Fasting intermittently is known to be healthy as long as you eat at sunset, you may only feel tired and yeah, if your body is accustomed to tobacco and coffee, it will come with some discomfort.

The reason for the fast is what I m really interested in, why fast a year, is it a culture or a religious practice or just a hunch to?

Yes a couple of people have done this, going for 7days on water, or fruits. This wasn't her first time either. A racing heart was just a compensatory action of the body trying to shift into a hypermetabolic state. Often times, it could be dangerous if the heart is not in the best of conditions.
But the adrenaline surge causes that and sets the body on high function so it can breakdown other sources of energy.

It could be worse, yeah.

More worse when the fast is over, the stomach walls are highly fragile and sore from acid secretion and reduced protective layers most of which may have also be broken down during the catabolic state.
What is eaten after that matters.
Nothing peppery, nothing hot, nothing solid. In Nigeria its best to go with pap, bland pap. This prevents ulcer, gastritis, or even a chronic inflammation can ensue which is a risk factor for perforation or even stomach cancer.