Liquid fat, like oils and melted butter that are incorporated into the bread dough before kneading will inhibit the gluten formation. Gluten is the (protein) elastic bands that allow the bread to raise. If your recipe calls for you to add melted butter or oil to the dough, knead the dough first to develop the gluten. In fact, recipes that require no fat will raise higher than recipes that add fat before the gluten is developed.
You can have the best of both worlds, all the benefits of the added fat and a high loaf. How and when the fat is added to the dough is the secret. You can use this trick with any recipe, just add the fat at a different time in the mixing process.
It's hard to get a light loaf using 100% whole grain but using a method developed by the French to add the butter after the gluten has been developed really helps when making 100% whole grain breads.
What I do is develop the gluten with the mixer or by hand and the last couple of minutes of kneading, I add cold butter which has been cut up into small pieces to the dough. I poke holes with my fingers in the dough and insert a piece of cold butter into each. Then I put the dough hook back on the mixer and knead or continue kneading by hand. At first the dough will start to fall apart but as you incorporate the cold butter the dough will pull together and the finished result is amazing.
Think of the cold butter like putting cream rinse on your long hair (gluten strands). The butter de-tangles the developed gluten strands which allows them to stretch to the fullest extent, and the bread to raise to new heights.
Next time your struggling with a recipe that for some reason never seems to raise very well try this technique. With bread making the ingredients are simple but sometimes the techniques make the difference between a brick and a high light loaf.