In this case you should follow the rules from your physicianlabo . If they clearly ask not to drink or eat anything before you take their test , it ' s better to do it like that . In general , glucose finger prick in the morning fast is done before breakfast . Test for HbA 1 c doesn ' t require you fasting . In my experience 25 years with type 1 coffee has no impact on glucose levels . Black coffee doesn ' t increase blood sugar . If you doubt about the nutrional value of something you drink , take a look at the label . I guess it looks like this are gonna be interested in carbohydrates , which also includes sugars . Carbohydrates are the elements in food and drinks that increase your blood sugar . F . i . Coca Cola Light 0 g carbohydrates so it won ' t increase your sugar . F . i . Chocovit 12 g per 100 ml . 11 g sugar 1 g CH fibers so it will increase your sugar . Probably diet soda won ' t increase your blood sugar .