When is a Calorie, not a Calorie?

Most of us think a food calorie puts out the uniform amount of energy. Understanding why this is NOT a truism is key to weight control. Under most conditions, the body’s metabolism operates more than 99% aerobically and perhaps less than 1% anaerobically. That is to say, when we take a single glucose molecule and process it through our highly evolved and efficient aerobic power machinery, just about every last drop of energy is wrung out. Coming out the other end are 38 ATP. ATP are the handy, cell-sized energy packets most enzymes use to get the work accomplished. Think of them like pennies. The body ingests large denominations of paper money which get broken down into small change-size coins at the cell level.

This process of breaking down single sugar molecules is the job of the metabolic disassembly machinery. The initial step of breaking apart sugars like glucose or fat takes place just inside the cell. The next step takes place in power plants within each cell, specialized structures called mitochondria.

The mitochondria house the aerobic metabolic machinery, whose job is to wring the most ATP energy packets from each food molecule. At the very end of the dismantling process, oxygen from the bloodstream latches onto the final waste end product. The resulting carbon dioxide moves out to the bloodstream to be expired through the lungs. Even most who studied college biochemistry may gloss over this point: Aerobic respiration metabolism happens at the cell level, not inside the lungs. The lungs function to exchange oxygen for carbon dioxide. The term aerobic refers to the metabolic process and is often confused with cardio, such as aerobic exercise.

When oxygen is plentiful at low exertion levels, almost all of the cells in the body operate in this highly efficient mode, producing 38 ATP for every molecule of glucose. Therefore, someone who is walking, sitting, or driving a car, i.e. 99% of the time for a couch potato, is relying on efficient aerobic metabolism.

Consider the other branch of metabolism. Of those 38 ATP, two were produced before getting into the mitochondria, in the process not requiring oxygen, known as anaerobic metabolism. These two ATP result from neatly cleaving glucose inside the cell, but outside of the mitochondria. Some of you know the end product of anaerobic metabolism — lactate. Now lactate can be further used as fuel, but only in a roundabout and relatively inefficient manner.

Without getting into unnecessary details, we can see that aerobically produced 38 ATP is a lot more than anaerobically made ATP! Each glucose molecule can be used very efficiently or very inefficiently. When trying to lose weight, the obvious choice is to make use of the inefficient pathway. How can we do that on a practical basis?

The largest potential energy consumer is skeletal muscle. If we exercise at a low exertional level, never getting out of breath or straining, then the body exclusively is in aerobic metabolic mode. Only when lifting heavy objects, moving heavy loads or exercising at harder-to-sustain levels, do we engage the inefficient anaerobic metabolic system.

That’s why running a mile consumes more energy than walking the same mile. When it comes to exercising to lose weight, making your body convert calories to energy as a lower efficiency helps considerably. Now you know another reason why higher intensity exercise matters!

Available on Amazon.com

William Shang, M.D.
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