Flax seed fiber contains a better natural healthy balance ratio of insoluble fiber to soluble fiber than psyllium. Flax has a balance of 65-75% insoluble and 25-35% soluble fiber. Psyllium has much higher levels of soluble fiber, approximately 20% insoluble to 80% soluble fiber. Therefore, psyllium tends to gel more easily when mixed with water than flax fiber. This is effective to relieve constipation because psyllium increases water in the stool; making is softer and easier to pass. When taking psyllium, make sure you drink enough water otherwise it can cause constipation or worsen existing constipation and cause swelling in the throat and choking. There is less of a chance of flax causing constipation.
Flax contains antioxidant-rich lignans, which are naturally occurring chemical compounds found in many plants. Lignans have been extensively studied for possible prevention of cancer, especially breast and colon cancer. Flax actually contains 100 times more lignans than any other source. Psyllium does not contain any lignans.
Psyllium contains a lot more fiber per tablespoon than flax. Psyllium provides 12 grams of fiber per tablespoon; compared to 2 grams of fiber in flax. Flaxseeds are also rich in Omega-3 fatty acids, magnesium, and potassium and provide a good source of the B vitamins, protein, and zinc. They are low in saturated fats and calories, and contain no cholesterol.