Buckwheat (Fagopyrum esculentum) blooming. Buckwheat is not a cereal grain but is a seed related to rhubarb and sorrel. It does not contain gluten and is a suitable substitute for cereals. Buckwheat's beneficial effects are due in part to its rich supply of flavonoids, particularly rutin. Flavonoids are phytonutrients that protect against disease by extending the action of vitamin C and acting as antioxidants. Buckwheat's lipid-lowering activity is largely due to rutin and other flavonoid compounds.