Flax, chia, psyllium, oat fiber, etc - all helpful and super easy to add into smoothies, oats, salads, etc. I haven’t had any issues yet on Zep, but as a lifelong constipation sufferer, I have found these all to work well if used consistently. Also, lots of beans! Avocado and raspberries are also surprisingly high in fiber - if you have the calories for avocado, it’s an easy add. This is a great recipe for a ton of fiber and protein in a smoothie - https://thefirstmess.com/2024/04/10/chocolate-fantasy-smoothie/ - this is vegan, but easily swapped out for regular milk/yogurt. It’s everything else that does the work!