1) Check out the book "The Paleodiet" (book and website by a female medical anthropologist). Apparently, for over 99% of the time humans have existed, their diet contained very little satfat, most fat coming from plant sources, few sugars other than from whole fruit, and few simple starches (like white flour or polished rice, as opposed to whole grains when they could get them). As has been noted earlier in this thread, the modern Western diet as the unaware follow it is linked to many illnesses.
2) I advised earlier against being a vegetarian due to the neccessary nutrients that are difficult to obtain in adequate quantity and quality.
3) Agreed that a thruhike is not the ideal time to make all kinds of changes in one's diet to make it more healthy. NOW is the time to start eating healthily, so to make it a lifelong pattern. An aspiring thruhiker would do best to be as healthy as possible BEFORE starting his hike; that means not only weight and exercise fitness, but nutrition.
4) Whole-grain macaroni CAN have sauces added to it. Spice up what you mix with it, and you'll not have an issue with its taste. I now prefer skim milk and whole-wheat bread over the greasy/nutrient-reduced the nutritionally unaware commonly eat.