I also have an unusually low pulse around 46 for which the MD's cannot figure
Many years ago, I had a very slow pulse in the 40s, too. Doctors couldn't figure out the cause. It took me awhile to solve the mystery, but I finally did. The slow pulse rate was caused by a calcium defiency -- but that wasn't the root cause of the problem.
The root cause of the problem was that my diet was deficient in vitamin B6, pyridoxine hydrochloride. Pyridoxine hydrochloride is the building block of hydrochloric acid. Because I was low in pyridoxine hydrochloride, my stomach wasn't producing enough hydrochloric acid to digest calcium (which requires an acid stomach environment to be digested and assimilated). Therefore, even though my diet had sufficient calcium, I wasn't absorbing it and developed a secondary calcium deficiency. One of the symptoms of calcium deficiency is slow pulse rate, although obviously there can be other causes.
I would get a blood test to see if you are low in calcium, especially if other symptoms support a possible calcium deficiency. For example, you may find that your muscles tend to cramp easily. Other symptoms of calcium deficiency are joint pains, heart palpitations,insomnia, and accelerated tooth decay.
One other thing: eating a lot of sweets (even natural sweets like honey) and carbs increases your need for B vitamins, including B6. So your diet may have sufficient amounts of B complex vitamins for someone who eats a balanced diet, but if you're a sweetaholic you are automatically increasing the amount of B6 (and the other B vitamins) you need to eat beyond the norm in order to avoid developing a deficiency.
Being super-athletic also lowers the pulse rate, but 46 seems abnormally low.
Best wishes,
Tumbleweed