It is harmful but just like the drug companies they are more concerned about making their millions than if some people have bad effects from it. It turns to formaldahyde in your system. I know lots of people who insist on drinking diet soda and using their Splenda and other sweeteners and they have very poor health... shakes, poor memory, vision problems, migraines, etc. that worsened more after they gave up the sugar for aspartame.
http://www.wnho.net/aspartame_causes_brain_tumors.htm
http://www.mercola.com/article/aspartame/dangers.htm
Aspartame: Drs Olney and Roberts on brain cancer
This might also help you as a diabetic
Xylitol sweetener : by Ray Sahelian, M.D. Xylitol product side effects
Stevia is a good alternative. Agave nectar and some Asian sweetener that I can't recall the name are suppost to be the best alternatives.
Also there are a few things that are good for diabetics but you have to monitor your insolin more and keep up with doctor check ups if you try these since they can normalize your sugar. These are what I remember
1 Teaspoon of cinnamon in the morning
Cherry juice
Chromium picolinate
Prickly pear
Prickly Pear Cactus Medicine (Treatments for Diabetes, Cholesterol, and the Immune System)
Omega, flax oil, olive oil
NEXUS: The Diabetes Deception
Avoid transfats, margarine, cooking oils,