It’s from Chapter 19, pg. 207-208 –
“I sometimes wonder if Latter-day Saints realize that the Word of Wisdom has been given to us for our exaltation; not only for our temporal blessing, but to prepare us for spiritual life …
“We are told that the glory of God is intelligence [see D&C 93:36], and we all admire intelligent men and women, therefore it should be our desire to lay the foundation for increased mental power and not do anything to weaken it. It is evident in the lives of some that they deprive themselves of the brain power they might enjoy, by the continued use of things our Heavenly Father has said are not good; they become less intelligent as a result, and fail to make the preparation for Eternal life that should be their ambition.”
Of course, following the Word of Wisdom improves intelligence. All you have to do is eat a fast-food meal every meal for a few days and feel how groggy it makes the brain. (No, I’m not saying that Fast-food is totally against the Word of Wisdom, rather making a point at how it alters your ability to think clearly, as do other ingestable substances.)
One of the great promises of the commandment is that those who live it, will gain “wisdom and great treasures of knowledge, even hidden treasures” [D&C 89:19].
Plus, it makes my body and mind feel so much clearer when I eat healthy foods. Now, maybe if I commit to living it more fully – not necessarily the “thou shalt nots”, but the “thou shalts” my mind will be more focused and my life more productive and thus, I can become more of what I ought.