Fasting, while a change from one's normal habits, doesn't actually have to put much strain on the human body. Indeed, it can often help to clear toxins.
One potential trouble spot comes at sundown when it's time to break the fast. Because there's a lack of food in your stomach, it produces less acid than it would normally. However, the thought of food can cause your stomach to start producing it again but with nothing to digest.
This becomes a problem when, upon breaking the fast, a large amount of often greasy food is eaten. Fatty foods are often linked to heartburn and this can create a situation where heartburn is very likely.
The solution? Eat carefully upon breaking your fast, don't eat too much and avoid overly greasy food. Once your stomach has adjusted to having food in it, though, feel free to dig into whatever you like!