Why are whole grains better for you than refined grains?

Refund grains have had fiber and nutrients processed out leaving only a few nutritious parts behind

Whole grains contain the entire grain kernel the bran, germ, and endosperm. Examples of whole grains are oatmeal, brown rice, bulgur, whole wheat flour, etc. 

But on the other hand refined grains have been milled, a process that removes the bran and germ. This is done to give grains a finer texture and improve their shelf life, but it also removes dietary fiber, iron, and many B vitamins. Some examples of refined grain products are white flour,
de-germed cornmeal, white bread, and white rice.

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