Pie fillings range from tart (lemon or Key lime) to fruity (apple, berries, etc.) to nutty (pecan) to creamy (chocolate cream) to veggie-like (pumpkin and sweet potato).
When you're trying to lighten pie filling, you've got two things to look out for: sugar and fat.
Many pie filling recipes call for a cup of sugar. That adds up to about 100 calories per serving, just from the sugar in your filling! You can cut the sugar calories in half either by using half the sugar the recipe calls for (this usually works well in a fruit filling) or by substituting Splenda for half the sugar.
Then there’s butter. One sweet potato pie recipe I looked at called for 1/2 cup of butter. That’s a tablespoon per serving, adding about 100 calories and 12 grams of fat per serving — and that doesn’t even include the crust!
You can usually trim the butter in fillings to 2 tablespoons, then add in a few tablespoons of orange juice, rum, or even maple syrup (especially if you’ve cut the sugar in half). Some fruit pie recipes don’t include butter in the filling, but call for dotting the top of the filling with butter. This is, in a word, unnecessary. Don’t dot, and save yourself the fuss and calories.
Creamy and custard pie fillings usually call for evaporated milk — not to be confused with sweetened condensed milk, which pops up in fancier pies like Key lime and Kahlua cream pie. You can use evaporated skimmed milk in recipes that call for evaporated milk, and fat-free sweetened condensed milk in recipes that call for sweetened condensed. Either switch will trim about 25 calories per slice (if you get eight slices per pie), and in a pie, every little bit helps!
Cream cheese is another creamy filling ingredient that can be replaced with a lower-fat variety. I personally I find the color and texture of fat-free cream cheese unappetizing, so you may want to go for low-fat or light cream cheese instead. If your filling calls for an 8-ounce package of cream cheese, you can shave about 37 calories and 5.5 grams of fat per serving (when 8 servings per pie) by using light cream cheese.