In order for you to lighten-up the fat or sugar in a recipe you need to understand how fats and sugar affects baked goods.
Wheat flour contains proteins that when mixed with liquid form tough strands called gluten. Fat tenderizes baked goods by coating the pieces of flour so that the liquid ingredients cannot get to them, so to speak. This interferes with the gluten formation, and shortens the strings of gluten to allow a more tender structure.
This is why removing the fat from baked goods often makes them tough and/or rubbery.
Many recipes call for mixing fat and sugar together which mixes in air bubbles and then during the baking process, these air bubbles are part of what makes the baked goods rise.
Fruit purees are often used as fat substitutes... unsweetened applesauce being the most common. Applesauce doesnt impart a lot of flavor and contains more pectin than other fruit purees, which helps to retain the moistness of baked goods. You may also use pumpkin, banana, or prune purees.
Pumpkin and prune purees are wonderful in brownies as they bring out the flavor of the chocolate. You may purchase canned pumpkin if you like or simply place a sugar pumpkin in the oven on a baking sheet and bake at 350 degrees for about an hour. Let cool, scrap out the seeds (and roast them later to snack on!) remove the skin, then puree in a blender or food processor. For the prune puree, place some pitted prunes in a small saucepan with a little water, cover, cook until softened and puree.
At first, eliminate only half the fat in a recipe, and the next time you make the recipe, try replacing even more fat, if desired. For example if you are making cookies that call for 1 cup of butter, use 1/2 cup butter and 1/2 cup of applesauce.
Sugar also interferes with gluten formation; so many fat-free and low-fat recipes contain additional sugar! Lower gluten flours like whole-wheat pastry flour, oat flour, oat bran, rolled oats, and cornmeal better choices for low fat baking as well as being less processed and contain more nutrients than white flour. You can generally reduce the amount of sugar a recipe calls for by 1/3 to 1/2 without compromising flavor or texture.
Other useful tips
-- Stirring batter excessively develops gluten and toughens the texture of baked goods, so minimize mixing in many recipes.
-- Reduced-fat baked goods often bake more quickly than their high-fat counterparts so to prevent over-baking, reduce oven temperatures by 25 degrees, and check the product for doneness a few minutes before the end of the usual baking time.
Patty James. M.S. is a Vital Health Educator and Nutrition Coach who founded the first certified organic cooking school and nutrition center in America.