The Word of Wisdom. Actually there are many.

Readings:

1. a. 
   Doctrine and Covenants 89 (the Word of Wisdom).
2. b. 
   Doctrine and Covenants 49:19–21; 59:15–21; 88:124(supplemental scriptures).

Within The Church of Jesus Christ of Latter-day Saints, there is a common phrase: "the word of wisdom."   This is more that just one word, and it refers to the general code of health. I have a tender spot in my heart for these particular doctrines of the gospel.  Because our spirit and our body make up our soul, it stands to reason that we need to take care of both and usually taking care of one helps take care of the other.  Both body and spirit were given to us from a loving Heavenly Father and given for us to maintain and care for, so I don't think it comes as a surprise that He didn't leave us without some recommendations to help in the care of our bodies.

Mormons are known for a few things, for better or worse, among those things are: we don't drink coffee, tea or smoke/use tobacco products.  These guidelines come from the 89th section of the Doctrine and Covenants.  Emma Smith, the prophet Joseph's wife did not like cleaning up the tobacco after meetings, which led to Joseph asking Heavenly Father about health practices.  Section 89 is a revelatory response concerning the issue regarding tobacco as well as other healthy practices.

The Lord says the Word of Wisdom is given to specific people: the weak.  This doesn't mean those who are not strong or those who do not have self-control.  The weak in this sense are those who are humble, those who are ready to learn and apply this principle.  I suppose it could mean that those who seem weak or silly in the eyes of the world:

 3 Given for a principle with promise, adapted to the capacity of the weak and the weakest of all saints, who are or can be called saints.
 4 Behold, verily, thus saith the Lord unto you: In consequence of evils and designs which do and will exist in the hearts of conspiring men in the last days, I have warned you, and forewarn you, by giving unto you this word of wisdom by revelation—

Other items of note in these two verses regarding the purpose of the word of wisdom are 1) It is a principle of promise - the Lord gives us many commandments which we are blessed for in one way or another, but these principles come with explicit and many time rather obvious promises, which I will list later. 2) The Lord gives a general code of health because He knows there are and will be conspiring people who will use the violation of the body in one way or another to destroy our spirits.

So here are the items listed in DC 89 that are NOT GOOD for the body:

1. a. 
   D&C 89:5–7. (Wine or strong drink.)
2. b. 
   D&C 89:8. (Tobacco.)
3. c. 
   D&C 89:9. (Hot drinks, defined as tea and coffee.)

Along with these things, more recent revelation instructs us not to use any illegal drugs or substances containing illegal drugs, any habit-forming substance, unless under the care of a competent physician, or the mis-use of prescription or other drugs.  I will not list the reasons behind this as there are plenty of scientific studies indicating the harmful consequences of the use of all of the above, and there are plenty of stories indicating the harm they have done to persons and families. I will point out though, that the Lord knew all of this and revealed it to Joseph Smith long before scientific studies proved such ill effects.

Here are the items listed in DC 89 that are GOOD for the body:

1. a. 
   D&C 89:10. (Wholesome herbs—nourishing vegetables and plants.)
2. b. 
   D&C 89:11. (Fruit.)
3. c. 
   D&C 89:12; see also D&C 49:19. (Flesh of beasts and fowls.)
4. d. 
   D&C 89:14–17. (Grains.)
5. e. 
   D&C 88:124. (Retiring to bed early and rising early—getting adequate sleep.)

A good guideline for the purpose and amount of foods is given in DC 59:18-20

 18 Yea, all things which come of the earth, in the season thereof, are made for the benefit and the use of man, both to please the eye and to gladden the heart;
 19 Yea, for food and for raiment, for taste and for smell, to strengthen the body and to enliven the soul.
 20 And it pleaseth God that he hath given all these things unto man; for unto this end were they made to be used, with judgment, not to excess, neither by extortion.

Here are the promises from the Lord for obeying these words of wisdom and warning:

1. a. 
   D&C 89:18. (Physical health.)
2. b. 
   D&C 89:19. (Wisdom and great treasures of knowledge.)
3. c. 
   D&C 89:20. (The ability to run and not be weary and walk and not faint.)
4. d. 
   D&C 89:21. (Protection from the destroying angel.)

There are many things that can be debated here, as there are many that have had different experiences and effects of using or eating a special kind of food or substance or herb that helps them or hinders them.  Perhaps a specific food or herb or oil or exercise or medicine helps you and you couldn't live without it.  That's just fine.  I think what the Lord is telling us in section 89 that all of these principle apply to the huge majority of the popualtion - drugs, alcohol, tobacco, addictive substances, etc are not good for merely ALL people.  Eating a wholesome, balanced diet is good for ALL people.  If you have needs or deficiencies or issues that require a different balance that the majority of the population than it is up to you to ask the Lord for help in obtaining that balance, as he will help the individual as well as the general population.

I think though that there are underlying principles here that are sometimes overlooked in the written words of section 89.  I think the Lord is trying to teach us, by way of "the word of wisdom", patience, temperance, self-control, good judgement, resistance, balance, the importance of proper education, and to always look to the creator of all things.  These things are hard to learn in a world (especially in the U.S.) where food, medication and drugs are abundant - even a hobby, they are fairly easily accessible and information regarding them abound (both accurate and inaccurate).  All of these learning principles apply to our spiritual nourishment as well and therefore benefits both body and spirit.  What better way to help our spiritual progress that to make sure we have a learning experience that is as constant as taking care of our bodies?