A cup of joe...

My name is Ja and I'm an addict. Do I have any intention of signing up for a twelve step programme in the near future? Not on your life. My daily vice is the rather innocuous drug of the masses; a freshly brewed cup of strong coffee. So aside from the minor dent in my personal finances created by regular doses of over-priced brew, it hasn't had an overly adverse effect on my day to day life.
As a T1 diabetic however there are some considerations to take into account when it comes to the cup of joe. First off, as a carb counter I generally stick to straight forward Americano or espresso. The carb content in a black espresso is nil and the added carbs of a dash of milk in my espresso is negligible. Of course I do partake in the odd cappuccino or even a mocca now and again, but they're always the more considered options as they generally necessitate a bolus. A cappuccino would usually be accompanied by one unit of insulin; the chocolate in a mocca might require a bit more.
So that's as far as if goes for me in terms of coffee and bg levels. However, there seems to be a broader range of experiences amongst both diabetics and those involved in the treatment and management of diabetes and there is a degree of debate out there around the effects of coffee on blood-sugar levels.
While my first concern is the influence of coffee on bg levels among those like myself who have already been diagnosed with T1 diabetes, it would be remiss not to point out that various studies concur that regular consumption of coffee can lead to a lower risk of Type 2 diabetes developing.
When it comes to T1 results of studies around the influence of coffee are less conclusive. That consumption of my daily tipple can prevent diabetes is not the case when it comes to Type 1, an auto-immune condition that thus far has not been shown to be preventable. But went comes to the bg levels of those with T1, the effects of coffee is a subject of debate. There seems to be a small degree of evidence to support the view that coffee can cause a rise in bg levels, even in the absence of added milk/cream/sugar (http://www.diabetesselfmanagement.com/blog/the-mystery-of-coffee-and-diabetes). While my own personal experience doesn't back this view I guess it is worth occasional monitoring.
Perhaps more worthy of consideration in the long term is the degree to which regular consumption of coffee can lead to reduced sensitivity to insulin. While this is also the subject of debate, there seems to be a degree of evidence to suggest that regular consumption can, over time, lead to reduced sensitivity (http://care.diabetesjournals.org/content/25/2/399.full). My own limited knowledge of the research tells me that the studies have been somewhat inconclusive and certainly won't have me rushing to give up my daily fix just yet.
See also
http://teeccino.com/images/uploads/pages/File/diabetes.pdf

http://www.mayoclinic.org/diseases-conditions/type-2-diabetes/expert-answers/blood-sugar/faq-20057941