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I recently learned about Kleiber's law describing how metabolic rate is related to an animal's mass. One way this law gets (over)simplified is by saying that all animals have the same number of heartbeats and small animals just use them up faster than large animals.

This got me wondering- how does exercise rate affect this? If I exercise a lot do I use up my total number of heartbeats faster or slower?

Or, phrased a bit more scientifically, does exercise increase or decrease my total average lifetime heartrate? I know that exercise causes it to increase during exercise and decrease during rest compared to someone who doesn't exercise. Does the increase on one side cancel out the decrease on the other side or does it on average increase or decrease?

I suspect there is some good academic research on this but I couldn't find it. Hopefully one of you with more expertise in the field will have a better idea of what terms to throw into Google.

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2 Answers 2

I had the same thought when I watched some documentary about that. Unfortunately I couldn't find any references either. What I did, though, was to start studying medicine, and things started to make sense.

When you exercise there is an acute increase in heart rate. That increase is short lasting, and depends on your oxygen debt (thats what exercise physiologists call it) during and after training. Once that oxygen debt has been repaid, there is a slight increase above baseline as the body is in a higher metabolic state (anabolism to repair and to build muscles, organelles and the like).

When you compare the total number of increased heart beats during exercise and after to the lower number of heart beats throughout the day and night, due to increased fitness and oxygen utilization efficiency, the result is a lower total number of heart beats.

I do not have any studies to back this up. However, you can read about these phenomena in exercise physiology books. For one side of the coin you can compare heart rates of athlete populations and sedentary individuals (I did just that; kind of like a meta-study), and for the other you can yourself measure (with a pulsmeter) your total increased heart beats, from baseline, for a training session, and you will see that it adds up. Furthermore, the last 6 years I have been recording my heart rate several times per day, together with every kilogram I've lifted in the gym, and every meter I have run or sprinted. Having gone through periods of both maximal fitness and injury periods with total rest, I have found the total number of heart beats to indeed be lower the more you exercise (unless you become overtrained as I was for about 3 months). This could be a good study to publish. However, since athletes differ widely in their physiologies and types of training, a case study wouldn't be as good as a prospective study with at least 10 participants.

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Your gross oversimplification is roughly OK when you are comparing one species against another. It breaks down completely when you try to apply it individually. To provide a parallel, BMI correlates to body fat in a way that is statistically significant, but when applied individually may give the wrong impression about the amount of fat someone carries. In other words, BMI works for producing statistices of the number of obese people state by state, but when applied to just you can paint a picture that is very wrong.

The truth is, people have a wide range of life spans (since this is what you are concerned about):

  • Life spans can be cut short due to disease, accident, or intentional actions (by the person or someone else).
  • Even when comparing people who die of old age, there are a number of factors that have a big impact on longevity that it is near impossible to accurately predict how many "heartbeats" a given person has.
  • Using the oversimplification, a person with class III obesity would outlive an average height person with normal body fat. Obesity carries with it greater risk of health complications and ultimately shortens your life.

The bottom line is this:

Exercise and active lifestyles improve almost all health markers.

There are some caveats, as exercise can be taken too far and people can get too lean to be healthy. However, any activity that improves your health will in turn improve your lifespan. NOTE: exercise won't improve your health if you have pre-existing medical conditions and you exceed your body's ability to handle exercise.

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