Yams are a good source of vitamin C. It provides a good amount of fiber, potassium, manganese, and metabolic B vitamins. The content of vitamin B6 (pyridoxine; 16% of the daily value) is good for shrinking the effects of homocysteine, which can do real damage to cell walls that could lead to heart attacks and/or stroke. 
The crunchy edible root is chiefly composed of complex carbohydrates and soluble dietary fiber. 