Ultralight food? Well yes and no. Food is a very personal subject, and before you start messing with your menu you need to be sure of what you're doing. I can't imagine a better picture of Hell than being on Day 7 of a trek, days out from re-supply, and having only that purported "miracle food" combo of corn pasta and spirulina tablets. I'd be chewing pine cones out of desperation.
The main thing to consider when selecting your menu is whether it will keep you happy, healthy and provide enough nutrients and energy to keep you on course. Within those constraints, there are some basic steps you can take to best balance your body's needs.
If you've been following this site awhile, you're probably familiar with the tale of Fruit Boy from one of my Grand Canyon treks. Fruit Boy did not believe in dried or cooked foods, and his 100+lbs of load was largely fresh fruit, including two 15-lb watermelons. (No, really! I saw it with my own eyes.) This for a 5-day trek across demanding terrain. Ignore for now the obvious problems of fresh fruit in a hot desert environment. The core problem here is subtle at first, obvious when pointed out.
It costs energy to carry food. If the energy payoff of consuming the food is less than the energy cost of carrying it to the point at which you eat it, you've incurred an energy debt simply by the act of carrying that item, not to mention the rest of your load. That debt has to be covered somewhere, and unless the answer is another food item that creates an energy profit, your body starts metabolizing what it can to pay the outstanding energy bill. On a short trip or maintaining only a minor deficit, this may not be a problem. On longer journeys or with steep energy deficits, disaster results. Mind you, the food you bring not only has to cover at least the cost of carrying it in, but also your body's efforts from transporting the entire load that day as well. So a food item that doesn't even cover its own cost of transport had better be a rare treat.
Granted, Fruit Boy was an extreme case, but it helps drive home the point - aside from the occasional splurge items, your menu should cover all aspects of your nutritional needs. For most of us this is never a problem, but as we begin to examine paring down food weight I thought it was worth underscoring the issue.
One quick note - along the JMT at the Vermilion Valley Resort, there are two "donation cans" (55-gallon drums) provided for backpackers to swap food. The two largest categories of food filling the cans were trail mix and instant oatmeal. Take whatever lesson from that you wish. :-)
In addition to your "ready to eat" snacks, most people prefer 2-3 core meals on the trail. Given that cooking requires fuel and stove, there's a temptation to leave the stove behind and bring entire meals that are ready to eat. If that really works for you, great, but if you're doing it solely to save weight, you're probably pursuing false economy. Given the viability of such simple stoves as the Esbit, the Pocket Rocket and others, the additional water weight of those no-cook meals is likely to far outweigh a lean cooking system, particularly on longer trips. Plus there really is something nice about a hot meal on a cold night/morning.
One thing you'll notice is that much of the food I bring isn't particularly "light" (peanut M&Ms Joe?). However it typically is either lightweight (cream of rice, dehydrated rice dinner) or very calorie-dense and darn tasty (to me). Follow those split goals and you'll probably find your personal Food Nirvana. The results of my meal tinkering worked fabulously on my John Muir Trail thru-hike Aug/Sept 2000. I ended up using a good number of store-bought freeze-dried dinners, not for any nutritional reasons, but because I have a hard time creating my own Sweet and Sour Shrimp instant dinners in my kitchen. ;-) My home-made dinners made up about 1/3 of them. On the JMT my snacks were mainly Red Vines (red licorice made from wheat flour and corn syrup), Raisinettes (chocolate-covered raisins - AWESOME!), Chex Mix, "fun size" Snickers Bars, Pop Tarts, and Clif Bars. Full details are on my JMT Trip Plan Page.
Between the Cream of Rice breakfasts, Moose Goo w/ tortillas, snacks and the rice-based dinners, I never ran out of energy on the trail, never bonked, and never felt run down. In fact one day I powered through from Rae Lakes to Lone Pine (with a stop on Whitney) in just under 24 hours, 60+ miles according to my Tom Harrison maps. My goal was to run some fun munchies/supplies back to friends still on the JMT, but the !%!$! trail turned out to be closed to inbound Whitney Portal foot traffic. Doh! Anyway, if the food wasn't adequate, that would have been an absolute death march as opposed to the total blast I had doing it!
Instant Cream of Rice (not Cream of Wheat) for breakfast is one of my key "secrets" now. The stuff has an amazing amount of complex carbs per serving. Two servings, along with sugar/raisins to sweeten, provides me with an incredible amount of oomph for the morning's portion of the trail. Your body may react differently, but at the moment I'm swearing by it. Nice warm friendly texture too. Just make sure you cook it long enough, and with plenty of water! The stuff seems to be hard to find in some parts of the world, but you can mail-order it from Amazon.com. This link might not last long, but you can go into their Gourmet Foods section and search for "cream of rice" when the link goes stale.
Note: 3500 daily calories is for trips with long days. For a more relaxed schedule, 2500 calories is a good target for me.