Concerning nutrition, the Lord said the following.
And again, verily I say unto you, all wholesome herbs God hath ordained for the constitution, nature, and use of man—
Every herb in the season thereof, and every fruit in the season thereof; all these to be used with prudence and thanksgiving. (D&C 89:10-11)
The Lord said to use herbs and fruits (and vegetables) in their season. Today, we have food preservation methods and fast transportation that didn't exist in the 19th century, and we are able to use food when it is out of season. However, there are good reasons to use when possible food fresh from the gardens and orchards. As an example of this, grocery stores don't carry tree-ripened fruit due to the long transportation and storage requirements of the modern food industry. Fortunately, farmers-markets and roadside food stands do provide tree-ripened fruits and other food fresh from their gardens and orchards. Another example is that of apples. It is common for the producers of apples to dye apples, especially lower quality apples, to make the apples appear more inviting to shoppers. The next time you buy red apples from a grocery store, take a bite and then look at the white meat to see if the red dye has soaked through the skin into the white meat.
Concerning grains, the Lord said
All grain is ordained for the use of man and of beasts, to be the staff of life, not only for man but for the beasts of the field, and the fowls of heaven, and all wild animals that run or creep on the earth;
All grain is good for the food of man; as also the fruit of the vine; that which yieldeth fruit, whether in the ground or above the ground—
Grains are rich sources of vitamins, minerals, and protein. In addition, we know through modern nutrition the importance of having sufficient fiber (or "roughage" as my mother called it) in our diets.
I think of vitamins, minerals, fats, proteins, and carbohydrates as the fuel that keeps our bodies going. All functions of our bodies are related to chemical activities, and the chemicals are provided by our intake of food and drink. Probably the best source of nutrition is natural foods and drinks. However, people are taking supplements instead of eating proper foods, because the supplements are easier to take and require less time to consume. In addition the nutritional quality of our food is declining, due to depletion of the soils used in agriculture and attempts by the food industry to produce food and drinks at a lower cost and with longer shelf-lives, and people take supplements to overcome these deficiencies in their food and drink.