How to Defat
Defatting is to remove as much of the fat from a sauce as possible. You can do this by using a large spoon to skim fat from the top of a hot sauce, by pouring the sauce into a fat separator or by refrigerating or freezing the sauce and then lifting off the congealed fat from the top of the cold sauce before reheating.
Always defat your sauce before thickening as you can end up with an emulsion of fat and sauce. You can control the amount of fat you remove from the sauce. There is no law that says you have to remove all or most of the fat from your sauce. It is up to you.
You should keep in mind not only dietary restrictions but also the body of the sauce. Too much fat can leave you with a greasy mouthfeel while too little can sometimes leave you with a very thin sauce without much depth.