LTT, you said it yourself: Heart Failure.

Exhaustion, simply. Used 50 times a day could mean 5 times an hour, 12 minutes each.

I can almost hear their begging now. "PLEASE JUST A MOMENT'S REST?!" with their eyes wide and their muscles tearing. What kind of pain would that sort of fatigue bring with continued physical activity?

Aside from the heart, other forms of organ failure could be their technical end. Even the brain itself might just cease functioning if it's not allowed to recharge.

As for proper nutrition... simply not possible. Unless the nutrition in question is some sort of miracle cure or invincibility potion.