complex. At any given moment, processes such as hormone secretion, enzyme activity and gene activation — to name just a few — play a decisive role in fat metabolism. So does the type and duration of the exercise you’re engaging in.

All of which leads us back to our familiar refrain when it comes to diet and exercise. That is, moderation and consistency. Eat a healthy diet, steer clear of junk food and simple carbs, do a mix of aerobic exercise and strength training, and get enough sleep. These will serve you better than trying to game the system with a tricky diet or exercise routine.

For those of you who decide to give fasted cardio a try, be sure that you’re still drinking plenty of water.

And don’t forget to eat a balanced breakfast after you’ve finished working out.

Eve Glazier, M.D., MBA, is an internist and associate professor of medicine at UCLA Health. Elizabeth Ko, M.D., is an internist and assistant professor of medicine at UCLA Health. Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.

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