The National Institutes of Health (NIH) National Center for Complimentary and Alternative Medicine (NCCAM) reviewed the safety and effectiveness of 6 dietary supplements commonly used by people with diabetes.
You can view their report here:
Research Report: Treating Type 2 Diabetes with Dietary Supplements
Its publication date is June 2005, so it may not reflect the most recent findings for these supplements. Nonetheless, it provides a good overview of each supplement along with a summary of research findings.
The 6 supplements reviewed were:
Omega-3 Fatty Acids
The agency's recommendations, in short:
"The evidence that is available is not sufficiently strong to prove that any of the six supplements discussed in this report have benefits for type 2 diabetes or its complications."
"The six dietary supplements appear to be generally safe at low-to-moderate doses. However, each can interact with various prescription medications, affecting the action of the medications."
"Consult your health care provider before starting a supplement, especially if you are pregnant or nursing, or considering giving a supplement to a child."