Shihan Dan Hayes of the Island Budokan brought the following article to my attention.  Apparently diet soft-drinks are not the healthier alternative that they are made out to be.

They found adults who drink one or more sodas a day — diet or regular — had about a 50 percent higher risk of metabolic syndrome — a cluster of risk factors such as excessive fat around the waist, low levels of “good” cholesterol, high blood pressure and other symptoms.

Reading into the article, it may be that diet soft drinks in themselves are not the problem.  Instead it may be that those who drink them are likelier than the average person to have a diet high in saturated fats, trans fats and a lifestyle lower in exercise. 

Another theory is that the colouring used to give softdrinks the caramel look may promote insulin resistance, thereby leading to metabolic syndrome.

So its not certain that soft drinks are a direct contributor, but there is certainly a statistical correlation.  Its important to note that the American Beverage Association, the lobby group for the soft drink industry, refutes the idea.

I am a big believer that we need to be drinking a lot of water, and that hydration is an important aspect of our health.  Green tea is the only real drink that is a good substitute for water, and with its antioxidants is something that should be factored into every diet.  For me, I have all but weaned myself off softdrinks over the past year, and allow myself a single diet soft drink per week, and make an exception if I need a caffeine hit.  But for each can of soft drink I consume, I add an additional 500ml of water or green tea.

No calories, same taste (and heart risks) – Health – TODAYshow.com.