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Saturday
May042013

When Does Cardio Become Too Much of a Good Thing?

This post is Part 2 of the Mastering the Art of Stress and Recovery Series

What do you do for cardio? What do you think it adds to your exercise routine? To your health? In the fitness world, the terms aerobic training, cardio and endurance training are often used interchangeably. Although a professional triathlete would argue that his/her training is dramatically different than someone trying to lose his/her muffin top working on a stepmill, what we’re really referring to in both cases is aerobic activity. You may remember from The Three A’s of Intensity, aerobic exercise burns mainly fat for fuel and relies heavily on type I (slow twitch) muscle fibers. As part of a balanced fitness program, cardio has a lot to offer. When we rely on it too much, however, or pursue it to the exclusion of other kinds of exercise, we’re hitting up against the law of diminishing returns and even compromising our overall fitness.

Chances are, if you include aerobic exercise in your workout program, it’s for one or some of the following reasons:

  • To optimize fat utilization and lower body fat levels
  • To improve muscular and/or cardiovascular endurance
  • To improve cardiovascular health
  • To improve recovery from anaerobic activity or resistance training
  • For enjoyment

The Upside: Health Benefits and Fat Utilization

Most people think of their aerobic exercise as a chance to burn a bunch of fat. In truth, a single exercise session doesn’t burn that much more energy than being sedentary. It does technically burn more (varying by activity and intensity), but it’s generally not the huge leap most people imagine. Instead, the biggest benefit of aerobic exercise comes from its role as stimulus or “stressor,” as we talked about in Part 1 of Mastering Stress and Recovery.

A properly designed aerobic exercise program provides multiple and significant benefits, including:

  • increasing the efficiency of the heart
  • increasing aerobic exercise capacity (measured as VO2 Max)
  • increasing the size and number of mitochondria in the muscle cells
  • increasing the body’s ability to store carbohydrates as glycogen
  • reducing resting heart rate (In fact, studies show an individual’s resting heart rate can drop by one beat per minute each week he or she exercises for the first eight to twelve weeks.)

Are there other ways to achieve those health benefits, however? Yes, it appears that high-intensity interval training, or even resistance training, can deliver most, if not all, of those health benefits faster and in a shorter amount of time. However, these activities quickly add to the body’s overall stress load, which means they can’t be done as often as aerobic exercise can be. Additionally, the intensity is often too much to expect for someone just getting started in a fitness regimen.

Nonetheless, some people simply enjoy aerobic exercise more than resistance or anaerobic training. Part of the enjoyment likely comes from the euphoria people feel during aerobic activity. A “runner’s high” is a genuine buzz because of the feel-good opioids released by the body. These opioids are released as a response to the stress of the activity and appear to be associated with reductions in blood sugar levels and elevations in cortisol. These reactions lead to an important question: when can it become too much of a good thing?

Look at it this way. The reason you feel so good during aerobic exercise is because your body is trying to keep you from feeling the pain and fatigue of the activity by making you feel good—so good that you may end up doing more aerobic exercise than you should to get the health benefits you’re looking for.

The Downside: Stress, Muscle Loss, and Fitness Imbalance

Yes, there can be downsides to excessive cardio—just as there are disadvantages to excessive anaerobic and resistance exercise. That said, it’s not a justification for skipping cardio altogether but an incentive to balance it out as part of your fitness program rather than an entire fitness program.

First, the type of exercise you choose should help to balance out the responses of your sympathetic and parasympathetic nervous system. High-intensity exercise and resistance training tend to stimulate your sympathetic (“fight or flight”) nervous system, while aerobic activity usually stimulates your parasympathetic nervous system. Stimulating either branch too much can reduce recovery and lower performance. Chronic endurance training can lead to an overly-stimulated parasympathetic nervous system, which can leave people feeling pretty flat. Resting heart rate becomes unusually low, metabolic rate may slow down, and even one’s thinking may become less clear and crisp. Again, too much parasympathetic stimulation says to the body “slow…yourself…down…” Over time, you may just feel like you’ve lost a little spring in your step.

In addition to the overall slowing of many of the body’s functions, long-term endurance training has been shown to lower testosterone levels. The drop can be especially dramatic in men, who have several-fold higher levels of testosterone than women do. As of yet, it’s unclear what the long-term effects of the reduced testosterone may be. Testosterone is important for maintaining libido, maintaining or increasing lean body mass and bone density, and supporting recovery among other functions.

Long-term, high levels of aerobic exercise can also lead to lower levels of lean body mass. Type II (fast twitch) muscle fibers grow or are maintained in response to heavy loads on the body. Resistance training is the greatest stimulator for increasing or even maintaining lean body mass. In fact, some studies show that aerobic exercise plus a weight-loss-oriented nutrition program can lead to faster losses of muscle than if someone didn’t exercise at all and just changed his or her nutrition. Resistance training, coupled with changes in nutrition, helps to maintain muscle.

This is probably the most important reason not to rely on aerobic exercise alone for your workout program. Far too often, people “go on a diet” and then spend their exercise time running, walking, cycling, using the elliptical or stepmill, or doing some other form of aerobic activity. This kind of exercise puts little to no stress on the muscular system. As a result, people do lose scale weight, but they lose as much muscle, if not more muscle, than they lose body fat. To avoid significant losses of lean body mass, everyone should include some resistance training in his or her program.

Interestingly, the same mechanism that tells your body to get rid of type II muscle fibers when you do a lot of aerobic exercise may also reduce the effectiveness of your heart muscle as well, especially if your diet is low in branched-chain amino acids, found in high-quality proteins like whey, meat, poultry, fish and eggs. Though there isn’t a significant amount of research on it yet, I expect in coming years to see comparisons of those who do mainly aerobic activity against those who do mixed training or resistance-only exercise. I also expect to see those who include resistance training to have healthier hearts and live longer lives.

The amount of aerobic exercise each person can and should take part in really depends on genetics, exercise preferences, and health and fitness goals. Personally, I’m plenty happy lifting weights for 45-60 minutes four times per week, walking on most days and doing some occasional sprints. I have friends who get the greatest amount of joy from the miles they log in their running shoes or the distance they cover on the seat of their bikes. A big part of life is enjoyment. So, to a certain extent at least, do the kind of exercise you enjoy.

Keep in mind, however, that research shows the body significantly increases cortisol levels after 90 minutes of exercise—at intensities as low as 25% of VO2 Max, which is a very low level of intensity. Cortisol is raised higher and faster with increasing intensity levels. The greatest stimulator of stress hormones seems to be the buildup of lactate, which occurs with training at anaerobic threshold, and/or a drop in blood glucose levels.

Again, the key to managing the fuels you burn during exercise is to monitor your heart rate. Provided you’re really working aerobically—and not trying to keep up with your friend who is in better shape—you shouldn’t be building up lactic acid or exhausting your available glucose.

The Take Home Lesson

The keys to maximizing the benefits of aerobic exercise—without compromising overall fitness—include the following:

  • Only do enough aerobic exercise to stimulate the changes you’re looking for in the body. (Keep in mind the changes occur during the recovery period, not during the workout.)
  • Include resistance training in your program—even if it means reducing the amount of aerobic exercise you’re able to do.
  • Use your heart rate monitor to make sure you’re actually working aerobically and not crossing the line into long-duration, anaerobic activity.
  • Periodically get your hormones checked, and make sure you’re staying healthy on the inside, not just getting fitter on the outside.

Finally, aerobic exercise can be great, but it shouldn’t be the only kind of exercise you get. Instead, look at it as a way to complement or enhance your anaerobic or resistance training for general health or weight management. Even if you’re a serious endurance athlete, make it a point to include plenty of resistance training in your overall fitness program.

Share your thoughts, ask questions and keep the conversation going below.

Written by Tom Nikkola – Sr. Director of Nutrition & Weight Management 

This article is not intended for the treatment or prevention of disease, nor as a substitute for medical treatment, nor as an alternative to medical advice. Use of recommendations in this and other articles is at the choice and risk of the reader. 

 

 

 

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