Determine the length of your fasting experience, whether this is for 1 meal, 1 day, 3 days or a week (Jesus and Moses fasted for a period of 40 days, but that is no sign that anyone else might do that). You may wish to try a shorter fast, and start slowly at first, if you have not previously fasted. You can also pray and ask the Holy Spirit to reveal to you how long you should fast.
Spending time in prayer and fasting is not automatically effective in accomplishing the desires of those who fast. Fasting or no fasting, God only promises to answer our prayers when we ask according to His will. 1 John 5:14-15 tells us, "This is the confidence we have in approaching God: that if we ask anything according to his will, he hears us. And if we know that he hears us - whatever we ask - we know that we have what we asked of him." In the prophet Isaiah's time, the people grumbled that they had fasted, yet God did not answer in the way they wanted (Isaiah 58:3-4). Isaiah responded by proclaiming that the external show of fasting and prayer, without the proper heart attitude, was futile (Isaiah 58:5-9).
How It Works: This one’s easy: Eat very little one day, and eat like normal the next. On the low-calorie days, that means one fifth of your normal calorie intake. Using 2,000 or 2,500 calories (for women and men, respectively) as a guide, “fasting” (or “down”) day should be 400 to 500 calories. Followers can use this tool to figure out how many calories to consume on “low-calorie” days.
Those desiring to receive Holy Communion keep a total fast from all food and drink from midnight the night before (see Eucharistic discipline). The sole exception is the Communion offered at the Easter Sunday midnight liturgy, when all are expressly invited and encouraged to receive the Eucharist, regardless of whether they have kept the prescribed fast.
Carbs are only problematic when eaten excessively, beyond what your body requires. As long as you are eating low calories then carb intake will make very little difference in respect to fat loss. In other words, two diets of equal calories with varying levels of carb intake will result in the same level of fat loss. If you are still unsure then you can read this brilliant article by Lyle McDonald, Low Carbohydrate Diets Have No Metabolic Advantage. In addition, carbs in the main meal serve very important functions such as increasing leptin (improved satiety and metabolic rate), refilling depleted muscle glycogen stores and triggering the release of serotonin which will improve your quality of sleep and make you feel good.
In a physiological context, fasting may refer to the metabolic status of a person who has not eaten overnight, or to the metabolic state achieved after complete digestion and absorption of a meal. Several metabolic adjustments occur during fasting. Some diagnostic tests are used to determine a fasting state. For example, a person is assumed to be fasting once 8–12 hours have elapsed since the last meal. Metabolic changes of the fasting state begin after absorption of a meal (typically 3–5 hours after eating).