One of the LDS church's more respectable doctrines is the 89th section of the Doctrine & Covenants, more commonly called the "Word of Wisdom". It's been called the "Lord's law of health." It is essentially a set of fairly simple guidelines about what is good for the body and what is not. LDS often use the Word of Wisdom as evidence of Joseph Smith, Jr.'s gift of prophecy. They say that he could not have known about the dangers of tobacco or coffee in his day; therefore, preaching against the use of these substances means he had direction from the Almighty (source).
In any case, I find the Word of Wisdom an interesting part of the LDS church. Some of its segments are specific in their meaning (e.g., "tobacco is not for the body"), while others are extremely vague. However, the leaders of the Church have somewhere established clearer definitions for some of the vague terms (i.e., "hot drinks" means black or green teas and coffee). And still, for some of the sections that are just as vague, they have left the decision up to the individual member. For example, when asking a bishop if caffeine is prohibited by the Word of Wisdom, the member will be told that it is between God and his or herself (here's a similarly vague response).
That's quite the statement. In other words, what the Church does there is state that God might have a different set of rules and consequences for each person. That is, what may be a sin for one believing member may not be a sin for another.
I've always seen this as problematic. For example, the temple recommend question simply asks, "Do you obey the Word of Wisdom?" I have never understood how an obese person could answer affirmatively to that question. If we are commanded (though it has been a slowly-developing commandment) to take care of our bodies, and told our bodies are temples (1 Cor. 6:19; John 2:21), how can somebody say they follow the commandment while obese, or even overweight? Can you imagine a bishop with a scale in his office, weighing each member during an interview, and then tearing up his or her temple recommend if his or her body mass index were over the limit set by the Brethren?
Couldn't obesity be compared to alcoholism (check this out)? Aren't they just two different forms of harmful behavior to the body? But then if a person can ingest trans and saturated fats every meal and still be temple worthy, why can't a member who has an occasional cigarette after dinner, or a beer on the weekends say, "this one's between God and me"?
Why would God have some universal commandments, and then some commandments that are individualized? Really, the only solution I see is to either list out the Lord's recommended daily caloric intake, and publish a list of approved ingredients and unapproved ingredients for the members who so badly want to be righteous. Either that, or leave the whole thing between members and God as the Doctrine and Covenants state.