Cardiovascular Exercise – Intensity and Fat Burning

By Grant Roberts

Many misconceptions exist as to what method of cardiovascular training provides the most efficient and effective results for fat oxidation (energy consumed or extracted from fuel) or the more commonly referred term: fat burning.

Before we get started, I feel it is important to point out that this article was created specifically to address the best methods of cardiovascular training for the purpose of burning fat, not improving athletic performance, endurance or oxygen utilization.

That being said, cardiovascular training is not the most effective method of exercise for burning fat. The most effective form of exercise for burning fat is resistance (weight) training. Unlike cardiovascular training, resistance training when performed properly preserves or is capable of increasing the amount of metabolic enhancing lean muscle tissue.

Regardless the overwhelming popularity of cardiovascular training and the fact that it does remains an essential part of an overall fitness program, here is my answer to one of the most commonly asked questions: What is the most effective method for burning fat through cardiovascular training?

Points to Consider: What time of day…How long…how often…and what of level intensity?

Time of Day:
The most effective time of day to perform cardiovascular training for optimum fat burning would be first thing upon waking and consuming your first meal following the cardio session.

How Long…How Fast…How Often:
As for duration, pace and frequency, the answer is plain and simple… slow, steady and sporadic wins the race. Maintaining a heart rate of approximately 65% of *maximum capacity provides optimal fat burning potential. Typically 45 minutes to 1 hour sessions 2 to 3 times a week is sufficient. Sessions should be a minimum of 20 minutes to an absolute maximum of 75 minutes and never more than 4 days a week when burning excess body fat is the goal.

Explanation:
Due to stored fuel depletion within the human body performing a cardiovascular activity for greater than 75 minutes may cause the body to start breaking down muscle tissue to continue fueling the motion. Muscle is a great metabolic booster so you want to protect the muscle tissue you have, loosing muscle mass will ultimately slow your metabolism. Likewise for any exercise to be optimally effective it should be perceived as a shock to the body for the best response. Partaking of cardio activities beyond 20 minutes regardless of variety or type of cardio exercise should not exceed 4 sessions a week. Beyond that and your body may start adapting to the load being placed on it by sending an anticipatory signal to slow the metabolism. Remember the human body has a genetic predisposition that has been programmed over the last 3 million years to store fat for times of famine.

This then is the reason to shock the body with portioned or appropriately spaced cardio sessions so that the body does not become accustomed to or adjust to the demands of a regular schedule of exercise.

Intensity:
Many people believe the faster they run, the faster they will lose fat…it may sound logical but it does not take into account how the body utilizes stored fuels…namely the macro nutrients Fat, Protein and Carbohydrates

It is important that the reader understand that we are talking about High Intensity Cardiovascular Training and not High Intensity Resistance (Weight) Training – these two types of exercise are diametrically opposed and the principles outlining the negative aspects of High Intensity Cardio on fat burning do not apply to High Intensity Resistance training. High Intensity resistance training is very beneficial for fat burning provided optimum rest periods between sets and muscle groups between training sessions are provided…thus the reason cardiovascular training is synonymous with aerobic training (with oxygen) and resistance or weight training is categorized as anaerobic (without oxygen) but that is another article.

A popular misconception is that High Intensity Cardiovascular Training (HICT) burns more calories in a shorter period of time and keeps the metabolism revved up post workout. The first problem in this statement is that the theory suggests HICT burns more calories…not fat. I can not stress this point enough, calories do not exist! Calories are simply the name used to describe a unit of measure of energy which is indiscriminate of the source of energy. In this case the topic of concern is fat oxidation not simply utilizing energy in a time efficient manner regardless of source of macronutrient namely carbohydrates, protein or fat.

Therefore I suggest no attention be paid to the little gauge on the cardio machine that ticks away the number of calories used during the exercise increasing the rate of calories alledgly consumed the faster you go. The explanation is quite simple, fat requires more time and energy to digest than the other macronutrients and is also the most energy dense nutrient (which also explains the reason why the human body stores fat – simple economics of space. Imagine how large people could be if we stored carbohydrates with less than half the energy /caloric density of fat per gram) so too does this density apply to the oxidation or burning of fat through exercise. Slow and steady wins this race every time. As soon as you elevate your heart rate beyond the optimum 65% capacity the body will convert from burning energy (calories) from fat as a fuel to accessing energy from stored carbohydrates (glycogen) or protein (muscle).

The explanation is based on the simple fact that fat requires an oxygen rich environment to burn. When oxygen is not abundant – meaning when the heart rate is elevated to a level which creates a greater demand for oxygen to fuel the motion ( faster pace = faster heat rate = more oxygen consumed) therefore as an economic measure the body will cease burning fat and commence accessing the more efficiently combustible fuel source of glycogen (carbohydrate) stores.

If one was to maintain a high heart rate pace and deplete all of the stored glucose the body will then access and commence burning the next most efficient fuel source which would be protein by breaking down muscle tissue – something you never want to do. This is the reason why you frequently see marathon athletes intake some form of high glycemic drink or snack at intervals during a race to replenish the fuel in attempt to spare muscle from being catabolized to provide fuel.

It is for this reason that I recommend exercise does not exceed 75 minutes in length as the readily available and accessible fuel sources are typically depleted and the body will then be left with no alternative but to commence breaking down protein (muscle).

* To determine an individual’s maximum capacity heart rate a simplistic and widely accepted formula is 220 beats per minute – minus your age. E.g. 220 – (age) = max heart rate – Multiply this number by 65% to determine target heart rate.

Scientific Support:
An Intensity Study on Fuel Utilized through endurance orientated exercise (cardio)

Comparison of various exercise intensities on plasma glucose (blood sugar) concentrations after 30 minutes of cycling found that as intensity increased, plasma glucose concentration increased. Specifically, the mean values were 77, 98, and 147 milligrams per deciliter of plasma glucose concentrations for 25, 65, and 85 % V02 max intensities.

This facilitated a higher reliance on glucose, as carbohydrate utilization increased as intensity increased. In the 25 % V02 max condition, carbohydrate oxidation provided approximately 7.5 % of the fuel used. Further, muscle glycogen stores did not contribute significantly to this number, suggesting almost complete reliance on peripheral glucose.

In the 65 % condition, carbohydrate utilization provided approximately 50 % of the fuel utilized, with 80 % of this coming from intramuscular glycogen stores.

During the 85 % V02 max condition, 75 % of the energy was derived from carbohydrate utilization, with 80 % coming from muscle glycogen stores.

The proportion of fat oxidation, showed an inverse relationship with exercise intensity. Over 85 % of the fuel during 25 % V02 max was provided by fatty acids from adipose, while 7.5 % was provided by intramuscular TGs (triglycerides). In the 65 % condition, 50 % of the fuel was derived from lipid (fat) oxidation, in equal measure from adipose and intramuscular stores. The lowest percentage was found in the 85 % condition, with only 25 % of the fuel coming from lipid oxidation from equal measure of adipose and intramuscular TGs

From the above findings it should be noted that total fat oxidation is greatest at moderate intensity, while total glycogen depletion is greatest at high intensity exercise. Fat utilized from adipose tissue was greatest in the low intensity condition, while glycogen depletion was lowest at this intensity. This suggests, that based on lipid oxidation, that 65 % V02 max is optimal.