Do You Burn Fat or Muscle First When Exercising?

Your body begins breaking down proteins once your available fat stores have been consumed.
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Your body relies on three key macronutrients to fuel itself through strength- and cardio-based workouts, according to an August 2020 research article in Nutrients: carbohydrates, fats and protein.

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Your body uses all three for energy, and if you're looking to build muscle mass while decreasing your body's fat composition, it's important to understand how to use these three macronutrients to optimize fat loss without losing muscle mass.

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What Happens to Body Fat and Muscle When You Exercise?

When you exercise, your body uses carbohydrates, fat and protein for fuel, in that order. (We'll get into in more detail about this in the next section!)

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"Your body has fat cells stored in reservoirs throughout your body," says registered dietitian Kylee Van Horn, RDN, co-founder of FlyNutrition nutrition consulting.. "When you are exercising, your body starts 'freeing' those fat stores, so to speak — this is called fat oxidation. Fat oxidation leads to the fat cells being broken down into an energy source that you can use to help fuel exercise."

Your body will always use carbohydrates and a little bit of fat stores each workout to give itself enough energy to complete your planned session, Van Horn says. But it doesn't "like" to use muscle, which is a protein, as a fuel source for workouts as it's hard to break down and doesn't provide as many calories of energy as fat.

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However, when your body's carbohydrate and fat stores are too depleted, muscle can be broken down to be used as fuel, which is generally not desirable for anyone.

When you're well-fueled and only using carbohydrates and fat as fuel, your muscles should be focused on propelling you through whatever movements are needed for your desired activity. If you're running or biking, your muscles should be energized from carbohydrates and fat to move your legs. If you're strength training, your muscles should use carbohydrates and fat fuel to contract and hoist weights in specific movements.

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Your body uses carbohydrates, fat and protein to fuel itself, but all fuel sources are not created equally. Let's dive into how and why the body prioritizes fuel sources.

Which Burns First: Fat or Muscle?

The first fuel your body breaks down for energy is carbohydrates, according to Colorado State University. After a meal, your body is in the "fed" state and preferentially breaks down carbohydrates because they're easily accessible and turned into energy.

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After your body has used up the carbohydrates from a meal or snack, your cells begin to break down glycogen, which is glucose stored in your muscles and liver.

Glycogen stores vary from person to person, but are typically depleted within 24 hours, at which point your body has to begin breaking down other compounds for energy, according to the National Library of Medicine.

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When glucose and glycogen are not available, your body preferentially breaks down fatty compounds known as triacylglycerols, which are present in adipose or fat tissue. Because fat is a high-energy source with 9 calories per gram, fat is an efficient fuel source.

Additionally, your body metabolically prefers to preserve muscle mass and, when possible, breaks down fat stores for fuel. Only when your fat stores are extremely low or depleted does your body then have to break down muscle mass to create useable proteins.

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"It is well-established knowledge that if you are in a deficit of 500 to 1,000 calories a day, you are at risk to begin breaking down muscle for fuel," Van Horn says. "This is why simply cutting calories is not necessarily the way to approach altering your diet. You need to take into consideration which macronutrients are being cut in those calories, too."

When glucose and fat stores are depleted, your body will turn to muscle to break it into individual amino acids for energy. Unlike carbohydrates and fat, your body doesn't store amino acids, which is why muscle breakdown is the only way to release amino acids for fuel.

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Under normal conditions when you're eating on a regular basis, your body doesn't use muscle for energy. Typically, protein is used for fuel only when you're depleted of carbohydrate and fat sources.

Because you need muscle tissue to survive and move, the natural tendency of metabolism is to spare muscle tissue and break down carbohydrates and fats first.

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How to Burn Fat and Not Muscle During a Workout

The first step to burning fat and not muscle during a workout is to ensure you're fueling yourself properly before and after a workout.

In order to burn fat — and keep it from coming back — you do need to be operating in a calorie deficit, but it's advised that you chat with a nutritionist instead of cutting out calories (and potentially important macronutrients) on your own.

Additionally, to burn fat and not muscle during a workout, it's important to understand the mechanisms by which fat loss occurs.

Brandon Howard is a Kentucky-based ACE-certified Health Coach, Fitness Nutrition Specialist and Medical Exercise Specialist who often works with clients looking to lose fat in a healthy way.

"Often, people will look at fitness classes that claim to be 'fat burning' workouts and think that's the solution to their desire to burn fat," says Brandon Howard, an ACE-certified health coach, fitness nutrition specialist and medical exercise specialist. "This type of marketing can be misleading"

A "fat-burning" class tends to be a class with high-intensity movements that will result in you burning a lot of calories, putting you in a caloric deficit and having burned fat for fuel, Howard explains. If you execute a safe diet and attend that fat-burning class for many weeks in a row, you may indeed lose some fat. But going to a "fat-burning" class just one time is likely not going to show you measurable fat loss results, he says.

Burning fat for weight loss and not muscle during a workout is a balance of operating in a safe, healthy calorie deficit that still provides the right amount of protein and finding workouts that resonate with your schedule, interests and physical abilities to burn carbohydrates and fat.

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There are no one-time quick fixes that lead to sustainable weight loss, Howard says. He notes that he's had clients who will attend multiple "fat-burning" classes, follow a crash diet, lose weight for a couple of months and then gain it back due to the unsustainable high intensity of the "fat-burner" classes and lack of appropriate macronutrients in the crash diet.

It's best to talk with with a registered dietitian and certified personal trainer to find the exact methodology that will allow you to optimize fat burning and minimize muscle loss over time.

How to Fuel for a Workout in Order to Burn Fat and Not Muscle

Your body will "pretty much always" burn carbohydrates and at least a little bit of fat during a workout session of a significant duration such as 30 to 60 minutes, Van Horn says.

To burn fat and keep it off, fueling yourself is a critical part of the weight loss puzzle. Many people conflate a calorie deficit with meaning they should fast before their workouts. Van Horn warns this may cause more harm than good.

"There is no substantial research that shows that fasted workouts result in greater weight loss or an increase in lean muscle mass," Van Horn says. "The risk of working out fasted is that it can increase your cortisol, which can promote mid-belly fat storage and muscle protein breakdown — the exact opposite of what you're trying to achieve."

Working out fasted can also lead to faster depletion of glycogen stores and available energy from fat, which can lead to your body using muscle for fuel much faster than if you fueled appropriately.

A small November 2014 study in ‌Taylor and Francis Online‌ concluded there was no measurable difference in body composition between groups that fueled and groups that fasted before their workouts.

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Another small July 2019 study in the ‌Journal of Strength and Conditioning Research‌ found when participants omitted breakfast before a workout, their ability to perform actions like weighted squats decreased, while those who consumed breakfast before the same activity completed the weighted squats at the same or better than their previous benchmark.

About 45 minutes before a workout, eat a light, carbohydrate-rich snack with a bit of protein, Van Horn advises. If you're doing a strength-focused workout, you can increase the protein amount a bit, she says.

It's recommended to consume 225 to 325 grams of carbs per day, per the Mayo Clinic. For protein, you should get about 0.8 grams per kilogram of body weight (or 0.36 grams per pound), according to Harvard Health Publishing.

For a pre-workout snack, 200 to 300 calories made up of carbohydrates and 10 to 15 grams of protein is a good starting place, per Van Horn.

Fueling after a workout is also important for retaining muscle mass and recovering from the workout itself.

"A key piece of retaining muscle mass that's often overlooked is combining carbohydrates with protein to maximize muscle protein synthesis post-workout," Van Horn says. "Protein provides even more benefits when mixed with carbs than just eating protein alone."

To that end, Van Horn recommends consuming 20 grams of high-quality protein such as dairy, eggs, meat and soy products with 45 to 60 grams of carbs as soon as possible after your workout.

Can You Maintain Muscle Mass in Weight Loss?

The short answer: Yes, you can maintain muscle mass in weight loss.

For people moving through a weight loss plan, some muscle loss may be inevitable, but it can be mitigated by consuming enough protein, according to Howard.

"Maintaining muscle mass during weight loss comes down to two main things: protein consumption and strength training," Howard says. "Protein is so important for maintaining muscle mass, and yet many people tank their protein consumption when they begin operating in a calorie deficit."

Along with consuming enough protein, strength training two or three times a week is the minimum Howard recommends if you want to retain or increase your muscle mass.

When trying to maintain muscle mass in weight loss, Howard says both cardio and strength training are critical. He recommends adding strength training to your routine three to four times a week for 30 to 45 minutes per session. You can do cardio as many times a week as you want for 30 to 45 minutes per session as well.

Howard explains that a cardio session will generally burn more calories than a weightlifting session, but you won't build as much muscle as you would during a weightlifting session.

Howard cautions people not to think of cardio or a "calories burned" metric as a rewards and punishment system, but rather as a key component to a successful weight loss and muscle- building program.

A healthy caloric deficit combined with a hybrid of cardio and strength training is the winning formula when it comes to maintaining muscle mass during weight loss.

Why We Lose Muscle and Signs You're Losing Muscle

By the time your body burns protein (aka muscle), that means it has essentially run out of carbohydrates and accessible fat stores to use as its preferred energy sources. This is not an ideal or sustainable scenario and can lead to you losing muscle.

You'll lose muscle by functioning in a caloric deficit that has removed too much protein from your diet. It's possible to create a healthy, safe caloric deficit that still meets your protein needs, and it's best to meet with a registered dietitian about what a sustainable caloric deficit for weight loss, but not muscle loss, might look like for you.

In addition to a protein-deficient caloric deficit, focusing too much on cardio can result in muscle loss, too, Howard says.

"Due to the higher caloric burn cardio provides, if you're doing only cardio every day of the week and operating in a caloric deficit with low protein, you're going to burn through your muscle stores faster than someone who is using a more balanced fueling and training plan," Howard says.

Signs of muscle loss can include sagging skin in your arms and legs and you may have difficulty completing weight-based exercises at weights you were previously able to lift, Howard says.

As you age into your 60s and 70s, your rate of muscle loss, or sarcopenia, doubles from what your rate may have been in your 30s and 40s, Howard notes.

"In your mid-30s to 40s, you will lose about 5 to 10 percent of your muscle mass per decade," Howard says. "As you move into your 60s and 70s, that rate doubles because your body isn't as efficient as it used to be, and you may need to consume even more daily protein than you did in your younger years."

A January 2014 study in ‌Current Opinion in Clinical Nutrition and Metabolic Care‌ found that consuming 30 grams of protein per meal when you're 60 and older presents a "promising strategy" older adults who want to maintain muscle mass and lose body fat.

Howard describes strength training as your "muscle 401K," meaning that weightlifting and other high-resistance activities in conjunction with a healthy diet that includes appropriate protein intake helps your body plan for older, or retirement, age and keeps you operating for years to come.

"You are never too late to begin strength training and eating well," Howard says. "We want to make sure the 'savings account' of muscle mass and health eating that we build when we are younger is enough to sustain us for the rest of our lives."

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