Although this question is a bit faulty, I have heard it a lot. The rumor gets around that the LDS church doesn't let their members drink a certain carbonated soft drink. Now I personally have never heard any official statement from the church telling us not to drink it, but I think I understand where this notion comes from.
In 1833, the Prophet Joseph Smith received a revelation commonly known as "The Word of Wisdom" (D&C 89). It is a law of health received from God. The principles taught in The Word of Wisdom outline the things we should and shouldn't take into our bodies. In it, the Lord tells us that "hot drinks are not for the body or belly" (v. 9). Through modern revelation we learn that the term "hot drinks" refers to coffee and tea. Now, these substances are definitely against the Word of Wisdom and must be avoided to follow the counsel of God and His prophets. Other than alcohol, no other drink is prohibited in the Word of Wisdom.
But, many believe we are told to abstain from coffee & tea because of the high amounts of caffeine found therein. In addition to this many sources in and out of the LDS church have found caffeine to be a harmful and addictive substance (for examples see here & here). But, the Church of Jesus Christ of Latter-Day Saints has never, to my knowledge, made an official statement advising us not to drink caffeinated beverages.
In short, Latter-Day saints are a health-conscious people. Some don't drink caffeinated drinks or soda because it's simply not good for you.
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