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The Farce of Fasted Cardio

Athlete runner trail running on summer beach. Fit body silhouettSo, you just picked up the latest fitness magazine and read that fasted cardio can help you get leaner. The article you read claims that the body burns more fat while exercising in a fasted state compared to doing cardio later in the day after eating. As a result, you decide to take your morning spin class on an empty stomach, thinking you will burn more fat and build more muscle. But does the science prove this? For years this idea has been accepted as fact by fitness gurus, professionals in the industry, and gym rats. However, it is only half true because it is only half of the story. While it does work and will utilize more fat as energy, it is not an optimal way to train because it breaks down muscles and is ultimately detrimental to building lean muscle mass and balancing hormones.

Part of the issue is that we, as consumers, tend to focus on what seems to be working without investigating why it works. There is so much information out there; however, a lot of it is conjecture and not based on scientific fact. It is important to make informed choices, to not accept every article as fact, and not to believe everything the 5% body fat bodybuilder or freak athlete preaches about. When you read an article, it is important to check the sources. It is actually more important than the author itself.

Genetics play a role in how our bodies function, and there are some people that get results no matter how they badly they eat or how they poorly they train. Unfortunately, this is not the norm. The point of this article is to demonstrate that fasted cardio is NOT optimal for fat loss, muscle building, or muscle maintenance, since it doesn’t make sense to burn extra fat at the expense of catabolizing muscle. Let’s fill in some educational gaps to learn the science behind this.

Types of Cardio Training

There are different types of cardio workouts, which affect how the body burns energy and what type of fuel is utilized. Cardio training can be HIIT (High Intensity Interval Training) cardio, low intensity long duration, or moderate intensity cardio. It is generally understood that more calories are potentially burned during exercise sessions of longer duration vs. HIIT. However, even though you burn more calories DURING a session of long exercise, HIIT will burn more calories POST- WORKOUT due to the process of EPOC (exercise post oxygen consumption). This is also known as “after burn.” Therefore, your ability to burn total calories will be more with HIIT because calorie burn continues even after you are done exercising! As a result of more calories being burned, weight loss is more likely.

Bioenergetics Basics

Every activity, from breathing to reaching for a glass of water, requires energy, which comes from calories. We burn calories all day, and even at night while we sleep. We ingest calories as fats, carbohydrates, and protein. Fat yields more energy per gram than carbohydrates because it is much denser in calories. Carbohydrates and protein are only 4 calories per gram while fat is 9 calories per gram. Fat and carbohydrates are the preferred fuel sources for energy, while protein is spared and used for enzyme synthesis and muscle creation.

After meals, carbohydrates are readily available in the bloodstream as glucose and can be stored as glycogen in the muscles and liver. Once these glycogen stores are full, the body will convert the excess glucose into fat and store it in fat cells. This is why eating too many carbohydrates can make us fat! There is a finite amount of storage space for extra carbohydrates, while there is an infinite amount of storage space for fat in the body. As a result, all excess energy that isn’t utilized by the body is stored as fat.

Proteins are made up of amino acids. The body must ingest 9 essential amino acids to create usable protein for cellular uptake in the body. Think of protein like a chain, and each essential amino acid as a link. If there is a link missing, then the protein is incomplete, which forces the body to break down muscle to get the missing amino acids.

Since protein is needed for higher-level functions, it is broken down for energy only as a last resort. Unlike fats and carbohydrates, protein cannot be stored or manufactured in the body, so it is “use it or lose it!” This is the reason why complete proteins must be ingested every few hours. Research shows that protein is only available for synthesis for a limited time. Since everyone is different and all proteins digest at different rates, that time varies from person to person. For this reason, research suggests ingesting protein every 2-5 hours. If you are very active and only eat small amounts of easily digested protein, you will probably need to eat more frequently. In general, a good rule of thumb is to eat protein every 3-4 hours to prevent muscle catabolism. If protein is not ingested during this window, your muscles are broken down into the amino acids the body needs and the unused excess is converted to carbohydrates. If that energy is not used, then the body converts that excess energy into stored fat.

A popular misconception is that protein is only needed for building muscle. As stated, protein has many functions in the body, including hormone production, fluid balance, growth & tissue maintenance, nutrient transportation, and enzyme & protein synthesis. According to research, the average male needs 56 grams of protein per day while a woman needs 46 grams. These requirements are higher for people who are athletic. The more intense the activity, the more protein is needed.

There are dozens of published opinions on the correct amount of protein that is needed for an active person/athlete. Some claim that 56g for men and 46g for women isn’t enough, and that they actually require at least 50% more protein than that. Others state it should be one gram per .6-1.1 grams of protein per pound of body weight. For example, if you weigh 100lbs and choose to ingest .6 grams of protein per pound, the daily requirement would be 100 x .6 = 60 grams per day.

If you are unsure about how much protein to consume, a good starting place is .8g/kg of body weight of protein for both men and women, and for athletes 1.2-1.7 g/kg. The amount of protein that can be used for protein synthesis in the body is debated among articles; however, the theories range from 15- 40 grams per meal. A good guideline to follow is men should aim to eat 20-25 grams of protein per meal per meal and women 10-15 grams. If you feel bloated and full after a meal very high in protein, then it is too much. If the amount of protein you eat does not cause GI stress, then you can slowly increase your protein intake until you reach your goal. However, do not consume more than 35 grams per serving without talking to your doctor.

It is important to note that when we exercise, we are never burning just one of these nutrients for fuel; it is always a mix of carbohydrates, fat, and protein in different proportions. To achieve our goal of being lean, we strive to utilize a ratio of more fat, moderate carbohydrates, and minimal protein. When we do an activity that is long term, we can potentially use more fat since we have time for it to be broken down. However, when we need to access energy quickly, like when we are sprinting to catch a train before it departs, your body will need to use the fuel source it can access the fastest, which is carbohydrates. Carbohydrates are more readily available than fats during exercise since they are easier to break down into energy than fat. Basically, the body uses the glucose in the blood and breaks down the glycogen stored in the liver and muscles. Once the body has depleted its stores of glycogen, it will tap into fat stores and convert fat to glucose. This is where the idea for fasted cardio originated. When you wake up and have not eaten, your blood and stored glycogen are low, which forces the body to start to burn fat during exercise.

New Age Dieting – Hormonal Not Caloric

Many diets teach the widespread formula, which states that 3500 kcal equals one pound of fat. Often, dieters strive to decrease their calories accordingly while trying to lose weight. Unfortunately, frequently people mistakenly assume that when their Fitbit states they burned 3500kcal, that this was exclusively fat loss. It is important to understand that a pound of body mass was lost, not just fat, which is a big difference. The goal of dieting and exercise is not to lose weight. It is important to lose the CORRECT type of weight, which should be predominantly of fat. People are so obsessed with caloric intake focused diets that they neglect the types of foods they eat and the nutrients contained in what they ingest. It is more important to eat in a way which will create a hormonal environment optimal for fat loss. When you eat carbohydrates, you secrete insulin, which will negate your ability to utilize fat stores optimally. If you keep eating carbohydrates all day, you are telling your body to use those carbohydrates for energy. By eating a diet consisting of complete proteins, healthy fats, and low carbohydrates, all while meeting your micronutrient and phytonutrient requirements, you will create an environment ideal for fat loss, muscle building and/or maintenance.
What Happens During Fasted Cardio?

Basically, the reasoning behind fasted cardio is that if carbohydrates are needed during cardio, then after an overnight fast, the body will need to convert fat to carbohydrates when carbohydrate sources are depleted. The process of converting fats to carbohydrates requires energy from the body, and thus burns extra energy. Therefore, the theory is that a person can burn more calories and use fat as a source of fuel in a fasted state. The only problem is that muscle loss is not factored into this equation.

Exercise is considered a type of stress, and we need energy quickly when the body is stressed. Additionally, exercise independently requires energy for movement, plus cortisol is secreted at times when we need energy. Therefore, we have increased cortisol levels during times of exercise. Cortisol stimulates gluconeogenesis, which allows the production of glucose (the building blocks of carbohydrates) from sources other than carbohydrates. It also increases free fatty acid blood levels, minimizes glucose utilization for the body, and saves glucose for the brain, since glucose is the only energy source utilized by the brain.

So far, cortisol seems great! However, it also increases protein catabolism, which means it breaks down muscle tissue. The body starts to metabolize protein in times of stress because it thinks it is in a survival situation (fight or flight is the nervous system response), and the muscular system, immune system, and digestive system all need protein to function. This is the exact opposite of what we want to do! We need to preserve muscle, because the more muscle in our bodies, the higher our metabolism and the better our ability to burn calories. People with more muscle mass have higher metabolic rates since it takes more fuel to move a bigger machine.

The main goal of exercise and healthy dieting is to increase our metabolism to lose weight. Since we can only burn so much fat during exercise, building muscle is extremely important during weight loss. If your exercise is depleting your muscle tissue and therefore decreasing your metabolism, does it make sense to do fasted cardio? Another negative effect of breaking down protein is that it may create an acidic environment in our bodies due to the creation of ammonia during protein catabolism. Research has shown that an acidic environment promotes inflammation, disrupts hormones, and makes a person more likely to get sick.

In modern society, cortisol is secreted during psychosocial stress instead of a survival situation, which is detrimental to our bodies. Our body is then working to create extra energy when we don’t need it. For example, your cortisol may spike when you are sitting at your cubicle and racing to finish your work for a deadline. There are two negative issues that may arise by being in a constant state of stress with excessive cortisol. First, during this state of catabolism, people under eat, which causes them to lose lean muscle. Second, the catabolism decreases metabolism, so even if the person eats the same amount as usual, they will gain weight because they are not burning the calories. The body is conserving the calories for the times of stress. Most often there is a lack of protein intake, which causes the muscle to break down to fuel the body with the amino acids it needs.

Recent research has shown that this is exacerbated due to imbalances in the hormones leptin and ghrelin, which cause people to overeat and gain weight. Again, as mentioned, each person is different. The take-home message is that your hormones need to be controlled in order to properly fuel your body and control your weight.

So yes, fasted cardio will burn more fat, but it does so at the expense of burning muscle.


The solution is NOT to truly fast before you work out, and to make sure that you include a protein source as soon as you wake up. Ideally, it is best to consume protein 30 minutes prior to working out, but as long you do not eat within 15 minutes of a workout, your body will reap the benefits. Men should consume about 15 grams prior to a cardio session and over 20 grams for weight lifting, while women should consume about 10 grams prior to cardio and over 15 grams if lifting weights. For those trying to build muscle or doing extended intense cardio bouts, branch chain amino acids (BCAAs) should be consumed 45 minutes into the session. BCAAs are the essential amino acids that the body needs for cellular function and muscle tissue preservation. They will help prevent any catabolism of muscle tissue during training sessions.

What Happens Next?

By following these guidelines, you will not burn through your muscle tissue for protein. You can then reap all the benefits of fasted cardio by utilizing more fat for fuel, while avoiding all the cons, such as high cortisol levels and muscle breakdown. The end result will be building lean muscle and burning fat while increasing resting metabolic rate.
Protein Sources

As discussed, it is important to consume a source of protein right before exercise. Many people choose to drink a protein smoothie or eat a small meal right before exercise. Your choice of protein source is important. A protein source is needed, but not at the expense of bogging the digestive system down with a heavy meal consisting of bacon & eggs on a roll or oatmeal. Remember that the goal is to preserve muscle during training by giving your body a very simple protein to synthesize during activity. Therefore, a light shake consisting of only protein and water is optimal. Another good suggestion is egg whites because they are easily absorbed and do not require much energy for digestion. In addition to the traditional whey shake, there is also an egg white based shake from a company called Egg Whites International. It is high in protein and tasteless, and it is a great way to boost protein intake. For traditional whey and vegan proteins, recommended brands are Jay Robb, Isopure and Orgain

Charles DeFrancesco
Gio Principe, BS, CSCS
Denise Groothuis, MS, RD


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