Metabolic rate in humans can be approximated using the Penn State equation.
According to Kleiber's law, metabolism scales across species proportionally to the mass by a power of 3/4 (also by 2/3 or the [2/3, 3/4] range for other models).
Now, despite the known relationship between temperature, mass and metabolism, the relationship between heart rate and metabolism is less clear. Heart rate also scales with body mass approximately by a power of -1/4.
Usually, as a human ages, both its metabolic rate and heart rate decrease. Also, there are many examples of animals with a very high metabolism that have a very fast heart rate, like the hummingbird or the etruscan shrew.
I want to know if metabolism, after controlling for mass, is related to heart rate. I find it suprising that nobody has studied this relationship. I guess more data is needed to generate a model (I found this site, which could be useful, beats-per-life).
Heart rate is related to the distribution of nutrients and oxygen to all organs in the body. I think a faster metabolism would require/imply a faster distribution. We know this is true at least within the same individual organism, for example, during the elevated metabolic demands of exercise. Despite all of this, I haven´t found examples of it being used in relation to metabolism.
Any ideas about why this might be the case? What's the relationship between metabolic rate after controlling for mass, and heart rate, if there is any?
What about cardiac output? Cardiac output is stroke volume times heart rate. A bigger body tends to have a bigger heart, a bigger heart likely implies a greater stroke volume. Maybe if you adjust stroke volume by weight, and combine it with heart rate, you get an even better indicator of metabolism.