I am no expert but basically there are two types. Type 2 is to do with insulin resistance. It can generally be managed with diet. Type 1 is more to do with direct damage to the pancreas for various reasons. And then there is so called pre-diabetes further back on the spectrum.
Nutritional supplements which help include chromium, cinnamon, purified fish oil, ALA oils (these fatty acids help with cell function) and bitter melon. There are many cases of diabetes being healed in 30 days through diet and exercise.
This is but one example of what can be done, here.