The modern Church uses the phrase "the Lord's law of health" to describe the Word of Wisdom. For example, Chapter 29 of Gospel Principles is called "The Lord's Law of Health." However, the Word of Wisdom itself states that it was given:
In consequence of evils and designs which do and will exist in the hearts of conspiring men in the last days. (v4)
Suggesting that it has more to do with avoiding addiction (or, perhaps, choosing freedom) than actual health. There are a great many valiant Church members who suffer from disability, terminal disease, and even chronic disease/pain to easily rationalize the simplicity of "obey the Word of Wisdom to be blessed with good health."1
I don't believe that the Church has ever taught that obedience to the Word of Wisdom guarantees perfect health — or even suggests that physiological tragedy (e.g., cancer, etc.) will be avoided because of said obedience. The Word of Wisdom itself simply states that the following blessings will come due to obedience (v18-21):
shall receive health in their navel and marrow to their bones;
And shall find wisdom and great treasures of knowledge, even hidden treasures;
And shall run and not be weary, and shall walk and not faint.
And I, the Lord, give unto them a promise, that the destroying angel shall pass by them, as the children of Israel, and not slay them.
I believe (but do not know if it is doctrine) that (1) has the same context as Prov. 3:7-8 in that it's a spiritual blessing, not a physical blessing, (3) has the same context as Isa. 40:28-31 in that it's a blessing of enthusiasm and productivity rather than bodily health, and (4) uses the reference from the story of Moses to describe avoiding the Lord's vengeance during judgement. (I skipped #2 because it doesn't appear to influence this question.)
Thus, whenever I hear the phrase used, I wonder that it's a bit of an oversimplification. I am therefore studying the section and its history and would appreciate some help.
Question: What is the earliest use of the phrase "the Lord's law of health" in regard to the Word of Wisdom?
1 An example of the problem at its worst comes from my time as a Branch President when a member asked me why I hadn't excommunicated another member for violating the Word of Wisdom. She was obese and had been for many years. It was a long discussion that's summarized as, "the Word of Wisdom doesn't actually work that way."