Do fiber supplements have carbohydrates in them?

I'm starting the Atkins diet and for the first week you can't have any carbs so you can get to ketosis or whatever. I'm scared I might get constipated so i was gonna take Metamucil but it says it has like 8 carbs or something i cant remember, but it says like 4 of the carbs are like insoluble fiber and the other are something else but there arent any sugars or anything in it. is it okay to use it or will it mess up the diet? if i jest drink a lot of water will i not get constipated?