Granola bars often have a positive effect on digestion as the fiber helps with regulating the digestive tract. Additionally, if the bar is made from oats, it contains a good source of prebiotics, which are food for the good bacteria in your gut. Be mindful of granola bars with artificial sweeteners such as xylitol, sorbitol, and aspartame, as they are not broken down in the digestive system and may cause stomach upset.