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Exercise: Aerobic Exercise

Aerobic exercise goes by many names. It’s often called “endurance exercise” because it’s done over long periods of time. Another common name is “cardio” because of its benefit to the cardiovascular system. Whatever you call it, it’s an integral part of any exercise routine and should be a part of your daily life in some form or another. So what exactly is aerobic exercise and how can we use it to improve our health?

First, science

Aerobic is a combination of the Greek words for “air” and “life” and, in the context of exercise, it’s any exercise that relies primarily on aerobic metabolism (which is fueled by oxygen) for its energy. In simple terms, the process breaks down sugar (what we consume) and molecular oxygen (what we inhale) and produces carbon dioxide (what we exhale) and energy. If you want to know more about aerobic energy production, read part 2 of this series that covers the process in more depth. 

A consequence of this reliance on oxygen for energy production is heavy breathing to get more oxygen to the lungs and an elevated heart rate to get more oxygen from the lungs to the rest of the body. This dual activation of the respiratory and cardiovascular systems is the origin of the term cardio-respiratory exercise — cardio, for short.

What’s so great about cardio?

There are lots of great benefits to cardio workouts, the most obvious being improved heart health. As our bodies gulp down oxygen in order to fuel our exercise, our heart is busy pumping the oxygen-carrying blood throughout our bodies. The heart is a muscle, so this extra work makes it stronger and more efficient, just like a normal muscle responds to exercise. A stronger heart can pump more blood, improving our circulatory efficiency. 

In response to persistent exercise, the body also produces more blood vessels (angiogenesis). These new capillaries improve circulatory efficiency, allowing for more nutrients and more oxygen to be delivered throughout the body. These new blood vessels have the added benefit of reducing our blood pressure.

Regular cardio exercise also induces the body to produce more blood. No one is certain why the body does this, but researchers think there are at least two factors at play. 

  • The first is due to a process called exercise-induced hemolysis. Hemolysis is when red blood cells are destroyed. When it happens during aerobic exercise, it’s primarily caused by repeated impacts to our feet. This loss of red blood cells triggers the body to produce more.
  • The second factor believed to be important is exercise-induced osteogenesis. Exercise causes physical strain to our bones. In response, our bodies produce cells that form new bone cells called osteoblasts. These osteoblasts are thought to help in the regulation of red blood cell formation in the bone marrow. 

Despite not knowing the exact cause of the increased blood volume, its benefits are much more obvious. One of the unsung functions of blood is its utility as a heat transfer mechanism. More blood also means more blood plasma which provides more thermal regulation since plasma is where all our sweat comes from. And of course, more blood means more oxygen-carrying hemoglobin to fuel the new demands we’re putting on our body. 

How to cardio

This aerobic exercise thing sounds pretty good, so what does it actually look like? All we have to do is work out any muscle or group of muscles long enough that we burn through our short-term energy reserves and start using oxygen to keep us moving. That’s about 20 minutes of exercise. If we want to start seeing all of those benefits, it’s recommended that we do at least 150 minutes of cardio throughout the week, but more is always better. We can even break it up into 10-minute chunks — as long as we’re active enough to get our heart pumping, we should be good.

The best part about aerobic exercise is there’s no best workout; whatever fits best into our life will work just as good as any other aerobic workout. If the best we can do is a brisk walk for 30 minutes, that counts. If running is too intense for our knees, then we can do swimming or cycling instead.

Awesome! Let’s lose some fat!

Yeah, about that. Simply put, aerobic exercise is not a magic bullet to lose weight. A review of scholarly literature in the journal Progress in Cardiovascular Diseases found that even after four to six months of following an aerobic exercise program of at least 150 minutes per week (a little over 20 minutes per day), research participants only lost around 2 kg (~4.4 lbs) of weight. 

A 2000 study from the Annals of Internal Medicine did find significant weight loss could be achieved from aerobic exercise, but in that case, the study participants maintained an isocaloric diet (consumed the same amount of calories every day) and burned 700 calories a day on a treadmill (about one hour of running at 6 mph). In other words, these participants were putting in around 420 minutes per week on low- to moderate-intensity cardio. 

So what’s going on here? Although we can’t know for certain the mechanisms at play in these two studies, we can make some educated guesses. According to the University of Michigan Medical School, the body’s first sources of energy are creatine phosphate and sugar. We burn through creatine phosphate pretty quickly and switch to sugar (in the form of glucose) to power us for the next 20 to 30 minutes. It’s not until after we’ve burned through these initial energy stores that we begin using the fat in our bodies as a fuel source. So, if we’re using cardio as only one part of our exercise routine, we should save it for last.

Further complicating things is the intensity level at which we exercise. If we don’t exercise with sufficient intensity, we may not burn enough calories to see significant weight loss. On the other hand, if we exercise with too much intensity, our bodies switch to a more efficient fuel source than fat: carbohydrates. Beyond a certain intensity threshold, the body converts carbs into sugar to fire its furnaces. So where is the magic cutoff for optimal fat burning? Around 75% of your maximum heart rate.

If working out at that intensity seems impossible, there’s still a ray of hope for those of us who are only ready to embark on a lower-intensity workout for now. A 2017 study in Health and Quality of Life Outcomes found that even a low-intensity workout, consisting of three 60-minute sessions on the treadmill, is enough to meaningfully alter our body composition. Study participants in the low-intensity group lost an average of 5 cm (~2 inches) around their waist and significantly improved their cardiorespiratory health.

All that’s left for us to do now is find a way to incorporate aerobic exercise into our daily routine. Next time we’ll be looking at anaerobic exercise and how it can improve our health.