Fasted cardio is an attempt to burn stored fat

  • Scant research exists on the benefits of fasted cardio, which involves a heart-pumping workout after not eating for a long stretch of time, such as 12 hours or overnight.

    Scant research exists on the benefits of fasted cardio, which involves a heart-pumping workout after not eating for a long stretch of time, such as 12 hours or overnight. Stock Photo

By Dr. Eve Glazier and Dr. Elizabeth Ko
Posted1/9/2021 10:00 AM

Q: 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?

A: 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 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, because 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.

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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 you're still drinking plenty of water. And don't forget to eat a balanced breakfast after you've finished working out.

You should consider getting a COVID-19 vaccine, even if you have a history of severe allergic reactions to an unrelated vaccine.
You should consider getting a COVID-19 vaccine, even if you have a history of severe allergic reactions to an unrelated vaccine. -
COVID vaccines

We want to address some of the questions we're getting regarding the coronavirus vaccine.

• A licensed registered nurse who is in line to get the vaccine soon had a good question: "Today is the first day of the COVID-19 vaccine being administered," she wrote. "I am scheduled for my second Shingrix (against shingles) shot this coming Friday. Should there be a lapse between the two vaccines, or does anybody know?"

The answer to your question isn't yet clear. But let's take a look at the common side effects of the coronavirus vaccine, which are pain at the injection site, fatigue, headache, muscle pain, chills, joint pain and mild fever. Some of these overlap with side effects of the Shingrix vaccine. In order to differentiate between the two vaccines, and to understand which is the one causing any side effects that you may experience, our recommendation is to separate the two injections by at least a few days.


• Several of you have written with questions about getting vaccinated when there are existing allergies. "Can a person who is allergic to penicillin and aspirin take the vaccine safely?" a reader asked. When it comes to an allergy to penicillin, or to aspirin or other nonsteroidal anti-inflammatories, the data collected during the clinical trials for the vaccines doesn't report either as a reason to avoid the vaccine. However, the Centers for Disease Control and Prevention recommends that anyone who is allergic or hypersensitive to any of the components of the vaccine should not receive it. Your doctor, or the vaccine provider, can help you with that information.

Everyone who gets the vaccine should be monitored on-site for at least 15 minutes post-injection. Individuals with a history of severe allergic reactions to any other vaccines, or to other injectable therapies, should consult with their doctors before getting the vaccine. CDC guidelines state that people with a history of severe allergic reactions that are not related to vaccines or injectable medications, such as to foods, pets, venom or environmental or latex allergies, may still get vaccinated.

When getting the coronavirus vaccine, be sure to disclose any existing allergies; whether you have a fever, are immunocompromised or are on a medicine that affects your immune system; are pregnant, planning to become pregnant or are breastfeeding; and whether you are on a blood thinner or have a bleeding disorder. And if you have already received another COVID-19 vaccine, it's important to disclose that as well.

• A reader who is worried about a possible adverse reaction to the vaccine wonders how she will be safe while getting her shot. The answer is that the CDC has asked all vaccination providers to have on hand the appropriate medications and equipment to deal with a possible allergic reaction. This includes epinephrine, antihistamines, stethoscopes, blood pressure cuffs and timing devices to check your pulse.

• Dr. Eve Glazier is an internist and associate professor of medicine at UCLA Health. Dr. Elizabeth Ko is an internist and assistant professor of medicine at UCLA Health. Send your questions to

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