Ben's answer above is good. I wish I could give him more than just one upclick.
Rules of thumb:
Carbs and proteins run 3.5 to 4 calories per gram. Fats run aobut 9 calories per gram.
Working hard, especially in cold weather, you can tolerate a lot of fat in your diet.
When planning food for teenagers, I figured on 4000 calories per day.
This is sufficient for days with 8-9 working hours per day. (Pretty hard core compared to most recreational use.)
With a 40% fat content diet, each hundred grams of food with provide 40 * 9 + 60 * 4 = 360 + 240 = 600 calories. So it would take 700 grams of food per day -- about a pound and a half. Our meals tended to be lower fat than that, and a rough rule of thumb was 2 lbs dry weight per person per day.
Two pounds per day means that the groceries for a 15 day trip are 30 lbs. With the high fat option (LOTS of nuts, lots of cooking oil) There really isn't getting around that.
So, as others have pointed out, you need to make the rest of your gear light, and minimize the parasitic weight (packageing) of your food.
If you do trips frequently, set up your food in a spread sheet. I had one in which I figured on the weight/volume per serving, had a constant for the number of people in the trip, another constant for the class of trip, and the spread sheet would figure out the packing weight/volume for everything. This makes things a BUNCH easier when packing for an expedition of 30 people for 3 weeks in the wild waters of northern Saskatchewan.
One aside: An external frame pack is considerably easier when handling lots of weight. They tend to be wider and flater, so keep the load closer to your own center of mass. They are however a true PITA in brushy country, as the extra width and exposed corners catch.