Most of the fiber in whole grain oats comes from the oat bran. Oat bran provides several health benefits to the cardiovascular and digestive systems, and helps to prevent weight gain. The soluble fiber in oat bran also helps keep blood sugar in check.
Before processing, whole oat grains consist of a lipid-rich germ surrounded by a starchy endosperm. The germ and endosperm are encased in the bran, which is high in fiber.
Raw oat bran retains this high fiber, and is a good source of zinc, magnesium, phosphorus, vitamin B1, vitamin E and potassium. According to the USDA Nutrient Database, one cup of raw oat bran has 231 calories, about 16 grams of protein, 6.6 grams of fat and 14.5 grams of fiber.