I have just noticed something similar. I started intolerances to wheat/gluten 9 months ago. My unfit HR is 49bpm, and superfit is 47bpm. However I checked my HR recently after not doing so for 12 months and found that it is now 40/41. What the hell??? I'd love a resting HR that low, but I think it is un-naturally low due to something linked to coeliac disease. Has anyone spoken with doctors about this?