I lost 20lb last summer, in that time my resting heart rate was in the 50s, over winter I let go and ate (and drank) what I wanted, my heart rate was consistently high 60s. I've snuck on 7lb so started calorie counting again and within a few days my heart rate has dropped into the 50s again for the first time since last summer. I drink a lot of caffeine when calorie counting (diet energy drinks and coke...I know I know, rest of my diet is very healthy, it's my vice when I'm limiting food)
I'm assuming this isn't a coincidence, does diet affect heart rate so quickly? I don't eat too badly usually, more chocolate! (Fitbit data).