If you're thinking about food plots, mineral licks and supplemental feed when trying to improve nutrition for whitetails, then you have missed the most influential part of a deer's diet… the natural browse. Think about it, a deer is only in a food plot a small portion of the day and will visit a feeding station occasionally, but they are constantly interacting with natural browse. Therefore, the majority of a deer's diet is composed of natural browse, making it is the most important area to manage.
The next time you step into your hunting area, try to see it as a deer would, by looking at the amount of green vegetation that is 5 feet and below. If the vegetation is mainly above 5 feet then it is out of the reach of a deer and won't provide nutritional benefits. However, if there is a great deal of vegetation on the forest floor and below 5 feet, then in most cases this means that the deer will have ample amounts of food available.
The reason that I say "most cases" is because green vegetation doesn't always mean good vegetation, for example pine plantations that have not been maintained mechanically or chemically can become dominated with sweet gum, a plant of low preference to whitetails, meaning even though woods are bursting with green vegetation, it doesn't mean that it will be beneficial for deer. But it isn't just the South that has this problem, in some areas of the country when forests mature, they have a closed canopy, meaning that light cannot reach the forest floor and no light means no plants growing where deer can eat them!
To correct, remove some trees, plants or bushes that will allow more light to reach the valuable vegetation. The best trees to remove are those that provide little benefits to wildlife while leaving mast producing trees.