Over the years I have witnessed many old fitness myths become ‘debunked’. Well, now I think it’s time to debunk one of my own. One idea in particular that has often been mischaracterized is the concept of ‘burning fat’ while exercising. We have historically been lead to believe that once we step outside of the fat burning zone (65-80% of maximum heart rate), we have stopped burning fat. Because of advances in exercise science leading to a better understanding of exercise physiology, we have been able to put to rest some of these tired training trends. Although 65-80% is, in fact, a fat burning zone, I would like to introduce a non-traditional concept that fat burning happens at all levels. More importantly, if your desired goal is to lose fat, I will introduce a training method to utilize more calories while moving in and out of the traditional fat burning zone. To truly understand the fat burning process, however, we need to take a quick trip down the avenue of nutrition and better understand how the body utilizes fuel.
The Fueling Process
Our bodies utilize three main sources of fuel: Fats, carbohydrates and proteins. For the purpose of energy production, fats are converted to fatty acids and are utilized in the presence of oxygen. (The word aerobic actually means with oxygen.). If not used for fuel, fat is stored in adipose tissue. Carbohydrates, on the other hand, are processed differently than fat in the body. First, carbohydrates are broken down into simple sugar called glucose, some of which is readily available in the bloodstream and some of which is stored for fuel in the muscles as glycogen. However, again, if the body has more glucose than it can store in the muscles and liver, it too will be stored in adipose tissue as fat. Lastly, we have proteins. Proteins are broken down as amino acids and are used primarily for growth and repair of tissues. If not used, amino acids will be stored in adipose tissue just like the rest of our excess fuel. Although they can be used as fuel, proteins are not commonly used by the body during exercise, as the process of utilizing amino acids for fuel is far more complex than, and not as readily available as, the other two sources of fuel (fatty acids and glucose).
Fuel Converted to Energy
Because we are (hopefully) constantly breathing, the body is constantly using fat as a fuel source. However, an average resting heart rate is around 60-75 beats per minute (which is in the range of 30-40% of an adult’s maximum heart rate). Therefore, the amount of calories being expended and the amount of fuel needed to create this energy is very low. As the heart rate increases, however, a body will utilize more fat as fuel. A range of 65-80% of the adult maximum heart rate has been adapted as the “fat burning zone” because oxygen is moving through the body at a high enough rate that fat “burning” (i.e. using fatty acids as an energy source) is substantial.
In the first few minutes of cardiovascular exercise, the body is actually in oxygen debt. Oxygen debt means that once exercise has begun, the heart has not yet been able to pump enough oxygen to the muscles to utilize fatty acids as fuel. Therefore, glycogen that is stored in the muscles (which can be converted to energy in the absence of oxygen) acts as the primary source of fuel for the body until the aerobic process can take over. You may notice when you first begin cardiovascular exercise a slight burn in your muscles. That is because lactic acid, a byproduct of using glycogen as fuel, has built up in your muscles. Once the oxygen demand has been met, however, you begin sailing nicely into your aerobic zone, utilizing fatty acids for fuel.
For each gram of fat ‘burned’ as fuel, the body will expend 9 kilocalories of energy. Because of this, while staying in your aerobic zone, the higher you go in this zone, the more calories you will burn, thereby, the more fat you will also burn (18 calories equals two grams of fat, 27 calories equals three grams of fat, etc.).
The exception to this fat-burning rule is that once your body has climbed past a certain heart rate (typical anaerobic threshold is between 86-92% of your maximum heart rate), you will again be in oxygen debt. This is known as ‘anaerobic training’ and, once again, you will be using primarily glycogen stores to fuel your muscles.
The Plot Thickens and the Trick to ‘Burning’ More is Revealed
This is where misinformation starts to rear its ugly head. It has been said that once the body is in the anaerobic zone, you cease burning fat. This is the myth we must bust wide open today. Once your body has begun burning fat, the Krebs cycle (the cycle used in the breakdown of fatty acids), is in a perpetual cyclical motion. Upon entering into anaerobic training, the muscles then switch over to glycogen for fuel. However, that does not mean the fat burning process comes to a halt. In other words, glycogen will kick in as the ready fuel source, but the Kreb’s cycle keeps spinning. Consequently, the body is burning a full-size portion of calories utilizing glycogen, and meanwhile the Kreb’s cycle is whizzing away chewing up fatty acids. You are getting a big burn on!
After a short burst of anaerobic activity, however, you will need to bring the heart rate back down to a sustainable aerobic level allowing the body to recover. By doing so, the Kreb’s cycle to will have an opportunity to gain momentum and the quickly depleted glycogen stores will be replenished. A good rule of thumb: Recovery period should be double the time spent in anaerobic interval. Therefore, the key to utilizing maximum calories in fat and carbohydrates is to go in and out of this cycle of anaerobic/aerobic activity, which you can do several times (before exhaustion sets in).
The average amount of time spent in the anaerobic interval can vary between 30 seconds to two minutes. As the body becomes more fit, it will need to be challenged to an even greater degree to achieve this anaerobic training zone. A new exerciser, and those less fit, should heed caution to this type of cardiovascular training. A newer exerciser will need shorter interval bursts with longer recovery time in between.
What Does this Type of Workout Look Like?
- Warm up – Warm the muscles with very light aerobic activity and then lightly stretch the muscles to prevent injury. After your stretches, slowly bring the heart rate to 65% of your maximum heart rate. (Five minutes.)
- Begin training – Stay in the aerobic zone, gradually increasing your heart rate to 80% of your maximum. (Fifteen minutes.)
- Intervals – Five sets of anaerobic/recovery intervals. (Anywhere from 7 ½ minutes to 15 minutes depending on your fitness level).
- Cool down – Lowering the heart rate to full recovery (65% or lower). Finish off with static stretches. Stretching is imperative after this type of exercise. You will have ample lactic acid buildup in your muscles which stretching will help to dissipate. (Seven to ten minutes).
The above plan is for a 35-40 minute workout. If you desire a one-hour cardiovascular training session, repeat steps 2 and 3 for one additional set before cool down.
This workout will leave you feeling spent because you have burned so much of your body’s fuel sources. It is essential after a high energy anaerobic training period to replace the depleted blood glucose with carbohydrates. (Bananas are great recovery food. A medium banana yields 27 wholesome carbohydrates as well as much needed potassium.) This training method is a great way to kick-start a weight loss program or help knock you off a training plateau. Enjoy your workout! Enjoy the ‘burn’.