In particular, it was reinstituted by commandment with the restoration of the Gospel in this dispensation.
There are 118 separate references to fasting in the scriptures (that's not instances of the word, that's separate discussions of the topic). Seems pretty important, I think.
Members of the Church of Jesus Christ of Latter-day Saints are encouraged to fast once a month, usually on the first Sunday of the month, and whenever they feel their faith could use an extra boost. A typical fast consists of abstaining from food and beverages for a period of time, usually enough to skip two consecutive meals.
I used to like to do 24-hour fasts - I found it to be a more spiritual experience to make my body subject to my spirit for that long. But these days I've been less disciplined. I'd like to get back to doing 24-hour fasts.
Elder L. Tom Perry of the Quorum of the Twelve Apostles stated:
"The law of the fast has three great purposes.Let me explain those a little.
First, it provides assistance to the needy through
the contribution of fast offerings, consisting of the
value of meals from which we abstain. Second, a
fast is beneficial to us physically. Third, it is to
increase humility and spirituality on the part
of each individual."
When we fast, we also give a fast offering - that is, we donate to the Church an amount that approximates what we would've spent on the food we didn't eat. It's the convenient equivalent of actually giving our food to the needy and going without ourselves. The money donated stays at the local level and is distributed to the needy in our area by the Bishop of the ward and the Stake Presidency.
Second, periodic fasting has been shown to be a good way to cleanse and rejuvenate the body.
And lastly, it is a great way to put mind over matter - to subject the body to the spirit and exert your will over the physical. It is like training for resisting temptation. And the brief deprivation can help you feel closer to the Lord. The minor suffering involved helps you learn to be more humble. And it helps you to count your blessings. It teaches self-mastery and it strengthens you - builds character and self discipline. And our Savior set the example by fasting Himself.
But fasting is not just "going without food and water." It must be combined with sincere prayer. It must have a conscious purpose.
Sometimes, a ward or a stake may be called upon to fast for a mutual purpose, such as for the well being of a member who was in an accident, or for rain in times of drought. Doing this helps focus the resolve of the group by making a sacrifice together for the benefit of another as we diligently pray for the same outcome. It's a kind of a group faith effort.
When I fast, I always begin with a prayer in which I establish the purpose and officially begin the fast. I often will have a dual purpose - something with which I need help in my own life, and a prayer for another. I also will mentally set an end point for the fast, so that I have a goal to work toward.
Then I refrain from eating and drinking until the appointed time, and before breaking my fast I will once again pray, thanking Heavenly Father for helping me through it, and revisiting the purposes for which I had fasted, offering up my prayers for those things.
I am grateful for the inspired guidance we receive from our Prophet, President Thomas S. Monson. I know that Joseph Smith was a prophet of God. I know that by keeping the commandments and following the example of Jesus Christ, I can be happier and healthier and be able to do more good in the world, and feel closer to my Father in Heaven.
I testify of that the principle of fasting is an important component of discipleship.