I've been struggling with that question, even if the heart rate is not low enough to be called bradychardia. I was asking why Ray Peat was saying that people with higher hear rates at 85 would be healthier than people with heart rates at 70.
Since the answer is harder to come by from studies, I'm just going by reasoning it out. Let's assume we're talking about people who have equal body temperatures, so that we're not talking about a higher body temperature being the cause of faster heart rates. After all, metabolism is not only used to increase body temperature. It is used for many functions in the body. If the body is very functional, meaning all systems are go, it would have to be more active than when the body is sub-functional, where some systems are impaired and not working 100%.
When a system is impaired, it becomes in itself a bottleneck on the larger system. The more systems are impaired, the more bottlenecks there are in the larger system. In a state where there are many bottlenecks, there would be less energy used. With less energy being used, less output results. The output we talk about here are all the things the body produces or does that makes it healthy. Like detoxifying, healing, destroying cells that have outlived their usefulness, regenerating tissues, fighting infections, killing cancer cells. If the body is healthy, it knows how to produce energy efficiently, and it can use this energy well to maintain all the processes the body needs, including helping itself produce energy efficiently and making it effective in sustaining itself.
So, a higher heart rate is indicative of the body being self-directed in making and using energy to keep itself fully functional, with the minimum of bottlenecks in its own factory of a body.
The key to tying heart rate to health is in knowing that the heart rate reflects a healthy demand for energy, not an unhealthy demand for energy coming from bottlenecks that make the heart work extra hard. I would say brachycardia is an unhealthy lack of demand for energy, while tachycardia is an unhealthy and inefficient use of energy by the body. Having a healthy heart rate involves the body being able to both produce and use energy effectively to keep itself at an optimal state. It is able to be full of energy, more than enough to survive and to function well, but to make itself outwardly presentable - nice skin, nice crop of hair, and a happy disposition.
Good and reasonable demonstrable asserts. Please tell me if I am hearing this correctly. So if bradycardia is a primary bottleneck known as a normal symptom of hypothyroidism, established with labs, (whose "ranges" I personally feel are suspect to the degree there exist so many variables in each individual), and that body is still unhealthily making demands for energy which it cannot muster (caloric is 2K-3K days with zero activity, nutrition is fortified, yet 75% of life in bed), then other hormonal (adrenal? other?) bottlenecks may still be influencing and remain undiscovered?
My personal decline was sudden as an event 4+yrs ago and proceeds swiftly without significant rise in temperature or heartrate WITH the ingestion of thyroxine-(diagnosed hypo and neuropathy, no diabetes). I have over 4 years of clinical intakes@40HR. So, lets go here, since I just unloaded misc boxes and furniture from the back of a full, full-size pickup and reached for my stethoscope and thermometer. I got to 98.2 (yay!) but my feet are ice cold, I have goosebumps but a hot face and HR 48. I made demands on my heart with stairs and fairly active work. The cold, numb feet hurt so much its back to bed horizontal on a heating pad for relief in an 80 degree room; the "nerve chills" are all over me. The thyroxine has been tried from 50mg to 125, both synthetic and not, with waves of nerve chills and musclespasms at the higher end dose absolutely intolerable. If "bradycardia is an unhealthy lack of demand for energy", how do I explain the spurts of strength and high energy output at intervals which appear inexplicable? I am the only one here hauling firewood and shoveling snow and gardening. Maybe a better question for this thread is "What kind of clinician would a person seek in the USA to perform an in-depth assessment of heart rate, temps, labs towards discovering the bottlenecks that might allow function in a tolerable range where bradycardia and hypo appear significant but unresolved? Beentheredonethat with Neuros, Cardio, and Internal Medicine at great travel/time/expense, and no, I don't get warm after a nice big piece of grassfed beef. I am losing my feet, hands, and coordination rapidly, then suddenly not. This needs a drilldown approach. Where is a Bottleneck opener?