White grits are hominy grits--grits made from grinding hominy which is corn treated with lye to make it puff up--then ground. Michael has the explanation correct. Yellow "grits" is just cornmeal, and once again, Michael has it right--basically polenta.
A cousin runs the last family owned mill in SC which makes all sorts of flour and meal products--supplies the grits to the finer restaurants throughout the South--and even Charlie Trotter in Chicago.
There is nothing inherently wrong with carbohydrates.
For further info--
Hominy grits, or just plain grits, are an institution here in the South, though they can be hard to find in northern states. Hominy is made from flint or dent corn,varieties with hard kernels that are dried on the cob then removed and soaked in a solution of baking soda, lime, or wood ash. This process causes the hulls to soften and swell. The kernels are then hulled and degermed using friction, then dried. Grits, coarse whitish grains, are ground from hominy, as is masa harina, the flour used to make corn tortillas. If you really want to start from scratch, Mountain Laurel has instructions
for the whole process, including making the lye solution with wood ashes.
It's interesting that the alkaline soaking process also unbinds necessary niacin in the corn, and has an effect on the protein balance.
Though the overall available protein is decreased, the relative availability of the lysine and tryptophan are increased. The alkaline process has been used for centuries where corn was a native food, but in areas where corn was introduced as a new staple, the process was not. Pellagra, a niacin and tryptophan deficiency, became common disease in areas where corn was the main source of food, as in the early South. One has to wonder how ancient civilizations discovered the process which made corn a more balanced source of nutrition.
The word grits comes from the Old English. "grytt", for "bran", but the Old English "greot" also meant something ground. Some cookbooks refer to grits as hominy because of regional preference for the name. Americans have been using the term "grits" since at least the end of the 18th century.