I'm an advocate of the Zone diet, which recommends eating carbs and protein together, and I have a set of carbohydrate and protein combos I find easy and compact to carry for small periodic snacks every 20 miles or so. One favorite is fat free pretzels (Snyder Sourdough Nibblers) with a yogurt drink called Kefir, the fat free version.

FYA, other choices are an apple and some walnuts (good source of omega-3), a mash of Kellogg's Special K and some non-fat yogurt, or a Zone Perfect bar or two. I even bring my own snacks on organized centuries with food stops.