Metabolic Oxidation for fat loss goals

In the post about the metabolic continuum, I talked about how intensity and duration can affect the metabolic pathways. I wanted to focus a bit on the metabolic oxidation pathway, or aerobic metabolism, for the people with fat loss goals.

It seems to reason with most common folk that simply not over eating, exercising regularly, and staying consistent will help you maintain a healthy weight. While this is true, I am a firm believer that, if we gain understanding of the physiological processes, we can in turn learn to control them and harness our full potential. By doing so, we are able to achieve our goals in shorter timeframes.

In terms of the fat loss goals, the objective is to control the fuel sources used at a cellular level. This does not mean nutritionally. Fat has the ability to store excessive calories worth of macronutrients, and it is capable of doing so almost limitlessly. When it stores these nutrients, however, they are converted into fatty acids and triglycerides. It is this reason why fats contain more calories gram from gram versus protein or carbs. One pound of adipose tissue has the capacity to store approximately 3500 calories worth of fat. Therefore, if we can stimulate the fat cells to release their stored energy and engage the appropriate metabolic pathway, we can strive to burn off say 3500 kcal worth of fat stores.

In order to gain control over these pathways, we must learn to listen to our bodies’ and what it is saying. For instance, the heart and lungs have a direct relationship with one another in terms of the rates of function. The lungs are charged with the task of enriching the blood with oxygen, and the heart is charged with the task of pumping that oxygen-rich blood to the working muscles. Did you know, though, why you’re heart rate and breathing becomes so labored at the onset of aerobic activity?

You may think that it’s because it is providing additional O2 to the working muscles. Partially true, BUT it is actually because the muscles are telling the brain that they are not getting the O2 that they need. As a result the HR and rate of respiration increases in an attempt to compensate. Until the muscles’ demands are met, and the HR subsides, the metabolic pathways will be primarily anaerobic and will be using glycogen stores as the primary fuel source. (See the post on “metabolic continuum” for details about how intensity affects the pathways and fuel sources)

So, what does all this mean in terms of burning fat? The word oxidation literally means ‘the addition of oxygen’. During glycolysis an enzyme simply adds O2 to split the glucose molecule into 2 separate pyruvate molecules to begin synthesizing ATP. A similar process occurs when breaking down fatty acids into pyruvate, except the process is much more elaborate…

Not trying to give a chemistry lesson here, but I want to introduce it briefly to give an illustration of why this process is much more elaborate, which will explain the increased demand for O2. Glycolysis is the process of splitting a six-carbon saccharide chain into two three-carbon pyruvate molecules (simply splits it in half). Since this requires only one chemical reaction to dismantle the glucose structure, the process is fairly simple and only uses up one single O2 molecule per glucose molecule. Fatty acids on the other hand contain and average of 18-carbon atoms making up the fatty acid chain. As a result, the lipolysis process requires 3 x the amount of O2 for breaking down fatty acids into pyruvate for energy production. You notice the key factor here is O2? Without it, oxidation cannot occur.

So, hopefully, we are beginning to see why O2 uptake is so important for burning fat. Simply jumping on a treadmill and running will not engage oxidative metabolism to the extent you’ll want it to, if your goal is to gain or maintain lean body mass while losing body fat. Because the great the intensity the more anaerobic (without oxygen) the pathways will be, a “run” will keep you closer to or past the aerobic/anaerobic threshold. As a result, the primary fuel source will not be fatty acids. The easiest way to know where you’re at is by monitoring your HR.

The average conditioning intensity for developing the aerobic phase in healthy adults is 60-70% of their functional capacity, referred to as maximum HR (max HR = 220-age). For me, that range is between 113-134 BPM. Staying within this percentage will ensure that oxygen uptake is optimized for the oxidation of fatty acids.

It is generally recommended that this mode of aerobic work be sustained for at least 40-45 minutes per session. A larger percentage of energy during this type of activity comes from mobilized fat. The long duration of this activity allows fatty acids to perfuse even more extensively into the smallest of capillary beds feeding the working muscles, thus optimizing fat loss and oxidation. It is important to note that the muscles’ cellular furnaces, called mitochondria, become more efficient at burning fatty acids for ATP as the duration continues.

This thread was written solely by me and in my own words. As a reference, I used the “NFPT Standard Personal Trainer Manual, vol 1, fourth edition” and "Understaning Nutrition, 10th edition, by Ellie Whitney & Sharon Rady Rolfes.