Walnuts are a good source of calcium, which is needed if you’re on the low FODMAP diet. If you don’t wish to use walnuts for any reason, you have other options. Almonds are a stellar source of calcium and protein, and many other nuts should work fine in their stead.  The only nut that you really need to be cautious over is the pistachio. Pistachios are high in FODMAPS. Thus, they are best avoided and left out of this granola recipe.